It is an established fact that human-induced greenhouse gas emissions have led to an increased frequency and/or intensity of some weather and climate extremes since pre-industrial time, in particular for temperature extremes. Evidence of observed changes in extremes and their attribution to human influence (including greenhouse gas and aerosol emissions and land-use changes) has strengthened since AR5, in particular for extreme precipitation, droughts, tropical cyclones and compound extremes (including dry/hot events and fire weather). Some recent hot extreme events would have been extremely unlikely to occur without human influence on the climate system.
Regional changes in the intensity and frequency of climate extremes generally scale with global warming. New evidence strengthens the conclusion from SR1.5 that even relatively small incremental increases in global warming (+0.5°C) cause statistically significant changes in extremes on the global scale and for large regions. In particular, this is the case for temperature extremes (very likely), the intensification of heavy precipitation including that associated with tropical cyclones, and the worsening of droughts in some regions. The occurrence of extreme events unprecedented in the observed record will increase with increasing global warming, even at 1.5°C of global warming. Projected percentage changes in frequency are higher for the rarer extreme events.
Methods and Data for Extremes
Since AR5, the confidence about past and future changes in weather and climate extremes has increased due to better physical understanding of processes, an increasing proportion of the scientific literature combining different lines of evidence, and improved accessibility to different types of climate models. There have been improvements in some observation-based datasets, including reanalysis data. Climate models can reproduce the sign of changes in temperature extremes observed globally and in most regions, although the magnitude of the trends may differ. Models are able to capture the large-scale spatial distribution of precipitation extremes over land. The intensity and frequency of extreme precipitation simulated by Coupled Model Intercomparison Project Phase 6 (CMIP6) models are similar to those simulated by CMIP5 models. Higher horizontal model resolution improves the spatial representation of some extreme events (e.g., heavy precipitation events), in particular in regions with highly varying topography.
The frequency and intensity of hot extremes have increased and those of cold extremes have decreased on the global scale since 1950 (virtually certain). This also applies at regional scale, with more than 80% of AR6 regions showing similar changes assessed to be at least likely. In a few regions, limited evidence (data or literature) prevents the reliable estimation of trends.
Human-induced greenhouse gas forcing is the main driver of the observed changes in hot and cold extremes on the global scale (virtually certain) and on most continents (very likely). The effect of enhanced greenhouse gas concentrations on extreme temperatures is moderated or amplified at the regional scale by regional processes such as soil moisture or snow/ice-albedo feedbacks, by regional forcing from land use and land-cover changes, or aerosol concentrations, and decadal and multidecadal natural variability. Changes in anthropogenic aerosol concentrations have likely affected trends in hot extremes in some regions. Irrigation and crop expansion have attenuated increases in summer hot extremes in some regions, such as the U.S. Midwest. Urbanization has likely exacerbated changes in temperature extremes in 12 cities, in particular for night-time extremes.
The frequency and intensity of hot extremes will continue to increase and those of cold extremes will continue to decrease, at both global and continental scales and in nearly all inhabited regions with increasing global warming levels. This will be the case even if global warming is stabilized at 1.5°C. Relative to present-day conditions, changes in the intensity of extremes would be at least double at 2°C, and quadruple at 3°C of global warming, compared to changes at 1.5°C of global warming. The number of hot days and hot nights and the length, frequency, and/or intensity of warm spells or heat waves will increase over most land areas (virtually certain). In most regions, future changes in the intensity of temperature extremes will very likely be proportional to changes in global warming, and up to 2–3 times larger. The highest increase of temperature of hottest days is projected in some mid-latitude and semi arid regions, at about 1.5 times to twice the rate of global warming. The highest increase of temperature of coldest days is projected in Arctic regions, at about three times the rate of global warming. The frequency of hot temperature extreme events will very likely increase non-linearly with increasing global warming, with larger percentage increases for rarer events.
Heavy Precipitation and Pluvial Floods
The frequency and intensity of heavy precipitation events have likely increased at the global scale over a majority of land regions with good observational coverage. Heavy precipitation has likely increased on the continental scale over three continents: North America, Europe, and Asia. Regional increases in the frequency and/or intensity of heavy precipitation have been observed with at least medium confidence for nearly half of AR6 regions, including WSAF, ESAF, WSB, SAS, ESB, REF, WCA, ECA, TIB, EAS, SEA, NAU, NEU, EEU, GIC, WCE, SES, CNA, and ENA.
