Weather Extremes and Climate Change

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ouragan irma - ouragans - evenements meteo extremes - extreme weather events - climate change

How do extreme weather events change with climate change? Are they becoming more frequent? More intense? Is it man’s responsibility? This article presents the state of scientific knowledge on this subject. Recent and future trends in temperature extremes, hydrological extremes, tropical cyclones and extra-tropical storms are discussed, accounting for the uncertainties associated with natural climate variability and numerical modelling. Climate change is already modifying and will continue to modify the probabilities of weather hazards, making some extreme events more frequent and/or intense, and others less so. However, one should not want to hold humans alone responsible for any meteorological event, but rather wonder, in probabilistic terms, how human activities have changed the risk of the occurring event.

1. A difficult link between weather and climate

Heat waves, cold spells, heavy rains, droughts, hurricanes, storms and other extreme weather events are regularly in the news, particularly because of their significant impacts on societies and the environment. The question of the link between the occurrence of such events and climate change is legitimately asked to scientists, and is the subject of increasing work. The answer is not always trivial, especially since the increasingly systematic media coverage of these phenomena and the sometimes increased vulnerability of populations to weather hazards can give a false impression of climate trends.

temperature moyenne globale monde - evolution of global average temperature
Figure 1. Scenarios for the evolution of the global average temperature (in ºC) based on 4 trajectories of greenhouse gas (GHG) concentration. The zero reference is the average over the end of the 20th century (1986-2005). The uncertainty around each curve includes the choice of climate model and meteorological hazard. The number of simulations used for each scenario and period is indicated. [Source: Figure SPM7 of the 5th IPCC report, ref[1])
In the context of anthropogenic climate change, the scientific challenge is twofold: to isolate the human footprint in observed events on the one hand, and to predict changes into a warmer climate on the other. This second point is studied using numerical simulations of a future climate, or “climate projections” (Figure 1, [1]). The projections currently used consider four possible trajectories of greenhouse gas concentration for the 21st century (“Radiative Concentration Pathways”, RCP), associated with different socio-economic scenarios of greenhouse gas emissions. With the low scenario RCP2.6, the Earth’s surface warms by 1ºC (± 0.7) during the 21st century (i.e. by 1.6ºC compared to the pre-industrial period); with the high scenario RCP8.5 by 3.7ºC (± 1.1), i.e. by 4.3ºC compared to the pre-industrial period.

The study of recent or future trends in extreme weather events is generally done for each type of event. Thus, this article will focus successively on temperature extremes, hydrological extremes, tropical cyclones (generic term including hurricanes and typhoons) and extra-tropical storms. We will see that several studies show that changes are already detectable in the frequency and/or the intensity of some types of extreme events, and that others may appear or increase during the 21st century. These qualitatively robust results should not mask two major difficulties in the study of extreme events:

  • The first is the predominant role of natural climate variability in these phenomena, which tends to make the anthropogenic signal unclear and requires high quality observations and/or a large number of simulations to be able to detect changes.
  • The second is the imperfect nature of climate models, hence the need to further develop and evaluate them and to rely on a multiplicity of models that are as independent as possible.

2. Temperature extremes

The global warming observed over more than a century affects not only the average temperature, but also the entire statistical distribution of temperatures, i.e. the entire range of possible temperatures in one place and one moment. At the extremes of this distribution are the rarest events – the cold and hot extremes – whose occurrence is generally accompanied by significant socio-environmental impacts. Extremes are traditionally defined in terms of their frequency of occurrence, or, in statistical terms, by the quantiles of their distribution. For example, it is common practice to describe a day as unusually hot when its temperature is in the top 10% of the expected values for that day (i.e., it is said to “exceed the 90th percentile – or 9th decile – of the distribution”). By construction, over the period used as a reference, 10% of the days are considered unusually hot. A symmetrical definition is used for abnormally cold days, and the seasonal temperature cycle is generally taken into account, so that we can talk about warm days in winter or cold days in summer. To study the evolution of these extremes, we consider either the evolution of their frequency (number of days below or above the current temperature decile) or the evolution of their intensity, defined for example as the average temperature of the 10% of extreme days.

2.1. Occurrence of extreme days

temperature estivale france - temperature moyenne ete france - summer temperature of metropolitan france 1900-2018
Figure 2. Top: distribution of summer temperature (average June-July-August) in metropolitan France over the period 1900-2018 (Météo-France data). Each vertical line corresponds to one summer: the lower axis gives their temperature (in °C), the upper axis the anomaly of this temperature compared to the average over the whole period (in °C). Extreme cold and hot summers, i.e. those belonging respectively to the first and last deciles of the distribution, are highlighted, particularly the heat waves of 2003 and 2018. The black curve is the Gaussian distribution closest to the data. Bottom: schematic representation of a future change in this distribution, which would follow a simple translation (homogeneous heating from the black curve to the red curve). For the illustration, the future climate is taken as a climate for which summer 2003 would have become the average (cf. new upper axis). [© Julien Cattiaux]
In a warming climate, the entire temperature distribution shifts in case of a leading order translation to warmer conditions (Figure 2). Depending on the definition used for extremes, cold extremes are expected to become less frequent and hot extremes more frequent (for a given temperature threshold), or cold extremes to become less cold and warmer (for a given frequency threshold). And that is what is observed.

annnual frequency abnormally warn and cold
Figure 3. Observed annual frequency of unusually warm (top) and unusually cold (down) days on average over the globe (continents only). A fixed temperature threshold is considered (first and last deciles of the 1961-1990 reference distribution). The curves are centred on the 1961-1990 period and are derived from 3 data sets (colors). [Source: Adapted from Figure 2.32 of the IPCC 5th report, ref.[1]]
On a global scale, there has been a significant increase in the number of unusually warm days since 1950 (Figure 3,[1]), with hot days more frequent during the 1990ies and the years 2000 than before. Symmetrically, we measure a decrease in the number of unusually cold days. At the same time, there is an overall trend to break more frequently warm than cold daily and monthly records. These results are also observed at the regional level, particularly in Europe.

2.2. Heat and cold waves

Beyond daily statistics, heat waves – several consecutive hot days – tend to be more frequent, more intense and/or longer, while the number of cold waves has decreased substantially since 1950. While recent heat waves, such as the one in August 2003 in Western Europe, correspond qualitatively to what is expected from a warmer climate, the few recent winter cold spells observed in Europe (winter 2009/10, December 2010, February 2012) may seem to contradict the idea of global warming.

There is in fact no paradox in the fact that cold episodes can occur locally and occasionally in a context of global warming. This is the difference between meteorological hazard and a climate trend on the long term.

