Chapter 11: Weather and Climate Extreme Events in a Changing Climate

Weather and Climate Extreme Events in a Changing Climate.

Box 11.2 | Changes in Low-likelihood, High-impact Extremes

Since the IPCC Fifth Assessment Report (AR5), there have been important new developments and knowledge advances on changes in weather and climate extremes, in particular regarding human influence on individual extreme events.

Some recent hot extreme events would have been extremely unlikely to occur without human influence on the climate system.

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.

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 (high confidence), of about 7% per 1°C of global warming.

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-emissions scenarios.

With increasing global warming, some compound events with low likelihood in past and current climates will become more frequent, and there is a higher chance of occurrence of historically unprecedented events and surprises.

The IPCC risk framework articulates clearly that the exposure and vulnerability to climatic impact-drivers, such as extremes, modulate the risk of adverse impacts of these drivers.

Rarity of an event with a fixed magnitude also changes under human-induced climate change, making events that are unprecedented so far rather probable under present conditions, but unique in the observational record – and thus often considered as ‘surprises’.

Multiple concomitant extremes can lead to stronger impacts than those resulting from the same extremes had they happened in isolation.

The frequency of hot temperature extremes, the number of heatwave days and the length of heatwave seasons in various regions also scale well, but nonlinearly, with global mean temperatures.

In each region changes in extremes are indicated by colour (orange – increase in the type of extreme; blue – decrease; both colours – changes of opposing direction within the region).

While the water vapour feedback always amplifies the initial temperature increases (positive feedback), the lapse rate feedback amplifies near-surface temperature increases (positive feedback) in mid- and high latitudes but reduces temperature increases (negative feedback) in tropical regions.

The extra latent heat released within storms has been shown to increase precipitation extremes by strengthening convective updrafts and the intensity of the cyclonic circulation.

Dynamic processes are often indirect responses to thermodynamic changes, are strongly affected by internal climate variability, and are also less well understood.

Changes in weather systems come with changes in the frequency and intensity of extreme winds, extreme temperatures, and extreme precipitation, on the backdrop of thermodynamic responses of extremes to warming.

King et al. (2016b) show that aerosol-induced cooling delayed the timing of a significant human contribution to record-breaking heat extremes in some regions.

Finally, extreme events may also regionally amplify one another.

Extreme temperatures are defined as the maximum daily temperatures that were exceeded on average once during a 10-year period (10-year event, blue) and once during a 50-year period (50-year event, orange) during the 1850–1900 base period.

In the case where the signal in the observations is still relatively weak but the physical processes underlying the changes in extremes in response to human forcing are well understood, confidence in the projections would be higher than in the attribution.

Highest increase of temperature of coldest days is projected in Arctic regions, at about three times the rate of global warming.

Highest increase of temperature of coldest days is projected in Arctic regions, at about three times the rate of global warming (high confidence).

High confidence that concurrent heatwaves and droughts will continue to increase under higher levels of global warming, with higher frequency/ intensity with every additional 0.5°C of global warming.

The low confidence does not by itself exclude the possibility of such events occuring, rather it indicates a poor state of knowledge.

As warming continues, the climate moves further away from its historical state we are familiar with, resulting in an increased likelihood of unprecedented events and surprises.

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 historically unprecedented events and surprises.

In the case of temperature extremes, changes in magnitude have been shown to often depend linearly on global surface temperature, while changes in frequency tend to be nonlinear and can, for example, be exponential for increasing global warming levels.

In regions with very limited station density (e.g., Africa), the gridded values are closer to point estimates of extremes.

Caution is needed when reanalysis data products are used to provide additional information about past changes in these extremes in regions where observations are generally lacking.

Natural archives may be sensitive only to intense environmental disturbances, and so only sporadically record short-duration or small spatial-scale extremes.

Recent observed drought extremes in some regions – such as the eastern Mediterranean Levant, California in the USA, and in the Andes – do not have precedents within the multi-century periods reconstructed in these studies, in terms of duration and/or severity.

In regions, such as Europe and China, that have rich historical flood documents, there is strong evidence of high-magnitude flood events over pre-instrumental periods.

The probability of finding an unprecedented extreme event increases with a longer length of past record-keeping, in the absence of longer-term trends.

These highly conditional approaches have also been called ‘storylines’ and can be useful when applied to extreme events that are too rare to otherwise analyse.

Extreme events characterized by atmospheric dynamics that stretch the capabilities of current-generation models limit the applicability of the probability-based approach of event attribution.

Since these local forcings are not represented, and their future changes are difficult to project, these can be important caveats when using GWL scaling to project future changes for these regions.

Overall, signals for extremes emerge very early for TXx, already below 0.2°C in many regions (Figure 11.8a,b), and at around 0.5°C in most regions.

For many (but not all) climate variables and CIDs, the response pattern for a given GWL is consistent across different scenarios.

Thus, the GWL approach isolates the uncertainty in the regional climate response from the global warming uncertainty induced by scenario, global mean model response and internal variability.

The close agreement of (g) and (h) demonstrates that the mean temperature response at 2°C is not sensitive to the rate of warming.

Thereby, a given GWL is potentially reached a few years earlier or later in different realizations of the same model due to internal variability.

