Best Climate Change FAQs Answered in 2022

FAQ: https://www.ipcc.ch/report/ar6/wg1/resources/frequently-asked-questions/

Report: UN IPCC AR6 WG1 Report’s Summary for Policymakers

The evidence for climate change rests on more than just increasing surface temperatures. A broad range of indicators collectively leads to the inescapable conclusion that we are witnessing rapid changes to many aspects of our global climate. We are seeing changes in the atmosphere, ocean, cryosphere, and biosphere. Our scientific understanding depicts a coherent picture of a warming world.

We have long observed our changing climate. From the earliest scientists taking meteorological observations in the 16th and 17th centuries to the present, we have seen a revolution in our ability to observe and diagnose our changing climate. Today we can observe diverse aspects of our climate system from space, from aircraft and weather balloons, using a range of ground-based observing technologies, and using instruments that can measure to great depths in the ocean.

Observed changes in key indicators point to warming over land areas. Global surface temperature over land has increased since the late 19th century, and changes are apparent in a variety of societally relevant temperature extremes. Since the mid-1950s the troposphere (i.e., the lowest few km of the atmosphere) has warmed, and precipitation over land has increased. Near-surface specific humidity (i.e., water vapour) over land has increased since at least the 1970s. Aspects of atmospheric circulation have also evolved since the mid-20th century, including a poleward shift of mid-latitude storm tracks.

Changes in the global ocean point to warming as well. Global average sea surface temperature has increased since the late 19th century. The heat content of the global ocean has increased since the 19th century, with more than 90% of the excess energy accumulated in the climate system being stored in the ocean. This ocean warming has caused ocean waters to expand, which has contributed to the increase in global sea level in the past century. The relative acidity of the ocean has also increased since the early 20th century, caused by the uptake of carbon dioxide from the atmosphere, and oxygen loss is evident in the upper ocean since the 1970s.

Significant changes are also evident over the cryosphere – the portion of the Earth where water is seasonally or continuously frozen as snow or ice. There have been decreases in Arctic sea ice area and thickness and changes in Antarctic sea ice extent since the mid-1970s. Spring snow cover in the Northern Hemisphere has decreased since the late-1970s, along with an observed warming and thawing of permafrost (perennially frozen ground). The Greenland and Antarctic ice sheets are shrinking, as are the vast majority of glaciers worldwide, contributing strongly to the observed sea level rise.

Many aspects of the biosphere are also changing. Over the last century, long-term ecological surveys show that many land species have generally moved poleward and to higher elevations. There have been increases in green leaf area and/or mass (i.e., global greenness) since the early 1980s, and the length of the growing season has increased over much of the extratropical Northern Hemisphere since at least the mid-20th century. There is also strong evidence that various phenological metrics (such as the timing of fish migrations) for many marine species have changed in the last half century.

Change is apparent across many components of the climate system. It has been observed using a very broad range of techniques and analysed independently by numerous groups around the world. The changes are consistent in pointing to a climate system that has undergone rapid warming since the industrial revolution.

The dominant role of humans in driving recent climate change is clear. This conclusion is based on a synthesis of information from multiple lines of evidence, including direct observations of recent changes in Earth’s climate; analyses of tree rings, ice cores, and other long-term records documenting how the climate has changed in the past; and computer simulations based on the fundamental physics that governs the climate system.

Climate is influenced by a range of factors. There are two main natural drivers of variations in climate on time scales of decades to centuries. The first is variations in the sun’s activity, which alter the amount of incoming energy from the sun. The second is large volcanic eruptions, which increase the number of small particles (aerosols) in the upper atmosphere that reflect sunlight and cool the surface–an effect that can last for several years (see also FAQ 3.2). The main human drivers of climate change are increases in the atmospheric concentrations of greenhouse gases and of aerosols from burning fossil fuels, land use and other sources. The greenhouse gases trap infrared radiation near the surface, warming the climate. Aerosols, like those produced naturally by volcanoes, on average cool the climate by increasing the reflection of sunlight. Multiple lines of evidence demonstrate that human drivers are the main cause of recent climate change.

The current rates of increase of the concentration of the major greenhouse gases (carbon dioxide, methane and nitrous oxide) are unprecedented over at least the last 800,000 years. Several lines of evidence clearly show that these increases are the results of human activities. The basic physics underlying the warming effect of greenhouse gases on the climate has been understood for more than a century, and our current understanding has been used to develop the latest generation climate models (see FAQ 3.3). Like weather forecasting models, climate models represent the state of the atmosphere on a grid and simulate its evolution over time based on physical principles. They include a representation of the ocean, sea ice and the main processes important in driving climate and climate change.

Results consistently show that such climate models can only reproduce the observed warming (black line in FAQ 3.1, Figure 1) when including the effects of human activities (grey band in FAQ 3.1, Figure 1), in particular the increasing concentrations of greenhouse gases. These climate models show a dominant warming effect of greenhouse gas increases (red band, which shows the warming effects of greenhouse gases by themselves), which has been partly offset by the cooling effect of increases in atmospheric aerosols (blue band). By contrast, simulations that include only natural processes, including internal variability related to El Niño and other similar variations, as well as variations in the activity of the sun and emissions from large volcanoes (green band in FAQ 3.1, Figure 1), are not able to reproduce the observed warming. The fact that simulations including only natural processes show much smaller temperature increases indicates that natural processes alone cannot explain the strong rate of warming observed. The observed rate can only be reproduced when human influence is added to the simulations.

Moreover, the dominant effect of human activities is apparent not only in the warming of global surface temperature, but also in the pattern of warming in the lower atmosphere and cooling in the stratosphere, warming of the ocean, melting of sea ice, and many other observed changes. An additional line of evidence for the role of humans in driving climate change comes from comparing the rate of warming observed over recent decades with that which occurred prior to human influence on climate. Evidence from tree rings and other paleoclimate records shows that the rate of increase of global surface temperature observed over the past fifty years exceeded that which occurred in any previous 50-year period over the past 2000 years (see FAQ 2.1).

Taken together, this evidence shows that humans are the dominant cause of observed global warming over recent decades.

The effects of substantial reductions in carbon dioxide emissions would not be apparent immediately, and the time required to detect the effects would depend on the scale and pace of emissions reductions. Under the lower-emissions scenarios considered in this Report, the increase in atmospheric carbon dioxide concentrations would slow visibly after about five to ten years, while the slowing down of global surface warming would be detectable after about twenty to thirty years. The effects on regional precipitation trends would only become apparent after several decades.

