A new peer reviewed paper published gives us the most up-to-date and accurate record of atmospheric CO2 levels over the past 66 million years.
Knowing what has happened in the past gives us information as to what we can expect as GHG levels continue to rise at accelerating rates.
For example, 51 million years ago CO2 levels peaked at 1600 ppm and global average temperature was over 12C warmer than today.
Today’s CO2 levels in the atmosphere (420 ppm) are higher than anything until we go back to near the Miocene Climate Optimum (MCO) about 16 million years ago. When we surpass 480 ppm, we will have to go back to roughly 28 million years or so.
By comparing these paleo-CO2 findings with global temperatures we can see that the Equilibrium Climate Sensitivity (ECS) and Equilibrium System Sensitivity (ESS) were higher in the past.
I should note that this paper used the mainstream IPCC ECS of 3C (for doubling of CO2) when the latest work from James Hansen argues that the sensitivity is a much higher 4.8C (with a range from 3.6C to 6.0C) which is a closer match to the higher sensitivities in the past.
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by paulhenrybeckwith