Warming is a background process, superimposed on the noise of the natural internal variability of the climate system [read Climate Variability: the example of the North Atlantic Oscillation]. Though variability may continue to cause cold weather events, they are expected to be less frequent and/or less intense due to the global warming [see focus Attribution of singular weather events to climate change: the 2003 heat wave]. Once again, this is exactly what we are seeing: the coolness of recent cold waves is only relative compared to the freezing winters of 1939/40 and 1962/63, which were similar in terms of atmospheric circulation.

Finally, it cannot be ruled out that the warming of recent years, which has been particularly pronounced in the Arctic, may have temporarily disrupted the circulation of air masses at our latitudes, and increased the probability of weather situations producing cold waves over Europe. However, this point is still very much debated within the scientific community.

2.3. Evolutions over the 21st century

Whatever the scenario, global warming will continue – more or less strongly – during the 21st century. Thus, not surprisingly, the trends in temperature extremes observed over the recent period are confirmed in climate projections: hot extremes increasingly frequent and intense, and cold extremes increasingly rare and less marked. However, the magnitude of these changes is largely dependent on the choice of the greenhouse gas emission scenario. It also varies from one model to another for a given scenario and remains strongly modulated by internal climate variability.

extremes climatiques monde - frequency of abnormally warm and cold days
Figure 4. Future projections of the annual frequency of unusually warm (left) and unusually cold (right) days, according to 3 scenarios. The frequency is by definition 10% over the 1961-1990 reference period. [Source: Figure WGI-AT9 of the 5th IPCC report[1]]
On average over the different climate models, and at global and annual scales, the probability of observing a daily temperature above the current 90th percentile thus increases from 10% in the current climate (one day in ten, by construction) to 25% (one day in four) in scenario RCP2.6, or even 60% (more than one day in two) in scenario RCP8.5, by 2100. Conversely, the frequency of unusually cold days falls to 4% in RCP2.6 and 1% in RCP8.5 (Figure 4). This evolution also affects very rare events: for example, a hot event that occurs on average every 20 years in the current climate – referred to as a return period of 20 years – would occur on average every other year by 2100 in RCP8.5. In the same scenario, its cold equivalent has its return period increasing from 20 years to more than a century. Despite these trends, the occurrence of extreme cold is still possible. Even in a strong scenario, it is to be expected that some cold records will be broken locally in the 21st century, but much less frequently than warm records. For example, in the United States, the ratio between the occurrence of hot records and the occurrence of cold records is currently 2:1: it is estimated at 20:1 around 2050 and 50:1 in 2100 in the moderate scenario.

In addition to the uncertainties associated with the scenario selection and with the internal climate variability, the future evolution of temperature extremes is sensitive to the choice of the numerical climate model, particularly at regional and seasonal scales. In Europe, in the RCP8.5 scenario, the probability of exceeding the current 90th percentile (or 9th decile) of summer temperature – which is by construction 10% on the current climate – ranges from 30% to 90% depending on the model on average over the future summers 2070-2100.

These uncertainties also affect the characteristics of multi-day events. For example, although future projections agree on the increase in the frequency, intensity and duration of European summer heatwaves, their evolution by 2100 varies by a factor of three, for a given scenario, depending on the numerical model considered.

temperature extreme chaud france - summer temperature france
Figure 5. Schematic representation of a change in the summer temperature distribution in France with, in addition to a simple translation (warming, from black to red curves, see Fig. 2), a widening (increase in variability from black to blue curves). In this latter case, the probability of hot extremes further increases. Same legend as Figure 2. [© Julien Cattiaux]
Finally, if changes in temperature extremes primarily depend on the amplitude of the mean warming (shift of the distribution), they are modulated by changes in variability (distribution shape and width, see Figure 5). In Europe, future projections suggest a small increase in variability in summer, making warm extremes even more likely, and a small reduction in winter, making cold extremes even less likely. These behaviours are respectively linked to a drying of the soil in summer and a decrease in snow cover in winter.

temperature estivale europe - evolution summer temperature in europe
Figure 6. a) 1961-1990 departure from normal summer temperature (June, July, August) in Europe (blue rectangle on the map) for observations (black curve), historical simulations (31 model simulations, blue curve) and future simulations with RCP2.6 (18 model simulations, green curve) and RCP8.5 (28 model simulations, red curve) scenarios. The averages are indicated by thicker lines. The anomaly observed in 2003 (2.8°C, marked by a star and the dashed threshold) becomes cold in RCP8.5 from 2040 onwards, but remains warm in RCP2.6 until 2100. b) Daily temperature of the 2003 summer (black circles) averaged over central France (red hatched rectangle on the map), compared to the observed mean temperature over 1961-1990 (thick black curve) and to the distribution of percentiles over 2070-2099 in all RCP8.5 simulations (blue zone delimited by the first and tenth percentiles for cold days, red zone by the 90th and 99th percentiles for warm days, see scale on the right). Early August 2003 would remain warm at the end of the century, even in this (strongest) scenario. [Source: Figure by Boucher et al., ref.[2])
Is the 2003 hot summer a prototype of the 21st century European summers? At the scale of Western Europe and for the summer season, the 2003 heat wave has a temperature anomaly of around 3°C compared to the 1961-1990 average temperature. It corresponds to an average summer of the 2040s and even becomes an extreme cold one in 2100, according to the RCP8.5 scenario (Figure 6a, [2]). However, in the RCP2.6 scenario it remains an unusually hot summer until 2100 . Moreover, locally (in France) and on a scale of a few days, the hottest days of August 2003 remain unusually hot even in 2100 in the RCP8.5 scenario (Figure 6b, [2]). The answer therefore depends on the scenario and on the spatial and temporal scale considered.

3. Hydrological extremes

Apart from the temperature increase, the increase in the greenhouse effect is likely to disrupt the global hydrological cycle (water exchanges between the atmosphere, the ocean and continents; read: Are we at risk of water shortage? ) and its extreme events (heavy rainfall and droughts in particular), for several reasons:

  • Surface warming promotes evaporation, especially in regions where water is always available (oceans and humid continents).
  • In accordance with the Clausius-Clapeyron relationship (read Thermodynamics a plot of a rising air parcel in a cumulonimbus), a warmer atmosphere has its maximum water vapour content increased by about 7% per degree of warming, potentially allowing a larger atmospheric water reservoir to be mobilized in a warm climate when weather conditions are favourable to precipitation.
  • Finally, rainfall patterns can be impacted by possible changes in atmospheric circulation since it is this circulation that transports most of the water vapour that will contribute to precipitation in a given location.

The response of hydrological extremes is particularly difficult to understand because of its spatio-temporal heterogeneity and of the direct influence (apart from climate change) of humans on continental water flows and storage.

