Trends of measured climate forcing agents

Abstract

The growth rate of climate forcing by measured greenhouse gases peaked near 1980 at almost 5 W/m(2) per century. This growth rate has since declined to approximately 3 W/m(2) per century, largely because of cooperative international actions. We argue that trends can be reduced to the level needed for the moderate "alternative" climate scenario ( approximately 2 W/m(2) per century for the next 50 years) by means of concerted actions that have other benefits, but the forcing reductions are not automatic "co-benefits" of actions that slow CO(2) emissions. Current trends of climate forcings by aerosols remain very uncertain. Nevertheless, practical constraints on changes in emission levels suggest that global warming at a rate +0.15 +/- 0.05 degrees C per decade will occur over the next several decades.

Figure 1

Estimated climate forcings; error bars are partly subjective 1σ uncertainties.

Figure 2

Growth rate of atmospheric CO₂ and CH₄ based on ice core measurements of Etheridge et al. (20, 21), in situ CO₂ observations initiated by C. D. Keeling (11), and National Oceanic and Atmospheric Administration (NOAA) CO₂ and CH₄ observations made available by the NOAA Climate Monitoring and Diagnostics Laboratory (ftp://ftp.cmdl.noaa.gov/ccg).

Figure 3

Growth rate of climate forcing for gases controlled by the Montreal Protocol (CFC-11, CFC-12, CFC-113, CFC-114, CFC-115, CCl₄, CH₃CCl₃, HCFC-22, HCFC-141b, HCFC-142b, HCFC-123, CF₂BrCl, CF₃Br) based on IPCC (appendix II of ref. 3), National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory Global Flask Network (ftp://ftp.cmdl.noaa.gov/hats/), and data from Montzka et al. (26).

Figure 4

Growth rate of climate forcing by well-mixed greenhouse gases (5-year mean, except 3-year mean for 1999 and 1-year mean for 2000). O₃ and stratospheric H₂O, which were not well measured, are not included.

Figure 5

CO₂ emissions based on data of Marland and Boden (27).

Figure 6

U.S. percentage of global fossil fuel CO₂ emissions (27).

Figure 7

Climate forcing scenario for 2000–2050 that yields a forcing of 0.85 W/m² (colored bars), including small forcings from stratospheric ozone recovery and trace gases (Table 1).