Replying to both Don's concern about Amazon deforestation, and Bob's about volcanic contributions- this is exactly my point. No one knows what the atmospheric CO2 threshold is before a runaway greenhouse effect begins. No one knows if, once started, it can be slowed or stopped. No one knows if it will take hundreds of years or only a few, before the planet is no longer hospitable to life as we know it. If the temperature increase trend is a gentle slope, there may be hope. If it increases geometrically once triggered, then the writing is on the wall, as far as humanity is concerned. Clearly, in the absence of a thorough understanding of the development and progression of the greenhouse effect on a planetary environment, man-caused emissions of greenhouse gases must stop completely as soon as possible. Some of the timescales I've seen for reduction of greenhouse gas emissions, by various world governments, are measured in decades, with only partial reductions. My gut feeling is that this will be too little, too late. I hope my gut is wrong. Don, only 200 years ago, much of North America was covered by forest. Europe lost it's primeval forests over a timescale about 4x-5x longer than that, but desertification over large portions of Africa is a fairly recent phenomenon. Even parts of Asia (southeast, particularly) have lost vast forested regions in recent history. The Amazon isn't unique in it's loss of forested areas. The major portion of the earth's land-based CO2 sinks is gone. Recent data shows that ocean sinks are already at or very near capacity- something that up until recently was thought to be hundreds of years away. Once the eccosystem can't sequester anymore CO2, it just won't matter if humanity or the sun is a major or minor contributor.