Science Briefs

Trends of Measured Climate Forcing Agents

The Earth's climate fluctuates from year to year without any forcing mechanism, as the atmosphere and ocean are chaotic fluids that are constantly sloshing about. However, the climate also responds to forcings, which are imposed perturbations of the planet's energy balance. For example, brighter sun is a positive forcing that warms the Earth. On the other hand, a volcanic eruption that blasts fine particles into the stratosphere is a negative forcing, as the particles reflect sunlight and tend to cause cooling.

These natural climate forcing agents have been joined in the past century by human-made agents, most notably "greenhouse" gases such as carbon dioxide (CO2). By trapping the Earth's thermal (heat) radiation, the added gases cause a forcing of about 2.5 watts per square meter (W/m2), about 1% of the 240 W/m2 of energy that the Earth absorbs from the sun. There is strong circumstantial evidence that this positive forcing is responsible for the observed global warming of about 3/4 °C in the past century.

It has commonly been assumed that the rate of growth of greenhouse gases is continuing to accelerate. That may appear to be a plausible assumption, as the world's population continues to increase each year. Global energy use, the primary source of these gases, also continues to increase. Thus climate scenarios have been dominated by the "business as usual" scenario in which the annual increment of greenhouse gas climate forcing continues to get larger and larger, leading to a specter of imminent climate disaster.

Figure 1: See caption and explanation in main text

Fig. 1: Growth rate of climate forcing by well-mixed greenhouse gases (five-year mean, except three-year for 1999 and one-year for 2000). O3 and stratospheric H2O, which were not well-measured, are not included.

A closer look at actual data is given in Figure 1. The picture of accelerating growth was valid from end of World War II until 1980, by which time the greenhouse gas forcing was increasing at a rate of 5 W/m2 per century. It is no wonder that climate models in the 1980s began to predict the possibility of dramatic climate effects in the 21st century. A continuation on that path would have led to the equivalent of doubled CO2, a forcing of 4 W/m2, by about 2025.

In reality, although greenhouse gases continue to increase, the growth rate has slowed to about 3 W/m2 per century. A big factor has been the phase-out of chlorofluorocarbons (CFCs), which was accomplished by cooperative international actions. The growth rate of methane (CH4) has fallen by two-thirds, at least in part due to slowing of the growth of sources. The growth rate of CO2 flattened out in the past 25 years, as the rate of growth of fossil fuel use declined from 4% per year to about 1% per year.

These trends provide hope that the proposed "alternative scenario" for climate change (see references) may be a feasible target. The objective in this scenario is to keep the added CO2 forcing in the next 50 years at 1 W/m2 and to have no net increase of the other anthropogenic forcings. The expected global warming for that forcing is about 3/4 °C.

In rough or approximate terms, achievement of this alternative scenario requires that fossil fuel CO2 emissions in the next 50 years average about what they are today and the net growth of other human-made forcing agents must be halted. The bulk of these other forcings, specifically black carbon (soot) aerosols, tropospheric ozone and methane, are prime ingredients of air pollution. A halt or even reversal of their growth would have other benefits, for human health, agricultural productivity, and environmental quality, which, together with the slowing of climate change, justify the actions needed to achieve the alternative scenario.

In the near-term, i.e., during this decade, it should be possible to keep fossil fuel use flat by means of concerted efforts in energy efficiency, fuel switching, and development of renewable energy sources. For the long-term, it is important to encourage development now of technologies that will lead to reduced emissions or will capture and sequester CO2 at power plants, in order to be able to stabilize atmospheric composition in the second half of the century.

Finally, improved measurements of all climate forcings are needed to develop optimum policies, which can be adjusted as our understanding develops. Climate change is a long-term problem.


Hansen, J., M. Sato, R. Ruedy, A. Lacis, and V. Oinas 2000. Global warming in the twenty-first century: An alternative scenario. Proc. Natl. Acad. Sci. 97, 9875-9880.

Hansen, J.E., and M. Sato 2001. Trends of measured climate forcing agents. Proc. Natl. Acad. Sci. 98, 14778-14783.