Greenhouse gas growth rates

Abstract

We posit that feasible reversal of the growth of atmospheric CH(4) and other trace gases would provide a vital contribution toward averting dangerous anthropogenic interference with global climate. Such trace gas reductions may allow stabilization of atmospheric CO(2) at an achievable level of anthropogenic CO(2) emissions, even if the added global warming constituting dangerous anthropogenic interference is as small as 1 degrees C. A 1 degrees C limit on global warming, with canonical climate sensitivity, requires peak CO(2) approximately 440 ppm if further non-CO(2) forcing is +0.5 W/m(2), but peak CO(2) approximately 520 ppm if further non-CO(2) forcing is -0.5 W/m(2). The practical result is that a decline of non-CO(2) forcings allows climate forcing to be stabilized with a significantly higher transient level of CO(2) emissions. Increased "natural" emissions of CO(2), N(2)O, and CH(4) are expected in response to global warming. These emissions, an indirect effect of all climate forcings, are small compared with human-made climate forcing and occur on a time scale of a few centuries, but they tend to aggravate the task of stabilizing atmospheric composition.

Fig. 1.

Growth rates of climate forcing by trace gases. (A) Growth rate of climate forcings for 13 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, and CF₃Br). CFC, chlorofluorocarbon; HCFC, hydrochlorofluorocarbon; HFC, hydrofluorocarbon. Measured abundances of 10 of these gases are reported by the National Oceanic and Atmospheric Administration Climate Monitoring and Diagnostics Laboratory (ftp://ftp.cmdl.noaa.gov). Abundances of CFC-114, CFC-115, and HCFC-123 are from IPCC (3) projections. (B) Climate forcing by trace gases for the 21st century baseline scenario (A1B) of IPCC (3).

Fig. 2.

Growth rates of atmospheric CO₂ (ppm/year) (A), CH₄ (parts per billion per year) (B), and N₂O (parts per billion per year) (C) based on ice core and in situ observations (see Supporting Text). A1B is the midrange baseline IPCC (3) scenario. ppb, parts per billion.

Fig. 3.

Annual growth rate of climate forcing by well mixed GHGs.

Fig. 4.

Five-year mean of the growth rate of climate forcing by well mixed GHGs (CO₂,CH₄,N₂O, MPTGs, and OTGs). O₃ and stratospheric H₂O, which are neither well mixed nor well measured, are not included.

Fig. 5.

Measures of atmospheric CO₂ growth rate. (A) CO₂ airborne fraction, i.e., the ratio of observed atmospheric CO₂ increase to fossil fuel CO₂ emissions. (B) Global fossil fuel CO₂ emissions and division into portions that remain airborne and are soaked up by the ocean and continents. (C) Ratio of soaked-up CO₂ to the excess CO₂ in the air (atmospheric CO₂ - 285 ppm). Atm., atmospheric.

Fig. 6.

Antarctic ice core records. (A) Temperature CO₂ (Upper) and CH₄ (Lower) time series (thousands of years before present) from the Vostok Antarctic ice core (21, 24). (B) Correlation of the temperature in A with the gas amounts as a function of a temporal shift of the curves.

Fig. 7.

Scatter plots of CO₂ (A) and CH₄ (B) in the Vostok ice core (21, 24) as a function of global temperature change, approximated as half of the polar temperature change.