Reducing emissions of carbon dioxide (CO2) – the most important greenhouse gas emitted by human activities – would slow down the rate of increase in atmospheric CO2 concentration. However, concentrations would only begin to decrease when net emissions approach zero, that is, when most or all of the CO2 emitted into the atmosphere each year is removed by natural and human processes (see FAQ 5.1 and FAQ 5.3). This delay between a peak in emissions and a decrease in concentration is a manifestation of the very long lifetime of CO2 in the atmosphere; part of the CO2 emitted by humans remains in the atmosphere for centuries to millennia.

Reducing the rate of increase in CO2 concentration would slow down global surface warming within a decade. But this reduction in the rate of warming would initially be masked by natural climate variability and might not be detected for a few decades (see FAQ 1.2, FAQ 3.2 and FAQ 4.1). Detecting whether surface warming has indeed slowed down would thus be difficult in the years right after emissions reductions begin.

The time needed to detect the effect of emissions reductions is illustrated by comparing low- and high-emissions scenarios (FAQ 4.2, Figure 1). In the low-emissions scenario (SSP1‑2.6), CO2 emissions level off after 2015 and begin to fall in 2020, while they keep increasing throughout the 21st century in the high-emissions scenario (SSP3‑7.0). The uncertainty arising from natural internal variability in the climate system is represented by simulating each scenario ten times with the same climate model but starting from slightly different initial states back in 1850 (thin lines). For each scenario, the differences between individual simulations are caused entirely by simulated natural internal variability. The average of all simulations represents the climate response expected for a given scenario. The climate history that would actually unfold under each scenario would consist of this expected response combined with the contribution from natural internal variability and the contribution from potential future volcanic eruptions (the latter effect is not represented here).

FAQ 4.2, Figure 1 shows that the atmospheric CO2 concentrations differ noticeably between the two scenarios about five to ten years after the emissions have begun to diverge in year 2015. In contrast, the difference in global surface temperatures between the two scenarios does not become apparent until later – about two to three decades after the emissions histories have begun to diverge in this example. This time would be longer if emissions were reduced more slowly than in the low-emissions scenario illustrated here and shorter in the case of stronger reductions. Detection would take longer for regional quantities and for precipitation changes, which vary more strongly from natural causes. For instance, even in the low-emissions scenario, the effect of reduced CO2 emissions would not become visible in regional precipitation until late in the 21st century.

In summary, it is only after a few decades of reducing CO2 emissions that we would clearly see global temperatures starting to stabilize. By contrast, short-term reductions in CO2 emissions, such as during the COVID-19 pandemic, do not have detectable effects on either CO2 concentration or global temperature. Only sustained emissions reductions over decades would have a widespread effect across the climate system.

Deliberate removal of carbon dioxide (CO2) from the atmosphere could reverse (i.e., change the direction of) some aspects of climate change. However, this will only happen if it results in a net reduction in the total amount of CO2 in the atmosphere, that is, if deliberate removals are larger than emissions. Some climate change trends, such as the increase in global surface temperature, would start to reverse within a few years. Other aspects of climate change would take decades (e.g., permafrost thawing) or centuries (e.g., acidification of the deep ocean) to reverse, and some, such as sea level rise, would take centuries to millennia to change direction.

The term negative carbon dioxide (CO2) emissions refers to the removal of CO2 from the atmosphere by deliberate human activities, in addition to removals that occur naturally, and is often used as synonymous with carbon dioxide removal. Negative CO2 emissions can compensate for the release of CO2 into the atmosphere by human activities. They could be achieved by strengthening natural CO2 sequestration processes on land (e.g., by planting trees or through agricultural practices that increase the carbon content of soils) and/or in the ocean (e.g., by restoration of coastal ecosystems) or by removing CO2 directly from the atmosphere. If CO2 removals are greater than human-caused CO2 emissions globally, emissions are said to be net negative. It should be noted that CO2 removal technologies are unable, or not yet ready, to achieve the scale of removal that would be required to compensate for current levels of emissions, and most have undesired side effects.

In the absence of deliberate CO2 removal, the CO2 concentration in the atmosphere (a measure of the amount of CO2 in the atmosphere) results from a balance between human-caused CO2 release and the removal of CO2 by natural processes on land and in the ocean (natural ‘carbon sinks’; see FAQ 5.1). If CO2 release exceeds removal by carbon sinks, the CO2 concentration in the atmosphere would increase if CO2 release equals removal, the atmospheric CO2 concentration would stabilize; and if CO2 removal exceeds release, the CO2 concentration would decline. This applies in the same way to net CO2 emissions – that is, the sum of human-caused releases and deliberate removals.

If the CO2 concentration in the atmosphere starts to go down, the Earth’s climate would respond to this change. Some parts of the climate system take time to react to a change in CO2 concentration, so a decline in atmospheric CO2 as a result of net negative emissions would not lead to immediate reversal of all climate change trends. Recent studies have shown that global surface temperature starts to decline within a few years following a decline in atmospheric CO2, although the decline would not be detectable for decades due to natural climate variability (see FAQ 4.2). Other consequences of human-induced climate change, such as reduction in permafrost area, would take decades; yet others, such as warming, acidification and oxygen loss of the deep ocean, would take centuries to reverse following a decline in the atmospheric CO2 concentration. Sea level would continue to rise for many centuries to millennia, even if large deliberate CO2 removals were successfully implemented.

‘Overshoot’ scenarios are a class of future scenarios that are receiving increasing attention, particularly in the context of ambitious climate goals, such as the global warming limits of 1.5°C or 2°C included in the Paris Agreement. In these scenarios, a slow rate of reduction in emissions in the near term is compensated by net negative CO2 emissions in the later part of this century, which results in a temporary breach or ‘overshoot’ of a given warming level. Due to the delayed reaction of several climate system components, it follows that the temporary overshoot would result in additional climate changes compared to a scenario that reaches the goal without overshoot. These changes would take decades to many centuries to reverse, with the reversal taking longer for scenarios with larger overshoot.

Removing more CO2 from the atmosphere than is emitted into it would indeed begin to reverse some aspects of climate change, but some changes would still continue in their current direction for decades to millennia. Approaches capable of large-scale removal of CO2 are still in the state of research and development or unproven at the scales of deployment necessary to achieve a net reduction in atmospheric CO2 levels. CO2 removal approaches, particularly those deployed on land, can have undesired side effects on water, food production and biodiversity.

