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. 2008 Feb 5;105(5):1425-30.
doi: 10.1073/pnas.0707386105.

Rates of change in natural and anthropogenic radiative forcing over the past 20,000 years

Fortunat Joos  1 , Renato Spahni
Affiliations

Rates of change in natural and anthropogenic radiative forcing over the past 20,000 years

Fortunat Joos et al. Proc Natl Acad Sci U S A. .

Abstract

The rate of change of climate codetermines the global warming impacts on natural and socioeconomic systems and their capabilities to adapt. Establishing past rates of climate change from temperature proxy data remains difficult given their limited spatiotemporal resolution. In contrast, past greenhouse gas radiative forcing, causing climate to change, is well known from ice cores. We compare rates of change of anthropogenic forcing with rates of natural greenhouse gas forcing since the Last Glacial Maximum and of solar and volcanic forcing of the last millennium. The smoothing of atmospheric variations by the enclosure process of air into ice is computed with a firn diffusion and enclosure model. The 20th century increase in CO(2) and its radiative forcing occurred more than an order of magnitude faster than any sustained change during the past 22,000 years. The average rate of increase in the radiative forcing not just from CO(2) but from the combination of CO(2), CH(4), and N(2)O is larger during the Industrial Era than during any comparable period of at least the past 16,000 years. In addition, the decadal-to-century scale rate of change in anthropogenic forcing is unusually high in the context of the natural forcing variations (solar and volcanoes) of the past millennium. Our analysis implies that global climate change, which is anthropogenic in origin, is progressing at a speed that is unprecedented at least during the last 22,000 years.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
Evolution of atmospheric CO2 (a), methane (b), and nitrous oxide (c), and sampling intervals (d) over the past 20,000 years. The gray bar denotes the range of the preindustrial, natural variability in the concentrations of CO2, CH4, and N2O as measured for the past 650,000 years. The green line in b Inset is a spline to the Greenland CH4 data only that preserves multidecadal variability. Sampling intervals for the Greenland CH4 data are shown by the dashed line in d. Data sources are given in SI Text.
Fig. 2.
Fig. 2.
Rates of change for the three greenhouse gases, N2O, CH4, CO2 and their combined radiative forcing for the last 22 ka. The Greenland CH4 data represent decadal-scale variations and inferred rates of change (dash) are directly comparable for different periods. The black lines/arrows show the peak in the rate of change in radiative forcing and concentrations after the anthropogenic signals of CO2, CH4 (no arrow shown), and N2O have been smoothed with a model describing the enclosure process of air in ice (17) (SI Text).
Fig. 3.
Fig. 3.
Decadal-scale rates of change for the three greenhouse gases N2O, CH4, CO2 and their combined radiative forcing over the past millennium. Rates were computed from the spline fits to the Law Dome (11) and atmospheric data shown in the main panels of SI Fig. 5. The thin lines at the top indicates the rate of change in forcing from CO2 (violet, dash), CH4 (green, dash-dot), and N2O (blue, dot).
Fig. 4.
Fig. 4.
Forcings and their rates of change during the last millenium. (Upper) Decadal-scale solar (15, 27) and volcanic (28) forcing during the last millennium expressed relative to 1750 AD. Data were splined with a 40-year cutoff period. The original reconstructions of volcanic forcing (28) (black) and of solar forcing from (15) (thin, magenta) are shown as well. (Lower) Rates of change in radiative forcing from explosive volcanism, solar changes, the three greenhouse gases (CO2, CH4, and N2O), sulfate aerosols, halocarbons, and SF6.
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