Regional trends in extreme events in the IPCC 2021 report

This summary is based on the Intergovernmental Panel on Climate Change (IPCC) Working Group 1 contribution to the 6th Assessment Report (AR6): “Climate Change 2021: The Physical Science Basis.” Around 1/3 of the report is dedicated to regional climate information, with an assessment of observed and projected changes in climatic impact-drivers which are physical climate system conditions (e.g., means, events, extremes) that affect an element of society or ecosystems. This is the physical science contribution to the assessment of climate-related risks, without anticipating whether their impacts provide potential opportunities or are detrimental (i.e., as for hazards). More detailed information on extreme events in a changing climate is available in the full report, especially in Chapter 11 (Weather and climate extreme events in a changing climate), Chapter 9 (Ocean, cryosphere and sea level change), Chapter 12 (Climate change information for regional impact and for risk assessment), and the Technical Summary as well as in the AR6 online interactive atlas. Syntheses of regional changes are also available in 2-page summaries for large regions (regional fact sheets).

Human-induced climate change is already affecting many weather and climate extremes in every region across the globe

During the last decade, the increase in global surface temperature has reached around 1.1 °C above 1850–1900 level. This observed warming is the best estimate of human caused warming. It is now unequivocal that human influence has warmed the climate system.

The evidence for observed changes and attribution to human influence has strengthened for many types of extreme events since the previous IPCC assessment was published in 2013 (AR5). This is particularly the case for heatwaves, extreme precipitation events, droughts, tropical cyclones, marine heatwaves, extreme sea levels, and compound extremes (Table 1).

Table 1. Summary table on observed changes in extremes, their attribution since 1950 (except where stated otherwise), and projected changes at +1.5 °C, +2 °C and +4 °C of global warming, on global and continental scales. (Source: AR6 WGI TS, Table TS.2)

Understanding about past and future changes in weather and climate extreme events has increased due to better observation-based datasets, physical understanding of processes, a greater proportion of scientific literature combining different lines of evidence, and improved accessibility to different types of climate models. New techniques and analyses drawing on several lines of evidence have provided heightened confidence when attributing changes in regional extreme events to human influence.

Figure 1. Changes in climate result in changes in the magnitude and probability of extremes, illustrated here for hot events (Source: AR6 WG1, Chapter 11, FAQ11.3)

In particular, event attribution is now an important line of evidence for assessing changes in extremes on regional scales. The attribution of extreme events has emerged as an important field for climate research with a growing body of literature. It provides evidence that greenhouse gases and other external forcings have affected individual extreme events by disentangling anthropogenic drivers from natural variability. The regional extreme events that have been studied are geographically uneven. A few events, for example, extreme rainfall events in the United Kingdom, heatwaves in Australia, or Hurricane Harvey that hit Texas in 2017, have been heavily studied. While many highly impactful extreme events have not been studied, particularly in the developing world where event attribution studies are generally lacking for various reasons, including the lack of observational data, of reliable climate models and of scientific capacity. Though the events that have been studied are not representative and results may also be subject to selection bias, the large number of such studies provide evidence that changes in the properties of these local and individual events are in line with expected consequences of human influence on the climate and can be attributed to external drivers.

At the global scale, hot extremes (including heatwaves) (Figure 1) have become more frequent and more intense across most land regions since the 1950s, while cold extremes (including cold waves) have become less frequent and less severe, with high confidence that human-induced climate change is the main driver of these changes (Figure 2).

Some recent hot extremes observed over the past decade would have been extremely unlikely to occur without human influence on the climate system. While cities intensify human-induced warming locally, no-till farming, irrigation and crop expansion have attenuated increases in summer hot extremes in some regions, such as central North America (medium confidence).

Marine heatwaves have approximately doubled in frequency since the 1980s, and since at least 2006 human influence has very likely contributed to most the increasing frequency.

The frequency and intensity of heavy precipitation events have increased since the 1950s over most land areas for which observational data are sufficient for trend analysis (in particular, North America, Europe and Asia), and human-induced climate change is likely the main driver (Figure 2).

