The Vostok Ice Core record now extends back some 400,000 years.
http://physics.gallaudet.edu/vostok/
Today's lecture in my Oceanography class touches on CO2 and global warming. So, class, here is today's lecture...
Here's the quick way of summarizing the rate-of-change issue:
Since the beginning of the industrial revolution, lets say 200 years ago (rounded) CO2 in the atmosphere has gone from about 280 parts per million (ppm) to almost 370 ppm. That is a rate of change of about 0.45 ppm per year.
According to ice core records, CO2 during the last
glacial period, ending 17,000 years ago (-ish) was about 190 ppm. It took some 6,000 years for CO2 concentrations to go from 190 ppm to 280 ppm, a rate of change of about 0.015 ppm per year, or about 30 times slower than what has been observed since the start of the industrial revolution.
Even during the previous warmest interglacial period, about 125,000 years ago, CO2 concentrations never went over 300 ppm.
We have actually burned enough CO2 in the form of fossil fuels that the atmospheric increase should have been twice what is observed. The best estimates for where the balance of the CO2 went is into the oceans. The capacity for the oceans to hold this extra CO2 in the long term is not well known. Ocean mixing times are very long, on the order of a thousand years, so it isn't clear at all what the consequences will be. It is the knowledge that it is unlikely that this fast rate of change has happen before that has scientist so concerned about the consequences.
Jeff