Human influence, in particular greenhouse gas emissions, is likely the main driver of the observed global scale intensification of heavy precipitation in land regions. It is likely that human-induced climate change has contributed to the observed intensification of heavy precipitation at the continental scale in North America, Europe and Asia. Evidence of a human influence on heavy precipitation has emerged in some regions.
Heavy precipitation will generally become more frequent and more intense with additional global warming. At global warming levels of 4°C relative to the pre-industrial, very rare (e.g., 1 in 10 or more years) heavy precipitation events would become more frequent and more intense than in the recent past, on the global scale (virtually certain) and in all continents and AR6 regions. The increase in frequency and intensity is extremely likely for most continents and very likely for most AR6 regions. At the global scale, the intensification of heavy precipitation will follow the rate of increase in the maximum amount of moisture that the atmosphere can hold as it warms, of about 7% per 1°C of global warming. The increase in the frequency of heavy precipitation events will accelerate with more warming and will be higher for rarer events, with a likely doubling and tripling in the frequency of 10-year and 50-year events, respectively, compared to the recent past at 4°C of global warming. Increases in the intensity of extreme precipitation at regional scales will vary, depending on the amount of regional warming, changes in atmospheric circulation and storm dynamics.
The projected increase in the intensity of extreme precipitation translates to an increase in the frequency and magnitude of pluvial floods – surface water and flash floods – as pluvial flooding results from precipitation intensity exceeding the capacity of natural and artificial drainage systems.
Significant trends in peak streamflow have been observed in some regions over the past decades. This includes increases in RAR, NSA, and parts of SES, NEU, ENA and decreases in NES, SAU, and parts of MED and EAS). The seasonality of river floods has changed in cold regions where snow-melt is involved, with an earlier occurrence of peak streamflow.
Global hydrological models project a larger fraction of land areas to be affected by an increase in river floods than by a decrease in river floods. River floods are projected to become more frequent and intense in some AR6 regions (RAR, SEA, SAS, NWS) and less frequent and intense in others (WCE, EEU, MED). Regional changes in river floods are more uncertain than changes in pluvial floods because complex hydrological processes and forcings, including land cover change and human water management, are involved.
Different drought types exist, and they are associated with different impacts and respond differently to increasing greenhouse gas concentrations. Precipitation deficits and changes in evapotranspiration (ET) govern net water availability. A lack of sufficient soil moisture, sometimes amplified by increased atmospheric evaporative demand (AED), results in agricultural and ecological drought. Lack of runoff and surface water result in hydrological drought.
Human-induced climate change has contributed to decreases in water availability during the dry season over a predominant fraction of the land area due to evapotranspiration increases. Increases in evapotranspiration have been driven by AED increases induced by increased temperature, decreased relative humidity and increased net radiation. Trends in precipitation are not a main driver in affecting global-scale trends in drought, but have induced drying trends in a few AR6 regions (NES; WAF, CAF, ESAF, SAM, SWS, SSA, SAS). Increasing trends in agricultural and ecological droughts have been observed on all continents (WAF, CAF, WSAF, ESAF, WCA, ECA, EAS, SAU, MED, WCE, WNA, NES, but decreases only in one AR6 region (NAU). Increasing trends in hydrological droughts have been observed in a few AR6 regions (MED; WAF, EAS, SAU). Regional-scale attribution shows that human-induced climate change has contributed to increased agricultural and ecological droughts (MED, WNA), and increased hydrological drought (MED) in some regions.
