Climate sceptics have long made the counter-intuitive claim that wind turbines don't actually reduce carbon emissions, but data analysed by the Guardian's environment blog suggests that as well as being counter-intuitive, it's just plain false.
The claim the Guardian's Chris Goodall and Mark Lynas investigated relies on the fact that wind power fluctuates with the weather. As a result, when the windspeed drops, gas power stations have to be spun up to provide electricity instead. It has to be gas, because other low-carbon sources of energy either have the same fluctuations as wind, or, like nuclear power, essentially can't be turned off.
The sceptics argue that the need to rapidly turn on the gas generators means that a type of power station known as a "gas-fired open cycle turbine" (OCGT) has to be used. Their key point is that these turbines, which are capable of being put into use extremely quickly, are less efficient than another type of gas generator, a "combined cycle gas turbine" (CCGT). The need to pick up the slack left by wind power, they say, means that so many of these inefficient turbines are used that carbon emissions actually go up compared to the scenario where all the electricity is generated with the more efficient CCGT type.
The Guardian's blog contains a lot of stats taken straight from the National Grid, all of which confirm the intuitive beliefs: wind power replaces fossil fuels. But for the specific claim that it still results in higher carbon emissions, the operative paragraph is the following:
Their arguments are not borne out by current statistics, however. If the sceptics were right, the recent windy conditions would have seen considerable use of less-efficient OCGT as wind input to the grid ramped up and down. In actual fact, during the entire June-September period, OCGTs and equally dirty oil-fired stations produced less than one hundredth of one percent of all UK electricity. In total they operated for a grand total of just nine half hour periods in the first 19 days of the month – and these periods had nothing to do with changing windspeeds.
In other words, the mechanism the sceptics suggested might be at work isn't. The authors go into greater detail about why that is, as well. In essence, it comes down to the fact that wind power isn't actually all that unpredictable. It comes and goes, but we usually have a good few hours warning about when it's going to die down; and it's spread throughout the country, allowing even more flattening of the peaks and troughs in supply.
The bigger reason for having to suddenly spin up big generators isn't sudden drops in supply, but sudden peaks in demand. Take, for example, a World Cup match, where England have made it through to sudden death penalties. The National Grid know there will be a spike in power consumption the minute the match ends, as people get up to put the kettle on – but they can't know when that spike will be until mere seconds before it actually happens. That sort of load will always be problematic to supply with renewables – but it's not much less problematic to supply with fossil fuel either.