There are several types of carbon budgets. Most often, the term refers to the total net amount of carbon dioxide (CO2) that can still be emitted by human activities while limiting global warming to a specified level (e.g., 1.5°C or 2°C above pre-industrial levels). This is referred to as the ‘remaining carbon budget’. Several choices and value judgements have to be made before it can be unambiguously estimated. When the remaining carbon budget is combined with all past CO2 emissions to date, a ‘total carbon budget’ compatible with a specific global warming limit can also be defined. A third type of carbon budget is the ‘historical carbon budget’, which is a scientific way to describe all past and present sources and sinks of CO2.

The term remaining carbon budget is used to describe the total net amount of CO2 that human activities can still release into the atmosphere while keeping global warming to a specified level, like 1.5°C or 2°C relative to pre-industrial temperatures. Emissions of CO2 from human activities are the main cause of global warming. A remaining carbon budget can be defined because of the specific way CO2 behaves in the Earth system. That is, global warming is roughly linearly proportional to the total net amount of CO2 emissions that are released into the atmosphere by human activities – also referred to as cumulative anthropogenic CO2 emissions. Other greenhouse gases behave differently and have to be accounted for separately.

The concept of a remaining carbon budget implies that, to stabilize global warming at any particular level, global emissions of CO2 need to be reduced to net zero levels at some point. ‘Net zero CO2 emissions’ describes a situation where all the anthropogenic emissions of CO2 are counterbalanced by deliberate anthropogenic removals so that, on average, no CO2 is added or removed from the atmosphere by human activities. Atmospheric CO2 concentrations in such a situation would gradually decline to a long-term stable level as excess CO2 in the atmosphere is taken up by ocean and land sinks (see FAQ 5.1). The concept of a remaining carbon budget also means that, if CO2 emissions reductions are delayed, deeper and faster reductions are needed later to stay within the same budget. If the remaining carbon budget is exceeded, this will result in either higher global warming or a need to actively remove CO2 from the atmosphere to reduce global temperatures back down to the desired level (see FAQ 5.3).

Estimating the size of remaining carbon budgets depends on a set of choices. These choices include: (1) the global warming level that is chosen as a limit (for example, 1.5°C or 2°C relative to pre-industrial levels); (2) the probability with which we want to ensure that warming is held below that limit (for example, a one-in-two, two-in-three, or higher chance), and (3) how successful we are in limiting emissions of other greenhouse gases that affect the climate, such as methane or nitrous oxide. These choices can be informed by science, but ultimately represent subjective choices. Once these choices have been made, to estimate the remaining carbon budget for a given temperature goal, we can combine knowledge about: how much our planet has warmed already; the amount of warming per cumulative tonne of CO2; and the amount of warming that is still expected once global net CO2 emissions are brought down to zero. For example, to limit global warming to 1.5°C above pre-industrial levels with either a one-in-two (50%) or two-in-three (67%) chance, the remaining carbon budgets amount to 500 and 400 billion tonnes of CO2, respectively, from 1 January 2020 onward. Currently, human activities are emitting around 40 billion tonnes of CO2 into the atmosphere in a single year.

The remaining carbon budget depends on how much the world has already warmed to date. This past warming is caused by historical emissions, which are estimated by looking at the historical carbon budget – a scientific way to describe all past and present sources and sinks of CO2. It describes how the CO2 emissions from human activities have redistributed across the various CO2 reservoirs of the Earth system. These reservoirs are the ocean, the land vegetation, and the atmosphere (into which CO2 was emitted). The share of CO2 that is not taken up by the ocean or the land, and that thus increases the concentration of CO2 in the atmosphere, causes global warming. The historical carbon budget tells us that, of the about 2560 billion tonnes of CO2 that were released into the atmosphere by human activities between the years 1750 and 2019, about a quarter were absorbed by the ocean (causing ocean acidification) and about a third by the land vegetation. About 45% of these emissions remain in the atmosphere (see FAQ 5.1). Adding these historical CO2 emissions to estimates of remaining carbon budgets allows an estimate of the total carbon budget consistent with a specific global warming level.

In summary, determining a remaining carbon budget – that is, how much CO2 can be released into the atmosphere while stabilizing global temperature below a chosen level – is well understood but relies on a set of choices. However, it is clear that, for limiting warming below 1.5°C or 2°C, the remaining carbon budget from 2020 onwards is much smaller than the total CO2 emissions released to date.

A warmer climate increases the amount and intensity of rainfall during wet events, and this is expected to amplify the severity of flooding. However, the link between rainfall and flooding is complex, so while the most severe flooding events are expected to worsen, floods could become rarer in some regions.

Floods are a natural and important part of the water cycle but they can also threaten lives and safety, disrupt human activities, and damage infrastructure. Most inland floods occur when rivers overtop their banks (fluvial flooding) or when intense rainfall causes water to build up and overflow locally (pluvial flooding). Flooding is also caused by coastal inundation by the sea, rapid seasonal melting of snow, and the accumulation of debris, such as vegetation or ice, that stops water from draining away.

Climate change is already altering the location, frequency and severity of flooding. Close to the coasts, rising sea levels increasingly cause more frequent and severe coastal flooding, and the severity of these floods is exacerbated when combined with heavy rainfall. The heavy and sustained rainfall events responsible for most inland flooding are becoming more intense in many areas as the climate warms because air near Earth’s surface can carry around 7% more water in its gas phase (vapour) for each 1°C of warming. This extra moisture is drawn into weather systems, fueling heavier rainfall.

A warming climate also affects wind patterns, how storms form and evolve, and the pathway those storms usually travel. Warming also increases condensation rates, which in turn releases extra heat that can energize storm systems and further intensify rainfall. On the other hand, this energy release can also inhibit the uplift required for cloud development, while increases in particle pollution can delay rainfall but invigorate storms. These changes mean that the character of precipitation events (how often, how long lasting and how heavy they are) will continue to change as the climate warms.

In addition to climate change, the location, frequency and timing of the heaviest rainfall events and worst flooding depend on natural fluctuations in wind patterns that make some regions unusually wet or dry for months, years, or even decades. These natural variations make it difficult to determine whether heavy rainfall events are changing locally as a result of global warming. However, when natural weather patterns bring heavy and prolonged rainfall in a warmer climate, the intensity is increased by the larger amount of moisture in the air.

