Reuters / Extreme weather events over the past decade have increased and were “very likely” caused by man-made global warming, a study in the journal Nature Climate Change said March 25.

Scientists at Germany’s Potsdam Institute for Climate Research used physics, statistical analysis and computer simulations to link extreme rainfall and heat waves to global warming. The link between warming and storms was less clear.

“It is very likely that several of the unprecedented extremes of the past decade would not have occurred without anthropogenic global warming,” said the study.

The past decade was probably the warmest globally for at least a millennium. Last year was the 11th hottest on record, the World Meteorological Organization said.

Extreme weather events were devastating in their impacts and affected nearly all regions of the globe.

They included severe floods and record hot summers in Europe; a record number of tropical storms and hurricanes in the Atlantic in 2005; the hottest Russian summer since 1500 in 2010 and the worst flooding in Pakistan’s history.

Last year alone, the United States suffered 14 weather events which caused losses of over $1 billion each.

Abnormal

The high amount of extremes is not normal, the study said.

Even between March 13 and 19 this year, historical heat records were exceeded in more than 7,000 places in North America.

For some types of extreme weather, there are physical reasons why they would increase in a warming climate. For example, if average temperature rises, then so will the number of heat records if all else remains equal, the study said.

Natural weather patterns like El Niño or La Niña can also cause highs in global temperature or increased precipitation which leads to floods.

“Single weather extremes are often related to regional processes, like a blocking high pressure system or natural phenomena like El Niño,” said Stefan Rahmstorf, co-author of the study and chair of the institute’s earth system analysis department.

“These are complex processes that we are investigating further. But now these processes unfold against the background of climatic warming. That can turn an extreme event into a record-breaking event.”

Recent years have seen an exceptionally large number of record-breaking and destructive heat waves in many parts of the world and research suggests that many or even most of these would not have happened without global warming.

Currently, nearly twice as many record hot days as record cold days are being observed both in the United States and Australia, the length of summer heat waves in western Europe has almost doubled and the frequency of hot days has almost tripled over the period from 1880 to 2005.

Extremely hot summers are now observed in about 10 per cent of the global land area, compared with only about 0.1–0.2 per cent for the period 1951 to 1980, the study said.

The link between storms and hurricanes and global warming is less conclusive but at least some of the recent rainfall extremes can be attributed to human influences on the climate, it added.

A report March 23 said the unprecedented heat wave across much of the U.S. set or tied more than 7,000 high temperature records.

“This heat wave is essentially unprecedented,” said Heidi Cullen of the non-profit science and communication organization Climate Central. “It’s hard to grasp how massive and significant this is.”

These records include daytime high temperatures and record-high low temperatures overnight, which in some cases are higher than previous record highs for the day, Cullen said.

“When low temperatures are breaking previous record highs, that’s when you see this is incredibly special,” she said.

Spring arriving earlier

Cullen noted that this warmth is part of a trend that is pushing the spring season earlier by an average of three days in the contiguous 48 U.S. states.

The date of first leafing — the day when buds burst open — has moved forward from March 20, where it was during the 30-year period from 1951 to 1980, to March 17, where it has been for the period from 1981 to 2010.

An online report (http://climatecentral.org) accompanying the briefing shows that some states have an even earlier spring, with Montana, Washington state, Oregon, Nevada, Utah and Connecticut seeing spring arrive five days or more ahead of the previous average.

This early wake-up call for plants and animals can have disastrous health consequences, especially for children, said Dr. Aaron Bernstein of the Center for Health and the Global Environment at Harvard Medical School.

Pollen counts are breaking records around the United States, Bernstein said, noting that allergies cost the U.S. economy between $6 billion and $12 billion annually.

The early heat stimulates growth in plants and the pollen season has become longer by one to two weeks over the last half-century, while the higher levels of carbon dioxide in the air stimulate pollen production in highly allergenic plants like ragweed, Bernstein said.

The rates of sensitization to pollen in the United States are also on the rise, he said, which means people who never suffered from pollen-related allergies may feel them now.

“As we juice these plants with carbon dioxide, we’re going to make people have greater allergy symptoms,” Bernstein said.