The fact that global warming will raise land temperatures, increasing the frequency of droughts and heat waves, is unavoidable, as is the fact that climate change will alter the average amount of precipitation on land. However, it has remained unclear until now under what conditions both extreme events, known as 'compound hot-dry-events,' will occur simultaneously.
These events were defined by the UFZ researchers as summers in which the average temperature was higher than in 90% of the summers between 1950 and 1980, while precipitation was lower than in 90% of those years.
"Drought and heatwaves were previously treated separately; however, there is a strong correlation between the two events, as evidenced by the extremes experienced in Europe in 2003 and 2018. The negative consequences of these compound extremes are frequently greater than the consequences of a single extreme "Dr. Jakob Zscheischler of the UFZ is the study's final author.
Until now, however, it was unknown what the future simultaneous occurrence of these extremes depended on; the uncertainties in the occurrences estimated using routinely used climate model simulations were too large to make firm predictions.
To reduce and better understand these uncertainties, the researchers used a novel model ensemble comprised of seven climate models. To account for natural climate variability, each model simulation was run up to 100 times.
They looked at the historical period between 1950 and 1980 and compared the results to a potential future climate that is two degrees warmer than preindustrial levels. "The advantage of these multiple simulations is that we have a much larger volume of data than with conventional model ensembles, allowing us to better estimate compound extremes," says first author and UFZ climate researcher Dr. Emanuele Bevacqua.
The researchers were able to confirm the previous assumption that the average frequency of compound hot-dry events will increase with global warming: while the frequency was 3 percent between 1950 and 1980, which is statistically an occurrence every 33 years, this figure will be around 12 percent in a two-degree warmer climate. This would be a fourfold increase over the studied historical period.
The climate scientists were also able to conclude from the simulations that the frequency of compound hot-dry events in the future will be determined by precipitation trends rather than temperature trends. The reason for this is that, even with a two-degree increase in temperature, the local temperature increase will be so large that, in the future, every drought anywhere in the world will be accompanied by a heatwave, regardless of the exact number of degrees by which the temperature rises locally.
The uncertainty in warming leads to a 1.5 percent uncertainty in the prediction of compound hot-dry event frequencies. This eliminates temperature as a deciding factor in uncertainty. However, for precipitation, the researchers calculated uncertainty of up to 48 percent.
"This demonstrates that local precipitation trends determine whether drought and heatwaves occur concurrently," Emanuele Bevacqua says. For Central Europe, for example, this means that in the case of a 'wet storyline' with increasing precipitation, concurrent droughts and heatwaves will occur on average every ten years, whereas they will occur at least every four years in the case of a 'dry storyline' with decreasing precipitation. These events would be expected every nine years ('wet storyline') and six years ('dry storyline') in Central North America. These regional precipitation trend storylines can be used to inform adaptation decisions, such as comparing best and worst-case scenarios.
Even if we know that precipitation trends are important in predicting the occurrence of concurrent droughts and heatwaves, it is still difficult to predict them with any degree of certainty: "Climate change may alter the distribution of precipitation in some areas.
The pattern of precipitation is determined by atmospheric circulation, which determines regional weather dynamics over large areas of the globe through numerous interactions "Emanuele Bevacqua says Because many of these processes' dynamics are still unknown, it is difficult to reduce these uncertainties any further.
This discovery, that a trend in one variable predicts the future occurrence of two simultaneous extreme events with a two-degree global temperature increase, could be applied to other compound extremes.
It can be used to model the interaction of tropical storms and heat waves, or the interaction of marine heat waves and acidity extremes in the oceans. "In these cases, the trend in storm frequency or ocean acidification is the deciding factor that determines the future concurrence rates of the two extreme events," says Jakob Zscheischler.
(Source: Helmholtz Centre for Environmental Research- UFZ)