Available in situ observations show that there is already an increase in the number and/or intensity of heavy rainfall in some regions of the world, particularly in Europe and North America where relatively long measurement series are available. For France, no systematic assessment of extreme precipitation trends is yet available.

3.1. “Mediterranean events”

episode pluie mediterranee - schema pluie meditarranee - mediterranean rainfall event
Figure 7. Illustration of a Mediterranean rainfall event [Source: Météo-France].
In the presence of a very humid Southerly to South-Easterly wind regime, the “Mediterranean events” observed in south-eastern France correspond to the highest precipitation events in mainland France (Figure 7). They are the subject of particular attention and some studies suggest a recent intensification of these events. However, it is more difficult to translate these changes into floods, as they are highly subject to increasing anthropogenic impacts (e.g. urbanisation, deforestation, agriculture) in many catchment areas (this is even more true at the global scale).

3.2. Droughts

secheresse france
Figure 8. Percentage of the French metropolitan territory affected by agricultural drought each year. The criterion used here is the first decile of soil moisture over the period 1961-1990, based on re-analysis data. [Source: © Météo France, Result of the ClimSec project, see Ref.[3]]
Droughts are caused by a more or less persistent water deficit. To characterize them, indicators are used. Meteorological droughts are the easiest to describe and show disparate trends from one region to another, sometimes largely influenced by natural climate variability (in Sahel or more recently in California for example). Recent trends in agricultural droughts are more difficult to assess in the absence of a global network of in situ measurements and despite progress in spatial observation. An alternative is to simulate the evolution of soil water content in response to observed variability in meteorological parameters. This method implemented by Météo-France on the metropolitan territory shows an increase in soil droughts in several regions since 1958 (Figure 8), particularly in the Mediterranean regions but also in West France [3].

3.3. Future global hydrological changes

Future projections suggest some qualitatively robust changes, including an increase in spatial and temporal contrasts in precipitation around the globe, often summarized by the motto “wet get wetter, dry get drier“. If this pithy motto is still the subject of debate within the scientific community, the drying up of the Mediterranean basin and more generally a poleward expansion of arid and semi-arid zones seems inevitable. As a result, the contrast between the European North (wettest) and South (driest) is expected to increase, and some recent work even suggests that most numerical climate models underestimate the summer drying of the northern hemisphere mid-latitudes.

These models also indicate a relatively widespread intensification of heavy precipitation events in response to global warming, including in areas that will experience an average drying. The exceptions to this increase in extreme rainfall are mainly in subtropical regions. In the simulations, this intensification of heavy rainfall is all the more pronounced as the scenario of increasing greenhouse gas concentrations is high. Also, the shorter the period of analysis of precipitation accumulation (daily to hourly accumulations), the more frequent and intense heavy precipitation is projected to be. This intensification occurs at a rate that sometimes exceeds the 7% per degree of average warming predicted and observed for atmospheric water vapour. However, these numerical results should be considered with caution due to the limited horizontal resolution of most models and their simplified representation of the processes associated with these extreme events.

The expected intensification of the hydrological cycle in warmer climates also results in an increased risk of drought in many parts of the world, including some regions where the average annual precipitation increases during the 21st century. This is due both to an increase in the temporal variability of precipitation (increase in the number of consecutive days without rain) and to an increase in evapotranspiration (evaporation of continental surfaces and transpiration from plants to the atmosphere). In summary, global warming therefore affects both ends of the precipitation distribution, making both intense rainfall events and drought episodes more likely.

Mediterranean-type climates (around the Mediterranean, but also some regions of Australia, South Africa, or America) are likely to be particularly affected by these hydrological changes, while more generally, a shift of arid zones towards mid-latitudes is expected. Although oversimplified, the paradigm “rich get richer, poor get poorer” thus reflects a predictable increase in inequalities in the climatic water supply. In France, a decrease in the quantity of water available in the soil is expected, as well as a decrease in the low levels of most rivers.

4. Tropical Cyclones

Tropical cyclones, also known as hurricanes (in the Atlantic) or typhoons (in the Pacific), are by far the most devastating meteorological events, both in terms of their power and of the population affected (read Tropical Cyclones: development and organization and Tropical Cyclones: impacts and risks). It was only since the 1970s that a systematic observation of cyclones has been possible with the advent of satellites. Thus, any trend estimated over the entire 20th century is questionable (Figure 9).

moyenne cyclones tropicaux atlantique - annual number tropical cyclones in atlantic
Figure 9. Annual number of tropical cyclones or hurricanes in the Atlantic (categories 1 to 5 on the Saffir-Simpson scale), and of the strongest hurricanes (categories 3 to 5 on the same scale). Data from the Hurricane Research Division of the United States Federal Oceanic and Atmospheric Administration (NOAA). [© Gilles Delaygue]
In addition, the observed cyclones observed are sensitive to natural climate variability affecting the ocean basins. The relative calm of cyclonic activity in the Atlantic basin during the 1970s and 1980s is probably linked to a cold phase of the Atlantic multi-decadal Oscillation (AMO), which also contributed to the major droughts in the Sahel at that time. This mode of variability corresponds to an oscillation of sea surface temperature anomalies in the North Atlantic on a time scale of several decades. The decades that followed were marked by greater hurricane activity in the Atlantic Ocean, which is difficult to compare with the previous positive phase of the AMO due to the lack of quality data, particularly satellite data. Thus, it is not easy to calculate long-term trends in tropical cyclone activity based on past observations. Some studies have attempted to highlight upward trends in the most intense cyclones over the past 40 years, but there has been little consensus in the scientific community.

Modelling tropical cyclones faces the difficulty of representing these phenomena in numerical climate models. Indeed, their small size (from a few tens to several hundred kilometres) requires the use of climate simulations on very fine calculation grids (around 50 km) in order to represent them realistically. The grid of a model can be thought of as a network of points of which the mutual distance limits the size of the smallest phenomenon that can be simulated.

Despite this limitation on their simulation, the work carried out to date converges on the trend of cyclonic activity for the end of the 21st century. Thus, climate models suggest that the total number of tropical cyclones would remain stable or even decrease in warmer climates, as conditions conducive to their onset would become somewhat rarer. On the other hand, once triggered, cyclones draw their energy from the heat content of the first 50 metres of the surface ocean. In a warmer world, the strongest cyclones would therefore see their intensity increase: stronger maximum winds and more intense associated rains. Cyclone Irma, which hit the islands of Barbuda, St. Martin and St. Barthélemy in 2017 as a category 5 cyclone (cover image), is a good example of these most intense cyclones, whose probability of occurrence is expected to increase with warming in the Atlantic basin. The most recent work also indicates a possible extension towards the poles (beyond the tropics) of regions affected by tropical cyclones.

