In any discussion about wind farms, or indeed energy policy, wait long enough and you'll hear the claim that they don't reduce greenhouse gas emissions. This idea has been comprehensively rebutted in a study of the effect of wind power on the British electricity grid, which shows that turbines prevented the release of 36 million tonnes (40 million tons) of carbon dioxide over six years, more than previous estimates.

Now that onshore wind is cheaper than fossil fuels in most locations, its critics, when not rattling on about baseless health effects, are focussing their attacks on wind's intermittency. The argument runs that wind farms need 24-hour backup from fossil fuels in case of calm, and that keeping these on standby releases as much carbon as relying on coal or gas in the first place. 

The idea isn't just one you encounter in online forums, it's been endorsed by America's President-elect. However, Dr Camilla Thomson of the University of Edinburgh has proven it wrong by tracking the effects of the UK's wind farms from 2008-2014. She used data on every source of electricity, broken down on a half-hourly basis, to reveal in the journal Energy Policy how production responded to variations in both demand and wind output.

“Until now, the impact of clean energy from wind farms was unclear,” Thomson said in a statement. “Our findings show that wind plays an effective role in curbing emissions that would otherwise be generated from conventional sources.”

Claims that wind farms do nothing to cut greenhouse emissions are obviously not true, but quantifying exactly how much they achieve is tricky. The benefits of wind varies depending on whether coal or gas is being displaced. Smaller grids are less able to smooth out variations in production over multiple wind farms, and so need more backup if they lack flexible hydropower.

The UK Advertising Standards Authority (ASA) ruled that wind farms could only claim to displace the average emissions of the British grid, which includes nuclear power. The ASA acknowledged that this was an approximation, but concluded more precise data was not available. The method is widely used, often without acknowledgment of its limitation. The ASA's method would estimate the savings at 32.4 million tonnes (35.7 million tons), rather than the 35.8 million tonnes (39.5 million tons) Thomson calculated.

The savings have risen as new farms have been built. In 2014 alone, more than 10 million tonnes of CO₂ was saved – enough to prevent the melting of 20 square kilometers (7 square miles) of Arctic ice.

Thomson also found that, contrary to widespread claims, increasing the proportion of wind in the system doesn't reduce the benefit provided by each new megawatt of capacity, at least at the levels reached so far.