windblown tree
© NickRH / getty images
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.

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.
pan evaporation australia
© Commonwealth of Australia 2005
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.

Conversely, the wind is getting faster around the poles and in certain coastal areas. In a perplexing twist, ocean winds also appear to be accelerating.

Several explanations have been proposed for the stilling.

Robert Vautard, who studies climate change at France's National Centre for Scientific Research, has a benign answer for some of the change: more vegetation, spurred by rising temperatures and carbon dioxide levels. It increases 'surface roughness', which slows the wind.

The planet's rising temperatures are another likely culprit.

One projected consequence of global warming is expansion of the 'Hadley cell', a planet-girdling double doughnut of atmospheric circulation in which warm air rises near the equator, loops towards the poles, cools and falls to the surface at around 30 or 40 degrees latitude, then heads back to its origin. This circulation, combined with the Coriolis effect of Earth's rotation, causes the consistent easterly trade winds found in the tropics and the prevailing westerlies of the middle latitudes. An expanding Hadley cell means many common storm tracks are slipping towards the poles, taking their high winds - and associated rainfall - away from the temperate regions.

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.

What does the drop in wind speed mean? The decrease in evaporation has immediate implications for the precision calculations used in modern irrigation, and more complex effects on rainfall patterns.

While less evaporation may be good for some plants in arid areas, stilling may make others less able to disperse wind-blown seed to suitable new habitats, and hence less resilient to climate change.

Less wind could also hurt city-dwellers. In what may be a taste of things to come, the winter of 2016/17 saw Europe becalmed, leading to smog so bad that Paris banned cars for six days, and the city of Skala in Poland briefly overtook Beijing atop the world's air-pollution tables.

Potential effects on wind power are another area of concern, though there does not appear to be anything to worry about in the short term. Stilling has so far been detected only at heights up to 10 metres, while turbines harvest their energy 50 to 150 metres above the ground.

"We certainly haven't seen anything that looks like stilling," says Keith Ayotte, the chief scientist of Australian wind power outfit WindLab, who monitors more than 100 sites across the world where the company has turbines.

Though these higher-altitude winds will change over the 21st century, Vautard has used climate simulations to project the effect on total wind power available across Europe is unlikely to be more than 5%. One difficulty with prediction is a lack of observations. As McVicar notes, accurate and consistent measurements only exist "for the past 40 or 50 years".

Cesar Azorin-Molina, a climate scientist at the University of Gothenburg in Sweden, has embarked on an EU-funded archival project with the effortful acronym STILLING: "TowardS improved undersTandIng of the worLdwide decline of wind speed in a cLImate chaNGe scenario."

His mission is "rescuing historical wind speed data" like logbooks from Ponta Delgada in the Azores and Blue Hill Observatory in the US that go back more than a century. The age of anemometers - the devices that measure wind speed - can affect readings but, by compiling a single set of quality-controlled data, Azorin-Molina hopes to determine whether stilling is purely a recent phenomenon or if similar declines have happened in the past.

For McVicar, the stilling of the planet's winds is a reminder that global warming has multiple and unpredictable flow-on effects. "We're dealing with climate change, not just rising temperatures."