A new paper in Nature by Carolyn Snyder called Evolution of global temperature over the past two million years comes to a startling conclusion—
... A comparison of the new temperature reconstruction with radiative forcing from greenhouse gases estimates an Earth system sensitivity of 9 degrees Celsius (range 7 to 13 degrees Celsius, 95 per cent credible interval) change in global average surface temperature per doubling of atmospheric carbon dioxide over millennium timescales.
This result suggests that stabilization at today’s greenhouse gas levels may already commit Earth to an eventual total warming of 5 degrees Celsius (range 3 to 7 degrees Celsius, 95 per cent credible interval) over the next few millennia as ice sheets, vegetation and atmospheric dust continue to respond to global warming.
For reasons Gavin Schmidt of Real Climate explains, Carolyn Snyder's conclusion here is almost certainly wrong on the high side.
I recently posted a summary of why you can’t constrain ‘Earth System Sensitivity’ (ESS, the long term response of the climate system, including ice sheets, vegetation etc.) just by looking at the regression between the forcing from CO2 (and other greenhouse gases) over the ice age cycles. That regression has been looked at before, and Snyder (2016) updates that with her new (and slightly higher amplitude) temperature reconstruction. Unfortunately, she then associates this regression with the Earth System Sensitivity (which it is not) to get a value of ~9ºC for a doubling of CO2.
In the previous post [same link as above], I outlined how the combination of carbon cycle feedbacks to the Milankovitch forcing and the climate system response to CO2 gives rise to this correlation and that – by itself – it can’t be used to define the latter term. Furthermore, because the regression is being defined over ice age cycles where the biggest changes are related to the (now disappeared) North American and Fenno-Scandanavian ice sheets, the regression might well be much less for situations where only Greenland and West Antarctica are “in play”.
So, what do we think the ESS is, and how does that impact our view of committed warming today?
Good question, Gavin!