Uncertainty Requirements in Radiative Forcing of Climate Change

Abstract

The continuing increase in atmospheric carbon dioxide (CO 2 ) makes it essential that climate sensitivity, the equilibrium change in global mean surface temperature that would result from a given radiative forcing, be quantified with known uncertainty. Present estimates are quite uncertain, 3 Ϯ 1.5 K for doubling of CO 2 . Model studies examining climate response to forcing by greenhouse gases and aerosols exhibit large differences in sensitivities and imposed aerosol forcings that raise questions regarding claims of their having reproduced observed large-scale changes in surface temperature over the 20th century. Present uncertainty in forcing, caused largely by uncertainty in forcing by aerosols, precludes meaningful model evaluation by comparison with observed global temperature change or empirical determination of climate sensitivity. Uncertainty in aerosol forcing must be reduced at least three-fold for uncertainty in climate sensitivity to be meaningfully reduced and bounded.

Statistically derived contributions of diverse human influences to twentieth-century temperature changes

“…Aerosols have a dominant role in the climate change observed during the industrial period, and the results of any study on attribution, climate sensitivity and model performance evaluation may be sensitive to the exact specification of this factor 40, 45,46 . The uncertainty in the radiative forcing from atmospheric aerosols has been estimated to be roughly of the same magnitude as the radiative forcing from greenhouse gases 45,46 . This section presents a sensitivity analysis of the results presented in this paper to an alternative series of the radiative forcing from atmospheric aerosols (direct effect).…”

“…Atmospheric aerosols are the largest contributors to the uncertainty in radiative forcing during the industrial period and the level of scientific understanding associated with this forcing is medium to low 45,25 . Aerosols have a dominant role in the climate change observed during the industrial period, and the results of any study on attribution, climate sensitivity and model performance evaluation may be sensitive to the exact specification of this factor 40, 45,46 .…”

Design and Application of a Pulsed Cavity Ring-Down Aerosol Extinction Spectrometer for Field Measurements

“…The uncertainty in aerosol radiative forcing needs to be reduced by a factor of 3 to 10 to support regulatory decisions related to global climate change (Schwartz, 2004). The large spatial and temporal variability of aerosol contribute significantly to the uncertainty in the aerosol radiative forcing.…”

Aerosol Light Extinction Measurements by Cavity Attenuated Phase Shift (CAPS) Spectroscopy: Laboratory Validation and Field Deployment of a Compact Aerosol Particle Extinction Monitor

“…The development of instruments that can adequately measure aerosol optical properties is a key to precisely quantify the direct radiative effect of atmospheric aerosols by reducing the uncertainty in estimates of particle single scattering albedo ω (Schwartz 2004), where ω = σ sp /σ ep . The σ sp coefficients are usually measured by nephelometry (Heintzenberg and Charlson 1996).…”