Climate Not as Sensitive to Carbon Dioxide

Sep 28th, 2014 | By | Category: Carbon, News, Research

The findings have generated vigorous international debate about an issue that remains a key area of uncertainty in climate ­science.

The paper, published in the journal Climate Dynamics, was prepared by US climate scientist Judith Curry and climate ­researcher Nic Lewis.

Dr Curry said the sensitivity of climate to increasing concentrations of CO2 was at the heart of the scientific debate on anthropogenic climate change, and also the public debate on the appropriate policy response to increasing ­carbon dioxide in the atmosphere.

She said climate sensitivity and estimates of its uncertainty were important to establishing the cost benefit of taking action to limit greenhouse gas emissions.

The Lewis and Curry paper does not claim to be the last word on the subject and said the major area of uncertainty was the role played by aerosols.

But the paper contains a much higher level of comfort than does the IPCC that the world will not exceed the two ­degrees warming threshold set by the UN.

The Lewis and Curry paper said the best estimate for equilibrium climate sensitivity — the change in global mean surface temperature at equilibrium that is caused by a doubling of the ­atmospheric CO2 concentration — was 1.64 degrees.

The temperature range given with a confidence level of 17 to 83 per cent was 1.25 to 2.45.

This range compares with a range of 1.5 to 4.5 given in the IPCC’s fifth assessment report for the same level of confidence.

Unlike the fourth assessment report, the IPCC’s most recent synthesis document did not give a best estimate for climate sensitivity. The Curry and Lewis paper’s best estimate for transient climate response — the temperature change at the time of CO2 doubling — was 1.33C with a range of 1.05C to 1.8C.

The IPCC range was 1.0C to 2.5C with no best estimate given.

The IPCC report acknowledges the scientific debate that continues over the issue of climate sensitivity and the different ­results between models and analysis based on observations.

To arrive at their lower climate sensitivity range than the IPCC, Lewis and Curry analysed the Earth’s observed temperature change, ocean heat uptake and the level of human greenhouse gas emissions and natural ­variability.

By contrasting the period 1859-82 with the period 1995- 2011 they estimated how much the Earth had warmed in association with human greenhouse gas emissions. Neither the Australian ­Science Media Centre nor the University of NSW Centre of ­Excellence Climate System Science commented on the Curry and Lewis paper yesterday.

Dr Curry said the paper was not the last word of climate sensitivity because it related only to the uncertainty in external forcing, surface temperature and ocean heat uptake.

It did not take account of solar influence or ­internal variability.

In an essay published this week, President Barack Obama’s former climate advisor Steven Koonin said today’s best estimate of the sensitivity was no different, and no more certain, than it was 30 years ago despite billions of dollars having been spent.

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