Proc. Natl. Acad. Sci. USA Vol. 95, pp. 12753–12758, October 1998 Perspective Climate forcings in the Industrial era James E. Hansen*, Makiko Sato, Andrew Lacis, Reto Ruedy, Ina Tegen, and Elaine Matthews National Aeronautics and Space Administration Goddard Institute for Space Studies, New York, NY 10025 Contributed by James E. Hansen, August 18, 1998 ABSTRACT The forcings that drive long-term climate change are not known with an accuracy sufficient to define future climate change. Anthropogenic greenhouse gases (GHGs), which are well measured, cause a strong positive (warming) forcing. But other, poorly measured, anthropo- genic forcings, especially changes of atmospheric aerosols, clouds, and land-use patterns, cause a negative forcing that tends to offset greenhouse warming. One consequence of this partial balance is that the natural forcing due to solar irradiance changes may play a larger role in long-term climate change than inferred from comparison with GHGs alone. Current trends in GHG climate forcings are smaller than in popular ‘‘business as usual’’ or 1% per year CO2 growth scenarios. The summary implication is a paradigm change for long-term climate projections: uncertainties in climate forc- ings have supplanted global climate sensitivity as the pre- dominant issue. A climate forcing is an imposed perturbation of the Earth’s energy balance with space (1). Examples are a change of the solar radiation incident on the planet or a change of CO2 in the Earth’s atmosphere. The unit of measure is Watts per square meter (Wym2), e.g., the forcing due to the increase of atmo- spheric CO2 since pre-Industrial times is approximately 1.5 Wym2 (2, 3). Climate change is a combination of deterministic response to forcings and chaotic fluctuations—the chaos being a con- sequence of the nonlinear equations governing the dynamics of FIG. 1. Principal anthropogenic GHGs in the Industrial era. the system (4). Quantitative knowledge of all significant climate forcings is needed to establish the contribution of Abundances of the principal human-influenced GHGs— deterministic factors in observed climate change and to predict CO2,CH4,N2O, CFC-11, and CFC-12—are shown in Fig. 1. future climate. We estimate chlorofluorocarbon (CFC) amounts before the We provide a perspective on current understanding of global first measurements by using Industrial production data and climate forcings, in effect an update of the Intergovernmental assuming atmospheric lifetimes of 50 and 100 years for CFC-11 Panel on Climate Change (IPCC; ref. 2). We stress the need and CFC-12, respectively. The early records of the other gases for a range of forcing scenarios in climate simulations. are based on ice core data (6, 7). Tabular data for Fig. 1 are available at www.giss.nasa.govydatayGHgases. Greenhouse Gases (GHGs) Climate forcings due to these trace gases can be computed accurately because the absorption properties are known within GHGs absorb and emit infrared (heat) radiation. Because '10%. Analytic fits to calculations based on the correlated temperature decreases with height in the Earth’s troposphere, k-distribution method (8) are given in Table 1. The accuracies increasing GHGs cause emission to space to arise from higher, are believed to be within 10% for abundances occurring in the colder levels, thus reducing radiation to space. This temporary Industrial era, including plausible amounts in the next century. imbalance with incoming solar energy forces the planet to The total climate forcing by the well-mixed GHGs in the warm until energy balance is restored. Industrial era is '2.3 Wym2 (Fig. 2). The forcing by CFCs Well-Mixed Gases. Changes of long-lived GHGs during the includes an estimate of 0.1 Wym2 for minor trace gases, which Industrial era are known accurately. These gases are reason- are mainly halocarbons (9, 10). ably well-mixed in the troposphere, and thus their changes can Inhomogeneously Mixed Gases. Climate forcing by ozone is be monitored at a small number of locations. The principal uncertain because ozone change as a function of altitude is not GHGs have been measured in situ for the past few decades and well measured. Ozone changes during the Industrial era in- their pre-Industrial abundances can be determined from an- clude long-term tropospheric ozone increase associated with cient air bubbles trapped in polar ice sheets (5). Abbreviations: GHG, greenhouse gas; CFC, chlorofluorocarbon. © 1998 by The National Academy of Sciences 0027-8424y98y9512753-6$2.00y0 *To whom reprint requests should be addressed. e-mail: jhansen@ PNAS is available online at www.pnas.org. giss.nasa.gov. 12753 Downloaded by guest on September 28, 2021 12754 Perspective: Hansen et al. Proc. Natl. Acad. Sci. USA 95 (1998) Table 1. Radiative forcings in industrial era Aerosols. Aerosols are fine particles suspended in the air. Gas Radiative forcing There are many aerosol sources and compositions (2, 17, 18). The anthropogenic component of atmospheric aerosols still is 5 2 5 1 CO2 F f(c) f(c0), where f(c) 5.04 ln[c poorly known and thus so is the aerosol climate forcing. 