Heat Waves & Drought

Drought and heatwaves are inextricably linked, and have devastating socio-economic and environmental impacts. This issue features a suite of articles outlining how these extreme events may increase in magnitude and frequency with anthropogenic warming, highlighting the increased need to mitigate and adapt to future conditions.

Recent studies have produced conflicting results about the impacts of climate change on drought. In this Perspective, a commonly used drought index and observational data are examined to identify the cause of these discrepancies. The authors indicate that improvements in the quality and coverage of precipitation data and quantification of natural variability are necessary to provide a better understanding of how drought is changing.

The dramatic switch from extreme drought to severe flooding in California, and the accompanying flip from atmospheric ridge to trough in the northeastern Pacific, exemplifies the pathways to an intensified water cycle under a warming climate.

Groundwater depletion the world over poses a far greater threat to global water security than is currently acknowledged.

Recent scientific advances make it possible to assign extreme events to human-induced climate change and historical emissions. These developments allow losses and damage associated with such events to be assigned country-level responsibility.

The African continent is one of the most vulnerable regions to future climate change. Research now demonstrates that constraining anthropogenic warming to 1.5 °C instead of 2 °C will significantly lower the risk of heatwaves to inhabitants.

Low soil moisture conditions can induce drought but also elevate temperatures. Detailed modelling of the drought–temperature link now shows that rising global temperature will bring drier soils and higher heatwave temperatures in Europe.

Deforestation often increases land-surface and near-surface temperatures, but climate models struggle to simulate this effect. Research now shows that deforestation has increased the severity of extreme heat in temperate regions of North America and Europe. This points to opportunities to mitigate extreme heat.

Heat waves have become increasingly frequent in the United States, but their occurrence is largely linked to natural variability. Model simulations reveal anthropogenically forced signals will first emerge in the western United States and Great Lakes regions by ~2030.

In 2003, Europe experienced a summer heatwave that resulted in tens of thousands of deaths. This study uses observation and model data to show that human influence is increasing the probability of extremely hot summers in Europe, with events now expected to occur twice a decade, compared with predictions of twice a century in the early 2000s.

Mean summer temperature in Eastern China has increased by 0.82 °C since the 1950s and five of the hottest summers have occurred since 2000. This study estimates anthropogenic influence to have caused a greater than 60-fold increase in the likelihood of extreme summer heat and projects that hot summers will continue to increase in frequency.

The 2010 Russia heatwave had devastating impacts, including loss of life, wildfire and drought. Model simulations reveal similar heatwaves may be amplified by up to 8 °C in the future as soil moisture becomes less able to suppress maximum temperatures.

Severe drought plagued Europe in 2003, amplifying heatwave conditions that killed more than 30,000 people. Assuming business as usual, such soil moisture deficits will become twice as frequent in the future and affect up to two-thirds of the European population.

Historical records show increased aridity over many land areas since 1950. This study looks at observations and model projections from 1923 to 2010, to test the ability of models to predict future drought conditions. Models are able to capture the greenhouse-gas forcing and El Niño–Southern Oscillation mode for historical periods, which inspires confidence in their projections of drought.

California recently experienced a rapid shift from multi-year drought to abundant rainfall. A large ensemble of climate model simulations suggests that the frequency of extreme wet-to-dry precipitation events will increase by 25% to 100% across California due to anthropogenic forcing.

The record hot year of 2015 in Africa had devastating impacts. The likelihood of future annual temperature extremes over Africa exceeding those of 2015 are 91% and 100% in 1.5 °C and 2 °C worlds, respectively, stressing the benefits of limiting future anthropogenic warming.

Limiting warming to 1.5 °C is expected to lessen the risk of extreme events, relative to 2 °C. Considering Australia, this work shows a decrease of about 25% in the likelihood of record heat, both air and sea surface, if warming is limited to 1.5 °C.

Climatic conditions that challenge human thermoregulatory capacity currently affect around a quarter of the world’s population annually. Such conditions are projected to increase in line with CO₂ emissions particularly in the humid tropics.

Deforestation in the northern mid-latitudes has generally been considered to cause biogeophysical changes that drive mean annual cooling in the region. Research now suggests that historical deforestation has led to substantial local warming of hot days.