An increased intensity and frequency of record-breaking daily rainfall has been detected for much of the land surface where good observational records exist, and this can only be explained by human-caused increases in atmospheric greenhouse gas concentrations. Heavy rainfall is also projected to become more intense in the future for most places. So, where unusually wet weather events or seasons occur, the rainfall amounts are expected to be greater in the future, contributing to more severe flooding.

However, heavier rainfall does not always lead to greater flooding. This is because flooding also depends upon the type of river basin, the surface landscape, the extent and duration of the rainfall, and how wet the ground is before the rainfall event. Some regions will experience a drying in the soil as the climate warms, particularly in subtropical climates, which could make floods from a rainfall event less probable because the ground can potentially soak up more of the rain. On the other hand, less frequent but more intense downpours can lead to dry, hard ground that is less able to soak up heavy rainfall when it does occur, resulting in more runoff into lakes, rivers and hollows. Earlier spring snowmelt combined with more precipitation falling as rain rather than snow can trigger flood events in cold regions. Reduced winter snow cover can, in contrast, decrease the chance of flooding arising from the combination of rainfall and rapid snowmelt. Rapid melting of glaciers and snow in a warming climate is already increasing river flow in some regions, but as the volumes of ice diminish, flows will peak and then decline in the future. Flooding is also affected by changes in the management of the land and river systems. For example, clearing forests for agriculture or building cities can make rainwater flow more rapidly into rivers or low-lying areas. On the other hand, increased extraction of water from rivers can reduce water levels and the likelihood of flooding.

A mix of both increases and decreases in flooding have been observed in some regions and these changes have been attributed to multiple causes, including changes in snowmelt, soil moisture and rainfall. Although we know that a warming climate will intensify rainfall events, local and regional trends are expected to vary in both direction and magnitude as global warming results in multiple, and sometimes counteracting, influences. However, even accounting for the many factors that generate flooding, when weather patterns cause flood events in a warmer future, these floods will be more severe.

Droughts usually begin as a deficit of precipitation, but then propagate to other parts of the water cycle (soils, rivers, snow/ice and water reservoirs). They are also influenced by factors like temperature, vegetation and human land and water management. In a warmer world, evaporation increases, which can make even wet regions more susceptible to drought.

A drought is broadly defined as drier than normal conditions; that is, a moisture deficit relative to the average water availability at a given location and season. Since they are locally defined, a drought in a wet place will not have the same amount of water deficit as a drought in a dry region. Droughts are divided into different categories based on where in the water cycle the moisture deficit occurs: meteorological drought (precipitation), hydrological drought (runoff, streamflow, and reservoir storage), and agricultural or ecological drought (plant stress from a combination of evaporation and low soil moisture). Special categories of drought also exist. For example, a snow drought occurs when winter snowpack levels are below average, which can cause abnormally low streamflow in subsequent seasons. And while many drought events develop slowly over months or years, some events, called flash droughts, can intensify over the course of days or weeks. One such event occurred in 2012 in the Midwestern region of North America and had a severe impact on agricultural production, with losses exceeding $30 billion US dollars. Droughts typically only become a concern when they adversely affect people (reducing water available for municipal, industrial, agricultural, or navigational needs) and/or ecosystems (adverse effects on natural flora and fauna). When a drought lasts for a very long time (more than two decades) it is sometimes called a megadrought.

Most droughts begin when precipitation is below normal for an extended period of time (meteorological drought). This typically occurs when high pressure in the atmosphere sets up over a region, reducing cloud formation and precipitation over that area and deflecting away storms. The lack of rainfall then propagates across the water cycle to create agricultural drought in soils and hydrological drought in waterways. Other processes act to amplify or alleviate droughts. For example, if temperatures are abnormally high, evaporation increases, drying out soils and streams and stressing plants beyond what would have occurred from the lack of precipitation alone. Vegetation can play a critical role because it modulates many important hydrologic processes (soil water, evapotranspiration, runoff). Human activities can also determine how severe a drought is. For example, irrigating croplands can reduce the socio-economic impact of a drought; at the same time, depletion of groundwater in aquifers can make a drought worse.

The effect of climate change on drought varies across regions. In the subtropical regions like the Mediterranean, southern Africa, south-western Australia and south-western South America, as well as tropical Central America, western Africa and the Amazon basin, precipitation is expected to decline as the world warms, increasing the possibility that drought will occur throughout the year (FAQ 8.3, Figure 1). Warming will decrease snowpack, amplifying drought in regions where snowmelt is an important water resource (such as in south-western South America). Higher temperatures lead to increased evaporation, resulting in soil drying, increased plant stress, and impacts on agriculture, even in regions where large changes in precipitation are not expected (such as central and northern Europe). If emissions of greenhouse gases are not curtailed, about a third of global land areas are projected to suffer from at least moderate drought by 2100. On the other hand, some areas and seasons (such as high-latitude regions in North America and Asia, and the South Asian monsoon region) may experience increases in precipitation as a result of climate change, which will decrease the likelihood of droughts. FAQ 8.3, Figure 1 highlights the regions where climate change is expected to increase the severity of droughts.

Evidence from the distant past shows that some parts of the Earth system might take hundreds to thousands of years to fully adjust to changes in climate. This means that some of the consequences of human-induced climate change will continue for a very long time, even if atmospheric heat-trapping gas levels and global temperatures are stabilized or reduced in the future. This is especially true for the Greenland and Antarctic ice sheets, which grow much more slowly than they retreat. If the current melting of these ice sheets continues for long enough, it becomes effectively irreversible on human time scales, as does the sea level rise caused by that melting.

Humans are changing the climate and there are mechanisms that amplify the warming in the polar regions (Arctic and Antarctic). The Arctic is already warming faster than anywhere else (see FAQ 4.3). This is significant because these colder high latitudes are home to our two remaining ice sheets: Antarctica and Greenland. Ice sheets are huge reservoirs of frozen freshwater, built up by tens of thousands of years of snowfall. If they were to completely melt, the water released would raise global sea level by about 65 m. Understanding how these ice sheets are affected by warming of nearby ocean and atmosphere is therefore critically important. The Greenland and Antarctic ice sheets are already slowly responding to recent changes in climate, but it takes a long time for these huge masses of ice to adjust to changes in global temperature. That means that the full effects of a warming climate may take hundreds or thousands of years to play out. An important question is whether these changes can eventually be reversed, once levels of greenhouse gases in the atmosphere are stabilized or reduced by humans and natural processes. Records from the past can help us answer this question.

