LIVERMORE, Calif.— Researchers at Lawrence Livermore National Laboratory, who examined temperature data from 1979 to 1999, have discovered that large volcanic eruptions cooled the lower troposphere (the layer of atmosphere from the Earth’s surface to roughly 8 km above it) more than the surface, and likely masked the actual warming of the troposphere.
This research helps explain the apparent difference in warming rates at the Earth’s surface and in the lower troposphere. While the surface has warmed markedly over the past 20 years, temperatures in the lower troposphere have shown little or no increase. This discrepancy has been the focus of considerable scientific and political attention.
The research is presented in "Accounting for the Effects of Volcanoes and ENSO in Comparisons of Modeled and Observed Temperature Trends," in the November edition of the Journal of Geophysical Research-Atmospheres . LLNL researchers Benjamin Santer, Charles Doutriaux, James Boyle, Sailes Sengupta and Karl Taylor teamed with scientists from the National Center for Atmosphere Research, NASA/Goddard Institute for Space Studies, the Climatic Research Unit at the University of East Anglia in the United Kingdom, and the Max-Planck Institute for Meteorology in Germany.
The paper attempts to quantify volcanic influences on surface and tropospheric temperatures. Volcano "signals" are themselves masked by the temperature changes caused by El Niño events. The eruption of El Chichón in Mexico in 1982 coincided with a large El Niño event during the winter of 1982-83, while the eruption of Mt. Pinatubo in the Philippines in 1991 occurred at the same time as El Niño of 1991-92. To study volcanic effects on temperature, El Niño influences must first be removed.
The atmospheric scientists used a statistical procedure to separate El Niño and volcanic effects in observed temperature records. They found that aerosol particles from El Chichón and Pinatubo cooled the lower troposphere and probably masked the actual warming of the troposphere. Volcanoes therefore supply at least part of the explanation for the different temperature trends at the Earth’s surface and in the troposphere.
This research undercuts claims by greenhouse skeptics that no warming has occurred during the last two decades. These claims are based on satellite measurements of temperatures in the lower troposphere, which show little or no warming since the beginning of the satellite record in 1979.
"Our recent work shows that some of the differences between warming rates at the Earth's surface and in the lower troposphere are due to the effects of volcanic eruptions and stratospheric ozone depletion," said Santer, who works in LLNL’s Program for Climate Model Diagnosis and Intercomparison and is lead author of the JGR-A paper. "Both of these factors probably cooled the lower troposphere by more than the surface, for physical reasons that are well understood. Without ozone depletion and the recent eruptions of El Chichón and Pinatubo, it is highly likely that the lower troposphere would have warmed over the last two decades."
Results from numerical models of the climate system reinforce these conclusions. Computer model experiments examined by the LLNL scientists suggest that it is important to include the effects of volcanoes and stratospheric ozone depletion (in addition to changes in other greenhouse gases, such as carbon dioxide and methane). Doing so brings simulated surface and tropospheric temperature changes into better agreement with the observations.