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Monday, February 24, 2014

Santer et al., Volcanic aerosols account for some of the overestimation of warming by climate models

by, February 23, 2014

New research suggests that climate models may have overestimated global warming because they do not include the impact of aerosols from volcanic eruptions. The implication is that this may be a partial explanation of the so-called global warming pause since volcanic materials dim the sun and cool the planet -- partially offsetting the warming effect of greenhouse gases. We also include (below) the text of news release issued by MIT.
Click to enlarge. Volcanoes eject material which forms aerosols in the stratosphere which makes the atmosphere more opaque, dims the sun, and cools the planet. The effect is thought to be contributing to the so- called global warming pause. Image courtesy: MIT.
Click to enlarge. Behaviour of overlapping 10-year trends in the ‘ENSO removed’ near-global (82.5 N–70 S) TLT data. Least-squares linear trends were calculated over 120 months, with overlap by all but one month; that is, the first trend is over January 1979–December 1988, the second trend over February 1979–January 1989, and so on. The last trend is over January 2003–December 2012. Courtesy: Santer et al. and Nature Geoscience.

Volcanic eruptions over the past decade or so have cooled global lower-atmosphere temperatures to a statistically significant degree, concludes an article published online in Nature Geoscience. Incorporating these volcanic influences into climate models reduces the difference between observed and computer-simulated surface temperature trends between 1998 and 2012 by up to 15%.

Benjamin Santer and colleagues analysed satellite data to show that volcanic aerosols released from several eruptions since 2000 had a discernable cooling effect on the lower layers of the atmosphere. The authors go on to estimate the magnitude of the effect in climate model simulations, and conclude that the lack of volcanic influences in model simulations of twenty-first-century climate can explain some of the overestimation of warming in these simulations of global mean surface temperatures, compared with observations.

The volcanic aerosols dim the sun and so reduce solar warming of the planet.

The authors state: “We identify statistically significant correlations between observations of stratospheric aerosol optical depth and satellite-based estimates of both tropospheric temperature and short-wave fluxes at the top of the atmosphere. We show that climate model simulations without the effects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998.”

By removing the effects of the El Nino Southern Oscillation (ENSO), the agreement between the observed and the model average temperature responses to major volcanic eruptions was improved, the authors report.

“When both ENSO and volcano influences are subtracted, the model and observed temperature residuals have very similar low-frequency changes up to the end of the twentieth century. After 1999, however, a 'warming hiatus' is still apparent in the observed residual TLT (temperature of the lower troposphere) time series, but the lower troposphere continues to warm in the CMIP-5 multi-model average,” they state.

The authors study the impact of the eruptions of El Chichón and Pinatubo and challenge the suggestion that the recent divergence between modelled and observed temperature changes provides evidence that climate models are on average two or three times too sensitive to human-caused changes in greenhouse gases.

“If this claim is correct, there is a serious error in present model-based estimates of the transient climate response (TCR) to greenhouse gas forcing. As both TCR and the volcanic signal decay time are related to the rate of ocean heat uptake, a large model error in ocean heat uptake would yield errors in the simulated temperature response to El Chichón and Pinatubo. The close agreement we find between the observed and model average TLT (temperature of the lower troposphere) responses to El Chichón and Pinatubo does not support the claim of a fundamental model error in climate sensitivity,” state the authors.

Prof Piers Forster, Professor of Climate Change at the University of Leeds, said: "This is a good paper and confirms the Intergovernmental Panel for Climate Change position that recent volcanoes contribute to the slowdown but cannot be the only cause.  Volcanoes give us only a temporary respite from the relentless warming pressure of continued increases in CO2."

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