May 19, 2013

expert reaction to new study of climate sensitivity

The downturn in the rate of climate warming over the past decade does not significantly change estimates of the eventual temperature rise in response to doubled atmospheric CO₂ concentrations, according to a study published in Nature Geoscience. 

Dr Richard Allan, Reader in Climate Science at the University of Reading, said:

“This work has used observations to estimate Earth’s current heating rate and demonstrate that simulations of climate change far in the future seem to be pretty accurate. However, the research also indicates that a minority of simulations may be responding more rapidly towards this overall warming than the observations indicate.”

“Sunlight reflected back to space by aerosol pollutant particles, which offsets some of the heating from greenhouse gases, is difficult to measure, as is the heating rate of the deep ocean. Both make it difficult to estimate the most realistic rate of future global warming, but they don’t change the overall picture and certainly don’t give us cause for complacency.”

“It is important to understand how much the planet will warm up in response to radiative forcing from rising greenhouse gas concentrations. This is often quantified as the total warming experienced in response to a doubling of the carbon dioxide concentration: how sensitive climate is to this heating effect (climate sensitivity).

“However, since the total response of the climate system can take hundreds of years to reach its final resting place (or equilibrium), more useful for making policy decisions involving adaptation strategies is the journey to this final resting place, or how quickly the climate responds (transient climate response).

“Are climate simulations, used to project future changes in climate, realistic in both of these respects?  To answer this question, Otto et al combine knowledge of the extra energy entering the climate system due to rising greenhouse gas concentrations and other factors (radiative forcing) with observations of surface temperature and of how heat is building up (primarily within the oceans).

“Despite the slow rate of surface warming in the recent decade, energy has continued to build up within the oceans and the authors find that the inferred sensitivity of climate to a doubling of carbon dioxide concentrations based on these observations (1.2-3.9 C total warming) is more or less consistent with the range from climate simulations (2.2-4.7 C).  However, the observations suggest that the rate of warming up to the point of doubled carbon dioxide concentrations over the coming decades may be slightly lower than predicted by a few of the climate models used to make future projections.

“The authors caution that uncertainties in the observations and the cooling effects of aerosol pollutant particles mean that it is difficult to precisely anticipate the most realistic rate of climate response over the coming decades, but with work like this our predictions become ever better.”

‘Energy budget constraints on climate response’ by Alexander Otto et al. published in Nature Geoscience on Sunday 19th May.