For at least the last 800,000 years, the Earth has followed cycles of gradual cooling followed by rapid warming caused by natural processes. During cooling phases, more and more ocean water is gradually deposited as snowfall, causing ice sheets to grow and sea level to slowly decrease. During warming phases, the ice sheets melt more quickly, resulting in more rapid rises in sea level (FAQ 9.1, Figure 1). Ice sheets build up very slowly because growth relies on the steady accumulation of falling snow that eventually compacts into ice. As the climate cools, areas that can accumulate snow expand, reflecting back more sunlight that otherwise would keep the Earth warmer. This means that, once started, glacial climates develop rapidly. However, as the climate cools, the amount of moisture that the air can hold tends to decrease. As a result, even though glaciations begin quite quickly, it takes tens of thousands of years for ice sheets to grow to a point where they are in balance with the colder climate.

Ice sheets retreat more quickly than they grow because of processes that, once triggered, drive self-reinforcing ice loss. For ice sheets that are mostly resting on bedrock above sea level – like the Greenland Ice Sheet – the main self-reinforcing loop that affects them is the ‘elevation–mass balance feedback’ (FAQ 9.1, Figure 1, right). In this situation, the altitude of the ice-sheet surface decreases as it melts, exposing the sheet to warmer air. The lowered surface then melts even more, lowering it faster still, until eventually the whole ice sheet disappears. In places where the ice sheet rests instead on bedrock that is below sea level, and which also deepens inland, including many parts of the Antarctic Ice Sheet, an important process called ‘marine ice sheet instability’ is thought to drive rapid retreat (FAQ 9.1, Figure 1, left). This happens when the part of the ice sheet that is surrounded by sea water melts. That leads to additional thinning, which in turn accelerates the motion of the glaciers that feed into these areas. As the ice sheet flows more quickly into the ocean, more melting takes place, leading to more thinning and even faster flow that brings ever-more glacier ice into the ocean, ultimately driving rapid deglaciation of whole ice-sheet drainage basins.

These (and other) self-reinforcing processes explain why relatively small increases in temperature in the past led to very substantial sea level rise over centuries to millennia, compared to the many tens of thousands of years it takes to grow the ice sheets that lowered the sea level in the first place. These insights from the past imply that, if human-induced changes to the Greenland and Antarctic ice sheets continue for the rest of this century, it will take thousands of years to reverse that melting, even if global air temperatures decrease within this or the next century. In this sense, these changes are therefore irreversible, since the ice sheets would take much longer to regrow than the decades or centuries for which modern society is able to plan.

As of 2018, global average sea level was about 15–25 cm higher than in 1900, and 7–15 cm higher than in 1971. Sea level will continue to rise by an additional 10–25 cm by 2050. The major reasons for this ongoing rise in sea level are the thermal expansion of seawater as its temperature increases, and the melting of glaciers and ice sheets. Local sea level changes can be larger or smaller than the global average, with the smallest changes in formerly glaciated areas, and the largest changes in low-lying river delta regions.

Across the globe, sea level is rising, and the rate of increase has accelerated. Sea level increased by about 4 mm per year from 2006 to 2018, which was more than double the average rate over the 20th century. Rise during the early 1900s was due to natural factors, such as glaciers catching up to warming that occurred in the Northern Hemisphere during the 1800s. However, since at least 1970, human activities have been the dominant cause of global average sea level rise, and they will continue to be for centuries into the future.

Sea level rises either through warming of ocean waters or the addition of water from melting ice and bodies of water on land. Expansion due to warming caused about 50% of the rise observed from 1971 to 2018. Melting glaciers contributed about 22% over the same period. Melting of the two large ice sheets in Greenland and Antarctica has contributed about 13% and 7%, respectively, during 1971 to 2018, but melting has accelerated in the recent decades, increasing their contribution to 22% and 14% since 2016. Another source is changes in land-water storage: reservoirs and aquifers on land have reduced, which contributed about an 8% increase in sea level.

By 2050, sea level is expected to rise an additional 10–25 cm whether or not greenhouse gas emissions are reduced. Beyond 2050, the amount by which sea level will rise is more uncertain. The accumulated total emissions of greenhouse gases over the upcoming decades will play a big role beyond 2050, especially in determining where sea level rise and ice-sheet changes eventually level off.

Even if net zero emissions are reached, sea level rise will continue because the deep ocean will continue to warm and ice sheets will take time to catch up to the warming caused by past and present emissions: ocean and ice sheets are slow to respond to environmental changes (see FAQ 5.3). Some projections under low emissions show sea level rise continuing as net zero is approached at a rate comparable to today (3–8 mm per year by 2100 versus 3–4 mm per year in 2015), while others show substantial acceleration to more than five times the present rate by 2100, especially if emissions continue to be high and processes that accelerate retreat of the Antarctic Ice Sheet occur widely (FAQ 9.1).

Sea level rise will increase the frequency and severity of extreme sea level events at coasts (see FAQ 8.2), such as storm surges, wave inundation and tidal floods: risk can be increased by even small changes in global average sea level. Scientists project that, in some regions, extreme sea level events that were recently expected once in 100 years will occur annually at 20–25% of locations by 2050 regardless of emissions, but by 2100 emissions choice will matter: annually at 60% of locations for low emissions, and at 80% of locations under strong emissions.

In many places, local sea level change will be larger or smaller than the global average. From year to year and place to place, changes in ocean circulation and wind can lead to local sea level change. In regions where large ice sheets, such as the Fennoscandian in Eurasia and the Laurentide and Cordilleran in North America, covered the land during the last ice age, the land is still slowly rising up now that the extra weight of the ice sheets is gone. This local recovery is compensating for global sea level rise in these regions and can even lead to local decrease in sea level. In regions just beyond where the former ice sheets reached and the Earth bulged upwards, the land is now falling and, as a result, local sea level rise is faster than the global rate. In many regions within low-lying delta regions (such as New Orleans and the Ganges–Brahmaputra delta), the land is rapidly subsiding (sinking) because of human activities such as building dams or groundwater and fossil fuel extraction. Further, when an ice sheet melts, it has less gravitational pull on the ocean water nearby. This reduction in gravitational attraction causes sea level to fall close to the (now less-massive) ice sheet while causing sea level to rise farther away. Melt from a polar ice sheet therefore raises sea level most in the opposite hemisphere or in low latitudes – amounting to tens of centimetres difference in rise between regions by 2100.

