This is from a paper I wrote, it should answer in detail the above questions.
Environmental Factors: CO2 Emissions
The biggest selling point for wind energy is the claim that it will help reduce or offset CO2 emissions; and that it can do this in a way that is harmless to the environment. This is what the wind industry wants you to believe but it is far from the truth. According to the AWEA, American Wind Energy Association
Based on the U.S. average fuel mix, approximately 1.5 pounds of CO2 is emitted for every kWh generated. This means that an average wind turbine prevents the emission of 2 million kWh x 1.5 pounds CO2/kWh = 3 million pounds of CO2 = 1500 tons of CO2 each year. According to Our Ecological Footprint, (Wackemagel & Rees, 1996), a forest absorbs approximately 3 tons of CO2 per acre of trees per year. Thus, a single 750kW wind turbine prevents as much carbon dioxide from being emitted each year as could be absorbed by 500 acres of forest. And the roughly 3 billion kWh that are produced each year by California's wind power plants displace CO2 emissions of 4.5 billion pounds (2.25 million tons), or as much as could be absorbed by a forest covering more than 1100 square miles. 4
Theoretically these numbers are correct but, unfortunately, they do not hold up in the real world. An electrical power grid works on a supply and demand basis. There are no storage capabilities due to the technological limitations of storing such vast amounts of electricity. The grid must have the ability to compensate for peak loads. Wind power cannot be counted on as part of the systems reserve capacity. It only produces when the wind is blowing not necessarily when there is peak demand on the grid.
A major problem is that wind generation is strongest at night and during the winter months when electricity demands tend to be lowest. Every megawatt of wind power capacity must be backed up by an equal amount of conventional power.
A numerical example will make this clearer. Let’s say we have a hypothetical country which has 100,000 MW of installed capacity. Because of Gov’t regulations and pressure from environmentalists this country decides to install several wind generating facilities. They use 1.5 MW GE turbines and install 5,000 turbines in various locations; theoretically supplying 7,500 MW of supplemental power to the grid (this number is closer to 1,875 using the 20 to 30% capacity factor but we will say 7,500 for the sake of simplicity).
It would now seem that this country would only need 92,500 MW of conventional power to supply the grid, and may even be able to shut down a conventional power plant. This would theoretically reduce emissions by 15 million tons (4 million kWh x 1.5 CO2/kWh=6 million pounds of CO2=3000 tons of CO2 x 5000 turbines=15 million tons). This number, if true, would certainly make the case for wind energy stronger. The startling fact is that it would not even reduce emissions by one ton of CO2 let alone 15 million tons. The reason is quite simple and one that proponents of wind energy like to overlook.
The grid, at all times, must have the ability to produce 100,000 MW of power at a moment’s notice to compensate for peak load. As stated earlier wind power cannot be counted on as baseload power but only as reserve capacity. If this country decided to shut down one of their conventional power plants they would still have the 100,000 MW required but 7,500 of that would be from wind power, which we know is intermittent and unreliable. If the demand reaches 100,000 MW at any given time we cannot count on the wind power being available at that time. Therefore, there would be a deficit of 7500 MW on the grid, resulting in rolling blackouts and loss of power to thousands of homes.
There is no reduction in CO2 because the conventional power plants must remain in state called spinning reserve mode. The conventional power plants cannot be shut down and turned back on at short notice. Spinning reserve mode means that the plant is still burning fossil-fuel but is not producing electricity. In fact, by going in and out of spinning reserve mode the conventional plants run at less than optimum efficiency, this in turn, causes a larger amount of CO2 to be released from each plant that is operating in this manner. These plants run at peak efficiency when they are continuously generating electricity and not being switched on and off depending on whether the wind is blowing. The above example is very simplistic and easy to understand, yet wind advocates seem to ignore the facts. The hard truth is that wind farms do not reduce the amount of CO2, and it is very likely they are causing an increase in emissions.