Of course, global warming, the universal boogey man, gets the blame
Near-surface wind speeds over landmasses across the planet have dropped by as much as 25% since the 1970s, and climate scientists are taking note. Michael Lucy reports.
The wind isn’t what it used to be. Scientists say surface wind speeds across the planet have fallen by as much as 25% since the 1970s. The eerie phenomenon – dubbed ‘stilling’ – is believed to be a consequence of global warming, and may impact everything from agriculture to the liveability of our cities. It has taken more than a decade for scientists to get a handle on stilling, a term coined by Australian National University ecohydrologist Michael Roderick in 2007.
Roderick had spent years studying a 50-year decline across Europe and North America of a climate metric called pan evaporation. It measures the rate at which water evaporates from a dish left outside. With his colleague biophysicist Graham Farquhar, he found the cause: the sunlight had dimmed due to air pollution. Less light equals slower evaporation.
In 2002, after publishing the explanation in the journal Science, Roderick received a query from Roger Beale, the head of Australia’s federal department for the environment. Was pan evaporation also declining in Australia? “To my embarrassment,” Roderick recalls, “I had to say I didn’t know, because I’d never looked.”
Two years later, he had an answer: the pan evaporation rate was also falling in Australia. It was puzzling, however, as air pollution levels on the continent were lower than those of Europe or North America.
Roderick went back to basics. The rate of evaporation depends on four factors: air temperature, humidity, the amount of solar radiation and wind speed. After another three years of combing through meteorological records, he had pinned down the culprit: “To my absolute surprise, we found the main reason for the drop in Australia was less wind – and by a lot.”
Roderick unearthed other local studies from around the world with similar findings, but till then no one had joined the dots.
He teamed up with Tim McVicar, a hydrologist at Australia’s national science agency, the CSIRO, who was looking for global wind patterns and their effects on evaporation. In 2012 this team – led by McVicar – compiled results from almost 150 regional studies to show stilling was taking place across much of the world.
In Australia in the 1970s, average wind speed a couple of metres above the ground was 2.2 metres per second: in 2017 it was 1.6 metres per second.
Over landmasses from as far north as Svalbard, 1,050 km from the North Pole, to as far south as the coast of Antarctica, “observations show that wind is stilling”, McVicar says.
Roderick takes a more telescopic view: air movements are powered by differences in temperature at different places. The bigger the difference between warm and cold air, the stronger the wind. One effect of global warming is to flatten those differences. The poles are warming faster than the equator, winters are warming faster than summers, and nights warming faster than days. “Everything becomes more uniform,” Roderick says.
Full report at Cosmosmagazine.com
h/t to Clyde Spencer