Atmospheric carbon dioxide, which causes 50% of greenhouse warming, is increasing by 0.5% per year. Reducing and reversing deforestation would slow the increase, but plantations the size of Europe would sequester only 20-30% of the accumulation. Methane's concentration is increasing at a rate of 1% per year, and 70-90% comes from biotic sources. Even if drastic reductions in the world's cattle population, slash and bum agriculture and fuelwood burning could be achieved, CH4 would probably continue to increase. Atmospheric nitrous oxide, with almost exclusively biotic sources, contributes 5% to global warming and is increasing at 0.2-0.3% per year. The causes are poorly understood, but reductions in the use of N fertilizer and careful soil management may be effective.