It should also be borne in mind that the damage caused by cyclones does not only depend on their intrinsic characteristics (intensity, trajectory, etc.), but also on associated phenomena, such as storm surge. The observed sea-level rise, projected for the 21st century, thus makes coastal regions increasingly vulnerable to cyclonic phenomena by submersion, such as the one caused by Cyclone Pam in 2015 in Vanuatu. In addition, a trend that seems to be well marked in climate projections concerns the intensity of rainfall associated with cyclonic phenomena. This intensity shows a significant increase in models for the end of the 21st century, sometimes beyond the 7% per degree of warming suggested by the Clausius-Clapeyron formula. The latter aspect of tropical cyclones is particularly important in a context where coastal cities are becoming denser and thus more vulnerable to flood risks. Recently, Cyclone Harvey (August 17 to September 2, 2017) dramatically illustrated the effects of heavy cumulative rainfall over the life of the system, worsened by the stagnation of the system over the city of Houston.

5. Extra-tropical storms

Storms – and more generally atmospheric circulation – in mid-latitudes are related to the temperature difference between the equator and the poles (read Atmospheric circulation: its organization). Their recent and future evolution therefore depends on the meridional contrasts of global warming. In the Northern Hemisphere, at the surface, the recent and projected melting of the Arctic ice pack results in a more pronounced warming at the pole, which reduces the temperature gradient between the pole and the equator. On the other hand, warming peaks at the top of the troposphere (about 10 km above sea level), reinforce this same gradient. The evolution of the atmospheric dynamics of the mid-latitudes, including depressions and storms, therefore depends on the competition in warming between these two regions, i.e. the upper tropical troposphere against the lower Arctic troposphere. More regional factors, such as the distribution of warming over the North Atlantic, can also modulate the evolution of lows over Europe. If we add to this the great natural variability of the climate system at these latitudes (read Climate variability: the example of the North Atlantic Oscillation), and the fact that only a fraction of the depressions evolve into real storms, we can understand that with the current state of knowledge, the effect of global warming on these phenomena remains very uncertain.

tempetes hivernales nord sud monde - winter storms northern and southern hemispheres
Figure 10. Projections of the change in frequency of winter storms in the northern (left) and southern (right) hemispheres, between the present climate 1986-2005 and the future climate 2081-2100 with the RCP8.5 scenario. In the southern hemisphere, models predict a decrease at mid-latitudes and an increase at higher latitudes: this reflects a poleward shift of storm tracks towards the South Pole. In the northern hemisphere, this signal is less clear, as it is offset by the particularly intense surface warming of the Arctic. The dotted regions are those where the used 29 models agree on the sign of change. [Source: Adapted from Figure 12.20 of the 5th IPCC report]
Studies based on atmospheric observations and re-analyses have nevertheless shown an increasing trend in the number of storms over Scandinavian countries during the 20th century. For other regions, particularly France, no reliable trends have been identified in the past. In the 21st century scenarios, while a majority of numerical models seem to favor a poleward shift of the activity of mid-latitude lows towards the poles (especially in the southern hemisphere) (Figure 10). In view of the complexity of the involved phenomena, this trend should still be considered with caution. It is therefore still too early to draw conclusions on the influence of anthropogenic warming on extra-tropical storms. However, it can be expected that the associated precipitation will be more abundant, due to the humidification of air masses and in accordance with the Clausius-Clapeyron relationship already mentioned above.

6. Conclusions: More frequent or intense extreme phenomena

The study of extreme weather events is a major scientific and societal challenge. Climate change is already modifying and will continue to modify the probabilities associated with weather hazards. But these changes do not always go in the same direction, as some oversimplified, alarmist or climatosceptical messages sometimes suggest. Thus, while global warming makes some extreme events more frequent and/or intense (heat waves, intense rainfall episodes, droughts), others are less likely (cold waves). The scientific message may even be more complicated:

  • For cyclones, the state of knowledge suggests a slight decrease in the total number but an increase in the number of cyclones of the strongest categories.
  • As for mid-latitude storms, their evolution with climate change remains largely uncertain (which does not mean “misunderstood”).

Beyond the knowledge already acquired, many scientific questions remain. The World Climate Research Programme has made this topic one of its priorities for the next decade. This challenge has multiple dimensions such as the establishment, operation and/or homogenization of the observation network, the improvement of numerical climate models and the evaluation of their ability to simulate extreme events, the development of statistical tools to detect, attribute and understand their climatic evolution, and research on their predictability at different time scales (season, decade, etc.).

Finally, it should be reiterated that even though climate change is expected to affect extreme weather events, it should not be systematically blamed when an event occurs. More specifically, we should not want to hold humans alone responsible for some meteorological event, but rather wonder, in probabilistic terms, how human impacts have changed the risk of the event.

7. Take home messages

  • Human-induced climate change is already modifying and will continue to modify the probability of weather hazards.
  • Most extreme events (e.g. heat waves, heavy rains, cyclones) become more frequent and/or intense, some become less likely (e.g. cold waves), while for others, the signals are contrasted (e.g. storms).
  • Humans cannot be held solely responsible for any observed event; on the other hand, we can estimate how human impacts have modified the probability of the event.
  • Climate variability and the heterogeneity of historical data make it difficult to detect trends in observations.
  • Improved numerical climate modelling tools will reduce the uncertainties associated with the future evolution of extreme events.

 


Notes and references

Cover image. Hurricane Irma on the Lesser Antilles (Sept. 6, 2017) [Source: © NASA/Goddard Space Flight Center Earth Science Data and Information System (ESDIS) project.]

[1] Intergovernmental Panel on Climate Change (IPCC), 2013, Fifth Assessment Report, Scientific Elements, Contribution of Working Group I (http://ipcc.ch/report/ar5/wg1/).

[2] Boucher et al (2015) Projection of future climate change, La Météorologie, n°88, p.56-68. Doi: 10.4267/2042/56362

[3] ClimSec project. See Soubeyroux et al (2012) Soil droughts in France and climate change: Results and applications of the ClimSec project. La Météorologie nº78, p.21-30. doi: 10.4267/2042/47512 and CNRM website: https://www.umr-cnrm.fr/spip.php?article605&lang=en


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: CATTIAUX Julien, CHAUVIN Fabrice, DOUVILLE Hervé, RIBES Aurélien (January 15, 2021), Weather Extremes and Climate Change, Encyclopedia of the Environment, Accessed December 24, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/en/climate/extreme-weather-events-and-climate-change/.

The articles in the Encyclopedia of the Environment are made available under the terms of the Creative Commons BY-NC-SA license, which authorizes reproduction subject to: citing the source, not making commercial use of them, sharing identical initial conditions, reproducing at each reuse or distribution the mention of this Creative Commons BY-NC-SA license.

