How sensitive is our climate?
What the science says...
Select a level... |
![]() |
![]() |
![]() | ||||
Net positive feedback is confirmed by many different lines of evidence. |
Climate Myth...
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)
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.
Basic rebuttal written by GPWayne
Update July 2015:
Here is the relevant lecture-video from Denial101x - Making Sense of Climate Science Denial
Last updated on 5 July 2015 by skeptickev. View Archives
1998,1995,2002,2007, 2010? I see that you are failing to differentiate between Charney feedbacks (transient climate response, Gregory and Forster 2005) and equilibrium climate sensitivity. Annan and Hargreaves, and others, show the PDF for EQS dropping off sharply below about 2.5 C. (On moderator's advice discussion started there is continued here) Well, let's suppose there is a first order low pass filter between "forcing" and "climate response" (lower troposphere temperature anomaly). If we apply a small step-like forcing ΔF to a climate system in equilibrium and the long term temperature anomaly response is ΔT = βΔF, than β is said to be the equilibrium sensitivity, right? The impulse response function of the filter in this case is (β/τ)e-t/τ for t > 0, zero otherwise, where t is the time variable and τ is a time constant characteristic to the relaxation time of the system. The response to a step-function is of course β(1-e-t/τ). Now, let's suppose the forcing is increasing linearly with time (instead of kicking in in a step-like fashion). With CO2 more or less this is the case, that is, ΔF = ft, where f ~ 0.006 year-1, if unit of forcing is CO2 doubling. The relation seems to hold pretty well at least during the last 70 years. The response of the low pass filter above to such a forcing is βf(t-τ). That is, the time constant τ has no effect other than introducing a delay in this case - or an additive constant, if we look at it the other way around. It has no influence on the trend whatsoever. Provided of course τ is not larger than several decades, that is, the pre-industrial flat part of the CO2 forcing curve has negligible effect beyond the start of satellite era (late 1978). Therefore my calculation is correct, the climate sensitivity is considerably less than 2°C (per CO2 doubling), for the reasons I've stated in the other thread. BTW, I think it is even lower, because satellite lower troposphere temperature anomalies are not reliable either. Back-calculation of temperature from narrow band radiances depends heavily on the atmospheric model used, especially on fine details of water vapor distribution, which is neither measured nor modeled properly. On top of that all climate variables behave like pink noise even in the unforced case, that is, they have large spontaneous fluctuations on all scales (this is characteristic of systems in a state of self-organized criticality).