Why wind speeds have slowed ?

Wind speeds have slowed over three decades across the Northern Hemisphere.

Increasing amounts of vegetation could be causing up to 60% of a slowing in wind speed across the Northern Hemisphere, according to researchers analysing three decades of wind-speed data in Nature Geoscience1 today. 

The decline is a potential concern for wind-turbine efficiency. But researchers cannot tell whether the effect, an average 10% slowdown, will make much difference to wind turbines — the slowing winds measured are at 10 metres above the ground, whereas turbines operate at 50–100 metres up, where there is little global data.

Several previous regional studies looking at the United States, Australia, China and parts of Europe have shown decreasing wind speeds just above the planet's surface. Climate change, afforestation and urban development had been mooted as possible causes. But, says Robert Vautard, at the University of Versailles Saint Quentin in Yvelines, France, "people always said the data were rubbish. There was no quality-controlled global archive of data." 

Vautard and his colleagues collected data from about 10,000 weather stations, although they removed all but 822 stations from their list because of incomplete records. That left records stretching back to 1979, taking in Europe, central Asia, eastern Asia and North America. 

Vautard had expected a study spanning such a large area to show speeds increasing in some areas and decreasing in others. But, he says, "we were surprised to see a very clear trend across the whole Northern Hemisphere". Annual wind speeds had declined at 73% of the stations, dropping by 5–15% over almost all of the land areas examined. The most pronounced effect was seen across Eurasia. The researchers also found that stronger winds have been affected more than weak ones. 

Brian Golding, director of forecasting research at the UK Met Office in Exeter, Devon, says that the observation is interesting. "However," he adds, "the timescales are very short for a meteorological trend — it's entirely possible that the previous 30 years would show a different trend."

Vautard counters that the few available records dating back to 1959 suggest that the trend had been developing since the 1960s.