We have left a clear climate change ‘fingerprint’ in the atmosphere, through CO2 emissions that have made air near the Earth’s surface warmer and caused cooling higher up. That’s according to Ben Santer from Lawrence Livermore National Laboratory (LLNL) in California, who started studying this fingerprint in the mid-1990s, and his expert team. They have strengthened the case by comparing satellite-recorded temperature data against the latest climate models including natural variations within Earth’s climate system, and from the sun and volcanic eruptions. Ben hopes that in the process their results will finally answer ill-tempered criticism his earlier work attracted, and lingering doubts over what causes global warming.
“There are folks out there even today that posit that the entire observed surface warming since 1950 is due to a slight uptick in the Sun’s energy output,” Ben told me. “That’s a testable hypothesis. In this paper we look at whether changes in the sun plausibly explain the observed changes that we’ve monitored from space since 1979. The very clear answer is that they cannot. Natural influences alone, the sun, volcanoes, internal variability, either individually or in combination, cannot explain this very distinctive pattern of warming.”
That pattern emerged when scientists in the 1960s did some of the first computer modelling experiments looking at what would happen on an Earth with higher CO2 levels in the air. “They got back this very curious warming in the lower atmosphere and cooling of the upper levels of the atmosphere,” Ben explained. The effect happens because most of the gas molecules in the atmosphere, including CO2, sit relatively near to Earth’s surface. CO2’s greenhouse effect lets heat energy from the Sun reach the Earth, but interrupts some of it getting back to the upper atmosphere and outer space. Adding more CO2 by burning fossil fuels therefore warms the lower atmosphere, or troposphere, and cools the stratosphere, 6-30 miles above the Earth’s surface.
In 1996, Ben and other scientists published data showing exactly this effect from 1963-1987, and suggested that humans were part of the cause. While many backed the findings, they proved controversial, with one critic calling for his dismissal from LLNL. Part of the anger also arose from the fact that Ben helped write the second assessment report of the UN Intergovernmental Panel on Climate Change (IPCC), published in 1995. Similarly to his 1996 paper’s findings, that was the first time the IPCC said ‘the balance of evidence suggests a discernible human influence on global climate’.
Some objected that Ben’s team selected its data specifically to get the results it did, while others said it overlooked uncertainties. And Ben admits that the tools the scientists had then hampered their efforts to respond. “Back then, we didn’t have this amazing scientific goldmine of simulations,” he recalled. “We had simulations from a couple of climate models that we used to look at the atmospheric temperature changes in response to human and natural influences, and we had weather balloon-based estimates of observed changes in atmospheric temperature.”
More recently, Ben has been able to turn to a database of 20 climate models called CMIP5, and temperature data from the atmosphere recorded by satellites since the 1970s. With these capabilities, he and his colleagues now have collected much fuller evidence. “It seemed important to bring that work into the modern era and be as complete as we possibly could in our analysis,” he said. “We wanted to scotch the early criticisms that these results were sensitive to the analysis period, so we looked comprehensively at different periods. We also look comprehensively at uncertainties in model simulations and observations of climate change and at the impact of different processing choices in how you compare models and observations.”
Last November Ben and his fellow scientists found a human-caused climate change fingerprint by comparing worldwide satellite observations against natural variability within the climate alone, determined from climate models. But now, in a paper published on Monday in Proceedings of the National Academy of Sciences of the USA, they’ve also included effects from the sun and volcanoes. Information on their effects is also measured by satellite back to the 1970s, and estimated indirectly from ‘proxy’ measurements back 1,000 years.
The researchers first ran global climate models focussing on this solar and volcano data, Ben said. “We can then ask whether these computer model estimates of the ‘world without us’ produce climate-change patterns similar to the ones we have actually observed since 1979,” he went on. They also consider simulations in which only human influences, like greenhouse gas and soot emissions, act on the climate system without solar or volcanic changes. The scientists can use these ‘human effects only’ simulations to estimate the climate-change fingerprint. Finally, the model simulations also gave Ben’s team estimates of year-to-year and decade-to-decade ‘noise’ of internal climate variability, arising from natural phenomena like the El Niño cycle.
From these simulations Ben’s team can find out the strength of the signal pattern in satellite observations from human effects only and by natural changes. If measured temperature changes are becoming more like the model ‘human effects only’ fingerprint and if natural variability patterns are unlike the ‘human effects only’ fingerprint pattern, they get high signal-to-noise ratios. The researchers calculate ratios for 1979 to 2012, the entire period for which satellite data are available, finding that they’ve been climbing since the early 1990s. Ratios above 3 show a highly significant correspondence between model fingerprint and satellite data, with natural variability unlikely to explain this pattern match. The ratio for variability data reaching back 1000 years has been above 3 since 2000. The ratio with more modern variability data has been above this level since 1998.
“Computer model estimates of the ‘human influence’ fingerprint are broadly similar to the observed pattern,” Ben said. “Model simulations of internal and total natural variability cannot produce the same sustained, large-scale warming of the troposphere and cooling of the stratosphere. So in current climate models, natural causes alone are extremely unlikely to explain the observed changes in the thermal structure of the atmosphere.”
Ben also stresses how important it is that these results hold true regardless of their choice of data for the comparison. “The ‘world without us’ simulations sample changes in volcanic and solar activity over the last 150 to 1,000 years,” he said. “Many of these changes are much larger than those we have observed since 1979. Even in this worst-case situation, using very large estimates of total natural variability, we still consistently detect a ‘human influence’ fingerprint. Coherent warming of the lower atmosphere and cooling of the upper atmosphere are still highly unusual.”
B. D. Santer, K. E. Taylor, T. M. L. Wigley, T. C. Johns, P. D. Jones, D. J. Karoly, J. F. B. Mitchell, A. H. Oort, J. E. Penner†, V. Ramaswamy, M. D. Schwarzkopf, R. J. Stouffer & S. Tett (1996). A search for human influences on the thermal structure of the atmosphere Nature DOI: 10.1038/382039a0
Benjamin D. Santer, Jeffrey F. Painter, Carl A. Mears, Charles Doutriaux, Peter Caldwell, Julie M. Arblaster, Philip J. Cameron-Smith, Nathan P. Gillett, Peter J. Gleckler, John Lanzant, Judith Perlwitz, Susan Solomon, Peter A. Stott, Karl E. Taylor, Laur (2012). Identifying human inﬂuences on atmospheric temperature Proceedings of the National Academy of Sciences of the United States of America DOI: 10.1073/pnas.1210514109
Benjamin D. Santer, Jeffrey F. Paintera, Céline Bonfils, Carl A. Mears, Susan Solomon, Tom M. L. Wigley, Peter J. Gleckler, Gavin A. Schmidt, Charles Doutriaux, Nathan P. Gillett, Karl E. Taylor, Peter W. Thorne, and Frank J. Wentz (2013). Human and natural influences on the changing thermal structure of the atmosphere Proceedings of the National Academy of Sciences of the United States of America DOI: 10.1073/pnas.1305332110