Climate Models Take the Stage

While weather forecasting models were still in their primitive operational stage in the late 1950s, meteorologists moved on to modeling the general circulation of the atmosphere. Although the original intent was to continue the development of atmospheric theory, it soon became evident to those involved that modeling the general circulation would lead to models of climate—the long-term manifestation of the weather that depends upon geographical location and local topography.

General circulation modeling was the mission of the Geophysical Fluid Dynamics Laboratory (GFDL), which was part of the U.S. Weather Bureau. Under the direction of Joseph Smagorinsky (1924-2005), an increasingly mathematically and physics-savvy group of young meteorologists sought to uncover the effects of changes in atmospheric chemistry, radiative transfer, and moisture content (humidity) on atmospheric behavior by running experiments on the computer. Unlike laboratory sciences such as physics, chemistry, and biology, meteorology has as its "laboratory" the atmosphere. Because it is impossible to recreate the entire global atmosphere indoors, meteorologists needed to find another way to see what happened when atmospheric variables such as temperature, pressure, and humidity were modified. They did it by "tweaking" these same variables in their computer programs, running them out over a predetermined number of computer "weeks," and analyzing the results.

The early general circulation-turned-climate models were not very good. Computers were not powerful enough to allow modelers to include more than two layers of the atmosphere in their calculations. Neither could they include the presence of the oceans—a major force in changing the composition of the atmosphere—and mountains. In those very early models, the Earth was flat and all land.

By the early 1960s, models included many atmospheric layers and meteorologists started to introduce a number of thermal (heat-related) processes such as changes in radiation, condensation, and heat transfer. Gradually they included the effects of CO2, water vapor, and ozone on the absorption of solar radiation. In so doing, modelers tried to examine how air of different temperatures became distributed throughout Earth's atmosphere—an important step in determining what factors most affect climate.

In 1967, Syukuro Manabe (1931- ) and Richard T. Wetherald (1936- ), both of the GFDL, published their paper "Thermal Equilibrium of the Atmosphere with a Given Distribution of Relative Humidity," which included the combined effects of radiation transfer and the hydrologic cycle. They considered the effect of changing amounts of solar radiation, the influence of cloudiness on radiative transfer, the extent to which Earth's albedo changed the energy budget, and ways that chemical constituents in the atmosphere acted as absorbers of radiation and affected surface temperatures. To include these factors, Manabe and Wetherald could only look at one small column of the atmosphere and redo the computations many times.

Since the time of Arrhenius at the end of the 19th century, questions about the effect of anthropogenic warming had come down to this one: What would the temperature be if the CO2 level doubled? Since the extrapolation of Keeling's curve showed this would probably happen in the 21st century, there was considerable interest in seeing just what a climate model would predict. In 1963, the German meteorologist Fritz Muller's (1906-83) climate model had predicted that doubling the CO2 level would raise the surface temperature a whopping 18°F (10°C). The new Manabe-Wetherald model predicted a still-substantial 4°F (2.3°C) increase. Because their model included enough of the atmospheric variables meteorologists thought necessary to give an accurate representation of climate, scientists took the predicted greenhouse warming effect seriously. (The term Greenhouse warming refers to the heat trapped by the glass of a greenhouse. In the atmosphere, gases such as CO2, water vapor, and methane do the trapping.) From this point on, climate models would take the lead as atmospheric scientists tried to determine Earth's future climate.

temperature trend up or down? Callendar said it was up. According to the U.S. Weather Bureau, it was down. Having worked through a massive amount of data, the bureau climatologist J. Murray Mitchell, Jr. (1928- ), concluded that global temperatures had risen through 1940 but had been falling since. Mitchell had apparently been prompted to undertake the temperature calculation because of preliminary work in Scandinavia showing that Arctic temperatures were leveling off. Because temperature change tends to be most obvious at higher latitudes, atmospheric scientists looked for slight variations there first. Given the concern over warming, Mitchell had decided that it was time for the bureau to make a detailed analysis of all the data. He did not have a reason to account for a cooling trend. The extra CO2 should be causing warming, and it certainly would not lead to cooling. Perhaps, Mitchell thought, the cooling was due to changes in the solar constant or to greater than average amounts of volcanic ash being carried in the upper atmosphere. Neither of these explanations seemed plausible. In his article "Recent Secular Changes of Global Temperature," Mitchell declared the situation "an enigma."

There is a fundamental difficulty with determining temperature trends. The calculations must be made for the entire Earth, not for just one location, nor even for several locations scattered around the world. This is not a trivial undertaking now, and it was even more difficult in the 1960s before the introduction of the World Weather Watch and other programs that ensured the systematic sharing, gathering, and processing of tens of thousands of observations made every day. It took scientists several years after the data were collected to determine what had happened. That is why the "warming" trend that everyone thought had occurred during the 1940s was actually a cooling trend, and why the "cooling" trend that Mitchell thought had occurred in the 1950s was actually a warming trend. The global temperature continued to rise in the last half of the 1950s and remained high, although cooling, during the 1960s. It would not be until the 1970s that questions—and concerns—about atmospheric cooling would push into the forefront of public awareness. In the 1960s, CO2 levels and warming were still a concern only in scientific circles. In those circles, climate change had become an issue that was not going away.

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