极端天气和气候变化

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ouragan irma - ouragans - evenements meteo extremes - extreme weather events - climate change

       极端天气事件是如何随着气候变化而变化?它们是否越发频繁?更加剧烈?这是人类的责任?这篇文章介绍了与这一主题相关的科学知识,探讨了极端温度、极端水文、热带气旋和特大热带风暴最近及未来的发展趋势,解释了自然气候变化和数字模拟的不确定性。气候变化已经在改变并将继续影响气象灾害出现的可能性,使一些极端事件发生得更加频繁且/或激烈;另一些极端事件则不然。然而,我们不应把任何气象事件的发生全部归因于人类活动,而是要用概率角度思考人类活动是如何改变事件发生的风险的。

1. 天气和气候之间很难联系起来

  热浪、寒潮、大雨、干旱、飓风、风暴以及其他极端天气事件经常出现在新闻中,特别是因为它们对社会和环境的巨大影响极端天气事件与气候变化之间存在何种联系是应该向科学家提出的问题。越来越多的研究也在关注这一主题。问题的答案并不是容易得到,特别是由于媒体对这些天气现象越来越系统化的报道以及人们对气象灾害脆弱性的增加,可能会给人们对气候变化趋势产生错误的印象。

环境百科全书-极端天气和气候变化-浓度路径的全球均温演变情景
图1.基于4种温室气体(GHG)浓度路径的全球均温(单位:摄氏度)演变情景。零基线代表20世纪末(1986-2005)均温。每个曲线附近的不确定性涉及气候模型的选择和气象灾害。每个情景和阶段使用的模拟数量在图中有所标注。
[来源:第五次政府间气候变化专门委员会评估报告的图 SPM7,见参考文献[1])

  在人为因素引起气候变化的背景下,科学挑战是双重的:一方面,在观测到的事件中分离出人类的影响痕迹;另一方面,要预测气候变暖的变化趋势。第二点的研究是使用对未来气候进行数字模拟,或“气候预测”(图1,[1])加以研究。目前的气候预测会考虑21世纪四种可能的温室气体浓度轨迹(“典型浓度路径”,RCP),该路径与温室气体排放的社会经济情景有关。在RCP2.6低排放的情景下,21世纪地球表面增温1℃(± 0.7)(即相比于工业化前水平增长1.6℃);在RCP8.5高排放的情景下,地表增温3.7ºC (± 1.1),即相比于工业化前水平增长4.3ºC 。

  对极端天气事件近期或未来趋势的研究通常针对每种类型的事件进行。因此,本文会依次研究极端温度、极端水文、热带气旋(包括飓风和台风)和特大热带风暴。我们将看到,一些研究表明,某些类型极端事件在频次和/或强度上已经可以检测到变化,其他类型的极端事件在21世纪可能会出现或增加。然而,这些定性的可靠的成果不应掩盖极端事件研究所面临的两大难题

  • 第一个是自然气候变率在这些现象中的主导作用,它往往使人为信号不明确,需要高质量的观测和/或大量的模拟才能探测到变化。
  • 第二个是气候模型的不完善性,因此需要继续完善、评估它们,并尽可能依靠多种独立的模型。

2. 极端温度

  一个多世纪以来观测到的全球变暖现象不仅影响均温,还会影响温度的整个统计分布,即某一时刻和某一地方全部可能出现的温度分布范围,在这一分布的两端意味着罕见的温度事件—极端低温和极端高温。极端天气的出现通常产生巨大的社会环境影响。传统上极端温度是依据其发生的频次,或者按照统计学术语,通过其分布的分位数来定义的。例如:通常的说法是,如果某天的温度位于期望值的前10%(即这个温度在分布上“超过了第90个百分位数或第9个十分位数”),将其描述位天气异常热。按照这种理解方式,我们可以认为在参考时期内,10%的天数都异常热。一个对称定义可用于异常寒冷的天气,季节性温度周期通常也会考虑在内。如此一来,我们便可讨论冬季的暖日或者夏季的冷天。为了研究两个极端事件的演变,我们要么考虑频次(低于或高于现在温度十分位数的天数)的演变,要么考虑强度的变化,例如将其定义为10%极端天数的平均温度。

2.1. 极端天气日的出现

环境百科全书-极端天气和气候变化-2018年夏季六、七、八月均温分布
图 2. 上图: 法国1900-2018年夏季六、七、八月均温分布(法国气象组织数据)。每条竖线代表一个夏季。下边的横轴给出了每个夏季对应的温度(℃),上边的横轴显示的该温度与全年均温的差值(℃)。极冷极高温的夏季(分别分布在十分位首尾),尤其是2003和2018年出现热浪的夏季被单独标出。黑色曲线是最接近数据的高斯分布。下图:显示未来温度分布变化的简图。温度分布会出现简单的转化(从黑色曲线到红色曲线的相同增温过程)。为进一步解释说明,把2003年夏季温度当成未来气候的平均值(请与上方新的横轴比较)。[© 朱利安·卡帝奥克斯]
环境百科全书-极端天气和气候变化-异常暖天、冷天
图 3. 观测到全球(仅限大陆)异常暖天的年均频次 (上图) 和异常冷天的频次(下图) 。 考虑到了固定的温度阈值(1961-1990基准分布中的首尾十分位数)。 曲线关注点在 1961-1990年,曲线来自3组数据 (颜色)。
[来源:改编自政府间气候变化专门委员会第五次报告图2.32,报告见参考文献[1]]

  随着气候变暖,整个温度分布就会发生变化(图2)。按照极端天气的定义,极冷天气出现的频次预计会减少,极端高温天气出现的频次会增加(既定的气温阈值)。或者,极冷天气变得没那么冷了(既定的频次阈值)。

  全球范围内,异常暖的天数自1950年开始就一直大幅增加(图3,[1]),其中20世纪90年代和21世纪前10年热天出现的频次比以往更多。对比来看,我们观测到的异常冷的天数减少了。与此同时,整体趋势是相较于低温日、月记录,高温日、月记录会更加频繁地被打破。这些结果在区域范围(尤其是在欧洲)也被观测到了。

2.2. 热浪与寒潮

  除了日统计数据之外,热浪(即连续数日的炎热天气)往往更频繁、更强烈和/或更长。但寒潮天数自1950年之后大幅减少。虽然最近的热浪,如2003年8月西欧出现的热浪,在性质上符合我们对气候变暖的预期,但近些年欧洲观测到的少数冬季寒潮(2009年10月冬季、2010年12月、2012年2月)似乎与全球变暖的观点相矛盾。