Slow-responding variables such as ice volumes of glaciers and ice sheets respond with a substantial delay and, due to their inertia, the response depends on when a certain GWL is reached.

The AR6 assessment shows that every bit of global warming matters and that changes in global warming of 0.5°C lead to statistically significant changes in mean climate and climate extremes.

Irrigation has been shown to be responsible for a cooling of hot temperature extremes of up to 1°C–2°C in many mid-latitude regions in the present climate, a process not represented in most of state-of-the-art ESMs.

While the magnitude of the observed trends in temperature-related extremes varies depending on the region, spatial and temporal scales, and metric assessed, evidence of a warming effect is overwhelming, robust, and consistent.

Warming of land mean TNn is even higher, with about 3°C of warming since 1960.

The increase in extreme minimum temperatures occurs in all seasons over most of Australia and typically exceeds the increase in extreme maximum temperatures.

Typical cold spells, such as that observed during the 2009–2010 winter, had an occurrence probability two times smaller currently than if climate change had not occurred.

Arctic annual minimum temperatures have increased at about three times the rate of global surface temperature since the 1960s.

The relative error estimates in the simulation of various indices of temperature extremes in the available CMIP6 models show that no single model performs the best on all indices, and the multi-model ensemble seems to outperform any individual model due to its reduction in systematic bias.

The land surface models used in the RCMs do not account for physiological CO2 effects on photosynthesis leading to enhanced water-use efficiency and decreased evapotranspiration.

As much as 75% of the moderate daily hot extremes (above 99.9th percentile) over land are due to anthropogenic warming.

Sun et al. (2014) found that human influence has caused a more than 60-fold increase in the probability of the extreme warm 2013 summer in eastern China since the 1950s.

Several studies of recent events from 2016 onwards have determined an infinite risk ratio (a fraction of attributable risk, or FAR, of 1), indicating that the occurrence probability for such events is close to zero in model simulations without anthropogenic influences.

Some specific recent hot extreme events would have been extremely unlikely to occur without human influence on the climate system.

In the Arctic winter, the rate of warming of the temperature of the coldest nights is about three times the rate of global warming.

Compared to the historical climate, warming will result in strong increases in heatwave area, duration and magnitude.

Future mid-latitude warm extremes, similar to those experienced during the 2010 event, are projected to become more extreme, with temperature extremes increasing potentially by 8.4°C (RCP8.5) over north-west Asia.

CMIP5 models do not project current 1-in-20-year annual minimum temperature extremes to recur over much of the continent.

The probability of temperature extremes generally increases nonlinearly with increasing global warming levels (high confidence).

Urbanization intensifies extreme precipitation, especially in the afternoon and early evening, over the urban area and its downwind region.

Often, sub-daily precipitation data have only sporadic spatial coverage and are of limited length.

With few exceptions, most South East Asian countries have experienced an increase in rainfall intensity, but with a reduced number of wet days.

Among all sub-regions, South-Eastern South America shows the highest rate of increase for rainfall extremes, followed by the Amazon.

In the Netherlands, the total precipitation contributed from extremes higher than the 99th percentile doubles per 1°C increase in warming.

The areal-reduction factor, the ratio between pointwise station estimates of extreme precipitation and extremes of the areal mean, can be as large as 130% at CMIP6 resolutions.

Additionally, CMIP5 models reproduced the relation between changes in extreme and non-extreme precipitation: an increase in extreme precipitation is at the cost of a decrease in non-extreme precipitation.

Simulated extreme precipitation in the tropics also appears to be too large, indicating possible deficiencies in the parametrization of cumulus convection at this resolution.

A still weak signal-to-noise ratio seems to be the main cause for the lack of robustness, as detection would become robust 20 years in the future.

There are multiple cases indicating that very extreme precipitation may increase at a rate more than the C-C rate (7% per 1°C of warming).

A small increment such as 0.5°C in global warming can result in a significant increase in extreme precipitation.

The CMIP5 model simulations show that the frequency for present-day climate 20-year extreme precipitation is projected to increase by 10% at the 1.5°C global warming level, and by 22% at the 2.0°C global warming level.

A half-degree difference in warming between the 1.5°C and 2.0°C warming levels can result in a detectable increase in extreme precipitation over the region.

Yet, there is large uncertainty in the increase because projected changes in dynamic processes lead to a decrease in Rx1day that can offset the thermodynamic increase over a large portion of the region.

The most intense precipitation events observed today in Europe are projected to almost double in occurrence for each 1°C of further global warming.

The increase in the frequency of heavy precipitation events will be non-linear with more warming and will be higher for rarer events (high confidence), with 10- and 50-year events to be approximately double and triple, respectively, at the 4°C warming level.

Water regulation and management have, in general, increased resilience to flooding, masking effects of an increase in extreme precipitation on flood probability in some regions.

This is in direct contrast to the global and continental scale intensification of short-duration extreme precipitation.

Regional models reproduce moderate and high flows reasonably well, but there are large biases for the most extreme flows, independent of the climatic and physiographic characteristics of the basins.

Attributing changes in heavy precipitation to anthropogenic activities cannot be readily translated to attributing changes in floods to human activities, because precipitation is only one of the multiple factors, albeit an important one, that affect floods.

These results suggest medium confidence in flood trends at the global scale, but low confidence in projected regional changes.

