Turbines in Texas mix up nighttime heat

Satellite measurements of temperatures near wind farms in Texas from 2003-2011 have suggested that wind turbines have mixed up the nighttime atmosphere, bringing warmer air down to the ground (Zhou et al, 2012). When looking at the physics it turns out that this suggests the chance of a (very, very small) global cooling effect.

The satellites measure that downwind from wind farms the surface is warming more than other places nearby, but only at night. In the windier Texan summer the night warming has been 0.73°C per decade, but the calmer winter months have only gone up at 0.46°C per decade.

The authors blame wind farms because the warming happened where turbines were built, as shown in Figure 1 below. The patterns also match expectations from physics: the effect is stronger when it's windier, downwind from turbines and at night.

Figure 1 - map of changes in temperature across Texas in degrees Celsius. Crosses represent places where there are wind turbines, and the prevailing wind is from the south. The area average temperature has been subtracted from each point, so a blue area doesn't necessarily mean it cooled, just that it warmed less quickly than the turbine areas.

At night the Earth's surface cools quickly as it radiates efficiently to space and the cold dense air right next to it can't rise. The air above is warmer and less dense so it floats on top and heat becomes 'trapped' (rather like the alcohol content in the layers of a B-52 cocktail) unless something helps mix it up - and this is where the chopping blades of wind turbines come in (stirring a B-52 does the same job).

During the day the Sun quickly warms the surface and the lower air heats up and rises. The extra chopping of the turbines doesn't make a difference because the atmosphere is already well mixed (to try this at home, try pouring a B-52 the wrong way round and too quickly, then see how much difference stirring it makes!).

What does this mean for climate change?

Lead scientist Liming Zhou from the University of Albany released a Q&A to explain that the effect is local, and that

Very likely, the wind turbines do not create a net warming of the air and instead only redistribute the air’s heat near the surface...fundamentally different from the large-scale warming effect caused by increasing atmospheric concentrations of greenhouse gases due to the burning of fossil fuels.

However, newspaper headline writers and political commentators disagree with physics, measurement and scientific experts and think that the results mean "Wind Farms Cause Global Warming". As Professor Zhou explained, this just ain't the case.

Tiny and local effect, but physics suggests wind farms cause small global cooling!

It's possible that the global effect of this would be a (very small) cooling. It works like this: the Earth's surface can emit in the atmospheric 'windows' where heat leaks easily to space. This is why the Earth's surface cools down faster at night than the atmosphere.

Typically the upper air stores this heat at night, but if it's mixed up by wind turbines, it loses it to the ground and the ground then leaks this heat efficiently into space. Like the radiator fins on a car: the radiator warms up, but it helps keep to cool the whole engine.

When contacted, Professor Zhou responded that "Your explanation is interesting and physically correct to me but the warming-induced emission is very small." He also commented that there might be other effects related to the efficiency of Earth's heat loss which could work in the opposite direction and that more research is needed. Regardless, the effect on global temperatures will be too tiny to measure.

Effect important locally, and a good reminder to stay skeptical

The nighttime warming measured by satellite is quite large compared with the global warming signal of ~0.2°C per decade. But it's only in a very small region and doesn't have much effect on global warming.

As Professor Zhou says, more research is needed. Is this effect widespread in other places? Does it explain some of the night warming measured by near surface thermometers? The area of atmosphere affected is too low for satellites, but maybe nearby weather balloons could be used to check these results.

The media confusion shows us how important it is to stay skeptical and try and think of the whole picture. The turbines are just mixing up heat that was already there and by only measuring where the heat is moving to you can easily get confused and draw the wrong conclusions.

Posted by MarkR on Thursday, 10 May, 2012

Creative Commons License The Skeptical Science website by Skeptical Science is licensed under a Creative Commons Attribution 3.0 Unported License.