  在全球变暖的背景下寒潮在局部偶尔会出现,这并非悖论。从长期看,这是气象灾害和气候趋势之间的区别。

  变暖是个背景过程,叠加在气候系统自然内部变率之上[阅读“气候变化:以北大西洋涛动为例”]。即使气候变化会继续造成寒冷天气事件,但由于受全球变暖影响,预计它们的频率和/或强度有望降低[参见“把单个天气事件归因于气候变化:2003年热浪”]。这恰恰又是我们再一次看到的:最近寒潮的凉爽只是相对于1939/40年和1962/63年的寒冬而言的,这两个冬天在大气环流方面是相似的。

  最后,不能排除最近几年的气候变暖(在北极尤为显著)可能会暂时扰乱我们所在纬度的气团循环,并增加了在欧洲产生寒潮的可能性。然而,关于这一点科学界存在颇多争议。

2.3. 21世纪的气候演变

  不管情景何,21世纪全球变暖或多或少地会继续下去。因此,近些年观测的极端温度趋势在气候预估中得以证实,则不足为奇:极端高温越来越频繁和强烈,极端低温越来越罕见和不那么明显然而,这些变化的程度主要取决于温室气体排放情景的选择。同一情景下,模型不同,变化的程度也不同,并且仍然受气候内部变率的强烈调节。

环境百科全书-极端天气和气候变化-根据三个情景预测
图4.根据三个气候情景预测的未来异常热天(左)和异常冷天(右边)的年频次。依据定义,频次比1961-1990年参考时期基准期高10%。
[来源: 政府间气候变化专门委员会第五次报告图 WGI-AT9,见参考文献[1]]

  在全球和年尺度上,平均而言,到2100年,从不同的气候模型模拟结果来看,观测到日温超过当前第90百分位数的概率从目前的10%(每十天有一天)增长到RCP2.6情景下的25%(每4天有一天),甚至在2100年达到RCP8.5情景下的60%(超过一半的天数)。相反,在RCP2.6和RCP8.5中,异常寒冷天气的频率分别降至4%和1%(图4)。这种演变还会影响非常罕见的事件。例如,当前气候下极端高温事件平均每20年出现一次(又称20年回归期),而在2100年RCP8.5场景下平均每两年出现一次。同样在RCP8.5情景下,极端低温事件的回归期从20年延长至100多年。虽然趋势如此,出现极端低温天气的可能性依然存在。即使在路径浓度高的情景下,在21世纪一些低温记录仍有望在局部区域被打破。但是相较于高温记录,低温纪录被打破的频次会低很多。例如,在美国,目前出现高温记录和出现低温记录的比例是2:1:在中等情景下,估计2050年左右为20:1,2100年为50:1。

  除了与情景选择和内部气候变率相关的不确定性外,极端温度的未来演变对数值气候模式的选择很敏感,特别是在区域和季节尺度上。在欧洲,RCP8.5情景下超过夏季温度第90个百分位数(或第9个十分位数),即当前气候的10%的概率在30%-90%之间。这取决于2070-2100年未来夏季的气候模型。

  这些不确定性还会改变多日事件的特征。例如,尽管未来的预测一直认为欧洲夏季热浪的频率、强度和持续时间会增加,然而既定情境下,它们到2100年的演变会因所采用的数字模型的不同而有三倍的差异。

环境百科全书-极端天气和气候变化-法国夏季温度分布
图 5.法国夏季温度分布变化简图,以及曲线的简单转化(变暖,从黑色曲线到红色曲线的平移,见图2)与增宽(从黑到红曲线,变化幅度增加)。在增宽的情况下,极端高温事件的可能性进一步增加。与图2类似。
[© 朱利安·卡帝奥克斯]

  最后,如果极端温度的变化主要取决于平均变暖的幅度(分布的变动),那么它们受到变率变化(分布形状与宽度,见图5)的调节。预测显示,在欧洲夏季变异的微增会使极端高温事件出现的可能性更大。冬季变率略有减少,就会使极端低温事件出现的可能性更小。这些现象分别与夏季土壤干燥与冬季雪量减少有关。

环境百科全书-极端天气和气候变化-图 6. a) 1961-1990年欧洲夏季温度
图 6. a) 1961-1990年欧洲(图中蓝色矩形框内)夏季(六七八月)温度与正常值的偏离,用于观测(黑色曲线)、历史模拟(31个模型模拟,蓝色曲线)以及未来RCP2.6(18个模型模拟,绿色曲线)、RCP8.5(28个模型模拟,红色曲线)情景下的模拟。较粗的线代表平均值。2003年观测到的异常值(2.8℃,以星号和虚线标出)在RCP8.5情景下从2040年开始温度降低,但在RCP2.6情景下一直到2100年会保持温暖。b) 与1961-1990年(黑色粗曲线)观测的均温和2070-2099年所有RCP8.5模拟中(蓝色区域代表第一和第十个百分位数之间的冷天,红色区域代表第九十和九十九个百分位数间的暖天,见右图标度)的百分位数分布相比,2003年夏季日温(黑色圆圈)是法国中部(图中红色阴影矩形区域)的均温。即使在这一(浓度最高)情景下,2003年8月初在该世纪末依旧保持高温。
[来源:布歇尔(Boucher)等,参考文献[2]]

  2003年的炎热夏季是21世纪欧洲夏季的典型代表吗?就西欧夏季而言,2003年热浪温度与1961-1990均温的差值约为3℃。根据RCP8.5情景(图6a[2]),它相当于21世纪40年代的普通夏季的情况。甚至在2100年成为一个极端寒冷的夏天。然而,在RCP2.6情景下,直到2100年,它仍然是一个异常热的夏季。此外,局部地区(即在法国)和以几天为尺度,在RCP8.5场景下(图6b,[2]),2003年8月最热的几天依旧会是异常炎热。因此,该问题的答案取决于情景选择和时空范围。

3. 极端水文事件

  温室效应除了会使温度升高,还有可能改变全球水文循环(大气、海洋和大陆间的水循环;阅读“我们是否面临缺水的危险?”)以及相关极端事件(尤其是强降雨和干旱)。原因如下:

  • 表面增温加剧蒸发,在水资源丰富的地区尤其如此(如海洋和湿润的大陆)。
  • 根据克劳修斯-克拉佩龙方程(阅读热力学:积雨云中上升的气团),一个较暖的大气温度每升高一度,其最大的水蒸气含量增加约7%。在温暖的气候条件下,当天气条件有利于降水时,可能会动用更大的大气水库
  • 最后,降雨模式会受到大气环流可能发生变化的影响,因为正是这种环流输送了大部分水蒸气,从而在特定地点导致降水。

