It is well known that climate change is causing more and more extreme precipitation, but the type is also changing – with problematic consequences, researchers warn: their climatic models illustrate how warming is increasingly turning snow into fast-flowing rain. As a result, the high-altitude regions of the northern hemisphere in particular are threatened by more and more flash floods with far-reaching potential dangers. The scientists therefore emphasize the importance of consistent climate protection. In the affected areas, people should now prepare for the growing threat.
The experiences of the last few years seem to drastically confirm the fears of climate researchers: Climate change is making the weather increasingly crazy. In addition to devastating drought and heat, more and more frequent heavy rainfall is causing problems for people and nature. Climate models also show how global warming is causing increasing amounts of water to fall from the sky in a short space of time in certain regions of the world. Since the ground cannot absorb them quickly enough, the enormous amounts of runoff can cause devastating floods, landslides and soil erosion.
The scientists led by Mohammed Ombadi from the Lawrence Berkeley National Laboratory in the USA have now devoted themselves to further research into this hazard potential and its future development using new climatic models. Their focus was on a special aspect: for the first time, they specifically investigated to what extent the increasing extreme precipitation will fall as rain or snow. The importance of this difference is obvious: even large amounts of snow do not have an immediate flooding effect, since they remain in place while heavy rain flows off immediately.
A shift in the precipitation pattern is becoming apparent
To model how climate change could lead to a shift in precipitation patterns, Ombadi and his colleagues analyzed data from global climate observations from 1950 to 2019. They then combined this information with future projections up to the year 2100 derived from Earth system models. As the scientists report, their analyzes show that the proportion of water that falls as snow decreases significantly, especially in the higher areas of the northern hemisphere, and that precipitation falls as rain instead.
Specifically, the model shows that for every one degree Celsius increase in global temperature, an average of 15 percent more rain can be expected at high altitudes. "This increase in extreme precipitation is not just something that's going to happen from now until the end of the 21st century — we're already seeing it," says Ombadi. “Because the same rate can be seen in the data from 1950 to 2019. The precipitation extremes in the mountains have already increased and will continue to change at this 15 percent rate," says the scientist.
A quarter of the world's population is affected
Their results also show in which regions the effect is particularly strong: While all mountain ranges in the northern hemisphere experience the change from snow to rain, the North American mountain regions on the Pacific, the Himalayas and the regions in the high latitudes are the strongest threatened by the extreme rainfall. One reason may be that a significant part of the snowfall typically falls at temperatures just below zero degrees Celsius. “The slightest change in air temperature turns that snowfall into rain. This is different from other mountain regions, where snowfall can occur even at very low temperatures below zero degrees," says Ombadi.
According to the researchers, the distinction between snow and rain should now be incorporated into global climate models to better assess the threat of heavy rain events. "A quarter of the world's population lives in or downstream of mountainous regions," emphasizes Ombadi. "The results should therefore now be taken into account in the planning and construction of the infrastructure in the particularly affected regions, so that they can withstand the negative consequences of increasing precipitation extremes," says the scientist.
Basically, however, the importance of climate protection becomes clear again. After all, it seems questionable whether the goal of limiting global warming to less than two degrees Celsius as part of the Paris Agreement can be achieved. Ombadi concludes: "Our results show a linear relationship between the degree of warming and the increase in extreme precipitation: One degree of warming causes 15 percent more rain, so three degrees leads to a 45 percent increase in precipitation".
Source: DOE/Lawrence Berkeley National Laboratory, Article: Nature: doi: 10.1038/s41586-023-06092-7