Climate Sensitivity Paper Hints Models Overestimate Warming

  • Date: 16/05/16
  • Reporting Climate 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.

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.C

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”.

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