Heavy rains have wreaked havoc here in Japan as well as abroad in 2021, as they have many times in recent years. The Mainichi Shimbun answers some common questions readers may have about the torrential rains that have become more frequent in the past decade or so.
Question: Is the number of heavy rain events increasing?
Answer: According to the Japan Meteorological Agency (JMA), in the 10 years since 2011, the country saw an annual average of 334 torrential rain events in which rainfall of at least 50 millimeters per hour, or “waterfall-like rain,” was recorded. This is 1.5 times the average recorded between 1976 and 1985, which was roughly 226 times a year. Experts have pointed out that this trend is likely caused by rising air temperatures due to global warming.
Q: Why do we have more torrential rains when the temperature increases?
A: The amount of moisture that the atmosphere can hold is believed to increase by approximately 7% with a temperature rise of 1 degree Celsius. The increased moisture means more rainfall, and rising temperatures create severe updrafts generating conditions for more cumulonimbus clouds to form. This all contributes to possibly causing heavy rain at levels never before recorded.
Q: How much of it is caused by global warming?
A: According to an analysis done by a supercomputer at the JMA’s Meteorological Research Institute, compared to a scenario where global warming had not advanced since 1850, the Setouchi region surrounding the Seto Inland Sea is roughly 3.3 times more likely to see heavy rain on par with the July 2018 rain disaster that hit an extended area of western Japan and claimed over 200 lives. Similarly, the supercomputer estimated 1.5 times more chance of torrential rain at about the same level as the July 2017 rain disaster that hit northern Kyushu in southwest Japan.
Q: What will happen if the Earth continues to warm up?
A: If no measures are taken, it is predicted that by the end of this century average global temperatures will rise by roughly 4 C compared to the pre-Industrial Revolution period. A temperature rise of 4 C is projected to cause heavy rainfall (at least 50 millimeters per hour) about 2.3 times more frequently compared to the end of the 20th century, while a 2 C increase is estimated to bring these rains 1.6 times more frequently.
According to a Kyoto University study, a 4 C rise will cause more rain disasters in Japan’s northernmost prefecture of Hokkaido and the Tohoku region in the northeast — areas that have seen relatively few rain-related natural disasters.
Not only do we need to prepare for torrential rains, but efforts to reduce carbon dioxide and other greenhouse gas emissions are crucial.
(Original by Mayumi Nobuta, Science & Environment News Department)
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