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Research: Volcanic Aerosols Contribute To The Pause

23.02.2014
23.02.2014 17:49 Age: 190 days

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 a 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.

 

Benjamin Santer and colleagues analysed satellite data to show that volcanic aerosols released from several eruptions since 2000 had a discernible 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."

 

 

News release

Here is the text of a news release issued by MIT regarding this research:

 

Study: Volcanoes Contribute to Recent Warming ‘Hiatus’

 

 

by Alli Gold Roberts, MIT Joint Program on the Science and Policy of Global Change

By the late 1990s, scientists had observed more than two decades of rapid global warming, and expected the warming trend to continue. Instead, despite continuing increases in greenhouse gas emissions, the Earth’s surface temperatures have remained nearly flat for the last 15 years. The International Panel on Climate Change verified this recent warming “hiatus” in its latest report.


Researchers around the globe have been working to understand this puzzle—looking at heat going into the oceans, changes in wind patterns, and other factors to explain why temperatures have stayed nearly stable, while greenhouse gas concentrations have continued to rise. In a study published today in Nature Geoscience, a team of scientists from MIT and elsewhere around the U.S. report that volcanic eruptions have contributed to this recent cooling, and that most climate models have not accurately accounted for the effects of volcanic activity. 


“This is the most comprehensive observational evaluation of the role of volcanic activity on climate in the early part of the 21st century,” says co-author Susan Solomon, the Ellen Swallow Richards Professor of Atmospheric Chemistry and Climate Science at MIT. “We assess the contributions of volcanoes on temperatures in the troposphere—the lowest layer of the atmosphere—and find they’ve certainly played some role in keeping the Earth cooler.” 


There are many components of the Earth’s climate system that can increase or decrease the temperature of the globe. For example, while greenhouse gases cause warming, some types of small particles, known as aerosols, cause cooling. When volcanoes erupt explosively enough, they enhance these aerosols—a phenomenon referred to as “volcanic forcing.” 


“The recent slowdown in observed surface and tropospheric warming is a fascinating detective story,” says Ben Santer, the lead author of the study and a climate scientist at Lawrence Livermore National Laboratory. “There is not a single culprit, as some scientists have claimed. Multiple factors are implicated. The real scientific challenge is to obtain hard quantitative estimates of the contributions of each of these factors to the so-called slowdown.”


The researchers verified the cooling phenomenon by performing two different statistical tests to determine whether recent volcanic eruptions have cooling effects that can be distinguished from the intrinsic variability of the climate. The team found evidence for significant correlations between volcanic aerosol observations and satellite-based estimates of both tropospheric temperature and sunlight reflected by the particles off the top of the atmosphere. 


“What’s exciting in this work was that we could detect the influence of the volcanic aerosols in new ways. Using satellite observations confirmed the fact that the volcanic particles reflected a significant amount of the sun’s energy out to space, and of course losing energy means cooling—and the tropospheric temperatures show that too,” explains Solomon, who is also a researcher with MIT’s Joint Program on the Science and Policy of Global Change. “There are still uncertainties in exactly how big the effects are, so there is more work to do.”


Alan Robock, a professor of environmental sciences at Rutgers University and a leading expert on the impacts of volcanic eruptions on climate, says these findings are an important part of the larger climate picture. “This paper reminds us that there are multiple causes of climate change, both natural and anthropogenic, and that we need to consider all of them when interpreting past climate and predicting future climate.”


“Since none of the standard scenarios for evaluating future global warming include volcanic eruptions,” Robock adds, “this paper will help us quantify the impacts of future large and small eruptions when they happen, and thus better interpret the role of humans in causing climate change.”


This research was led by a team at Lawrence Livermore National Laboratory and builds upon work Solomon conducted in 2011, finding that aerosols in an upper layer of the atmosphere—the stratosphere—are persistently variable and must be included in climate models to accurately depict climate changes.


The research was supported by the U.S. Department of Energy.

News release ends

 

Abstract

Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previously. Possible explanations for the slow-down include internal climate variability, external cooling influences and observational errors. Several recent modelling studies have examined the contribution of early twenty-first-century volcanic eruptions to the muted surface warming. Here we present a detailed analysis of the impact of recent volcanic forcing on tropospheric temperature, based on observations as well as climate model simulations. 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 eects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998. In two simulations with more realistic volcanic influences following the 1991 Pinatubo eruption, differences between simulated and observed tropospheric temperature trends over the period 1998 to 2012 are up to 15% smaller, with large uncertainties in the magnitude of the effect. To reduce these uncertainties, better observations of eruption-specific properties of volcanic aerosols are needed, as well as improved representation of these eruption-specific properties in climate model simulations.


Citation

Volcanic contribution to decadal changes in tropospheric temperature by Benjamin D. Santer, Céline Bonfils, Jerey F. Painter, Mark D. Zelinka, Carl Mears, Susan Solomon, Gavin A. Schmidt, John C. Fyfe, Jason N. S. Cole, Larissa Nazarenko,Karl E. Taylor and Frank J. Wentz PUBLISHED ONLINE: 23 FEBRUARY 2014 in Nature Geoscience DOI:10.1038/NGEO2098

Read the abstract and get the paper here.


Source

Nature press release and Nature Geoscience.

MIT press release here.

Notes

Headline change to better reflect story content 0700 24 February 2014.

This story was further updated to include the text of the MIT news release at 1030 24 February 2014.


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