The land area affected by increasing drought frequency and severity expands with increasing global warming. Several regions will be affected by more severe agricultural and ecological droughts even if global warming is stabilized in a range of 1.5°C-2°C of global warming, including WCE, MED, EAU, SAU, SCA, NSA, SAM, SWS, SSA, NCA, CAN, WSAF, ESAF and MDG. At 4°C of global warming, about 50% of all inhabited AR6 regions would be affected (WCE, MED, CAU, EAU, SAU, WCA, EAS, SCA, CAR, NSA, NES, SAM, SWS, SSA, NCA, CAN, ENA, WNA, WSAF, ESAF, MDG), and only two regions (NEAF, SAS) would experience decreases in agricultural and ecological drought. There is high confidence that the projected increases in agricultural and ecological droughts are strongly affected by ET increases associated with enhanced AED. Several regions are projected to be more strongly affected by hydrological droughts with increasing global warming (at 4°C of global warming: NEU, WCE, EEU, MED, SAU, WCA, SCA, NSA, SAM, SWS, SSA, WNA, WSAF, ESAF, MDG). There is low confidence that effects of enhanced atmospheric CO2 concentrations on plant water-use efficiency alleviate extreme agricultural and ecological droughts in conditions characterized by limited soil moisture and enhanced AED. There is also low confidence that these effects will substantially reduce global plant transpiration and the severity of hydrological droughts. There is high confidence that the land carbon sink will become less efficient due to soil moisture limitations and associated drought conditions in some regions in higher-emission scenarios, in particular under global warming levels above 4°C.
Extreme Storms, Including Tropical Cyclones (TCs)
The average and maximum rain rates associated with TCs, extratropical cyclones and atmospheric rivers across the globe, and severe convective storms in some regions, increase in a warming world. The average and maximum rain rates associated with TCs, extratropical cyclones and available event attribution studies of observed strong TCs provide medium confidence for a human contribution to extreme TC rainfall. Peak TC rain rates increase with local warming at least at the rate of mean water vapour increase over oceans (about 7% per 1°C of warming) and in some cases exceeding this rate due to increased low-level moisture convergence caused by increases in TC wind intensity.
It is likely that the global proportion of major TC (Category 3–5) intensities over the past four decades has increased. The average location where TCs reach their peak wind intensity has very likely migrated poleward in the western North Pacific Ocean since the 1940s, and TC translation speed has likely slowed over the conterminous USA since 1900. Evidence of similar trends in other regions is not robust. The global frequency of TC rapid intensification events has likely increased over the past four decades. None of these changes can be explained by natural variability alone.
The proportion of intense TCs, average peak TC wind speeds, and peak wind speeds of the most intense TCs will increase on the global scale with increasing global warming. The total global frequency of TC formation will decrease or remain unchanged with increasing global warming.
There is low confidence in past changes of maximum wind speeds and other measures of dynamical intensity of extratropical cyclones. Future wind speed changes are expected to be small, although poleward shifts in the storm tracks could lead to substantial changes in extreme wind speeds in some regions. There is low confidence in past trends in characteristics of severe convective storms, such as hail and severe winds, beyond an increase in precipitation rates. The frequency of springtime severe convective storms is projected to increase in the USA, leading to a lengthening of the severe convective storm season; evidence in other regions is limited.
Events, Including Dry/Hot events, Fire Weather, Compound Flooding, and Concurrent Extremes
The probability of compound events has likely increased in the past due to human-induced climate change and will likely continue to increase with further global warming. Concurrent heat waves and droughts have become more frequent and this trend will continue with higher global warming. Fire weather conditions (compound hot, dry and windy events) have become more probable in some regions and there is high confidence that they will become more frequent in some regions at higher levels of global warming. The probability of compound flooding (storm surge, extreme rainfall and/or river flow) has increased in some locations, and will continue to increase due to both sea level rise and increases in heavy precipitation, including changes in precipitation intensity associated with TCs. The land area affected by concurrent extremes has increased. Concurrent extreme events at different locations, but possibly affecting similar sectors (e.g., critical crop producing areas for global food supply) in different regions, will become more frequent with increasing global warming, in particular above 2°C of global warming.
Low-Likelihood High-Impact (LLHI) Events Associated With Climate Extremes
The future occurrence of LLHI events linked to climate extremes is generally associated with low confidence, but cannot be excluded, especially at global warming levels above 4°C. Compound events, including concurrent extremes, are a factor increasing the probability of LLHI events. With increasing global warming some compound events with low likelihood in past and current climate will become more frequent, and there is a higher chance of occurrence of historically unprecedented events and surprises . However, even extreme events that do not have a particularly low probability in the present climate (at more than 1°C of global warming) can be perceived as surprises because of the pace of global warming.