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Around the world, global warming is making unprecedented weather and climate events far more likely to occur, with the planet now teetering on the edge of a new era of routinely damaging global warming-related extremes, a new study found.
The study, published Monday in Proceedings of the National Academy of Sciences, found that in addition to making extreme events more likely across broad swaths of the globe, climate change is also making such events more severe.
The study is rooted in an emerging field of climate detective work, in which scientists seek to detect the role that human-caused climate change may have played in often damaging extreme events.
However, this study differs from many other so-called "climate attribution" studies by looking at how climate change is tipping the scales in favor of unprecedented events worldwide, rather than focusing on one or two extreme occurrences in particular.
Every weather event that occurs today takes place in an environment modified by human activities, with more greenhouse gases in the atmosphere making the air and seas warmer, pumping more moisture into the atmosphere and providing more energy to storm systems.
As a result, heat waves are becoming more intense and longer-lasting, and heavy precipitation events are occurring more frequently in many areas.
The new study takes such findings a step further by looking at how global warming is shifting the odds and severity of unprecedented climate events, such as the historically hottest month, hottest day, driest year, and wettest 5-day period on record for a given location.
What the researchers found was surprising: The influence of global warming is already clear globally, with the greatest certainty concerning hot weather records.
The study is important because policy makers and individuals need to make risk management decisions now to prepare for future climate impacts. And extreme weather events are some of the most costly ways — both in monetary terms and human lives — in which climate change is manifesting itself.
How they did it
The study is particularly ambitious in that it uses multiple methods to probe the climate for signs of global warming-related trends, from statistical examinations of climate data to the use of sophisticated computer models.
Using weather observations and a group of climate models known as an ensemble, the researchers from Stanford University and the University of California at Los Angeles found that the warming-to-date has already caused the severity and probability of the hottest month and hottest day of the year to increase at more than 80 percent of weather observing sites.
"The world isn’t at the point where every extreme hot event has a human component, but it’s getting close,” said Noah Diffenbaugh, a climate scientist at Stanford University and lead author of the study.
The study also found that global warming has boosted the probability of the driest year at 57 percent of studied areas, while the wettest 5-day period has become more likely at 41 percent of the observed areas. Diffenbaugh says the lower probabilities associated with precipitation extremes is due in large part to the greater variability, or "noisiness," in precipitation data.
As global warming continues it's becoming easier for locations to set all-time records, particularly temperature records, because the baseline climate is shifting so significantly and quickly.
This is similar to a basketball game in which the floor is steadily rising, making it easier for players to dunk the ball.
“With temperature events the mean [average] warming has been so strong... that it doesn’t take as big a departure from the new mean to set the record as it would’ve without that trend,” Diffenbaugh said.
The tropics have been especially hard-hit by global warming so far, with a four-fold increase in the probability of setting a record for hottest month, and at least a factor of 2 increase in the odds of the driest year.
Kevin Trenberth, a climate researcher at the National Center for Atmospheric Research in Boulder, Colorado, said he has "some qualms" about the new study, but that "it adds to the literature and is useful." Trenberth was not involved in the new study.
He criticized the computer models used in the study for their inability to simulate extreme events as well as other models can. "The results seem quite reasonable, although I suspect they are conservative (low)," he said in an email.
Katharine Hayhoe, a climate scientist at Texas Tech University who was not involved in the new research, said the study focuses on extreme events that have four different lines of evidence supporting their links to climate change, including statistically significant historical trends.
"Whenever an extreme event occurs, the number one question we scientists are asked is, is this natural or is it human induced climate change? And my answer always is: there is a human component in nearly every event these days, and that component is somewhere between zero and 100 percent," she said.
Hayhoe used a human health analogy to help explain the relationship between global warming and extreme weather and climate events.
"A heart attack is usually some combination of genetic risk and lifestyle choices," she said in an email.
"Similarly, these days, an extreme event is usually some combination of natural risk and lifestyle choices – but in this case, what matters is not what we eat, but rather how we get our energy."