Nobel Lecture: Physical modeling of Earth’s climate^*

Figure 1

It is a great honor to be chosen by the Royal Swedish Academy of Sciences to receive the Nobel Prize established through the generosity and foresight of Mr. Nobel. It is likewise a great pleasure to give a talk on global warming, the subject that I have enjoyed exploring throughout my career.

Figure 2

On this occasion, I would like to thank the late Joseph Smagorinsky, the inaugural director of Geophysical Dynamical Laboratory of National Oceanic and Atmospheric Administration, USA. It has been a great privilege and pleasure to work at the Laboratory, unraveling the secret of climate change.

Figure 3

The energy balance of our planet is maintained between the net incoming solar, shortwave radiation and outgoing, longwave radiation at the top of the atmosphere. According to satellite observation, the globally averaged value of outgoing longwave radiation is $\sim 240{\mathrm{W}\mathrm{m}}^{-2}$. Assuming that the Earth-atmosphere system radiates as a blackbody according to Stefan-Boltzmann’s law of blackbody radiation, one can estimate the effective emission temperature of the planet.

Figure 4

The temperature thus obtained is $-18.7°\mathrm{C}$, which is colder than $+14.7°\mathrm{C}$, which is the global mean temperature of Earth’s surface. This implies that Earth’s surface is warmer than it would be in the absence of the atmosphere by as much as 33 °C. In other words, the atmosphere has a so-called greenhouse effect that increases the temperature of Earth’s surface by as much as 33 °C. It is the satellite observation of outgoing longwave radiation that has provided the most convincing evidence for the existence of the greenhouse effect of the atmosphere.

Figure 5

In order to illustrate schematically the thermal structure and the greenhouse effect of the atmosphere, this figure is constructed. In this figure, the slanted line indicates schematically the idealized, vertical temperature profile of the troposphere, where temperature decreases almost linearly with height. The vertical line segment above the slanted line illustrates schematically the almost isothermal lower stratosphere. The dot in the middle troposphere indicates the effective emission center of the outgoing radiation from the top of the atmosphere. Its temperature is $-18.7°\mathrm{C}$, which may be compared with $+14.7°\mathrm{C}$, which is the global mean temperature of Earth’s surface. The latter is warmer than the former by about 33 °C, indicating the magnitude of the greenhouse effect of the atmosphere.

Figure 6

If a greenhouse gas such as carbon dioxide increases in the atmosphere, the infrared opacity of air increases, making it harder for the radiation emitted from the lower layer to reach the top of the atmosphere. Consequently, the average height of the layer, from which the outgoing longwave radiation originates, increases as the concentration of greenhouse gas increases in the atmosphere. In short, the more opaque the atmosphere is, the higher the effective center of the upward flux that reaches the top of the atmosphere. Since the effective center (A) of the outgoing radiation is located in the troposphere, where temperature decreases with increasing height, the temperature of the center decreases as it moves upward, thereby reducing the outgoing radiation from the top of the atmosphere.

Figure 7

Using the model, we obtained the vertical temperature profile of the atmosphere in radiative-convective equilibrium not only for the normal concentration of the atmosphere that is 300 ppm by volume, but also for two other concentrations, that is, 150 and 600 ppm. This figure shows vertical temperature profiles of the coupled atmosphere–Earth surface system, in radiative-convective equilibrium, which are obtained for these three concentrations. As explained already, temperature increases not only at Earth’s surface but also in the troposphere as the atmospheric concentration of carbon dioxide doubles from 150 to 300 ppm and 300 to 600 ppm, whereas it decreases in the stratosphere.

Figure 8

The magnitude of the warming in the troposphere is 2.3 °C in both cases and is practically identical to each other.

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In the box diagram shown here, the coupled model consists of three major components, which are the general circulation model of the atmosphere, that of the ocean, and a simple, heat- and water-balance model of continental surface. Although the initial version of the coupled model was constructed in the late 1960s, it took two more decades before the coupled model with realistic geography became ready for the global warming experiment conducted in the late 1980s. The result from the experiment was published in the late 1980s and was discussed extensively in the first report of the Intergovernmental Panel on Climate Change published in 1990. For further details of this study, see our book Beyond Global Warming recently published by Princeton University Press.

Figure 10

Global warming involves changes not only in temperature but also in the rate of evaporation and that of precipitation. If a greenhouse gas such as water vapor and carbon dioxide increases in the atmosphere, the downward flux of longwave radiation increases at Earth’s surface as explained already, thereby increasing the temperature of Earth’s surface. Since the saturation vapor pressure at a wet surface (e.g., ocean) increases with increasing surface temperature, it is expected that the rate of evaporation from Earth’s surface also increases so long as the relative humidity of the overlying atmosphere does not change systematically. Given sufficient time, global-scale increase in the rate of evaporation results in the corresponding increase in the rate of precipitation, thereby increasing the strength of the global water cycle.

Figure 11

For example, precipitation usually increases in many water-rich regions in high northern latitudes and heavily precipitating regions of the tropics, increasing river discharge and the frequency of floods. In contrast, soil moisture usually decreases in many relatively arid regions in the subtropics and other water-poor regions that are relatively dry, increasing the frequency of drought.

Figure 12

The implied amplification of the existing difference in water availability between water-poor and water-rich regions presents a very serious challenge to the water-resource managers of the world.