Climate Sensitivity Paper Hints Models Overestimate Warming

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. An underestimation of the amount of heat radiated from the tropics compared with observations can cause the the climate models “to give a substantial overestimation of the effective climate sensitivity” - that is, the amount of warming - according to study author professor J. Ray Bates. This graphic from the paper shows how the effective climate sensitivity – the amount of global warming expected from a doubling of atmospheric carbon dioxide levels - shown on the vertical axis realised by two climate models, varies as a function of the tropical radiative response coefficient b1 (horizontal axis) with the so called extratropical radiative response coefficient b2. The lower the estimate of tropical radiative response used in the models then the higher the sensitivity. Courtesy: author and Earth and Space Science.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. An underestimation of the amount of heat radiated from the tropics compared with observations can cause the the climate models “to give a substantial overestimation of the effective climate sensitivity” - that is, the amount of warming - according to study author professor J. Ray Bates. This graphic from the paper shows how the effective climate sensitivity – the amount of global warming expected from a doubling of atmospheric carbon dioxide levels - shown on the vertical axis realised by two climate models, varies as a function of the tropical radiative response coefficient b1 (horizontal axis) with the so called extratropical radiative response coefficient b2. The lower the estimate of tropical radiative response used in the models then the higher the sensitivity. Courtesy: author and Earth and Space Science.

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.

Latest

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.

Abstract

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.

Citation

J. Ray Bates; Estimating Climate Sensitivity Using Two-Zone Energy Balance Models; Earth and Space Science, DOI: 10.1002/2015EA000154

Source

Earth and Space Science.

 

4 Comments on "Climate Sensitivity Paper Hints Models Overestimate Warming"

  1. Alberto Knox | May 31, 2016 at 7:50 pm | Reply

    We need to be open to ALL research and not pre-judge (or appear to pre-judge) science based on our interpretation of what the results SHOULD be. Climate is complex and has many overlapping drivers. It is only through critical examination of data and conclusions that we can know the truth. If we dismiss papers that run counter to our opinions we are no better than the deniers who will embrace this paper as the Holy Grail.

  2. then the cycle of increased temperatures and decreased albedo may be an unstoppable cycle.

    Not at all, as we know over Million of years in Proxy T Data!

  3. Dan Aldridge | May 18, 2016 at 4:36 am | Reply

    “Bates’ Embarrassment: Sad and Sloppy Climate Sensitivity Study”

    http://m.dailykos.com/story/2016/05/16/1526954/-Bates-Embarrassment-Sad-and-Sloppy-Climate-Sensitivity-Study

  4. Philip Haddad | May 16, 2016 at 9:35 pm | Reply

    One only has to look at http://www.pbs.org/wgbh/warming/etc/graphs.html. to realize that global warming started around 1920 when CO2 was 300 ppm and figure out that CO2 was not the cause of global warming. This is the time when heat emissions from fossil fuels reached 1 terawatt and have been climbing steadily ever since. Naturally CO2 has also increased since 80% of our energy is being supplied by fossil fuels. the heat emissions from fossil and nuclear power is more than four times the amount that can be attributed to the rise in air temperature and is ~ 18 terawatts. The rise in air temperature has prevented the normal dissipation of the 44 TW of geothermal heat flow creating an increase in the heat stored in the oceans. Recent studies by Levitus et al 2012 indicate it is rising at a rate of 250-380 TW/yr. This is alarming, if true, since this exceeds the total of heat emissions and geothermal heat flow( as already indicated it is not from CO2). Let’s hope Levitus is wrong but as Glenn Tamblyn pointed out, that is only 0.2-0.3% of the total solar impact. This may be due to the decrease in reflected heat(albedo) caused from the melting of glaciers and snow cover. If this is the correct explanation, then the cycle of increased temperatures and decreased albedo may be an unstoppable cycle.

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