Earth to warm 4C by 2100 under business as usual

A new paper by Steven Sherwood et al. published in Nature this week is predicting a higher climate sensitivity of 3°C or more per doubling of CO2. The paper is called Spread in model climate sensitivity traced to atmospheric convective mixing.

Current projections of climate sensitivity fall in the range of 1.5-4.5°C. Part of the reason there is such a large range is because of the uncertainty in how clouds will behave in a warmer world. Low cloud cover has a net cooling effect on the globe through the reflection of incoming sunlight whereas mid- and high cloud clover has a net warming effect due to the absorption of long wavelength radiation.

Sherwood et al. find that a warmer world will produce fewer low-level clouds and so reduce the cooling effect that these clouds will have on our climate. Without this added reflectivity, the warming will be greater. The physical explanation for this is as follows: As air collects water from the ocean surface it rises, sometimes by a only few kilometres before returning back to the surface and at other times by 10-15 kilometres. The rise and descent of water vapour at the lower level is termed lower-tropospheric mixing and acts to pull water vapour away from layers where clouds that cool the climate form. Hence fewer clouds form in these layers.

Climate models that pitch climate sensitivity at less than 3°C do not incorporate this shallow circulation of water vapour and so, according to Steven Sherwood, are inconsistent with observations.

Steven Sherwood explains this all rather well in the following video:

For a more detailed explanation see A bit more sensitive… at Real Climate.

Some other articles about the same paper:
The Guardian: Planet likely to warm by 4C by 2100, scientists warn.

The SMH: Climate change: Planet to warm by 4 degrees by 2100.

2 thoughts on “Earth to warm 4C by 2100 under business as usual”

  1. Reblogged this on And Then There's Physics and commented:
    Since I’ve been keeping a low profile for the last few days and Rachel’s been keeping an eye on my blog, I hope she doesn’t mind me restarting my blogging by reblogging this post. It’s about a paper by Sherwood et al. suggesting that if one treats convection, and it’s influence on low-level clouds “correctly” then climate sensitivity is likely to be closer to the high-end of the range than the low. I haven’t had a chance to read the paper (hence my thought that reblogging this post would be better than writing my own). What I have heard are some who think this “alarmist” paper should be attacked as much as those that suggest climate sensitivity is low. Not having read the paper, I can’t really comment. All I can say is that just because the result are alarming, doesn’t make it an alarmist paper.

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