James Randerson et al. from UCI have looked at this question using the scientific method. There have been previous papers about the subject that came to a different conclusion because they essentially neglected the albedo. Randerson and his collaborators have made the analysis a bit more carefully.
Their answer is that the forest fires increase the amount of snow and the reflectivity of the surface. If you take the average over Earth, this effect roughly cancels the greenhouse effect. If you're further from the equator where snow is common, the albedo wins and cooling is what you get.
The study printed in Nov 17th issue of Science also implies that some policies meant to prevent fires were actually contributing to warming: see e.g. this activity in Canada. Well, such things occur constantly whenever politicians and zealots become more powerful than scientists and reasonable people.
And that's the memo.