The distinction of drought types is not absolute, as drought can affect different sub-domains of the Earth system concomitantly, but sometimes also asynchronously, including propagation from one drought type to another.

In general, AED is highest in regions where ET is lowest (e.g., desert areas), further illustrating the decoupling between the two variables under limited soil moisture.

In addition, soil moisture plays a role in drought self-intensification under dry conditions in which ET is decreased and leads to higher AED, an effect that can also contribute to triggering flash droughts.

For this reason, soil moisture deficits are the main driver of xylem embolism, the primary cause of plant mortality.

Drought events are the result of dynamic and/or thermodynamic processes, with thermodynamic processes being the main driver of drought changes under human-induced climate change (high confidence).

In several regions, AED increases have intensified recent drought events, enhanced vegetation stress, or contributed to the depletion of soil moisture or runoff through enhanced ET.

Drying has not only occurred in dry regions but also in humid regions.

In the Mediterranean region, there is high confidence in hydrological drought intensification.

There are stronger signals indicating observed increases in agricultural and ecological drought, which highlights the role of increased ET, driven by increased AED, for these trends.

ESMs are generally found to underestimate the severity of precipitation deficits and the dry day frequencies in comparison to observations.

These models display overly strong drying in early summer, resulting in an excessive decrease of latent heat fluxes, with potential implications for more severe droughts in dry environments.

Simulations of hydrological drought metrics show uncertainties related to the contribution of both GCMs and hydrological models, but hydrological models forced by the same climate input data also show a large spread.

There is evidence of substantial internal variability in long-term precipitation trends in the region, which limits the attribution of human influence on variability and trends of meteorological droughts from observational records.

Their results showed a disagreement in the original anthropogenic attribution in a number of precipitation deficit events, which increased uncertainty in the attribution of meteorological droughts events.

A global study with a single hydrological model estimated that human water consumption has intensified the magnitude of hydrological droughts by 20–40% over the last 50 years.

Several meteorological and agricultural and ecological drought events have been attributed to human-induced climate change, even in regions where no long-term changes are detected.

There are also substantial increases in drought hazard probability from 1.5°C to 2°C global warming and for further additional increments of global warming (high confidence).

In California, more precipitation variability is projected, characterized by increased frequency of consecutive drought and humid periods.

The benefits of the atmospheric CO2 for plant stress and agricultural and ecological droughts would be minimal precisely during dry periods given stomatal closure in response to limited soil moisture.

Increased AED is thus both a driver and a feedback with respect to changes in ET, complicating the interpretation of its role on drought changes.

This stresses the dominant influence of ET (as a result of increased AED) in intensifying agricultural and ecological droughts in the warm season in many locations, including mid- to high latitudes.

In western USA, a 22% reduction in winter snow water equivalent is projected at around 2°C of global warming, with a further decrease of a 70% reduction at 4°C global warming.

The enhanced AED associated with human forcing would increase plant water stress, with effects on widespread forest dieback and mortality, and stronger risk of megafires.

There is high confidence that the global land carbon sink will become less efficient due to soil moisture limitations and associated drought conditions.

A projected reduction in mean soil moisture by one standard deviation corresponds to soil moisture conditions typical of about 1-in-6-year droughts during 1850–1900 becoming the norm in the future.

This broad range of natural variability makes detection of anthropogenic effects difficult, and uncertainties in the projected changes of these modes of variability increase uncertainty in the projected changes in TC activity.

The first metric – the mean latitude where TCs reach their peak intensity – exhibits a global and regional poleward migration during the satellite period.

TC translation speed is a measure of the speed at which TCs move across the Earth’s surface, and is very closely related to local rainfall amounts (i.e., a slower translation speed causes greater local rainfall).

In combination with slowing translation speed, abrupt TC track direction changes – that can be associated with track ‘meanders’ or ‘stalls’ – have become increasingly common.

However, higher horizontal resolution does not necessarily lead to an improved TC climatology.

Models with horizontal resolutions of 10–60 km are capable of reproducing strong TCs with Category 4–5 and those of 1–10 km are capable of the eye wall structure of TCs.

The cause of the observed slowdown in TC translation speed is not yet clear.

These results imply that higher resolution, such as in a convective permitting 5 km or less mesh model, is required to obtain a robust anthropogenic intensification of a strong TC.

A best estimate from a regional climate and flood model is that urbanization increased the risk of the Harvey flooding by a factor of 21.

This disparity in the sign of the projected change in global TC frequency, and the difficulty in explaining the mechanisms behind the different signed responses, further emphasize the lack of process understanding of future changes in tropical cyclogenesis.

The strongest TC in the western North Pacific can be as strong as 857 hPa in minimum surface pressure with a wind speed of 88 m s–1 under warming conditions in 2074–2087.

A plausible mechanism is that, as the tropopause height becomes higher with global warming, the eye wall areas become wider because the eye walls are inclined outward with height to the tropopause.

Greenhouse gas forcing in CMIP5 and the Community Earth System Model Large Ensemble simulations, however, erodes the pattern and degrades the natural shear barrier along the USA coast.

There is also high confidence that most current climate models underestimate the number of explosive systems over both hemispheres.

They found that the number of extreme cyclones is projected to increase by at least 20% and as much as 50%, depending on the specific metric used to define extreme ETCs.

