Computer climate models relied on by scientists and governments to forecast the impact of climate change may be running too hot, according to the conclusions of a new paper, which says models may overestimate so called climate sensitivity.
This may mean that forecasts from the Intergovernmental Panel on Climate Change (IPCC) on the amount of global warming that we can expect may be overestimated by between a half and four times if the conclusions of this research are confirmed.
Climate sensitivity to carbon dioxide is a key factor in the computer models of the climate that are used to forecast future climate change and form the basis for climate policy by governments around the world. Climate scientists’ views on the rate of global warming due to carbon dioxide – the so called sensitivity of the climate – forecast by computer climate models need to be “reconsidered”, according to the conclusions of the new study published this month in Earth and Space Science.
The sensitivity values that computer models use “do not agree” with those lower values of climate sensitivity given by observations, according to study author, professor J. Ray Bates, from the School of Mathematics and Statistics, University College Dublin, Ireland. The paper suggests that the reason this occurs is because computer models systematically underestimate the amount of heat that is radiated into space from the tropics.
IPCC widened its climate sensitivity range in 2013
In 2013 the IPCC decided to widen its estimate on the range of the so called equilibrium climate sensitivity for a doubling in the level of carbon dioxide in the atmosphere to between 1.5°C and 4.5°C as compared with the previous range of between 2°C and 4.5°C published by the IPCC in 2007 – that is, a reduction in the lower limit. The implication of this paper is that the lower limit may need to be further reduced to around 1°C.
Bates, a former NASA scientist who studied in the past under the great meteorologist Jule Charney, examined the issue by analysing the behaviour of two different energy balance models of the atmosphere – known as two-zone energy balance models – which estimate the changes in the climate system from the behaviour of the energy budget of the Earth, essentially the difference between incoming solar radiation from the sun and the outgoing heat radiated by the Earth back into space.
Substantial overestimation of the effective climate sensitivity
Bates found that an underestimation of the amount of heat radiated from the tropics, as occurs in many current climate models, can cause the climate models “to give a substantial overestimation of the effective climate sensitivity”.
Among the other key findings of the paper, entitled “Estimating Climate Sensitivity Using Two-Zone Energy Balance Models”, is that feedbacks in the climate system affecting the sensitivity of the climate to increases in the concentration of carbon dioxide are “not always additive”.
“The central conclusion of this study is that to disregard the low values of effective climate sensitivity ( 1°C) given by observations on the grounds that they do not agree with the larger values of equilibrium, or effective, climate sensitivity given by GCMs, while the GCMs themselves do not properly represent the observed value of the tropical radiative response coefficient, is a standpoint that needs to be reconsidered,” Bates states in his paper.
This research is the latest in a number of studies that have suggested that estimates for climate sensitivity to increased carbon dioxide levels may be too high.
Estimates of 2 x CO2 equilibrium climate sensitivity (EqCS) derive from running global climate models (GCMs) to equilibrium. Estimates of effective climate sensitivity (EfCS) are the corresponding quantities obtained using transient GCM output or observations. The EfCS approach uses an accompanying energy balance model (EBM), the zero-dimensional model (ZDM) being standard. GCM values of EqCS and EfCS vary widely [IPCC range: (1.5, 4.5)°C] and have failed to converge over the past 35 years. Recently, attempts have been made to refine the EfCS approach by using two-zone (tropical/extratropical) EBMs. When applied using satellite radiation data, these give low and tightly-constrained EfCS values, in the neighbourhood of 1°C. These low observational EfCS/two-zone EBM values have been questioned because (a) they disagree with higher observational EfCS/ZDM values, and
(b) the EfCS/two-zone EBM values given by GCMs are poorly correlated with the standard GCM sensitivity estimates. The validity of the low observational EfCS/two-zone EBM values is here explored, with focus on the limitations of the observational EfCS/ZDM approach, the disagreement between the GCM and observational radiative responses to surface temperature perturbations in the tropics, and on the modified EfCS values provided by an extended two- zone EBM that includes an explicit parameterization of dynamical heat transport. The results support the low observational EfCS/two-zone EBM values, indicating that objections (a) and (b) to these values both need to be reconsidered. It is shown that in the EBM with explicit dynamical heat transport the traditional formulism of climate feedbacks can break down because of lack of additivity.