Arguments
How sensitive is our climate?
The skeptic argument...
Climate sensitivity is low
"His [Dr Spencer's] latest research demonstrates that – in the short term, at any rate – the temperature feedbacks that the IPCC imagines will greatly amplify any initial warming caused by CO2 are net-negative, attenuating the warming they are supposed to enhance. His best estimate is that the warming in response to a doubling of CO2 concentration, which may happen this century unless the usual suspects get away with shutting down the economies of the West, will be a harmless 1 Fahrenheit degree, not the 6 F predicted by the IPCC." (Christopher Monckton)
What the science says...
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Net positive feedback is confirmed by many different lines of evidence. |
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Climate sensitivity is the estimate of how much the earth's climate will warm in response to the increased greenhouse effect if we double the amount of carbon dioxide in the atmosphere. This includes feedbacks which can either amplify or dampen that warming. This is very important because if it is low, as some climate 'skeptics' argue, then the planet will warm slowly and we will have more time to react and adapt. If sensitivity is high, then we could be in for a very bad time indeed.
There are two ways of working out what climate sensitivity is. The first method is by modelling:
Climate models have predicted the least temperature rise would be on average 1.65°C (2.97°F) , but upper estimates vary a lot, averaging 5.2°C (9.36°F). Current best estimates are for a rise of around 3°C (5.4°F), with a likely maximum of 4.5°C (8.1°F).
The second method calculates climate sensitivity directly from physical evidence, by looking at climate changes in the distant past:
Various paleoclimate-based equilibrium climate sensitivity estimates from a range of geologic eras. Adapted from PALEOSENS (2012) Figure 3a by John Cook.
These calculations use data from sources like ice cores to work out how much additional heat the doubling of greenhouse gases will produce. These estimates are very consistent, finding between 2 and 4.5°C global surface warming in response to doubled carbon dioxide.
It’s all a matter of degree
All the models and evidence confirm a minimum warming close to 2°C for a doubling of atmospheric CO2 with a most likely value of 3°C and the potential to warm 4.5°C or even more. Even such a small rise would signal many damaging and highly disruptive changes to the environment. In this light, the arguments against reducing greenhouse gas emissions because of climate sensitivity are a form of gambling. A minority claim the climate is less sensitive than we think, the implication being we don’t need to do anything much about it. Others suggest that because we can't tell for sure, we should wait and see.
In truth, nobody knows for sure quite how much the temperature will rise, but rise it will. Inaction or complacency heightens risk, gambling with the entire ecology of the planet, and the welfare of everyone on it.
Last updated on 22 April 2013 by dana1981. View Archives
Fasullo and Trenberth 2012 (described here) appears to be much in the same vein as Spencer and Braswell 2011, where they examined how climate models matched observations, although S&B 2011 was clearly refuted due to poor technique and the exclusion of models they themselves tested which refuted their conclusions.
Assuming the climate is ever in equilibrium as the basis for a calculation is absurd. It is never, ever, in equilibrium. If it were, we would not see the changes that have occurred in the past.
Until science has a better handle on clouds (and many other second-order feedbacks), any attempts to quantify sensitivity are relying on guessing about past events, but not on understanding why.
Your comments on cloud feedbacks would have been justified for TAR but they are far better quantified now. Note also that models are doing a pretty good job of estimating temperature trends, even something as primitive as that used by the Broecker in 1975 ("Climatic change; are we on the brink of a pronounced global warming?" Science, v 189, n 4201, p 460-3, 8 Aug. 1975") which got temp for 2010 better good.