There is high confidence that snowfall associated with winter ETCs will decrease in the future, because increases in tropospheric temperatures lead to a lower proportion of precipitation falling as snow.

MCSs sometimes take a linear shape and stay almost stationary with successive production of cumulonimbus on the upstream side (back-building type convection), and cause heavy rainfall.

The uncertainty, however, arises from the balance between these environmental factors affecting severe storm occurrence.

The mean annual number of tornadoes has remained relatively constant, but their variability of occurrence has increased since the 1970s, particularly over the 2000s, with a decrease in the number of days per year, but an increase in the number of tornadoes on these days.

Even in finer-resolution convection-permitting models, it is difficult to directly simulate tornadoes, hail storms, and lightning, so modelling studies of these changes are limited.

Convective inhibition becomes stronger over most land areas under global warming, resulting mainly from reduced low-level relative humidity over land.

hail is almost eliminated at the surface in the future in most of the simulations, despite more intense future storms and significantly larger amounts of hail generated in-cloud.

An earlier study attributed the stilling to both changes in atmospheric circulation and an increase in surface roughness due to an overall increase in vegetation cover.

This is because multiple stressors can exceed the coping capacity of a system more quickly.

Many major weather- and climate-related catastrophes are inherently of a compound nature.

Such single driver analyses might underestimate flood probabilities if multiple correlated drivers contribute to flood occurrence.

This means that, even if drought occurrence is unaffected, compound hot and dry events will be more frequent.

Between 1979 and 2013, the global burnable area affected by long fire weather seasons doubled, and the mean length of the fire weather season increased by 19%.

The first focuses on concurrent extremes driven by variability in tropical Pacific sea surface temperatures (SSTs) associated with the 2015–2016 extreme El Niño.

It also left rivers with very low water levels and large sandbanks, preventing ship transportation of food, medicines, and fuels.

Overall, tropical forests were a carbon source to the atmosphere during the 2015–2016 El Niño-related drought, with some estimates suggesting that up to 2.3 PgC were released.

High CO2 release to the atmosphere associated with droughts and fires in several affected regions

The reported impacts included: 90 deaths from heat strokes in Quebec; 1469 deaths from heat strokes in Japan; and melting of roads in the Netherlands and the UK.

Hence, it is virtually certain that these 2018 concurrent events would not have occurred without human-induced global warming.

In cases when the evidence is strongly contradictory, for example, with substantial regional changes of opposite sign, ‘mixed signal’ is indicated.

This assessment is further supported by an understanding of how temperature extremes change with the mean temperature and overwhelming evidence of a human contribution to the observed larger-scale changes.

Medium to high confidence in drying was assigned in the assessment for arid regions if a signal was also identifiable in total soil moisture in addition to surface soil moisture.

One global-scale study with regional-scale information is available for the attribution of hydrological droughts (Gudmundsson et al., 2021), but only in a few AR6 regions.

On the contrary, changes in precipitation extremes (heavy precipitation) generally do not follow those in average precipitation, and can even move in the opposite direction.

At the local scale, average and extreme surface temperature changes are strongly related, while average and extreme precipitation changes are often weakly related.

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.

Scientists cannot answer directly whether a particular event was caused by climate change, as extremes do occur naturally, and any specific weather and climate event is the result of a complex mix of human and natural factors.

Additionally, attribution of certain classes of extreme weather (e.g., tornadoes) is beyond current modelling and theoretical capabilities.

Median increase of more than 3°C in the 50-year TXx and TNn events compared to the 1°C warming level.

The magnitude of projected changes increases with global warming.

The magnitude of projected changes increases with global warming.

Median increase of more than 3°C in the 50-year TXx and TNn events compared to the 1°C warming level.

Median increase of more than 4.5°C in annual TXx and TNn compared to pre-industrial.

Evidence of a human contribution to the observed increase in the intensity and frequency of hot extremes, and decrease in the intensity and frequency of cold extremes

Median increase of more than 2.5°C in the 50-year TXx and TNn events compared to the 1°C warming level and more than 4.5°C in annual TXx and TNn compared to pre-industrial.

Median increase of more than 2.5°C in the 50-year TXx and TNn events compared to the 1°C warming level (Li et al., 2021) and more than 4°C in annual TXx and TNn compared to pre-industrial.

Median increase of more than 3.5°C in annual TXx and TNn compared to pre-industrial.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation (Li et al., 2021; 11.SM).

Intensification of heavy precipitation: Extremely likely (compared with the recent past, 1995–2014) Virtually certain (compared with pre-industrial)

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

Median increase of more than 15% in the 50-year Rx1day and Rx5day events compared to the 1°C warming level.

No evidence that late onset of 2015 wet season in Nigeria was due to human contribution.

Robust evidence that recent meteorological drought events (in 2016 and 2017) are not attributable to anthropogenic climate change.

Inconsistent signal in observations vs models for 1951–2010 trends; no signal in single-model based study.

Robust evidence that recent drought events are not attributable to anthropogenic climate change.

High confidence: Increases in dryness (CDD, DF, NDD); slight but weaker increase in SPI.

Medium confidence that human influence has contributed to stronger recent meteorological drought.

Robust increase of drought frequency and severity (SPI-12).

