On June 23, 1988, Dr. James Hansen testified before a Senate Committee in Washington that a recent rising trend in global mean surface temperatures had exceeded the point where natural causes could adequately explain it, and was instead the result of a buildup of carbon dioxide in the atmosphere due to human activity. Hansen was the director of the Goddard Institute for Space Studies (GISS), a NASA facility situated in New York City. GISS had been founded in 1961 for the initial purpose of studying planetary atmospheres, and like most of the scientists who worked at GISS in the eighties, Hansen’s background was in planetary science. He co-wrote one of the definitive works on the scattering of radiation in planetary atmospheres, and he was one of the first scientists to realize that Venus once had oceans but lost them to a runaway greenhouse effect. His understanding of the greenhouse effect, coupled with the knowledge that levels of carbon dioxide were rising on Earth, eventually led him to focus his research on our own planet. But Hansen didn’t go to Washington to present a brand new scientific breakthrough; the paper that he and his team at GISS were working on at the time extended the work published in previous papers on the present and future state of the Earth’s climate and the role of feedbacks in climate change. Instead, he went to Washington to tell our government that action was required to prevent the worst consequences of global warming, including rising sea levels and more frequent droughts, from happening. This had the effect of turning global warming into a political issue, and as such it has, despite clear scientific evidence of its reality, remained controversial for three decades.
The paper that Hansen and the GISS team were preparing to publish at the time of Hansen’s trip to Washington is titled “Global Climate Changes as Forecast by Goddard Institute for Space Studies Three-Dimensional Model.” (It would be published in the Journal of Geophysical Research in August.) As the title suggests, the paper used a computer model to simulate temperatures over a hundred-year period, based on projected increases in carbon dioxide and other “greenhouse gases.” Other properties not known to influence or be influenced by temperature were held fixed. The simulations were done for three different scenarios. To quote from the paper: “Scenario A assumed that growth rates of trace gas emissions typical of the 1970s and 1980s will continue indefinitely; the assumed annual growth averages about 1.5% of current emissions, so that the net greenhouse forcing increases exponentially. Scenario B has decreasing trace gas growth rates, such that the annual increase of the greenhouse gas climate forcing remains approximately constant at the present level. Scenario C reduces trace gas growth between 1990 and 2000 such that the greenhouse climate forcing ceases to increase after 2000.” The paper goes on to say that “Scenario B is perhaps the most plausible of the three cases,” on the grounds that resource limitations and environmental concerns would make the growth rate of Scenario A increasingly hard to sustain. And indeed, an assessment published last year showed that Scenario B would have levels of carbon dioxide in the atmosphere of 401 parts per million (ppm) in 2016, close to the real value of 404 ppm. The scenario did not anticipate the strong reduction in ozone-depleting gases that also add to the greenhouse effect, so the overall imbalance in the planet’s energy budget was overestimated by about 10%.
Figure 1 (taken directly from the 1988 paper) shows the modeled forecast of how global temperatures would evolve, and Figure 2 shows the actual result from the GISS temperature record. Scenario B predicts a warming of a little over 1.1ºC relative to the 1951-1980 mean by 2020. A recent analysis of the GISS temperature record by Hansen and several collaborators (he retired from GISS in 2013) indicates that an 11-year running mean of the temperature data since 1970 is remarkably linear; given a trend of 0.18ºC per decade, the actual temperature rise relative to the 1951-1980 mean is approaching 0.9ºC. So the model projection somewhat overestimated reality over the last thirty years. The GISS model used in 1988 had a higher sensitivity of global temperatures to greenhouse gas concentrations than most current models do, but of course current models have the benefit of thirty more years of data.
One aspect of the model’s predictions that often gets overlooked is the natural variability in global mean temperature. Scenario B clearly does not show a steadily increasing curve, but an alternating series of periods of rapidly rising temperature and periods of relative stability. The paper included a caveat that the variability in the model slightly underestimated the real, observed natural variability. I have commented in a previous blog post that the real temperature record looks more like a stepladder than a smooth curve, and that this is not out of line at all with what models have predicted. The model suggests that this stepladder pattern will continue at least into the next decade, and I see no reason to think it won’t. People will suggest, as was done during an extended period of relative stability in global mean temperatures during the decade of the 2000s, that global warming has either stopped or gone on “hiatus.” Do not be fooled by that.
Thirty years ago today, the term “global warming” entered the public consciousness. It was already well-known to climate scientists, having been coined by Wallace Broecker in a seminal 1975 paper. The theory behind it was well understood, and by the late 1980s its signal could be clearly seen in the temperature record. That signal has only gotten stronger, as Jim Hansen and many other climate scientists have predicted it would. But once Hansen went to Washington and said that something needs to be done about it, global warming became a political issue at least as much as a scientific one. By the time I first went to GISS as a graduate student in 1996, Hansen had become fed up with the inertia in the political system; he felt that people in Washington simply weren’t listening. But if he was content at the time to go back to doing the science and communicating the results, that had changed by 2006 when the government started to interfere with his ability to discuss his results publicly. Since then he has remained very active on both the political and scientific fronts, and he currently heads the Climate Science, Awareness, and Solutions Program at Columbia University’s Earth Institute. And much more work needs to be done. The scientific debate on global warming was already more or less resolved by 1988, but thirty years later, politicians (especially here in the United States) are still too bogged down debating whether it is happening to get down to actually doing something about it. In the meantime, the planet has continued to warm.