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The Environmental Debate, 3rd Edition

Document 137: James Hansen Makes the Case for Climate Change (1988)

In June 1988 the U.S. Senate Committee on Energy and Natural Resources conducted hearings to examine the global warming trend attributed to increased atmospheric concentrations of carbon dioxide and other heat-retaining gases produced by the burning of fossil fuels and to consider its energy and environmental implications. James Hansen, who was then the director of the National Aeronautics and Space Administration (NASA) Goddard Institute for Space Studies, the premier climate research center in the United States, was among a number of scientists, economists, and others invited to testify at the hearings before the committee, one of whose members was Al Gore [see Document 155]. Hansen’s testimony, which presented three model scenarios for possible future emissions of greenhouse gases, raised the hackles of many people both in and out of government, including Dixy Lee Ray [see Document 138], who insisted that there was no proof of a connection between global warming and human activity. The hearings sparked an ongoing debate about global warming and a continuing controversy over the accuracy of Hansen’s models. Hansen is considered the father of climate change awareness.

I would like to draw three main conclusions. Number one, the earth is warmer in 1988 than at any time in the history of instrumental measurements. Number two, the global warming is now large enough that we can ascribe with a high degree of confidence a cause and effect relationship to the greenhouse effect. And number three, our computer climate simulations indicate that the greenhouse effect is already large enough to begin to effect the probability of extreme events such as summer heat waves.

My first viewgraph [Fig. 1] . . . shows the global temperature over the period of instrumental records, which is about 100 years. The present temperature is the highest in the period of record. The rate of warming in the past 25 years, as you can see on the right, is the highest on record. The four warmest years . . . have all been in the 1980s. And 1988 so far is so much warmer than 1987, that barring a remarkable and improbable cooling, 1988 will be the warmest year on the record.

Fig. 1. Global surface air temperature change for the past century, with the zero point defined as the 1951-1980 mean. Uncertainty bars (95% confidence limits) are based on an error analysis . . . ; inner bars refer to the 5-year mean and outer bars to the annual mean. The analyzed uncertainty is a result of incomplete spatial coverage by measurement stations, primarily in ocean areas. The 1988 point compares the January-May 1988 temperature to the mean for the same 5 months in 1951-1980.

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Now let me turn to my second point, which is causal association of the greenhouse effect and global warming. Causal association requires first that the warming be larger than natural climate variability and, second, that the magnitude and nature of the warming be consistent with the greenhouse mechanism. These points are both addressed on my second viewgraph [Fig. 2]. The observed warming during the past 30 years, which is the period when we have accurate measurements of atmospheric composition, is shown by the heavy black line in this graph. The warming is almost 0.4 degrees Centigrade by 1987 relative to climatology, which is defined as the 30 year mean, 1950 to 1980 and, in fact, the warming is more than 0.4 degrees in 1988. The probability of a chance warming of that magnitude is about 1 percent. So, with 99 percent confidence we can state that the warming during this time period is a real warming trend.

The other curves in this figure are the results of global climate model calculations for three scenarios of atmospheric trace gas growth. We have considered several scenarios because there are uncertainties in the exact trace gas growth in the past and especially in the future. We have considered cases ranging from business as usual, which is scenario A, to draconian emission cuts, scenario C, which would totally eliminate net trace gas growth by year 2000.

The main point to be made here is that the expected global warming is of the same magnitude as the observed warming. Since there is only a 1 percent chance of an accidental warming of this magnitude, the agreement with the expected greenhouse effect is of considerable significance. Moreover, if you look at the next level of detail in the global temperature change, there are clear signs of the greenhouse effect. Observational data suggests a cooling in the stratosphere while the ground is warming. The data suggest somewhat more warming over land and sea ice regions than over open ocean, more warming at high latitudes than at low latitudes, and more warming in the winter than in the summer. In all of these cases, the signal is at best just beginning to emerge, and we need more data. Some of these details, such as the northern hemisphere high latitude temperature trends, do not look exactly like the greenhouse effect, but that is expected. There are certainly other climate change factors involved in addition to the greenhouse effect.

Fig. 2. Global surface air temperature change at seasonal resolution for the past 30 years.

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Altogether the evidence that the earth is warming by an amount which is too large to be a chance fluctuation and similarity of the warming to that expected from the greenhouse effect represents a very strong case, [i]n my opinion, that the greenhouse effect has been detected, and it is changing our climate now.

