World proof hyperlinks the surge in excessive rainfall to man-made local weather change – cotton wool?
Gavin D. Madakumbura, University of California, Los Angeles; Alex Hall, University of California, Los Angeles; Chad Thackeray, University of California, Los Angeles, and Jesse Norris, University of California, Los Angeles
The Research Brief is a short version of interesting scientific work.
The big idea
Human activities, such as burning fossil fuels for transport and electricity, have worsened the intensity of extreme rainfall and snowfall over land in recent decades, not just in some areas but around the world, new research shows.
Previous studies have been able to attribute individual extreme events and long-term changes in some regions to climate change, but global assessments have been more difficult. We used a new technique to analyze rainfall records from around the world and found conclusive evidence of human impact on extreme rainfall in each one.
Scientists warn that rising global temperatures will lead to more extreme precipitation in the future, mainly because warm air “holds” more water vapor in the atmosphere and heats up storms.
Since the earth had been about 1 degree Celsius warmer since the beginning of the industrial age, we wanted to find out whether this change had already started.
Previous attempts to identify human influence in historical precipitation records usually required long time series with many consecutive years of data. However, precipitation is difficult to monitor from land or space over long periods of time, so such records are rare. We found another way.
We used artificial neural networks, a type of machine learning, to find extreme rainfall patterns in weather records. Once these neural networks understood what to look for, we could analyze shorter and different observation recordings.
The result is multiple lines of evidence that human activity has increased extreme rainfall in recent decades. Even if the data sets were very different, we could see the human influence.
The results were published on July 6, 2021 in the journal Nature Communications.
Why it matters
Understanding how humans influence extreme precipitation is important in order to interpret climate events today and to prepare cities and protective infrastructures for the changing world.
In recent years, devastating floods after exceptional rainfall have made headlines that historically would have been extremely rare. The 2017 hurricane season in Texas, Florida and Puerto Rico and the extreme monsoon rains over India and Bangladesh in 2017 are two examples. Our results show that precipitation has generally become more extreme around the world over the past few decades.
Perhaps more importantly, our results suggest that further warming of the planet in the 21st century is likely to exacerbate the most extreme precipitation events. Climate models predict that such intensification will occur this century, and they suggest that a similar, but less rapid, intensification occurred in the 20th century based on how much the planet has already warmed. Our results confirm this finding.
With greenhouse gas concentrations in the atmosphere still rising, the planet is expected to continue warming in the 21st century. How much it heats will depend on decisions made today regarding fossil fuel use and other important factors in climate change. That 1 degree warming could be 4 degrees by the end of the century if emissions remain high.
While we have clearly identified the human impact on extreme rainfall in the past, we have not yet isolated how much each type of human activity contributed. Greenhouse gas emissions, aerosols and land use changes can all have an impact. We plan to modify our machine learning method in the future to take these sources into account.
The machine learning method we use is also currently learning from data alone. We can still improve this by integrating climate physics into the algorithm. This would allow the machine to learn the physical processes that lead to an intensification of extreme precipitation. Other climate variables such as wind, clouds and radiation could be included to answer not only whether extreme precipitation is increasing, but why.
Gavin D. Madakumbura, Ph.D. Candidate in Atmospheric and Ocean Sciences, University of California, Los Angeles; Alex Hall, Professor and Director, UCLA Center for Climate Science, University of California, Los Angeles; Chad Thackeray, Assistant Researcher, UCLA Center for Climate Science, University of California, Los Angeles, and Jesse Norris, Project Scientist, Atmospheric and Oceanic Sciences, University of California, Los Angeles
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