Figure 2. Climate change is already affecting every inhabited region across the globe, with human influence contributing to many observed changes in weather and climate extremes (Source: AR6 WGI SPM, Figure SPM.3)

Figure 3. Projected changes in extremes are larger in frequency and intensity with every additional increment of global warming (Source: AR6 WGI SPM, Figure SPM.6)

Human-induced climate change has contributed to increases in agricultural and ecological droughts in drying regions due to increased land evapotranspiration (Figure 3).

It is likely that the global proportion of major (Category 3–5) tropical cyclone occurrence has increased over the last four decades, and that the latitude where tropical cyclones in the western North Pacific reach their peak intensity has shifted northward. These changes cannot be explained by internal variability alone. Confidence in long-term (multi-decadal to centennial) trends on the frequency of all-category tropical cyclones is low. Event attribution studies and physical understanding indicate that human-induced climate change increases the heavy precipitation associated with tropical cyclones (high confidence) but data limitations inhibit clear detection of past trends on the global scale.

Human influence has likely increased the chance of compound extreme events since the 1950s. This includes increases in the frequency of concurrent heatwaves and droughts on the global scale (high confidence); fire weather in some regions (e.g. southern Europe, northern Eurasia, the USA, Australia); and compound flooding in some locations, including the US coastlines (medium confidence).

Global mean sea level rose around 0.20 metres from 1901 to 2018, and the rate of rising has accelerated since the late 1960s. Regional sea-level change has been the main driver of changes in extreme still water levels across the quasi-global tide gauge network over the twentieth century. High tide flooding events that occurred five times per year during the period 1960–1980 occurred on average more than eight times per year during the period 1995–2014 (high confidence) (Figure 7).

Projected changes in extremes are larger in frequency and intensity with every increment of global warming. Every region will increasingly experience concurrent and multiple changes.

Future emissions cause future additional warming. A level of 1.5 °C of global warming (averaged over 20 years) relative to 1850–1900 is expected to be reached in the near term (2021–2040). If greenhouse gas emissions stay close to current levels for a few more decades, or increase, a level of 2 °C of global warming would be crossed in the mid-term (2041–2060), but reaching such a level of warming could be avoided with rapid and strong reductions in emissions of CO₂, methane and other greenhouse gases.

The frequency of extreme temperature and precipitation events in the current climate will change with every increment of warming, with warm extremes becoming more frequent (virtually certain), cold extremes becoming less frequent (extremely likely) and precipitation extremes becoming more frequent in most locations (very likely) (Table 1, Figures 3, 4, 5, 6). The projected increase in heavy precipitation extremes translates to an increase in the frequency and magnitude of pluvial floods.

Figure 4. Illustration of the spatial patterns of changes in the warmest three-month season temperature and annual mean precipitation, and extreme temperature and precipitation (simulations for 4°C global warming by 2100) (Source: AR6 WGI Chapter 11, FAQ 11.1)

Figure 5. Summary schematic of past and projected changes in tropical cyclone, extratropical cyclone, atmospheric river, and severe convective storm behaviour. (Source: AR6 WGI Chapter 11, Figure 11.20)

Figure 6. Projections of regional changes of the heat warning index and extreme rainfall for different levels of global warming (GWL). The heat warning index is the number of days per year averaged across each region at which a heat warning for human health at level “danger” would be issued according to NOAA. The maps of extreme rainfall changes show the percentage change in the amount of rain falling on the wettest day of a year (Rx1day, relative to 1995–2014) averaged across each region when the respective GWL is reached. (Source: AR6 WGI TS, Figure TS.6)

Figure 7. Observed and projected changes in marine heatwave probability ratio (Source: AR6 WGI Chapter 9, Box 9.2, Figure 1)

Some mid-latitude and semi-arid regions, as well as the South American Monsoon region, are projected to see the highest increase in the temperature of the hottest days, at about 1.5 to 2 times the rate of global warming (high confidence). The Arctic is projected to experience the highest increase in the temperature of the coldest days, at about 3 times the rate of global warming (high confidence).

