As you might imagine, there is a wide variety of estimates for these statistics because there is a great deal of uncertainty in making the calculation. Estimates must be made for the total area burned, the organic matter per unit area, the fraction of the organic matter that is above ground, the burning efficiency, and the average carbon content of the organic matter. Andreae, M. 0. This may possibly be, in part, due to human activitiy, and the changes will certainly produce significant impacts on we humans. Dear Carl, It is hard to give a precise answer on the state of the past climate because only the last few decades of data have been sufficiently comprehensive to give true global averages of the various atmospheric parameters - temperature, precipitation, cloudiness, and so on.
Instrumental records go back another century in some locations.
Such work is known as paleoclimatology. Geologists have identified a number of major ice ages during the earth's history, during which the climate was much colder than it is today. The last such episode peaked about 21,000 years ago, at which point the climate wasn't simply different - the current sites of many major cities were covered by thick ice sheets. More recently the period 1450 - 1890 AD was unusually cold, at least across North America and Europe. In contrast, the North Atlantic climate had been warm enough a century earlier to allow the Vikings to establish agricultural settlements in southern Greenland.