Several analysts (links below) have examined the consequences of fossil fuel–fired generators, particularly natural gas, having to modulate their output and frequently start and stop to balance the highly variable infeed from wind turbines so that electrical supply is stable and reliable. The question is to factor in the increased emissions from operating the generators in that way compared with operating them more steadily, i.e., if they were not required to cope with the fluctuating contribution from wind turbines: How do the extra emissions of running the generators less efficiently  compare with the emissions saved by running them less?
Since natural gas–fired generators are best able to respond quickly enough to balance wind energy, they have been added almost in parallel with wind (see graphs provided by the Department of Energy’s Energy Information Agency ), so it is not wind replacing coal-generated electricity, but wind plus its necessary partner natural gas (which, fracking and methane release aside, is much cleaner than coal). Might it not only be much cheaper and less land-intensive, but also even reduce emissions more to replace coal with natural gas only?
“The hidden fuel costs of wind generated electricity ” by K. de Groot and C. le Pair
“The impact of wind generated electricity on fossil fuel consumption ” by C. le Pair and K. de Groot
“Wind integration: Incremental emissions from back-up generation cycling (Part V: Calculator update) ” [with links to Parts I–IV] by Kent Hawkins
“Big wind: How many households served, what emissions reduction? (a case study) ” by Kent Hawkins and Donald Hertzmark
“Integrating Renewables: Have Policymakers Faced the Realities? ” by Kent Hawkins
“Integrating Wind Power: Wind Fails in Two Important Performance Measures ” by Kent Hawkins
“Analysis of Ontario’s electricity system ” by Kent Hawkins
“Air emissions due to wind and solar power ” by Warren Katzenstein and Jay Apt
“Cost and quantity of greenhouse gas emissions avoided by wind generation ” by Peter Lang
“Calculating wind power’s environmental benefit s” by Tom Hewson and David Pressman