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OKIsItJustMe

(19,933 posts)
Tue Sep 27, 2016, 02:32 PM Sep 2016

The Snyder Sensitivity Situation

http://www.realclimate.org/index.php/archives/2016/09/the-snyder-sensitivity-situation/#more-19634
[font face=Serif][font size=5]The Snyder Sensitivity Situation[/font]

26 September 2016

[font size=3]Nature published a great new reconstruction of global temperatures over the past 2 million years today. Snyder (2016) uses 61 temperature reconstructions from 59 globally diverse sediment cores and a correlation structure from model simulations of the last glacial maximum to estimate (with uncertainties) the history of global temperature back through the last few dozen ice ages cycles. There are multiple real things to discuss about this – the methodology, the relatively small number of cores being used (compared to what could have been analyzed), the age modeling etc. – and many interesting applications – constraints on polar amplification, the mid-Pleistocene transition, the duration and nature of previous interglacials – but unfortunately, the bulk of the attention will be paid to a specific (erroneous) claim about Earth System Sensitivity (ESS) that made it into the abstract and was the lead conclusion in the press release.

The paper claims that ESS is ~9ºC and that this implies that the long term committed warming from today’s CO₂ levels is a further 3-7ºC. This is simply wrong.

[center]
[font size=2]The Snyder (2016) reconstruction of global temperatures compared to Antarctic ice core temperature and CO₂, and deep water temperatures.[/font][/center]

I recently posted a summary of why you can’t constrain ‘Earth System Sensitivity’ (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, 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”.

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