The world is physically and culturally diverse, and the challenges posed by climate change vary by region and location. Because climate change affects so many aspects of people’s daily work and living, climate change information can help with decision-making, but only when the information is relevant for the people involved in making those decisions. Users of climate information may be highly diverse, ranging from professionals in areas such as human health, agriculture or water management to a broader community that experiences the impacts of changing climate. Providing information that supports response actions thus requires engaging all relevant stakeholders, their knowledge and their experiences, formulating appropriate information, and developing a mutual understanding of the usefulness and limitations of the information.

The development, delivery, and use of climate change information requires engaging all parties involved: those producing the climate data and related knowledge, those communicating it, and those who combine that information with their knowledge of the community, region or activity that climate change may impact. To be successful, these parties need to work together to explore the climate data and thus co-develop the climate information needed to make decisions or solve problems, distilling output from the various sources of climate knowledge into relevant climate information. Effective partnerships recognize and respond to the diversity of all parties involved (including their values, beliefs and interests), especially when they involve culturally diverse communities and their indigenous and local knowledge of weather, climate and their society. This is particularly true for climate change – a global issue posing challenges that vary by region. By recognizing this diversity, climate information can be relevant and credible, most notably when conveying the complexity of risks for human systems and ecosystems and for building resilience.

Constructing useful climate information requires considering all available sources in order to capture the fullest possible representation of projected changes and distilling the information in a way that meets the needs of the stakeholders and communities impacted by the changes. For example, climate scientists can provide information on future changes by using simulations of global and/or regional climate and inferring changes in the weather behaviour influencing a region. An effective distillation process (FAQ 10.1, Figure 1) engages with the intended recipients of the information, especially stakeholders whose work involves non-climatic factors, such as human health, agriculture or water resources. The distillation evaluates the accuracy of all information sources (observations, simulations, expert judgement), weighs the credibility of possible conflicting information, and arrives at climate information that includes estimating the confidence a user should have in it. Producers of climate data should further recognize that the geographic regions and time periods governing stakeholders’ interest (for example, the growing season of an agricultural zone) may not align well with the time and space resolution of available climate data; thus additional model development or data processing may be required to extract useful climate information.

One way to distil complex information for stakeholder applications is to connect this information to experiences stakeholders have already had through storylines as plausible unfoldings of weather and climate events related to stakeholders’ experiences. Dialogue between stakeholders and climate scientists can determine the most relevant experiences to evaluate for possible future behaviour. The development of storylines uses the experience and expertise of stakeholders, such as water-resource managers and health professionals, who seek to develop appropriate response measures. Storylines are thus a pathway through the distillation process that can make climate information more accessible and physically comprehensible. For example, a storyline may take a common experience like an extended drought, with depleted water availability and damaged crops, and show how droughts may change in the future, perhaps with even greater precipitation deficits or longer duration. With appropriate choices, storylines can engage nuances of the climate information in a meaningful way by building on common experiences, thus enhancing the information’s usefulness.

Forging partnerships among all involved with producing, exploring and distilling climate data into climate information is at the centre of creating stakeholder-relevant information. These partnerships can occur through direct interaction between climate scientists and stakeholders as well as through organizations that have emerged to facilitate this process, such as climate services, national and regional climate forums, and consulting firms providing specialized climate information. These so-called ‘boundary organizations’ can serve the varied needs of all who would fold climate information into their decision processes. All of these partnerships are vital for arriving at climate information that responds to physical and cultural diversity and to challenges posed by climate change that can vary region-by-region around the world.

Urban areas experience air temperatures that can be several degrees Celsius warmer than surrounding areas, especially during the night. This ‘urban heat island’ effect results from several factors, including reduced ventilation and heat trapping due to the close proximity of tall buildings, heat generated directly from human activities, the heat-absorbing properties of concrete and other urban building materials, and the limited amount of vegetation. Continuing urbanization and increasingly severe heatwaves under climate change will further amplify this effect in the future.

Today, cities are home to 55% of the world’s population. This number is increasing, and every year cities welcome 67 million new residents, 90% of whom are moving to cities in developing countries. By 2030, almost 60% of the world’s population is expected to live in urban areas. Cities and their inhabitants are highly vulnerable to weather and climate extremes, particularly heatwaves, because urban areas already are local hotspots. Cities are generally warmer – up to several degrees Celsius at night – than their surroundings. This warming effect, called the urban heat island, occurs because cities both receive and retain more heat than the surrounding countryside areas and because natural cooling processes are weakened in cities compared to rural areas.

Three main factors contribute to amplify the warming of urban areas (orange bars in FAQ 10.2, Figure 1). The strongest contribution comes from urban geometry, which depends on the number of buildings, their size and their proximity. Tall buildings close to each other absorb and store heat and also reduce natural ventilation. Human activities, which are very concentrated in cities, also directly warm the atmosphere locally, due to heat released from domestic and industrial heating or cooling systems, running engines, and other sources. Finally, urban warming also results directly from the heat-retaining properties of the materials that make up cities, including concrete buildings, asphalt roadways, and dark rooftops. These materials are very good at absorbing and retaining heat, and then re-emitting that heat at night.

The urban heat island effect is further amplified in cities that lack vegetation and water bodies, both of which can strongly contribute to local cooling (green bars in FAQ 10.2, Figure 1). This means that when enough vegetation and water are included in the urban fabric, they can counterbalance the urban heat island effect, to the point of even cancelling out the urban heat island effect in some neighbourhoods.

The urban heat island phenomenon is well-known and understood. For instance, temperature measurements from thermometers located in cities are corrected for this effect when global warming trends are calculated. Nevertheless, observations, including long-term measurements of the urban heat island effect are currently too limited to allow a full understanding of how the urban heat island varies across the world and across different types of cities and climatic zones, or how this effect will evolve in the future.

As a result, it is hard to assess how climate change will affect the urban heat island effect, and various studies disagree. Two things are, however, very clear. First, future urbanization will expand the urban heat island areas, thereby amplifying future warming in many places all over the world. In some places, the nighttime warming from the urban heat island effect could even be on the same order of magnitude as the warming expected from human-induced climate change. Second, more intense, longer and more frequent heatwaves caused by climate change will more strongly impact cities and their inhabitants, because the extra warming from the urban heat island effect will exacerbate the impacts of climate change.

In summary, cities are currently local hotspots because their structure, material and activities trap and release heat and reduce natural cooling processes. In the future, climate change will, on average, have a limited effect on the magnitude of the urban heat island itself, but ongoing urbanization together with more frequent, longer and warmer heatwaves will make cities more exposed to global warming.

