Human-caused climate change increased the severity of last year's heat waves in Asia, a new report found. But not all extreme weather events could be linked to global warming, as climate expert Thomas Peterson tells DW.
The report, published in the Bulletin of the American Meteorological Society, investigates the causes of a wide variety of extreme weather and climate events from around the world in 2013. Titled "Explaining Extreme Events of 2013 from a Climate Perspective" the 84-page document examines the causes of 16 individual extreme events - including heat waves, rain, flood, droughts and storms - that occurred on four continents in 2013.
Thirteen independent studies mentioned in the report - compiled by 92 scientists from around the globe – determined there was a link between extreme weather and the burning of fossil fuels. But while the authors found that human influence had substantially increased both the likelihood and severity of heat waves in places like Asia and Australia, it was more difficult to measure the influence of human activity on other events like droughts, heavy rain and storms.
Thomas Peterson, principal scientist at NOAA's National Climactic Data Center and one of the report's lead editors, says in a DW interview that while scientists could not identify a linkage between human-caused global warming and some extreme weather events, the data gathered provides evidence that human activity has increased the intensity and likelihood of heat waves in Asian countries such as China, Japan and Korea.
DW: Which extreme weather events did you focus on in Asia and why?
Thomas Peterson: Each of the four topics evaluated in Asia was selected by the author team for subjective reasons. Often because the event was of interest to them on a personal level, as it impacted them, their families and their friends. For instance, the report focuses on Japan, Korea and China which all experienced extremely hot summers in 2013.
The fourth topic focused on heavy rain in India. With an early arrival of monsoon-like atmospheric circulation in June, the heavy precipitation that occurred in northern India was a once-in-a-century event.
What were the key findings of the report?
The report found that long duration heat waves during the summer and prevailing warmth for annual conditions are becoming increasingly likely due to a warming planet, as much as 10 times more likely due to the current cumulative effects of human-induced climate change, as found for the Korean heat wave of summer 2013.
Extreme precipitation events of last year were found to have been much less influenced by human-induced climate change than extreme temperature events. Furthermore, prolonged cold waves have become much less likely, such that the severely cold 2013 winter over the United Kingdom was perhaps the most remarkable event of all those studied in 2013 - its probability of occurrence may have fallen 30-fold due to global warming alone.
However, there was no clear evidence of human influence on any of the three very intense storms examined, which included a surprising winter-like storm during autumn in the Pyrenees, an extreme blizzard across the US High Plains, and Cyclone 'Christian' that delivered damaging winds across northern Germany and southern Denmark.
To which extent did climate role play a role in extreme weather events in Asia last year?
All four events evaluated in Asia were found to indicate that human activity had influenced the strength of extreme weather or increased its likelihood. For instance, analyses of observed and simulated June precipitation provided evidence that human-caused climate change has increased the likelihood of much stronger precipitation in northern India, and made heat waves more likely to occur in Japan, Korea and Easter China.
How come human factors were found to have influenced some weather events, but not others?
Natural variability was a prime cause of all events, just the randomness of the weather. For some of these events, in addition to the natural causes, human influence helped make the events stronger or more likely. But for some events in other regions, the analyses that scientists conducted could not identify a linkage. For a few events, greenhouse gases actually made the extreme event less likely.
Three independent studies which examined Pacific Sea surface temperatures and atmospheric anomalies, found some, but not conclusive, evidence for the impact of human-caused climate change on the ongoing rainfall deficit in the US state of California. For example, one paper found evidence that atmospheric pressure patterns related to the drought increased due to human causes, but their exact influence on the California drought remains uncertain.
So, in your view, which weather events are more likely to be influenced by human factors than others?
Three papers with different methodologies looking at the Chinese, Korean and Japanese heat waves reached the same conclusion. This is a very powerful message. If increases in greenhouse gases are making an event more likely, then it implies that we should expect events like that more often in the future.
If some extreme weather events could be linked directly to climate change, what does this tell us about the urgency to tackle the issue?
It tells us that climate change is not just in the future but, for example, for the people in eastern China, Korea and Japan, climate change is in their own backyards.
Thomas C. Peterson is President of World Meteorological Organization's Commission for Climatology and the Principal Scientist at the National Climatic Data Center of the National Oceanic and Atmospheric Administration (NOAA) in the United States.