Researchers say heatwaves that previously occurred once every three years are now happening every 200 days thanks to global warming
Karl Mathiesen The Guardian 27 Apr 15;
Extreme heatwaves and heavy rain storms are already happening with increasing regularity worldwide because of manmade climate change, according to new research.
Global warming over the last century means heat extremes that previously only occurred once every 1,000 days are happening four to five times more often, the study published in Nature Climate Change said.
It found that one in five extreme rain events experienced globally are a result of the 0.85C global rise in temperatre since the Industrial Revolution, as power plants, factories and cars continue to pump out greenhouse gas emissions.
“A lot of us and our colleagues were surprised by how high these numbers are already now in the present day climate,” said Dr Erich Markus Fischer from the Swiss Federal Institute of Technology.
What represents an extreme day varies depending on the background climate. In the south-east of England, for example, temperatures used to reach 33.2C once every 1,000 days, but are now happening as much as once every 200 days.
Future warming will bring a more volatile, dangerous world, even if the world manages to keep temperature rises within a 2C limit to which governments have committed, Fischer’s research found. On average, any given place on Earth will experience 60% more extreme rain events and 27 extremely hot days.
Numbers of extreme weather events spiral even higher at a rise of 3C, a level of warming that the world is on track to exceed with current levels of manmade global greenhouse gas emissions.
Drawing links between specific weather events and climate change can erode the sense that climate change is something that will happen in the future, rather than causing havoc in the present. But the science, called attribution, has proved complicated.
Peter Stott, a scientist at the UK’s Met Office Hadley Centre, said the new study was an important step in attribution science.
“What has been lacking up to now is a robust calculation of how much more likely extreme temperatures and rainfall have become worldwide.”
The study shows warming of the atmosphere increases the number of times temperatures reach extreme levels and evaporates more water from the oceans. It is from this hotter, wetter background that extreme weather events emerge.
Longer events, such as heat waves and prolonged rainy periods, will also occur more often.
“When we talk about 15-day precipitation or 15-day heat waves rather than one-day cases, one very robust finding is the longer the period the higher the fraction that is attributable to warming,” said Fischer.
The study also found that the effects of warming will vary around the world. Weather events at the equator will become more extreme with 2C of warming, meaning tropical countries already dealing with frail infrastructure and poverty will experience more than 50 times as many extremely hot days and 2.5 times as many rainy ones.
But some already dry regions including the parts of the Mediterranean, North Africa, Chile, the Middle East and Australia will experience less heavy rain days.
“In the UK, for a one-in-a-thousand day, which is one in three years, we would probably be well adapted to that,” said Stott. “But I think we’ve shown that we are vulnerable to more extreme situations – those that happen once in a century. For example the wet winter we had in 2013-14. Or indeed the heatwave we had back in 2003 when many vulnerable, eldery people died. But in the tropics, in parts of the developing world, they are extremely vulnerable to one-in-three year events.”
Saleemul Huq, a Bangladeshi scientist who has been involved in the UN climate negotiations, said the developing world was already struggling to cope with extreme events.
“The increased probability of high rainfall events will enhance the adverse impacts of these events in many parts of the world, particularly for vulnerable communities. For example short bursts of intense rainfall in Dhaka already cause huge traffic jams and misery for its citizens,” he said.
Study blames global warming for 75 percent of very hot days
SETH BORENSTEIN Associated Press Yahoo News 28 Apr 15;
WASHINGTON (AP) — If you find yourself sweating out a day that is monstrously hot, chances are you can blame humanity. A new report links three out of four such days to man's effects on climate.
And as climate change worsens around mid-century, that percentage of extremely hot days being caused by man-made greenhouse gases will push past 95 percent, according to the new study published Monday in the journal Nature Climate Change.
Humans have not had as great an effect on heavy downpours, though. The Swiss scientists who did the study calculated that 18 percent of extreme rain events are caused by global warming. But if the world warms another two degrees Fahrenheit (1.1 degrees Celsius) — expected to happen around mid-century — about 39 percent of the downpours would be attributed to humanity's influence, according to the study. That influence comes from greenhouse gases, mostly carbon dioxide from the burning of coal, oil and gas.
"This new study helps get the actual probability or odds of human influence," said University of Arizona climate scientist Jonathan Overpeck, who wasn't part of the research. "This is key: If you don't like hot temperature extremes that we're getting, you now know how you can reduce the odds of such events by reducing greenhouse gas emissions."
Lead author Erich Fischer, a climate scientist at ETH Zurich, a Swiss university, and colleague Reto Knutti examined just the hottest of hot days, the hottest one-tenth of one percent. Using 25 different computer models. Fischer and Knutti simulated a world without human-caused greenhouse gas emissions and found those hot days happened once every three years.
Then they calculated how many times they happen with the current level of heat-trapping gases and the number increases to four days. So three of the four are human caused, the team said.
And when the scientists dialed up the greenhouse gases — using current pollution trends — to simulate a world about mid-century, they got 26 of those super-hot days, "almost a whole month," Fischer said.
The figures that Fischer and Knutti calculated are global estimates. The margins of error, plus or minus about 13 percent with current hot days, grow larger when smaller regions are considered. However, they found Africa and South America now have the highest percentages of unusual hot days that could be blamed on human influence, 89 percent and 88 percent respectively. Europe, at 63 percent, and North America, with 67 percent, come in at the lowest. By mid-century, if emissions continue at current pace, all continents will be able blame at least 93 percent of super hot days on humans.
Half a dozen outside scientists praised the study as valid, elegant and important.
When people ask if a single weird weather event is due to human activity or just natural variation, that's the wrong question because both factors are always involved, said Princeton University climate scientist Michael Oppenheimer, who wasn't part of the study but praised it heavily. This study, he said, asks the right question: "How much of the change is due to human activity and how much is natural variation?"
And once that percentage of damages, costs and deaths can be attributed to human influence, it's easier for governments to put a price on carbon dioxide emissions in an effort to control global warming, said Duke University climate scientist Drew Shindell.
Nature Climate Change: http://www.nature.com/nclimate