Climate change has already increased the magnitude and frequency of extreme hot events and decreased the magnitude and frequency of extreme cold events, and, in some regions, intensified extreme precipitation events. As the climate moves away from its past and current states, we will experience extreme events that are unprecedented, either in magnitude, frequency, timing or location. The frequency of these unprecedented extreme events will rise with increasing global warming. Additionally, the combined occurrence of multiple unprecedented extremes may result in large and unprecedented impacts.

Human-induced climate change has already affected many aspects of the climate system. In addition to the increase in global surface temperature, many types of weather and climate extremes have changed. In most regions, the frequency and intensity of hot extremes have increased and those of cold extremes have decreased. The frequency and intensity of heavy precipitation events have increased at the global scale and over a majority of land regions. Although extreme events such as land and marine heatwaves, heavy precipitation, drought, tropical cyclones, and associated wildfires and coastal flooding have occurred in the past and will continue to occur in the future, they often come with different magnitudes or frequencies in a warmer world. For example, future heatwaves will last longer and have higher temperatures, and future extreme precipitation events will be more intense in several regions. Certain extremes, such as extreme cold, will be less intense and less frequent with increasing warming.

Unprecedented extremes – that is, events not experienced in the past – will occur in the future in five different ways. First, events that are considered to be extreme in the current climate will occur in the future with unprecedented magnitudes. Second, future extreme events will also occur with unprecedented frequency. Third, certain types of extremes may occur in regions that have not previously encountered those types of events. For example, as the sea level rises, coastal flooding may occur in new locations, and wildfires are already occurring in areas, such as parts of the Arctic, where the probability of such events was previously low. Fourth, extreme events may also be unprecedented in their timing. For example, extremely hot temperatures may occur either earlier or later in the year than they have in the past.

Finally, compound events – where multiple extreme events of either different or similar types occur simultaneously and/or in succession – may be more probable or severe in the future. These compound events can often impact ecosystems and societies more strongly than when such events occur in isolation. For example, a drought along with extreme heat will increase the risk of wildfires and agriculture damages or losses. As individual extreme events become more severe as a result of climate change, the combined occurrence of these events will create unprecedented compound events. This could exacerbate the intensity and associated impacts of these extreme events.

Unprecedented extremes have already occurred in recent years, relative to the 20th century climate. Some recent extreme hot events would have had very little chance of occurring without human influence on the climate (see FAQ 11.3). In the future, unprecedented extremes will occur as the climate continues to warm. Those extremes will happen with larger magnitudes and at higher frequencies than previously experienced. Extreme events may also appear in new locations, at new times of the year, or as unprecedented compound events. Moreover, unprecedented events will become more frequent with higher levels of warming, for example at 3°C of global warming compared to 2°C of global warming.

FAQ: https://www.ipcc.ch/report/ar6/wg2/about/frequently-asked-questions

Report: UN IPCC AR6 WG2 Report’s Summary for Policymakers

Climate change impacts are increasingly being felt in all regions of the world with growing challenges for water availability, food production and the livelihoods of millions of people. We also know that impacts will continue to increase if drastic cuts in greenhouse gas emissions are further delayed – affecting the lives of today’s children tomorrow and those of their children much more than ours. But science is also clear: with immediate action now, drastic impacts can still be prevented.

The scientific assessment in the WGII Report addresses the near-term (up to 2040), mid-term (2041-2060) and the long-term (2081-2100). Today, the latter two milestones may seem far away, but children who were born in 2020 will be 20 years old in 2040 and 80 years old in 2100. The end of the century is less than a lifetime away. Actions taken now to reduce emissions of heat-trapping greenhouse gases drastically and adapt to a changing climate will have a profound effect on the quality of their lives and their children’s lives, as well as their health, well-being, and security. We also have to take into account that by 2050 almost 70% of the world’s growing population will live in urban areas, many in unplanned or informal settlements. As a result, today’s children and future generations are more likely to be exposed and vulnerable to climate change and related risks such as flooding, heat stress, water scarcity, poverty, and hunger. Children are amongst those suffering the most, as we see today.

But what is our children’s future going to look like, if we do not limit global warming to well below 2°C, preferably to 1.5°C, compared to pre-industrial temperature? Based on the Working Group II assessment, we know that global warming is already changing much of the world as we know it. Its impacts will intensify in the coming decades with profound implications for all aspects of human life around the world. Our food and water supplies, our cities, infrastructure and economies as well as our health and well-being will be affected.

For example: children aged ten or younger in the year 2020 are projected to experience a nearly four-fold increase in extreme events under 1.5°C of global warming by 2100, and a five-fold increase under 3°C warming. Such increases in exposure would not be experienced by a person aged 55 in the year 2020 in their remaining lifetime under any warming scenario.

Globally, the percentage of the population exposed to deadly heat stress is projected to increase from today's 30% to 48-76% by the end of the century, depending on future warming levels and location. If the world warms more than 4°C by 2100, the number of days with climatically stressful conditions for outdoor workers will increase by up to 250 workdays per year by century’s end in some parts of South Asia, tropical sub-Saharan Africa and parts of Central and South America. This would cause negative consequences such as reduced food production and higher food prices. In Europe, the number of people at risk of heat stress will increase two- to three-fold at 3°C global warming compared to warming levels of 1.5°C.

With ongoing global warming, today’s children in South and Southeast Asia will witness increased losses in coastal settlements and infrastructure due to flooding caused by unavoidable sea level rise, with very high losses in East Asian cities. By mid-century, more than a billion people living in low-lying coastal cities and settlements globally are projected to be at risk from coastal-specific climate hazards. Many of those will be forced to move to higher ground, which will increase competition for land and the probability of conflict and forced relocation.

Climate change will impact water quality and availability for hygiene, food production and ecosystems due to floods and droughts. Globally, 800 million to 3 billion people are projected to experience chronic water scarcity due to droughts at 2°C warming, and up to approximately 4 billion at 4°C warming, considering the effects of climate change alone, with present-day population. Children growing up in South America will face an increasing number of days with water scarcity and restricted water access, especially those living in cities and in rural areas depending on water from glaciers. As the Andean glaciers and snowcaps continue to melt, the amount of available water decreases as the glaciers shrink or disappear entirely. Countries in Central America will experience more frequent and stronger storms or hurricanes and heavy rainfall, causing river flooding.

