A Less Mighty Wind

Wind turbines wring energy out of a free-flowing fuel ­supply that may be losing some of its punch. Surface winds appear to be weakening across the Northern Hemisphere, including in the United States, Western Europe, and China—the world's top three markets for wind power. And climate change threatens to weaken them further during this century as faster warming over northern ­latitudes trims the temperature gradients that energize airflows.

China could be the hardest hit, according to modeling by University of Texas–Austin research scientist Diandong Ren in the November issue of the Journal of Renewable and Sustainable Energy. He projects a 4 to 12 percent decrease in wind speeds in China for the last three decades of the 21st century (compared to the corresponding decades of the 20th). Since the energy in wind increases with the cube of the wind speed, Ren estimates that the slower winds would trim power from Chinese turbines by at least 14 percent.

There is now little doubt that China's surface winds are already slowing. Independent analyses published in 2009 and 2010 found that recent readings from weather station anemometers were lower than those taken in the 1960s and 1950s. In both cases, the majority of Chinese stations reported slowing near-surface winds, and the largest declines occurred in the windiest regions—in the north, on the Tibetan Plateau, and along China's coastline.

Comparable stilling is occurring across the Northern Hemisphere, according to an October report by a team centered at France's Laboratoire des Sciences du Climat et l'Environnement (LSCE). Their report in the journal Nature Geosciences found that winds slowed by 5 to 15 percent over almost all continental areas in the northern midlatitudes between 1979 and 2008.

Experts in the wind-power industry pooh-pooh such warnings. Peter Thomas, a senior engineer with the wind energy consultancy GL Garrad Hassan, based in Bristol, England, concedes that the projections are of a scale that could "impact the economics of the wind-power industry." But he questions their veracity.

Thomas argues that data sets from anemometers are not robust enough to support such interdecadal comparisons, because measurement practices were poorly standardized as recently as the 1980s and may be corrupted by construction around weather stations, many of which are at airports or near cities. "It is important to separate these potential influences from the measured data before conclusions are drawn," says Thomas.

That data-quality critique is wearing thin, however, according to Jean-Noël Thépaut, who runs the data division for the European Center for Medium-Range Weather Forecasts in Reading, England, and is a coauthor of the French report. Thépaut says the team applied a stringent screen to remove questionable anemometer data, narrowing its analysis to reports from just 822 out of roughly 10 000 possible anemometers worldwide. "My colleagues from LSCE have been very careful with the quality control," says Thépaut.

Still, even Thépaut sees unanswered questions, starting with why winds are slowing and whether the stilling will continue. The French study identified climate change as the most likely cause of stilling over central Asia, which means Ren's modeling could well foretell a less productive future for wind power in China.

But the French modelers pegged forests as the primary culprit behind the stilling in other regions. Their modeling showed a correlation between wind reductions and forest regrowth. As they grow, trees increase the roughness of Earth's surface and could be responsible for up to 60 percent of the stilling observed over North America and Western Europe, the modelers estimate. However, the effect on the wind industry might be minimal, because industrial wind turbines tower above most trees, their hubs supported on structures that commonly stand 60 to 100 meters tall. The average anemometer tower is just 10 meters tall. Thépaut says his colleagues plan to figure out if turbines will really be above the fray in the months ahead by analyzing wind-speed data from air-balloon-based weather stations called radiosondes.

There is one source of waning wind that turbines cannot rise above: neighboring wind farms. Here, too, modeling reveals previously unforeseen impacts on wind speed. For example, Arno Brand, a wind modeler at the Energy Research Centre for the Netherlands, in Petten, projects that wind "shadows" behind installed wind farms will sap the productivity of some planned offshore wind projects in the Dutch zone of the North Sea.

Brand's modeling suggests that wind farms must be spaced at least 10 to 30 kilometers apart to keep speed reductions from such shadows below 0.5 meters per second—and even that reduction translates to a 14 percent power loss for a turbine seeing 9.5-m/s wind instead of 10 m/s. Brand says this Dutch problem could become a diplomatic dispute, because the United Kingdom has plans of its own to build what would be three of the world's largest offshore wind farms just upwind of Dutch waters. "These farms are going to produce considerable wind shadows that will affect the most important Dutch zones," says Brand.

While slowing winds could shave value off wind farms or complicate their planning, none of the modelers estimates that the impacts will eliminate the advantage that has made wind power the world's fastest-growing energy source: its supply of virtually carbon-free power at a cost that's comparable to that of fossil fuels. As Thomas points out, it is fossil fuels that are the truly unpredictable fuel source. Even before factoring in their likely contribution to global climate change, the economic cost of fossil fuels is already far harder to predict than the wind will ever be, he says. "In the last 10 years, the cost of a barrel of oil has varied between $20 and $150. The wind is free."