With additional global warming, the frequency of marine heatwaves will continue to rise (high confidence), particularly in the tropical ocean and the Arctic (medium confidence) (Figure 7).

A warmer climate increases moisture transport into weather systems, which intensifies wet seasons and events (high confidence). Increases in near-surface atmospheric moisture capacity of about 7% per 1 °C of warming lead to a similar response in the intensification of heavy precipitation from sub-daily up to seasonal time scales, increasing the severity of flood hazards (high confidence) (Figure 3). The average and maximum rain-rates associated with tropical and extratropical cyclones, atmospheric rivers and severe convective storms will therefore also increase with future warming (high confidence) (Figure 5). The interannual variability of precipitation and runoff over land is projected to increase at a faster rate than changes in seasonal mean precipitation all year round in the tropics and in the summer season elsewhere (medium confidence). Sub-seasonal precipitation variability is also projected to increase, with fewer rainy days but increased daily mean precipitation intensity over many land regions (high confidence). The proportion of intense tropical cyclones (categories 4-5) and peak wind speeds of the most intense tropical cyclones are projected to increase at the global scale with increasing global warming (high confidence) (Table 1).

Table 2. Observed and projected changes in low likelihood, high impact extreme conditions (Source: AR6 WGI Chapter 11, Box 11.2, Table 1)

Every additional 0.5 °C of global warming also causes clearly discernible increases in the intensity and frequency of agricultural and ecological droughts in some regions (high confidence) (Figure 3). Discernible changes in the intensity and frequency of meteorological droughts, with more regions showing increases than decreases, are seen in some regions for every additional 0.5 °C of global warming (medium confidence). Increases in frequency and intensity of hydrological droughts become larger with increasing global warming in some regions (medium confidence). Rainfall variability related to the El Niño/Southern Oscillation is projected to be amplified by the second half of the twenty-first century for intermediate or high greenhouse gas emissions, and global warming above 2 °C.

There will be an increasing occurrence of some extreme events unprecedented in the observational record with additional global warming, even at 1.5 °C of global warming. Projected percentage changes in frequency are higher for rarer events (high confidence) (Table 2). Many regions are projected to experience an increase in the probability of compound events with higher global warming (high confidence). In particular, concurrent heatwaves and droughts are likely to become more frequent. Concurrent extremes at multiple locations become more frequent, including in crop producing areas, at 2 °C and above compared to 1.5 °C global warming (high confidence). Some compound extreme events with low likelihood in past and current climate will become more frequent, and there will be a higher likelihood that events with increased intensities, durations and/or spatial extents unprecedented in the observational record will occur. The probability of compound flooding (storm surge, extreme rainfall and/or river flow) will continue to increase due to both sea-level rise and increases in heavy precipitation, including changes in precipitation intensity associated with tropical cyclones (high confidence) (Figure 8).

Figure 8. Observed change in extreme still water level. Defined as the 99th percentile of daily observed water levels over 1995-2014. (a) Percent change in occurrences over 1995-2014 relative to those over 1960-1980. (b-g) Annual mean sea level (blue) and annual occurrences of extreme still water levels over the 1995-2014 99th percentile daily maximum (yellow) at six selected tide gauge locations. (Source: AR6 WGI Chapter 9, Figure 9.31)

It is virtually certain that global mean sea level will continue to rise through 2100 and for centuries thereafter, and will remain elevated for thousands of years. The likely range of sea-level rise above 1995–2014 is from 0.15 to 0.30 m by 2050. Sea-level rise by 2100 strongly depends on future emissions, reaching around 0.40 m for very low emissions (global warming close to 1.5 °C), or around 0.8 m for very high emissions (global warming above 4 °C), and 1 m more if ice-sheet instability processes associated with deep uncertainty are triggered.