Today’s young people and future generations will also witness stronger negative effects of climate change on food production and availability. The warmer it gets, the more difficult it will become to grow or produce, transport, distribute, buy, and store food – a trend that is projected to hit poor populations the hardest. Depending on future policies and climate and adaptation actions taken, the number of people suffering from hunger in 2050 will range from 8 million to up to 80 million people, with most severely affected populations concentrated in Sub-Saharan Africa, South Asia and Central America. Under a high vulnerability-high warming scenario, up to 183 million additional people are projected to become undernourished in low-income countries due to climate change by 2050.

Africa is the continent with the world’s youngest population (40% of the population are under 15 years old). Here, climate change will significantly increase the number of children with severe stunting (impaired growth and development which often leads to limited physical and cognitive potential), by approximately 1.4 million by 2050 under 2.1°C of warming due to malnutrition. The lack of food and under-nutrition are strongly linked with hot climates in the sub-Saharan area and less rainfall in West and Central Africa. Climate change can undermine children’s educational attainment, thus reducing their chances for well-paid jobs or higher incomes later in life.

The concerning news is: all these projected impacts will not only reduce the prospects of sustainable development, but our Working Group II Report also projects an increase in poverty and inequality as well as increased involuntary migration of people due to climate change. These responses follow expected climate-driven increases in the frequency and strength of regional wildfires, increased floods and droughts, and an increase in temperature-related incidences of vector-borne, water-borne and food-borne diseases such as dengue, malaria, cholera and Rift Valley Fever.

In addition, we now know that multiple climate hazards will occur simultaneously more often in the future. They may reinforce each other and result in increased impacts and risks to nature and people that are more complex and more difficult to manage. For example, reductions in crop yields due to heat and drought, made worse by reduced productivity because of heat stress among farmworkers, will increase food prices, reduce household incomes and lead to health risks from malnutrition, as well as climate-related deaths, especially in tropical regions.

But there is also positive news: all these risks can be reduced substantially by taking urgent action to limit global warming and by strengthening our adaptation efforts – for example by protecting and conserving nature, and by improving planning and management of our cities (for details see FAQ 5). The youth movement, together with many non-governmental organizations, has led to a rising wave of public global awareness of climate change and its life-threatening impacts. To successfully secure our own future and the future of the coming generations, climate risks must be factored into each decision and planning. We have the knowledge and the tools. Now it is our choice to make.

Worldwide action to achieve a climate resilient, sustainable world is more urgent than previously thought. But what can be done? Our report highlights a solutions framework that we call Climate Resilient Development. It combines strategies to adapt to climate change with actions to reduce greenhouse gas emissions to support sustainable development for everyone. Action to implement this concept has to start now because making progress is already challenging at current global warming levels. If temperatures exceed 2°C of warming, climate resilient development will become impossible in some regions of the world.

We know that climate change presents risks to nature, people and infrastructure around the world. These risks will increase with every small increase in warming, and reducing them is made more complicated by other global trends such as over-consumption, population growth, rapid urbanization, land degradation, biodiversity loss, poverty and inequity, etc. In short: the world is facing a long list of complex and interacting challenges that need to be dealt with simultaneously.

Both the urgency and the complexity of the climate change crisis require actions at a new depth and scale. Our report provides a solutions framework that successfully combines strategies to deal with climate risks (adaptation) with actions to reduce greenhouse gas emissions (mitigation) which result in improvements for nature’s and people’s well-being – for example by reducing poverty and hunger, improving health and livelihoods, providing more people with clean energy and water and safeguarding ecosystems on land, in lakes and rivers and in the ocean. This solutions framework is called Climate Resilient Development.

In developing countries and in areas that are particularly exposed to climate change (e.g. in coastal areas, small islands, deserts, mountains and polar regions) climate impacts and risks can exacerbate vulnerability and injustices which can undermine efforts to achieve sustainable development, particularly for marginalized communities. But each country will follow its own path. Importantly, each country has different capacities and opportunities for Climate Resilient Development. Nevertheless, our report clearly shows that rapidly scaled-up, well-aligned investment facilitates Climate Resilient Development and that it advances more quickly with increased international cooperation and financial assistance.

Striving for Climate Resilient Development means reducing exposure and vulnerability to climate hazards, cutting back greenhouse gas emissions and conserving biodiversity are given the highest priorities in everyday decision-making and policies on all aspects of society including energy, industry, health, water, food, urban development, housing and transport. It is about successfully navigating the complex interactions between these different systems so that action in one area does not have adverse effects elsewhere and opportunities are harnessed to accelerate progress towards a safer, fairer world.

Climate Resilient Development isn’t achieved with a single decision or action. It’s the result of all of the choices we make about climate risk reduction, emissions reductions and sustainable development on a daily basis. New scientific evidence shows that addressing the risks and impacts of climate change successfully involves a more diverse set of actors than previously thought – it not just policy-makers but everyone in government, civil society and the private sector. For example, if we consider changes in agriculture, it takes a combination of effective government policy and regulation as well as informed daily decisions by farmers, traders and agricultural companies to lead to fundamental change which is required to adapt to a changing climate, reduce greenhouse gas emissions and secure lives and livelihoods not just of those directly involved but for wider society as well.

In brief: Climate Resilient Development involves everyone. The prospects for effective action improve when governments at all levels work with citizens, civil society, educational bodies and scientific institutions, the media, investors and businesses and form partnerships with traditionally marginalised groups, including women, youth, Indigenous Peoples, local communities and ethnic minorities. In such a societal setting, scientific, Indigenous and local knowledge and practical knowhow can come together to provide more relevant effective actions. In addition, different interests, values and worldviews can be reconciled if everyone works together.

Targeting a climate resilient, sustainable world involves fundamental changes to how society functions, including changes to underlying values, worldviews, ideologies, social structures, political and economic systems, and power relationships. This may feel overwhelming at first, but the world is changing anyway and will continue to change so Climate Resilient Development offers us ways to drive change to improve well-being for all – by reducing climate risk, tackling the many inequities and injustices experienced today, and rebuilding our relationship with nature.

The choices we make in the next decade will determine our future. Our report clearly states Climate Resilient Development is already challenging at a warming level of less than 1.5°C, and will become more limited by 2°C. In some regions, it will be impossible if the temperature exceeds 2°C, including low-lying coastal cities, settlements and small islands, some mountain areas and polar regions.

This key finding underlines the urgency for climate action and that focusing on equity and justice as well as on adequate funding, political commitment and partnerships lead to more effective climate change adaptation and emissions reductions.

The scientific evidence is unequivocal: climate change is a threat to human well-being and the health of the planet. Any further delay in concerted global action will miss a brief and rapidly closing window to secure a liveable future.