Human influence very likely contributed to the observed increase in the intensity and frequency of hot extremes, and decrease in the intensity and frequency of cold extremes.

Median increase of more than 5.5°C in annual TXx and TNn compared to pre-industrial.

Median increase of more than 5.5°C in annual TXx and TNn compared to pre-industrial.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Human influence likely contributed to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Human influence likely contributed to the observed intensification of heavy precipitation.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

Intensification of heavy precipitation: Virtually certain (compared with the recent past, 1995–2014) Virtually certain (compared with pre-industrial)

Median increase of more than 25% in the 50-year Rx1day and Rx5day events compared to the 1°C warming level.

Median increase of more than 20% in the 50-year Rx1day and Rx5day events compared to the 1°C warming level.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

Low confidence: Inconsistent trends across models and subregions for surface and total soil moisture.

Low confidence: Mixed signal between different metrics, including total and surface soil moisture, PDSI and SPEI-PM.

Difference in signal in CMIP6 vs CMIP5: CMIP6 models show drying in soil moisture, while CMIP5 models show wetting.

Difference in signal in CMIP6 vs CMIP5: CMIP6 models show drying in soil moisture, while CMIP5 models show wetting.

Difference in signal in CMIP6 vs CMIP5: CMIP6 models show drying in soil moisture, while CMIP5 models show wetting.

Most of the drying trend took place from 1990, with wetting trend beforehand.

Drought conditions in northern China (soil moisture and runoff) have been intensified by agriculture.

Overall poor climate model performance for South Asia monsoon in CMIP5 and CORDEX.

The equatorial Asia drought of 2015 has been attributed to anthropogenic warming effects.

Robust evidence of a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Human influence very likely contributed to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Median increase of more than 4°C in annual TXx and TNn compared to pre-industrial (11.SM)

Human influence likely contributed to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Human influence likely contributed to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

CMIP6 models project a robust increase in the intensity and frequency of TXx events and a robust decrease in the intensity and frequency of TNn events.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

CMIP6 models project an increase in the intensity and frequency of heavy precipitation (Li et al., 2021; 11.SM).

Lewis et al. (2020) supported an anthropogenic attribution of 2018 drought associated with more extreme temperatures that exacerbated atmospheric evaporative demand (AED) and evapotranspiration, and depleting soil moisture.

Medium confidence: Increased drying for some metrics or part of domain for soil moisture and SPEI-PM with stronger changes for SPEI-PM.

Enhanced AED-driven by extreme temperatures increased the severity of the 2019 drought.

Most subregions show a likely increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Strong evidence of changes from observations that are in the direction of model-projected changes for the future.

Medium confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Median increase of more than 3°C in the 50-year TXx and TNn events compared to the 1°C warming level and more than 4.5°C in annual TXx and TNn compared to pre-industrial.

Median increase of more than 4.5°C in annual TXx and TNn compared to pre-industrial.

At 4°C warming, an increase in the intensity and frequency of hot extremes is virtually certain compared with the recent past.

Increase in the intensity and frequency of hot extremes: Virtually certain (compared with the recent past, 1995–2014)

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

Insufficient data and a lack of agreement on the evidence of trends.

Evidence of a human contribution for some events (S. Li et al., 2020), but cannot be generalized.

Median increase of more than 8% in the 50-year Rx1day and Rx5day events compared to the 1°C warming level and more than 20% in annual Rx1day and Rx5day.

Median increase of more than 15% in the 50-year Rx1day and Rx5day events compared to the 1°C warming level.

Total soil moisture shows a median decrease in a large sample of CMIP6 simulations but substantial spread between models.

High confidence: Increase in drought severity with different metrics and high agreement between studies.

High confidence: Increase in drought severity with different metrics and high agreement between studies.

High confidence: Increase in drought severity with different metrics and high agreement between studies.

Low confidence: Mixed signals between studies and models.

All subregions show a very likely increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Median increase of more than 1°C in the 50-year TXx and TNn events compared to the 1°C warming level (Li et al., 2021) and more than 2.5°C in annual TXx and TNn compared to pre-industrial (11.SM)

Median increase of more than 3.5°C in the 50-year TXx and TNn events compared to the 1°C warming level and more than 5°C in annual TXx and TNn compared to pre-industrial.

Median increase of more than 5.5°C in the 50-year TXx and TNn events compared to the 1°C warming level.

Human influence likely contributed to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

CMIP6 models project a robust increase in the intensity and frequency of TXx events and a robust decrease in the intensity and frequency of TNn events.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

Median increase of more than 15% in the 50-year Rx1day and Rx5day events compared to the 1°C warming level.

High confidence in a human contribution to the observed intensification of heavy precipitation in winter.

Drought intensity and frequency increase with high confidence, particularly in the southern Mediterranean.

Large decreasing soil water availability during drought events compared to 1971–2000, even when accounting for adaptation to mean conditions; more than three times larger signal compared to response at +1.5°C.

Model-based assessment shows with medium confidence a human fingerprint on increased hydrological drought, related to rising temperature and atmospheric demand.

Very strong decrease (40–60%) of total runoff in spring-summer half-year in southern Europe.

Medium confidence: Increase in drying, mostly in western part of domain: summer season surface runoff compared to pre-industrial.

