Low latitude to mid to low altitude land surfaces cooled by an average of 5.8 ± 0.6 degrees C during the last glacial maximum (LGM), based on an analysis of rare gases dissolved in groundwater, according to a new study published in Nature.
The temperature estimates in the study are significantly lower than those shown by some notable low-level marine and terrestrial studies that have relied on various indicators to reconstruct past temperatures during the LGM, a period around 20,000 years ago that represents the most recent extended period of global stability. climate much cooler than the present.
“The real significance of our paper is that previous work severely underestimated the cooling of the last ice age, which has low estimates of the sensitivity of Earth’s climate to greenhouse gases,” the co-worker said. -author of the article Jeffrey Severinghaus, professor of geosciences. at the Scripps Institution of Oceanography, University of California at San Diego. “The main reason the earlier work was flawed was that it relied heavily on the abundance of species in the past. But just like humans, species tend to migrate where the climate suits them. , to the snowbirds that move from Canada to Arizona in the winter. So species are not very good thermometers. “
The paper broadly supports a recent marine proxy study by Tierney et al. published last year which revealed significantly greater low latitude cooling than previous efforts and, in turn, suggested greater climate sensitivity than previous studies. This previous article suggested that the response to the Earth’s average surface temperature equilibrium was 3.4 degrees C per doubling of atmospheric carbon dioxide, in line with the consensus range of estimates from advanced climate models, but a little higher than usual. best estimate of 3.0 degrees C.
“The rather high climate sensitivity our results suggest is not good news for future global warming, which could be stronger than expected using previous best estimates.” In particular, our global review strengthens the conclusion of several unique rare gas case studies that the tropics were much colder during the last ice maximum than today. The unpleasant implication for the future is that the hottest regions of the world are not immune from further warming, ”commented co-author Werner Aeschbach, professor at the Institute of Physics. Heidelberg environment. University, Heidelberg, Germany.
The paper used a technique in which measurements of rare gases dissolved in ancient groundwater allow direct and quantitative determination of past surface temperature. Noble gases in the atmosphere are chemically and biologically inactive and have no sinks or appreciable sources over the 40,000 years covered by this study. They dissolve in groundwater and their concentrations at equilibrium are highly temperature dependent. The authors compiled four decades of data on rare gases from groundwater from all continents except Antarctica, along with unpublished measurements of some key tropical locations to produce a global record of LGM’s noble gas (NGT) derived temperatures. .
“The noble gas paleo temperature records are so powerful because they are based on a physical principle and are not influenced much by life – which always complicates everything – and short-term extreme events.” said Martin Stute, co-author of the review article, professor in the Department of Environmental Sciences at Barnard College and associate principal investigator at the Lamont-Doherty Earth Observatory. “They provide an average temperature over hundreds to thousands of years. It is remarkable and gratifying for me to see how consistent rare gas paleo-temperature reconstructions are in low latitudes since the earliest studies I have done. conducted in the 1990s until the most recent. “
The study strengthens the rare gas analysis method to reconstruct paleo temperatures and gives more confidence in climate models, according to the authors.
“Another key objective of our study was to assess the overall accuracy of the so-called ‘rare gas paleo-thermometer’ in reconstructing temperatures on earth during the last ice age. Naturally, our ability to confidently use this tool to understand the past is By comparing modern temperature observations to independent estimates using noble gases in relatively young groundwater, we have found that the rare gas thermometer is remarkably accurate. over a wide temperature range of about 2 to 33 degrees C (36 This adds a lot of confidence to our estimates of cooling during LGM, “said lead author of the paper, Alan Seltzer, scientific assistant at the Department. of Marine Chemistry and Geochemistry from the Woods Hole Oceanographic Institution.
Seltzer added that the new analysis is important because climate models “are an important tool that policymakers can use to decide how to prepare for future environmental changes. This study alleviates concerns that, based on the rough data LGM, the models could over-predict the global average temperature response to carbon dioxide. In fact, based on both our study and the recent marine proxy compilation, it becomes clear that proxies and models paleoclimatic are in agreement.