Then, my third point. Finally I would like to address the question of whether the greenhouse effect is already large enough to affect the probability of extreme events, such as summer heat waves. As shown in my next viewgraph [Fig. 3], we have used the temperature changes computed in our global climate model to estimate that impact of the greenhouse effect on the frequency of hot summers in Washington, D.C. and Omaha, Nebraska. A hot summer is defined as the hottest one-third of the summers in the 1950 to 1980 period, which is the period the Weather Bureau uses for defining climatology. So, in that period the probability of having a hot summer was 33 percent, but by the 1990s you can see that the greenhouse effect has increased the probability of a hot summer to somewhere between 55 and 70 percent in Washington according to our climate model simulations. [In] the late 1980s, the probability of a hot summer would be somewhat less than that. You can interpolate to a value of something like 40 to 60 percent.

I believe that this change in the frequency of hot summers is large enough to be noticeable to the average person. So, we have already reached a point that the greenhouse effect is important. It may also have important implications other than for creature comfort.

Fig. 3. Estimate of the probability of the summer being “hot”, shown for two locations for scenarios A, B, and C. A “hot” summer is one in which the mean temperature exceeds a value which was chosen such that one third of the summers were “hot” in 1950-1979 observations. The estimated probability for hot summers in the 1990s is shown by the shaded region for the range of scenarios.

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My last viewgraph [Fig. 4] shows global maps of temperature anomalies for a particular month, July for several different years between 1986 and [2019], as computed with our global climate model for the intermediate trace gas scenario B. As shown by the graphs on the left, where yellow and red colors represent areas that are warmer than climatology and blue areas represent areas that are colder than climatology, at the present time in the 1980s the greenhouse warming is smaller than the natural variability of the local temperature. So, in any given month, there is almost as much area that is cooler than normal as there is area warmer than is normal. A few decades in the future, as shown on the right, it is warm almost everywhere.

However, the point that I would like to make is that in the late 1980s and 1990s we notice a clear tendency in our model for greater than average warming in the southeast United States and the midwest. In our model this result seems to arise because the Atlantic Ocean off the coast of the United States warms more slowly than the land. This leads to high pressure along the east coast and circulation of warm air north into the midwest or the southeast. There is only a tendency for this phenomenon. It is certainly not going to happen every year, and climate models are certainly an imperfect tool at this time. However, we conclude that there is evidence that the greenhouse effect increases the likelihood of heat wave drought situations in the southeast and Midwest United States even though we cannot blame a specific drought on the greenhouse effect.

Fig. 4. Annual mean global surface air temperature computed for trace gas scenarios A, B, and C described in reference 1. [Scenario A assumes continued growth rates of trace gas emissions typical of the past 20 years, i.e., about 1.5% yr-1 emission growth; scenario B has emission rates approximately fixed at current rates; scenario C drastically reduces trace gas emissions between 1990 and 2000.]. . . The shaded range is an estimate of global temperature during the peak of the current and previous interglacial periods, about 6,000 and 120,000 years before present, respectively. The zero point for observations is the 1951-1980 mean (reference 6); the zero point for the model is the control run mean.

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Therefore, I believe that it is not a good idea to use the period 1950 to 1980 for which climatology is normally defined as an indication of how frequently drought will occur in the future.

Source: Transcript of oral version of statement of Dr. James Hansen, “Greenhouse Effect and Global Climate Change,” U.S. Senate, Committee on Energy and Natural Resources, 100th Congress, 1st session, Senate Hearing 100-461, pt. 2, pp. 39-41. http://image.guardian.co.uk/sys-files/Environment/documents/2008/06/23/ClimateChangeHearing1988.pdf. Fig. 3, missing from transcript, from James Hansen et al., “Global Climate Changes as Forecast by Goddard Institute for Space Studies Three-Dimensional Model,” Journal of Geophysical Research, Vol. 93, D8 (Aug. 20, 1988), p. 9357, Fig. 6.

Citation Types

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Format
MLA 9th
"Document 137: James Hansen Makes The Case For Climate Change (1988)." The Environmental Debate, 3rd Edition, edited by Peninah Neimark & Peter Rhoades Mott, Salem Press, 2017. Salem Online, online.salempress.com/articleDetails.do?articleName=Envd3e_0148.
APA 7th
Document 137: James Hansen Makes the Case for Climate Change (1988). The Environmental Debate, 3rd Edition, In P. Neimark & P. R. Mott (Eds.), Salem Press, 2017. Salem Online, online.salempress.com/articleDetails.do?articleName=Envd3e_0148.
CMOS 17th
"Document 137: James Hansen Makes The Case For Climate Change (1988)." The Environmental Debate, 3rd Edition, Edited by Peninah Neimark & Peter Rhoades Mott. Salem Press, 2017. Salem Online, online.salempress.com/articleDetails.do?articleName=Envd3e_0148.