Weak increase in probability of low flow but low signal-to-noise ratio

Inconsistent changes, generally with drying in ESMs (CMIP5, CMIP6) and wetting in CORDEX.

The magnitude of projected changes increases with global warming.

Evidence of a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes.

Median increase of more than 4.5°C in the 50-year TXx and TNn events compared to the 1°C warming level and more than 5.5°C in annual TXx and TNn compared to pre-industrial.

Median increase of more than 5°C in the 50-year TXx and TNn events compared to the 1°C warming level.

High confidence in a human contribution to the observed increase in the intensity and frequency of hot extremes and decrease in the intensity and frequency of cold extremes

Median increase of more than 4°C in the 50-year TXx and TNn events compared to the 1°C warming level (Li et al., 2021) and more than 5°C in annual TXx and TNn compared to pre-industrial.

At the 4°C warming level, intensification of heavy precipitation is virtually certain compared with the recent past.

Evidence of a human contribution to the observed intensification of heavy precipitation.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

CMIP6 models project a robust increase in the intensity and frequency of heavy precipitation.

High confidence: Increase in meteorological drought severity in the majority of models.

Williams et al. (2015) and Griffin and Anchukaitis (2014) concluded that increased AED has had an increased contribution to drought severity over the last decades, and played a dominant role in the intensification of the 2012–2014 drought in California.

Human influence on surface soil moisture deficits due to increased evapotranspiration caused by higher temperatures.

Low confidence: Inconsistent trends between models, metrics and studies based on total and surface soil moisture.

Some evidence (medium confidence) for strong seasonality of trends, with decrease in summer and increase in winter.

Integrating human behaviour dynamics into flood disaster risk assessment.

A century of observations reveals increasing likelihood of continental-scale compound dry-hot extremes.

Meta-analysis reveals that hydraulic traits explain cross-species patterns of drought-induced tree mortality across

Tornadoes in Europe: An Underestimated Threat.

Multidecadal to multicentury scale collapses of Northern Hemisphere monsoons over the past millennium.

Recent intensification of Amazon flooding extremes driven by strengthened Walker circulation.

Climate models predict increasing temperature variability in poor countries.

Land–atmosphere feedbacks amplify aridity increase over land under global warming.

Higher probability of compound flooding from precipitation and storm surge in Europe under anthropogenic climate change.

Models are likely to underestimate increase in heavy rainfall in the extratropical regions with high rainfall intensity.

Hanging by a thread? Forests and drought.

The Response of Precipitation Minus Evapotranspiration to Climate Warming: Why the “Wet-Get-Wetter, Dry-Get-Drier” Scaling Does Not Hold over Land.

Diagnosis of the MJO modulation of tropical cyclogenesis using an empirical index.

Projected Significant Increase in the Number of Extreme Extratropical Cyclones in the Southern Hemisphere.

Anthropogenic warming has caused hot droughts more frequently in China.

Dramatically increasing chance of extremely hot summers since the 2003 European heatwave.

Evapotranspiration depletes groundwater under warming over the contiguous United States.

Present-day greenhouse gases could cause more frequent and longer Dust Bowl heatwaves.

Asian irrigation, African rain: Remote impacts of irrigation.

Choice of Irrigation Water Management Practice Affects Indian Summer Monsoon Rainfall and Its Extremes.

Growth and photosynthetic responses in Brassica napus differ during stress and recovery periods when exposed to combined heat, drought and elevated CO2.

More extreme precipitation in the world’s dry and wet regions.

Midlatitude daily summer temperatures reshaped by soil moisture under climate change.

Large-scale dieback of mangroves in Australia’s Gulf of Carpentaria: a severe ecosystem response, coincidental with an unusually extreme weather event.

The increasing intensity of the strongest tropical cyclones.

Indonesian fire activity and smoke pollution in 2015 show persistent nonlinear sensitivity to El Niño-induced drought.

Observed heavy precipitation increase confirms theory and early models.

Evidence linking Arctic amplification to extreme weather in mid-latitudes.

Anthropogenic Enhancement of Moderate-to-Strong El Niño Events Likely Contributed to Drought and Poor Harvests in Southern Africa During 2016.

Increasing risks of multiple breadbasket failure under 1.5 and 2°C global warming.

Potential Increase in Hazard From Mediterranean Hurricane Activity With Global Warming.

How unusual is the 2012–2014 California drought?

Australian tropical cyclone activity lower than at any time over the past 550–1,500 years.

Weak linkage between the heaviest rainfall and tallest storms.

Carbon starvation during drought-induced tree mortality – are we chasing a myth?

Intensification of hydrological drought in California by human water management.

Multi-hazard dependencies can increase or decrease risk.

Anatomy of an extreme event.

Fire carbon emissions over maritime southeast Asia in 2015 largest since 1997.

The July 2018 High Temperature Event in Japan Could Not Have Happened without Human-Induced Global Warming.

Examining why trends in very heavy precipitation should not be mistaken for trends in very high river discharge.

Behind the veil of extreme event attribution.

Quasi-Stationary Band-Shaped Precipitation Systems, Named as “Senjo-Kousuitai”, Causing Localized Heavy Rainfall in Japan.

Risks from Climate Extremes Change Differently from 1.5°C to 2.0°C Depending on Rarity.

