Whither U.S. Climate?
What's happening to our climate? Was the heat wave and drought in the Eastern United States in 1999 a sign of global warming?
Empirical evidence does not lend much support to the notion that climate is headed precipitately toward more extreme heat and drought. The drought of 1999 covered a smaller area than the 1988 drought, when the Mississippi almost dried up. And 1988 was a temporary inconvenience as compared with repeated droughts during the 1930s "Dust Bowl" that caused an exodus from the prairies, as chronicled in Steinbeck's Grapes of Wrath.
How can the absence of clear climate change in the United States be reconciled with continued reports of record global temperature? Part of the "answer" is that U.S. climate has been following a different course than global climate, at least so far. Figure 1 compares the temperature history in the U.S. and the world for the past 120 years. The U.S. has warmed during the past century, but the warming hardly exceeds year-to-year variability. Indeed, in the U.S. the warmest decade was the 1930s and the warmest year was 1934. Global temperature, in contrast, had passed 1930s values by 1980 and the world has warmed at a remarkable rate over the last 25 years.
|Fig. 1: Annual and 5-year mean surface temperature for (a) the contiguous 48 United States and (b) the globe, relative to 1951-80, based on measurements at meteorological stations.|
A picture of how U.S. climate change during the past half century compared with the rest of the world is shown in Figure 2. This map shows that the trend has been toward warmer temperatures in most of the world. There has been nearly ubiquitous warming in the tropics, especially in the Eastern Pacific Ocean, where the largest warming coincides with the location of more frequent strong El Niños. The strongest warming has been in Alaska and northern Asia. Warming in Alaska is often associated with El Niños. A suspicion of many climatologists — as yet unproven — is that an increasing greenhouse effect may cause more frequent and intense El Niños. Asia has long been predicted to show the largest warming due to increasing greenhouse gases, especially in the winter, and observations are consistent with that.
Yet in the U.S. there has been little temperature change in the past 50 years, the time of rapidly increasing greenhouse gases — in fact, there was a slight cooling throughout much of the country (Figure 2). We caution that linear trends, as in Figure 2, can mask temporal detail. Indeed, Figure 1(b) indicates that the last 20 years have seen a slight warming in the U.S. Nevertheless, our analysis (Hansen et al., 1999a), summarized in Figures 1 and 2, makes clear that climate trends have been fundamentally different in the U.S. than in the world as a whole.
Is this a temporary fluke, a chaotic regional climate fluctuation? If so, as the regional fluctuation reverses and global warming continues, will the U.S. experience dramatic, perhaps "disastrous" climate change in the next few decades? Or is there an understandable and continuing reason that the U.S. is warming less than the rest of the world? In that case, will the U.S. be relatively immune to global warming in the next several decades?
In order to answer such questions and predict future climate change reliably, a prerequisite is an understanding of the cooling of the past half-century in the U.S. Figure 2 suggests that the U.S. cooling is associated, at least in part, with cooling in the North Atlantic Ocean. Climate model simulations tend to confirm this, yielding cooling in the U.S. during the past 50 years when driven by observed ocean temperatures (Hansen et al., 1999b).
But that only changes the question: what is the cause of the Atlantic cooling? In part, the Atlantic cooling is a natural fluctuation, the North Atlantic Oscillation, that occurs on decadal time scales. Observations of the past few years, summarized in Figure 17 of Hansen et al. (1999a), suggest that the North Atlantic Oscillation is now moving into its warmer phase.
|Fig. 2: Change of surface temperature during 1950-98 based on local linear trends.|
However, North Atlantic cooling is also a predicted consequence of the transient growth of greenhouse gases. Climate models (Manabe and Stouffer, 1995; Russell and Rind, 1999) driven by increasing greenhouse gases yield increased precipitation at high latitudes, decreased ocean salinity in the North Atlantic, and thus a weakening of the ocean conveyor belt that transports heat to the North Atlantic.
Additional mechanisms may contribute to observed climate change. For example, in the decades after World War II, when the amount of aerosols (fine particles) in the air grew most rapidly in the Eastern U.S., the pattern of cooling showed a clear resemblance to the distribution of aerosols. Also changes of solar irradiance (the brightness of the sun) are difficult to dismiss as a mechanism of climate change, because there are observed correlations of solar variability and climate change.
The upshot is that we will be able to understand climate change well only with the help of global climate models that are able to incorporate all of these mechanisms on an equal footing. We will be able to test our understanding during the era of satellite measurements, when all of these forcing factors can be measured accurately.
In the meantime, we can venture two "predictions" on "whither U.S. climate". First, regarding U.S. temperature, we have argued (Hansen et al., 1999a) that the next decade will be warmer than the 1990s, rivaling if not exceeding the 1930s. The basis for that prediction is the expectation of continued greenhouse warming and probable slackening of regional ocean cooling. Second, regarding precipitation and drought, even without analysis of regional patterns of change, we can offer the probabilistic statement that the frequencies of both extremes, heavy precipitation and floods on the one hand and droughts and forest fires on the other, will increase with increasing global temperature. The rationale for this (Hansen et al., 1991) is that increased surface heating increases evaporation, and this increases the intensity of both precipitation and drought conditions where and when they occur.
More quantitative predictions should be possible soon based on global climate models and satellite observations. Model runs for the past 50 years suggest that it is possible to simulate regional climate trends realistically. Simulations for the future will depend upon estimates of trends in climate forcings derived from global satellite observations and analyses.
Hansen, J., D. Rind, A. Del Genio, A. Lacis, S. Lebedeff, M. Prather, R. Ruedy and T. Karl 1991. Regional greenhouse climate effects. In Greenhouse-Gas-Induced Climatic Change (M. Schlesinger, ed.). Elsevier, Amsterdam.
Hansen, J., R. Ruedy, J. Glascoe and M. Sato 1999. GISS analysis of surface air temperature change. J. Geophys. Res. 104, 30997-31022.
Hansen, J., R. Ruedy, A. Lacis, M. Sato, L. Nazarenko, N. Tausnev, I. Tegen and D. Koch 1999. Climate modeling in the global warming debate. In Climate Modeling: Past, Present and Future (D. Randall, ed.), pp. 127-164. Academic Press, San Diego.
Manabe, S. and R.J. Stouffer 1995. Simulation of abrupt climate change induced by freshwater input to the North Atlantic Ocean. Nature 378, 165-167
Russell, G.L., and D. Rind 1999. Response to CO2 transient increase in the GISS coupled model: Regional coolings in a warming climate. J. Climate 12, 531-539