Abstract for talk, "Global Carbon Emissions, the Historical Record" by R.A. Houghton

Changes in land use, including the conversion of natural ecosystems to croplands and pastures and the modification of ecosystems through logging, fire protection, and other forms of management, have reduced the area of the world’s forests by ~15% during the last 150 years and released an estimated 156 PgC to the atmosphere. The annual emissions of carbon from these changes in land use were larger than the emissions from combustion of fossil fuels until early in the 20th century. Current emissions from land-use change are 1-2 PgC/yr, almost entirely from accelerating rates of tropical deforestation. Outside the tropics, the emissions from forest clearing and logging are approximately balanced by the uptake of carbon in forests recovering from earlier logging and agricultural abandonment, although individual regions are small sinks and other small sources of carbon. Before the middle of last century, the emissions of carbon from land-use change in northern countries exceeded the emissions from the tropics.

Independent estimates of the net terrestrial carbon flux, from top-down analyses with atmospheric and oceanic data, are largely consistent with the estimated current source of carbon from the tropics. Outside the tropics, however, these top-down estimates indicate a net sink of 1-2 PgC/yr. In comparison to the nearly zero flux estimated from land-use change, this net sink suggests (1) that other factors besides land-use change (for example, CO2 fertilization, N deposition, or changes in climate) are responsible for a significant uptake of carbon, (2) that management practices not included in the analyses of land-use change have increased the accumulation of carbon on in northern lands, and/or (3) that northern forests today are recovering from a reduction in the rate or extent of ‘natural’ disturbances. Distinguishing among these possibilities is important for predicting whether the current net carbon sink will continue and for assigning credits under the Kyoto Protocol.