Climate change turns Australia’s 2013 big dry into a year of record-breaking heat.

The co-incidence of storm surges and extreme discharges within the Rhine–Meuse Delta.

Amplified Rossby waves enhance risk of concurrent heatwaves in major breadbasket regions.

Inconsistencies Between Long-Term Trends in Storminess Derived From the 20CR Reanalysis and Observations.

Hurricane Sandy before 1900 and after 2100.

Historical deforestation locally increased the intensity of hot days in northern mid-latitudes.

Recent Very Hot Summers in Northern Hemispheric Land Areas Measured by Wet Bulb Globe Temperature Will Be the Norm Within 20 Years.

Summer soil drying exacerbated by earlier spring greening of northern vegetation.

The Three Gorges Dam causes downstream lake shrinkage and severe droughts.

European summer temperatures since Roman times.

Extreme Hail Storms and Climate Change: Foretelling the Future In Tiny, Turbulent Crystal Balls?

Twentieth-century hydroclimate changes consistent with human influence.

Darcy’s law predicts widespread forest mortality under climate warming.

Less bluster ahead? ecohydrological implications of global trends of terrestrial near-surface wind speeds.

Colorado River flow dwindles as warming-driven loss of reflective snow energizes evaporation.

Broad threat to humanity from cumulative climate hazards intensified by greenhouse gas emissions.

Historically hottest summers projected to be the norm for more than half of the world’s population within 20 years.

Frequency of extreme precipitation increases extensively with event rareness under global warming.

Regional climate model simulations of extreme air temperature in Greece. Abnormal or common records in the future climate?

Sand beach ridges record 6000 year history of extreme tropical cyclone activity in northeastern Australia.

Flash droughts: A review and assessment of the challenges imposed by rapid-onset droughts in the United States.

Future temperature in southwest Asia projected to exceed a threshold for human adaptability.

Increasing risk of another Cape Town “Day Zero” drought in the 21st century.

Flash droughts present a new challenge for subseasonal-to-seasonal prediction.

A protocol for probabilistic extreme event attribution analyses.

Prein, A.F. et al., 2016b: The future intensification of hourly precipitation extremes.

Defining “Atmospheric River”: How the Glossary of Meteorology Helped Resolve a Debate.

Forecasted attribution of the human influence on Hurricane Florence.

The role of atmospheric rivers in compound events consisting of heavy precipitation and high storm surges along the Dutch coast.

Death from drought in tropical forests is triggered by hydraulics not carbon starvation.

When will unusual heat waves become normal in a warming Africa?

Multidimensional risk in a nonstationary climate: Joint probability of increasingly severe warm and dry conditions.

Amplification of mega-heatwaves through heat torrents fuelled by upwind drought.

Regional climate model projections underestimate future warming due to missing plant physiological CO2 response.

If precipitation extremes are increasing, why aren’t floods?

Storylines: an alternative approach to representing uncertainty in physical aspects of climate change.

Singh, D. et al., 2018: Climate and the Global Famine of 1876–78.

Warming and wetting signals emerging from analysis of changes in climate extreme indices over South America.

Aerosol versus greenhouse gas effects on tropical cyclone potential intensity and the hydrologic cycle.

Will drought events become more frequent and severe in Europe?

Ongoing Poleward Migration of Tropical Cyclone Occurrence Over the Western North Pacific Ocean.

Climate Science Needs to Take Risk Assessment Much More Seriously.

A Persistent Japanese Heat Wave in Early August 2015: Roles of Natural Variability and Human-Induced Warming.

Frequency of extreme Sahelian storms tripled since 1982 in satellite observations.

The Exceptional 2018 European Water Seesaw Calls for Action on Adaptation.

Extreme Eastern U.S. Winter of 2015 Not Symptomatic of Climate Change.

The twenty-first century Colorado River hot drought and implications for the future.

Drought in a human-modified world: Reframing drought definitions, understanding, and analysis approaches.

Water scarcity hotspots travel downstream due to human interventions in the 20th and 21st century.

Unraveling the influence of atmospheric evaporative demand on drought and its response to climate change.

Concurrent 2018 Hot Extremes Across Northern Hemisphere Due to Human-Induced Climate Change.

Quantitative attribution of climate effects on Hurricane Harvey’s extreme rainfall in Texas.

The Deadly Combination of Heat and Humidity in India and Pakistan in Summer 2015.

An underestimated record breaking event – why summer 1540 was likely warmer than 2003

Temperature as a potent driver of regional forest drought stress and tree mortality.

Recent Trends in Extreme Precipitation and Temperature over Southeastern South America: The Dominant Role of Stratospheric Ozone Depletion in the CESM Large Ensemble.

Slowdown of Typhoon Translation Speeds in Mid-latitudes in September Influenced by the Pacific Decadal Oscillation and Global Warming.

Increased Quasi Stationarity and Persistence of Winter Ural Blocking and Eurasian Extreme Cold Events in Response to Arctic Warming.

Increased atmospheric vapor pressure deficit reduces global vegetation growth.

Larger Drought and Flood Hazards and Adverse Impacts on Population and Economic Productivity Under 2.0 than 1.5°C Warming.

Urbanization exacerbated the rainfall and flooding caused by hurricane Harvey in Houston.

Wildfire Impact on Environmental Thermodynamics and Severe Convective Storms.