  因为其时空异质性和人类对大陆水流和储存的直接影响(除了气候变化之外),水文极端现象的响应特别难以理解。

   现有的观测表明,在世界某些地区,特别是在欧洲和北美,有较长的测量序列可用的地区,强降雨的次数和/或强度已经有所增加。对于法国,目前还没有对极端降水趋势的系统评估。

3.1. “地中海事件”

环境百科全书-极端气候和气候变化-地中海降水
图7. 地中海降水事件示意图
[来源:法国气象组织]

  法国东南部受极为湿润的南风和东南风影响,该区域观测到的“地中海事件”与法国本土最高降水事件一致(图7)。这些事件是关注的焦点。一些研究表明最近此类事件发生的强度在增加。然而,将这些变化转化为洪水更加困难,因为在许多集水区,它们受到日益增加的人为活动 (例如城市化、森林砍伐、农业) 的影响(在全球范围内更是如此)。

3.2. 干旱

环境百科全书-极端气候和气候变化-法国本土每年受农业干旱影响的比例
图8.法国本土每年受农业干旱影响的比例。这里的标准是基于再分析数据的1961-1990年土壤湿度第一个十分位数。
[来源: © 法国气象组织, “气候变动与安全威胁项目”结果, 参见参考文献[3]]

  干旱是由或多或少持续的缺水造成的。干旱指标可用于描述干旱特点。气象干旱是最容易描述和显示了地区间的不同趋势的,有时主要受自然气候变异影响(例如发生在萨赫勒地区,或者最近在加利福尼亚)。虽然空间观测工作取得了进展,但因为缺少全球原位测量网络,对农业干旱的近期趋势评估更加困难。另一种方法就是模拟土壤含水量随观测到的气象参数变化而发生的变化。这种由法国气象局在本国实施的方法表明,自1958年以来部分地区土壤干旱情况有所增加(图8),尤其是地中海区域,以及法国西部[3]

3.3. 未来全球水文变化

  预测显示,未来会出现一些质量上的强劲变化,包括全球降水时空差异增大。这可以用 “湿的越湿,干的越干”的格言来概括为。如果这些精辟的格言仍然是科学界争论的主题,那么地中海盆地的干涸以及更广泛地说,干旱和半干旱地区向极地的扩张似乎是不可避免的。结果是,欧洲北部(最湿)和南部(最干)之间的差异预期会继续增大。最近一些研究表明,大部分数字模型都低估了北半球中纬度地区的夏季干旱趋势。

  这些模式还表明,由于全球变暖,强降水事件将相对普遍地加剧,包括在将经历平均干旱的地区,但在亚热带地区例外。在模拟中,随着温室气体浓度增加的情景越来越高,这种强降雨的加剧更加明显。降水累积分析周期越短(日-时累积),强降水预测频率越高、强度越大。这种强化发生的速度有时超过预测和观测到的变化,这些变化表现为温度每升高一度大气水蒸气强度增加超过7%。然而,应该谨慎看待这些数据。因为大多数模型水平分辨率有限,而且对极端事件相关的过程描述也过于简化。

  因为降水时间变化增加(连续无雨的天数增多)以及蒸散加剧(地表蒸发和植物向大气的水汽输送),在更暖的气候条件下,水文循环增强也会导致世界很多地区干旱风险增加,包括一些21世纪出现年均降水增加的地区。总之,全球变暖因此影响降水分布的两端,使得强降水事件和干旱出现的可能性都增加了。

  地中海型气候(地中海附近,也包括澳大利亚、南非或美洲的部分地区)可能尤其受到这些水文变化的影响。但是总体上,干旱地区预计会向中纬度扩展。“富者愈富,穷者愈穷”的范式虽然过于简化,但反映出了气候水源供给量差距的拉大。在法国,预计土壤有效水分含量会减少,多数河流的低水位也会降低。

4. 热带气旋

  热带气旋,在大西洋也称飓风或在太平洋称台风。就其威力及影响的人口而言,热带气旋是目前为止最具破坏性的气象事件。(阅读“热带气旋:形成和结构”和“热带气旋:影响和危害”)。自20世纪70年代以来,随着卫星的出现,对气旋进行系统的观测才成为可能。因此,整个20世纪所估计的所有趋势都存在问题(图9)。

环境百科全书-极端气候和气候变化-大西洋地区热带气旋或者飓风每年出现的次数
图9.大西洋地区热带气旋或者飓风每年出现的次数 (萨菲尔-辛普森飓风量表的一到五级),以及最强飓风每年的次数 (同一量表上的三到五级)。来自美国联邦海洋和大气管理局(NOAA)飓风研究部门的数据。 [© 吉尔斯·德莱格(Gilles Delaygue)]

  此外,已经观测到的气旋对影响海盆的自然气候变异非常敏感。20世纪70、80年代,大西洋海盆的气旋活动相对平静,这可能与大西洋年代际振荡(AMO)的冷期有关,这也造成了当时萨赫勒地区的严重干旱。这种变化模式与北大西洋海表面温度几十年的异常振荡具有一致性。之后几十年里,大西洋海域的飓风活动表现出更加猛烈的特点,由于缺少高质量的数据,尤其是卫星数据,因此难以和此前大西洋年代际振荡的正位相相比较。因此,基于先前的观测,计算热带气旋活动长期趋势并非易事。一些研究试图强调过去40年里最强烈气旋呈上升趋势,但科学界对此几乎没有达成共识。

  模拟热带气旋的困难在于如何在数字模型中再现热带气旋。事实上,这些气旋规模小(几十到几百千米),需要在精细的计算网格上(约50千米)进行气候模拟,从而更真实地再现。可一个模型的网格可以看做是点构成的网络,点之间的距离限制了可模拟的最小气旋的规模。

  虽然模拟存在局限性,但迄今为止开展的工作集中在21世纪末的气旋活动趋势上。因此,气候模型表明,热带气旋的总数会保持稳定,或者随着气候变暖数量甚至会减少。这是因为此时适合气旋形成的条件减弱了。另一方面,一旦形成条件满足,气旋就会从海洋表面50米海水中吸收热量,获取能量。因此,在一个更加温暖的世界,最强气旋的强度会有所增加:最大风力更大,带来的降雨更强。2017年巴布达岛、圣马丁岛和圣巴泰勒米岛遭遇了5级飓风伊玛(Irma,封面图片)的袭击。伊玛是典型的强气旋,其出现的概率预计会随着大西洋海盆增温而增加。最近的研究还显示,受热带气旋影响的地区可能会向两极扩张(超出热带)。