0.0005c2] Our quantitative evaluation focuses on three aerosols that u 2 u 2 2 CH4 0.04 ( m m0) [g(m, n0) g(m0,n0)] probably contribute most to the anthropogenic aerosol optical g(m, n) 5 0.5 ln[1 1 0.00002(mn)0.75] depth: sulfates, organic aerosols, and soil dust. But we use u 2 u 2 2 N2O 0.14 ( n n0) [g(m0,n) g(m0,n0)] realistic aerosol absorption, and thus we also implicitly include 5 2 CFC-11 F 0.25 (x x0) the principal radiative effect of minor aerosol constituents 5 2 CFC-12 F 0.30 (y y0) such as black carbon (soot). t c, CO2 (ppm); m, CH4 (ppb); x, CFC-11 (ppb); and y, CFC-12 (ppb). Aerosol optical depth is defined such that a vertically incident solar beam is attenuated by the factor exp(2t). air pollution as well as stratospheric ozone depletion associ- Aerosol absorption effects are represented by the aerosol ated with anthropogenic chlorine and bromine emissions. single scatter albedo v. v is the fraction of sunlight intercepted Surface ozone over Europe increased by a factor of five in by the aerosol that is scattered (i.e., not absorbed). the past century (11), but measurements cover a small region Sulfate is the aerosol that has received most attention. The and calibration of early data is uncertain. Thus estimates of principal anthropogenic source is sulfur in fossil fuels, which is global tropospheric ozone change depend in part on chemistry emitted as SO2 during combustion and oxidized in the atmo- models (12), which yield an ozone change in Europe smaller sphere to become mainly sulfuric acid. Fig. 3A shows the than that reported (11). We calculate a global forcing of 0.3 annual global distribution of fossil fuel sulfate derived from a Wym2 for the modeled tropospheric ozone change of the past chemical transport model (19). century, based on an accurate radiative treatment (8) including Organic aerosols, i.e., aerosols containing organic carbon, realistic global distributions of clouds and other radiative are produced in combustion of biomass and fossil fuels (2, 17, constituents. Our estimated forcing and uncertainty in Fig. 2, 20, 21). The global distribution of organic aerosols is not well 0.4 6 0.15 Wym2, reflect the range of data and models. known, but field studies find them to be a large portion of total Ozone loss in recent years in the lower stratosphere and aerosol amount (22). Fig. 3B shows an estimated global tropopause region, labeled ‘‘stratospheric’’ in Fig. 2 and de- distribution of anthropogenic organic aerosols (20), with the rived from the most recent versions of satellite analyses, causes aerosols in Eurasia and North America based on fossil fuel a climate forcing 20.2 6 0.1 Wym2 (13, 14). This negative sources and those at low latitudes and in the Southern Hemi- forcing tends to offset the positive forcing due to increasing low sphere associated with biomass burning. level ozone. But this offset does not imply that ozone change Soil (mineral) dust aerosols are produced mainly in arid and has no climatic effect. Indeed, ozone change has a large effect semi-arid regions, especially when the soil or vegetation is on the temperature profile in the troposphere and stratosphere disturbed. Fig. 3C shows an estimated global distribution of (13). anthropogenic soil dust based on an atmospheric transport Climate forcing by stratospheric water vapor is not included model (23) with several regions of land-use disturbance. in Fig. 2 because it is small and water vapor change is measured These three aerosols may dominate anthropogenic aerosol globally only for the past few years (15). Tropospheric water optical depth, but the climate forcing also depends strongly on vapor, the strongest GHG, is not included either because this minor constituents, especially black carbon. In principal, the water vapor amount is a function of climate and thus represents absorption and scattering of all constituents can be added to a climate feedback, rather than a forcing (16). produce the total t and v that, along with the aerosol angular scattering pattern, determine the radiative effect. But this Other Anthropogenic Forcings approach yields a huge uncertainty in the climate forcing. One difficulty is that v is sensitive to whether black carbon is In principal, there are any number of anthropogenic climate externally mixed or contained within the dominant aerosols forcings in addition to GHGs. But forcings are likely to be (24) because internal mixing of black carbon increases its important only if they involve change of one of the other major absorptive effect. v and t also vary with humidity.