Land–atmosphere feedbacks exacerbate concurrent soil drought and atmospheric aridity.

AED is an upper bound for actual ET but also induces additional vegetation stress under dry conditions.

These indices are not intended to be a soil moisture or water-balance proxy.

High confidence (including likely, very likely, extremely likely and virtually certain changes) increases.

High confidence (including likely, very likely, extremely likely and virtually certain changes) increases.

High confidence (including likely, very likely, extremely likely and virtually certain changes) increases

High confidence (including likely, very likely, extremely likely and virtually certain changes) increases

Weather and Climate Extreme Events in a Changing Climate.

Box 11.2 | Changes in Low-likelihood, High-impact Extremes

Some recent hot extreme events would have been extremely unlikely to occur without human influence on the climate system.

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.

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 (high confidence), of about 7% per 1°C of global warming.

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-emissions scenarios.

With increasing global warming, some compound events with low likelihood in past and current climates will become more frequent, and there is a higher chance of occurrence of historically unprecedented events and surprises.

The frequency of hot temperature extremes, the number of heatwave days and the length of heatwave seasons in various regions also scale well, but nonlinearly, with global mean temperatures.

While the water vapour feedback always amplifies the initial temperature increases (positive feedback), the lapse rate feedback amplifies near-surface temperature increases (positive feedback) in mid- and high latitudes but reduces temperature increases (negative feedback) in tropical regions.

The extra latent heat released within storms has been shown to increase precipitation extremes by strengthening convective updrafts and the intensity of the cyclonic circulation.

Changes in weather systems come with changes in the frequency and intensity of extreme winds, extreme temperatures, and extreme precipitation, on the backdrop of thermodynamic responses of extremes to warming.

King et al. (2016b) show that aerosol-induced cooling delayed the timing of a significant human contribution to record-breaking heat extremes in some regions.

Finally, extreme events may also regionally amplify one another.

Extreme temperatures are defined as the maximum daily temperatures that were exceeded on average once during a 10-year period (10-year event, blue) and once during a 50-year period (50-year event, orange) during the 1850–1900 base period.

Highest increase of temperature of coldest days is projected in Arctic regions, at about three times the rate of global warming.

High confidence that concurrent heatwaves and droughts will continue to increase under higher levels of global warming, with higher frequency/ intensity with every additional 0.5°C of global warming.

The low confidence does not by itself exclude the possibility of such events occuring, rather it indicates a poor state of knowledge.

As warming continues, the climate moves further away from its historical state we are familiar with, resulting in an increased likelihood of unprecedented events and surprises.

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 historically unprecedented events and surprises.

In the case of temperature extremes, changes in magnitude have been shown to often depend linearly on global surface temperature, while changes in frequency tend to be nonlinear and can, for example, be exponential for increasing global warming levels.

Natural archives may be sensitive only to intense environmental disturbances, and so only sporadically record short-duration or small spatial-scale extremes.

Recent observed drought extremes in some regions – such as the eastern Mediterranean Levant, California in the USA, and in the Andes – do not have precedents within the multi-century periods reconstructed in these studies, in terms of duration and/or severity.

In regions, such as Europe and China, that have rich historical flood documents, there is strong evidence of high-magnitude flood events over pre-instrumental periods.

These highly conditional approaches have also been called ‘storylines’ and can be useful when applied to extreme events that are too rare to otherwise analyse.

Overall, signals for extremes emerge very early for TXx, already below 0.2°C in many regions (Figure 11.8a,b), and at around 0.5°C in most regions.

Thereby, a given GWL is potentially reached a few years earlier or later in different realizations of the same model due to internal variability.

Slow-responding variables such as ice volumes of glaciers and ice sheets respond with a substantial delay and, due to their inertia, the response depends on when a certain GWL is reached.

Warming of land mean TNn is even higher, with about 3°C of warming since 1960.

Arctic annual minimum temperatures have increased at about three times the rate of global surface temperature since the 1960s.

As much as 75% of the moderate daily hot extremes (above 99.9th percentile) over land are due to anthropogenic warming.

Several studies of recent events from 2016 onwards have determined an infinite risk ratio (a fraction of attributable risk, or FAR, of 1), indicating that the occurrence probability for such events is close to zero in model simulations without anthropogenic influences.

In the Arctic winter, the rate of warming of the temperature of the coldest nights is about three times the rate of global warming.

Urbanization intensifies extreme precipitation, especially in the afternoon and early evening, over the urban area and its downwind region.

The CMIP5 model simulations show that the frequency for present-day climate 20-year extreme precipitation is projected to increase by 10% at the 1.5°C global warming level, and by 22% at the 2.0°C global warming level.

In addition, soil moisture plays a role in drought self-intensification under dry conditions in which ET is decreased and leads to higher AED, an effect that can also contribute to triggering flash droughts.

This stresses the dominant influence of ET (as a result of increased AED) in intensifying agricultural and ecological droughts in the warm season in many locations, including mid- to high latitudes.

The first metric – the mean latitude where TCs reach their peak intensity – exhibits a global and regional poleward migration during the satellite period.

Many major weather- and climate-related catastrophes are inherently of a compound nature.

Hence, it is virtually certain that these 2018 concurrent events would not have occurred without human-induced global warming.