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Science Briefs

Climate Forcings in the Industrial Era

Figure 1

Fig. 1: A set of one-Watt light bulbs.

Everyone has heard of "greenhouse effect" — the warming of Earth caused by gases that trap heat emitted by Earth's surface. But how can we tell whether such human-made gases are changing the climate? How can we even visualize this problem in a quantitative way?

One way to think of human greenhouse heating is in terms of the number of miniature Christmas tree bulbs that would be needed to warm Earth's surface by an equivalent amount. All of the greenhouse gases created by humans and added to the air since the Industrial Revolution began cause a heating equal to that of two 1-Watt bulbs over every square yard of Earth's surface.

One fundamental question this raises is: how much will climate change in response to such a forcing? That basic issue has been a focus of climate research for 20 years, ever since the National Academy of Sciences' famous "Charney report" (see references below) in 1979 estimated that the world would warm between 1.5° and 4.5°C if the amount of carbon dioxide in the air were to double. Climate sensitivity is still an important topic, but we maintain that convincing evidence has accumulated in the past 20 years that climate is indeed very sensitive and that Charney's estimate of about 3°C for doubled CO2 (which causes a forcing of 4.3 Watts/m2 is approximately correct. The strongest evidence is the empirical fact that the warmth of the current interglacial period, with Earth 5°C warmer than during the ice age 20,000 years ago, is maintained by a forcing between 6 and 9 Watts/m2.

Figure 2

Fig. 2: Estimated climate forcings between 1850 and the present. The black bars represent estimated uncertainty associated with Earth climate forcing.

We argue in our paper "Climate Forcings in the Industrial Era" (Hansen et al. 1998), a paradigm shift has occurred. The principal issue is no longer global climate sensitivity, but rather climate forcings. What is the magnitude of the human climate forcing and how is it changing? The second figure, patterned after a chart of the Intergovernmental Panel on Climate Change, summarizes current knowledge of climate forcings.

The bar graph illustrates the role of greenhouse gases, as a positive (warming) forcing. It also emphasizes other anthropogenic forcings, especially changes of atmospheric aerosols and clouds, which cause a negative (cooling) forcing that partially offsets greenhouse warming. Although these forcings are not well-measured, it appears that the net climate forcing is probably much less than the greenhouse forcing. One implication of this is that a forcing such as solar irradiance is more important than one would surmise by simply comparing solar and greenhouse forcings.

The main conclusion that leaps out from the bargraph is the huge uncertainty in the forcings due to aerosols (fine particles in the air) and forced cloud changes. "Forced cloud changes" refers to human-made cloud changes. Such changes, including aircraft contrails, are mainly an indirect effect of aerosols, which serve as condensation nuclei for cloud drops and can alter the brightness and lifetime of clouds. It will be impossible to interpret current climate change or predict reliably future climate unless we develop a better understanding of the aerosol and cloud forcings. That is the objective of a new NASA project, the Global Aerosol Climatology Project. This project will obtain information on the aerosol and cloud forcings from existing and planned satellite observations. But evaluation of the climate effect will also require measurements with increased detail on cloud microphysical properties in association with aerosol properties. It will require a long-term research effort, including satellite monitoring, field measurements, and aerosol and climate modeling, to determine the role of aerosols in climate change.

Figure 3

Fig. 3: Growth rate of greenhouse climate forcing. Dashed line is the forcing for 1% per year increase of CO2.

Finally, we emphasize that even the best known climate forcing, that due to the greenhouse gases, presents major uncertainties when one attempts to predict the future. The final figure illustrates the rapid growth of greenhouse forcing between 1950 and the 1970s, and a subsequent leveling off and modest decline of that growth rate. It raises interesting questions about our understanding of even these greenhouse gases. Why has the CO2 growth rate leveled off in the past two decades, despite increased emissions and deforestation? Why has the growth rate of CH4 declined? As we discuss in a companion paper, until we develop an understanding of such issues it will be impossible to assess the effectiveness of policy options for dealing with long-term climate change.