Prehistoric Climate Change

Every time history repeats itself, the price goes up:

Foreshadowing potential climate chaos to come, early global warming caused unexpectedly severe and erratic temperature swings as rising levels of greenhouse gases helped transform Earth, a team led by researchers at UC Davis said Thursday.

The global transition from ice age to greenhouse 300 million years ago was marked by repeated dips and rises in the amount of carbon dioxide in the atmosphere and wild swings in temperature, with drastic effects on forests and vegetation, the researchers reported in the journal Science.

The provocative insight into planetary climate change counters the traditional view that global warming could be gradual and its regional effects easily anticipated.

Over several million years, carbon dioxide in the ancient atmosphere increased from about 280 parts per million to 2,000 ppm, the same increase that experts expect by the end of this century as remaining reserves of fossil fuels are burned.

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Ok, sow when the scientists tell us that CO2 has never been so high, that's wrong? And when they tell us that CO2 comes from human activity, that's suspect as well?

confused: when they tell us concentrations haven't been so high, they are referring to the past million years or two. 300MY ago, the positions of the continents were quite different than today, even the solar luminosity was a couple percent lower then.
The salient point is that climate epochs with highly variable climate are common in the geological record, we are likely perturbing our system out of a rare quasi-stable state.

I don't know of anyone predicting atmospheric concentrations of CO2 at 2000ppm by 2100; 550-900ppm seems in the more plausible range.

There is little to no doubt the increased concentrations of C02 are the result of human activities.
http://www.realclimate.org/index.php?p=87