[Will]

We have precise temperature records dating back to sometime around the turn of the 19th century. And there's also things like core samples of ice in Antarcica that gives us a very good estimate of concentrations of atmospheric gasses and a reasonably good estimate on what temperatures were (varying amounts of gas would be trapped based on the temperature at the time it was frozen, and crystal structure of the ice will vary based on the conditions when the water froze).

Data from around 1900 back is going to be a lot more coarse than data that we have today, so that does affect estimates of the impact of the greenhouse gasses, since more detailed analysis is not possible. All we really have is aggregate data averages; the temperature in a certain approximately 10 year range in the past, in the area the ice was formed, averaged X degrees. Correlate to percentages of greenhouse gasses trapped. Studies have found a correlation to the calculated levels of gasses and the calculated temperature.

The naysayers for global warming point out that the estimated release of greenhouse gasses by human actions (including farmed livestock, vehicles, factories, etc.) is calculated to be only a modest percentage of the natural release. The only problem with this criticism is what Phoenix-D pointed out: large portions of the population of the world is in danger if sea levels rise even a little. Even the modest percentage accelerates the temperature increase, which accelerates the melting of ice at the polar caps, which accelerates the rising of sea levels and the rising of sea temperatures. Higher ocean temperatures mean bigger more powerful storms (see this year's hurricane season). Higher sea levels mean more danger from those storms and flooding, tsunamis, etc. There are lots of costal areas that are technically below sea level, and only have essentially a small ridge holding back the ocean.

So, the point is, do we want to accelerate the cycle and make sure the peak temperature is even higher than it would have been? Or do we want to cut back on our emissions and make sure that the damage is minimized?