One of the arguments of global warming deniers is that CO₂ levels have been higher in the past than they are now, and that therefore it shouldn’t be a problem. Reuters has a good article which explains why this argument is false: it’s not really the absolute amount of CO₂ that’s the problem, but the rate of change. We’re currently pumping CO₂ into the atmosphere about 14,000 times faster than any stage in the last 600,000 years.

The average change in the amount of atmospheric carbon dioxide over the last 600,000 years has been just 22 parts per million by volume, Zeebe said, which means that 22 molecules of carbon dioxide were added to, or removed from, every million molecules of air.

Since the Industrial Revolution began in the 18th century, ushering in the widespread human use of fossil fuels, the amount of carbon dioxide in the atmosphere has risen by 100 parts per million.

That means human activities are putting carbon dioxide into the atmosphere about 14,000 times as fast as natural processes do, Zeebe said.

And it appears to be speeding up: the U.S. government reported last week that in 2007 alone, atmospheric carbon dioxide increased by 2.4 parts per million.

The natural mechanism will eventually absorb the excess carbon dioxide, Zeebe said, but not for hundreds of thousands of years.