  同时还要注意,气旋的危害程度,不仅取决于气旋本身内在的特点(强度、轨迹等),还取决于与气旋相关的现象,比如风暴潮。21世纪预计会观测到海平面上升,因此气旋发生时,沿海地区也易受到气旋现象的影响而被淹没。例如2015年发生在瓦努阿图的气旋帕姆就导致城市被淹没。另外,在气候预测中存在着一种似乎非常明显的趋势,该趋势涉及与气旋现象有关的降水强度。21世纪末,这种强度在模型中会大幅增加。根据克劳修斯-克拉贝龙方程,有时每升高一度,强度增加超过7%。当前沿海城市更加密集,更易受到洪水的威胁。因此,在此背景下,气旋危害程度的其它方面更为重要。最近,气旋哈维(2017年8月17日至9月2日)戏剧性的说明了在气旋系统生命期内大量累积降水的影响,并因气旋在休斯顿市上空的停滞而恶化。

5. 温带风暴

  中纬度地区的风暴——以及更普遍的大气环流——与赤道和两极之间的温差有关(参见大气环流:其组织)。因此,它们最近和未来的演变取决于全球变暖的经向对比。在北半球,从表面上来看,北极浮冰预计未来会继续融化,导致极地变暖更加明显,进而降低极地和赤道的温度梯度。另一方面,对流层顶(海拔约10千米)的增温峰值加大了这一温度梯度。因此,中纬度大气运动(包括低气压和风暴)的演变取决于这两个区域(热带对流层上部和北极对流层下部)在增温方面的差距。更多区域因素,如北大西洋变暖的分布,也会调节欧洲低气压区的演变。此外,如果把该纬度范围气候系统巨大的自然变异性(阅读“气候变动:北大西洋涛动”)考虑在内,以及只有一小部分低气压演变成真正的风暴这一事实,我们就可以理解,以目前知识,仍无法特别明确全球变暖对这些现象产生的影响。

       如果我们再加上这些纬度地区气候系统的巨大自然变异性(参见气候变异性:北大西洋涛动的例子),以及只有一小部分低气压演变成真正的风暴的事实,我们就可以理解,以目前的知识水平,全球变暖对这些现象的影响仍然非常不确定。

环境百科全书-极端气候和气候变化-RCP8.5情景
图10. RCP8.5情景下,南(右)北(左)半球冬季风暴频次从1986-2005年的当前气候到2081-2100年未来气候的变化预估。在南半球,模型预测在中纬度频次会减少,在高纬度会增加:这反映了风暴路径向南极移动的趋势。但是这种趋势在北半球就不如南半球明显,原因在于这种趋势被极其强烈的北极地表变暖所抵消。点状区域里,使用的29个模型对变化发生的标志达成共识。
[来源:改自政府间气候变化专门委员会第五次报告图12.20]

  基于大气观测的研究以及再分析表明,在20世纪,风暴在斯堪的纳维亚国家的发生次数呈上升趋势。在其他地区,尤其是法国,在以前没有发现任何可靠的趋势。在21世纪的情景中,虽然大多数数字模型似乎支持中纬度低气压活动会向两极转移(尤其是在南半球)(图10)。鉴于相关现象的复杂性,我们应该谨慎对待这种转移趋势。因此,关于人为变暖对热带外风暴产生的影响做出的结论还为时过早。然而,由于空气团的湿度增加,而且根据前边提到的克劳修斯-克拉贝龙方程,相关降水预计会更加充沛。

6. 结论:极端现象更加频繁或者更加强劲

  极端天气事件研究是一项重大的科学研究和社会挑战。气候变化正在改变,且将继续改变气象灾害相关的概率。然而,并非像一些危言耸听或气候怀疑论的观点所言:这些变化总是朝着同一个方向发展。所以,全球变暖会使一些极端事件更加频繁,且/或更加强烈(热浪、连续强降水、干旱),而让另一些事件(寒潮)出现的可能性降低。这些科学信息可能会更加复杂:

  • 就气旋而言,目前的知识状况表明,总数略有减少,但最强类别的气旋数量有所增加。
  • 至于中纬度风暴,它们随气候变化的演变在很大程度上仍然不确定(这并不意味着“误解”)。

       我们虽然已经掌握一定的知识,但还有许多科学问题尚未解决。世界气候研究计划已经将这一主题作为作为未来十年的优先事项之一。这一问题从不同层面构成了挑战,比如观测网络的建立、观测网络的运行和同质化、数字气候模型的改进以及评估其模拟极端事件能力,开发统计工具来检测、归因和理解其气候演变以及研究它们在不同时间尺度(季节、十年等)的可预测性。

  最后,应该再次强调,即使气候变化预计会影响极端天气事件,但是不能一出现极端事件,就全部归咎于气候变化。更具体来说,我们不应该把一些气象事件全部归咎于人类,更应该多思考人类的影响如何改变气象事件的风险

7. 要点

  • 人类导致的气候变化已经并继续影响气象灾害发生的概率。
  • 大多数极端事件(比如热浪、大雨、气旋)发生得越来越频繁,且/或更强烈,比如像寒潮一类极端事件不大可能出现,像风暴一类事件的情况则相反。
  • 我们不能把所有观测到的气象事件全都归咎于人类;另一方面,我们可以估算人类是如何影响这些事件发生的概率的。
  • 由于气候多变性和历史数据的异质性使得很难在观测中发现趋势。
  • 完善后的数字气候模拟工具将会减少预测未来极端事件演变中的不确定性。

 


参考资料及说明

封面照片小安的列斯群岛上的飓风艾尔玛 (2017.09.06) [来源: © 美国宇航局/戈达德太空飞行中心地球科学数据与信息系统 (ESDIS) 项目]

[1] Intergovernmental Panel on Climate Change (IPCC), 2013, Fifth Assessment Report, Scientific Elements, Contribution of Working Group I (http://ipcc.ch/report/ar5/wg1/).

[2] Boucher et al (2015) Projection of future climate change, La Météorologie, n°88, p.56-68. Doi: 10.4267/2042/56362

[3] ClimSec project. See Soubeyroux et al (2012) Soil droughts in France and climate change: Results and applications of the ClimSec project. La Météorologie nº78, p.21-30. doi: 10.4267/2042/47512 and CNRM website: https: //www.umr-cnrm.fr/spip.php?article605&lang=en


The Encyclopedia of the Environment by the Association des Encyclopédies de l'Environnement et de l'Énergie (www.a3e.fr), contractually linked to the University of Grenoble Alpes and Grenoble INP, and sponsored by the French Academy of Sciences.

To cite this article: CATTIAUX Julien, CHAUVIN Fabrice, DOUVILLE Hervé, RIBES Aurélien (March 14, 2024), 极端天气和气候变化, Encyclopedia of the Environment, Accessed December 24, 2024 [online ISSN 2555-0950] url : https://www.encyclopedie-environnement.org/zh/climat-zh/extreme-weather-events-and-climate-change/.

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