It is very likely to virtually certain that regional mean relative sea-level rise will continue throughout the twenty-first century, except in a few regions with substantial geologic land uplift rates. Approximately two-thirds of the global coastline has a projected regional relative sea-level rise within ±20% of the global mean increase (medium confidence), with projection data available at https://sealevel. nasa.gov/ipcc-ar6-sea-level-projection-tool. Due to relative sea-level rise, extreme sea-level events that occurred once per century in the recent past are projected to occur 20 to 30 times more frequently by 2050 and 160 to 530 times more frequently by 2100, and at least annually at 20-30% of tide gauge locations by 2050, and at 60 to 80% of all tide gauge locations by 2100 (high confidence). Relative sea-level rise contributes to increases in the frequency and severity of coastal flooding in low-lying areas and to coastal erosion along most sandy coasts (high confidence).

Further urbanization together with more frequent hot extremes will increase the severity of heatwaves (very high confidence). Urbanization also increases mean and heavy precipitation over and/or downwind of cities (medium confidence) and resulting runoff intensity (high confidence). In coastal cities, the combination of more frequent extreme sea-level events (due to sea level rise and storm surge) and extreme rainfall/riverflow events will make flooding more probable (high confidence).

Based on paleoclimate and historical evidence, it is likely that at least one large explosive volcanic eruption would occur during the twenty-first century. Such an eruption would temporarily affect many climatic impact-drivers (medium confidence). Such natural drivers and internal variability will modulate human-caused changes, especially at regional scales and in the near term, with little effect on centennial global warming. These modulations are important to consider in planning for the full range of possible changes.

Changes in extremes will be more widespread and pronounced for higher warming levels

At 1.5 °C global warming, heavy precipitation and associated flooding are projected to intensify and be more frequent in most regions in Africa and Asia (high confidence), North America (medium to high confidence) and Europe (medium confidence). Also, more frequent and/or severe agricultural and ecological droughts are projected in a few regions in all continents except Asia compared to 1850–1900 (medium confidence); increases in meteorological droughts are also projected in a few regions (medium confidence).

At 2 °C global warming, the level of confidence in and the magnitude of change in droughts and heavy and mean precipitation increase compared to those at 1.5 °C. Heavy precipitation and associated flooding events are projected to become more intense and frequent in the Pacific Islands and across many regions of North America and Europe (medium to high confidence). These changes are also seen in some regions in Australasia and Central and South America (medium confidence). Several regions in Africa, South America and Europe are projected to experience an increase in frequency and/or severity of agricultural and ecological droughts with medium to high confidence; increases are also projected in Australasia, Central and North America, and the Caribbean with medium confidence. A small number of regions in Africa, Australasia, Europe and North America are also projected to be affected by increases in hydrological droughts, and several regions are projected to be affected by increases or decreases in meteorological droughts with more regions displaying an increase (medium confidence). Region-specific changes include intensification of tropical cyclones and/or extratropical storms (medium confidence), increases in river floods (medium to high confidence), and increases in fire weather (medium to high confidence). There is low confidence in most regions in potential future changes in hail, ice storms, severe storms, dust storms, heavy snowfall, and landslides.

Figure 9. Projected change in the recurrence of extreme still water level (amplification factor) as a function of the emission scenario (very high, intermediate or low) by 2050 and 2100. (Source: AR6 WGI, chapter 9, Figure 9.32)

In the future, unprecedented extremes will occur as the climate continues to warm. Those extremes will be more intense – of greater magnitude – and will occur more frequently 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 (Table 2, Figure 9).

In the case of low or very low greenhouse gas emissions, compared to the case of intermediate, high or very high greenhouse gas emissions in the coming decades, changes in climatic impact-drivers would be substantially smaller beyond 2040. By the end of the century, the increase in the frequency of extreme sea-level events, heavy precipitation and pluvial flooding, and exceedances of extreme heat thresholds dangerous for agriculture and health (see Figure 5) and the number of regions where such exceedances occur would be more limited.

This summary highlights the current state of knowledge on specific extreme events, and the importance of preparing for such changes, through the distillation of regional climate information. It is a co-production between scientists, practitioners and users to support adaptation and risk management decisions.

References

IPCC, 2021: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press. In Press.