Reply to “Comments on ‘Rethinking the Lower Bound on Aerosol Radiative Forcing’”

This reply addresses a comment questioning one of the lines of evidence I used in a 2015 study (S15) to argue for a less negative aerosol radiative forcing. The comment raises four points of criticism. Two of these have been raised and addressed elsewhere; here I additionally show that even if they have merit the S15 lower bound remains substantially (0.5 W m–2) less negative than that given in the AR5. Regarding the two other points of criticism, one appears to be based on a poor understanding of the nature of S15’s argument; the other rests on speculation as to the nature of the uncertainty in historical SO2 estimates. In the spirit of finding possible flaws with the top-down constraints from S15, I instead hypothesize that an interesting—albeit unlikely—way S15 could be wrong is by inappropriately discounting the contribution of biomass burning to radiative forcing through aerosol–cloud interactions. This hypothesis is interesting as it opens the door for a role for the anthropogenic (biomass) aerosol in causing the Little Ice Age and again raises the specter of greater warming from ongoing reductions in SO2 emissions.

Download Full-text

Bounding global aerosol radiative forcing of climate change

10.5194/egusphere-egu2020-7745 ◽

2020 ◽

Author(s):

Nicolas Bellouin ◽

Keyword(s):

Radiative Forcing ◽

Review Paper ◽

Aerosol Radiative Forcing ◽

The Past ◽

Open Questions ◽

Substantial Progress ◽

Theoretical Considerations ◽

Present Understanding ◽

Lines Of Evidence ◽

Aerosol Radiative

Aerosol radiative forcing plays an important role in the attribution of past climate changes, estimates of future allowable carbon emissions, and the assessment of potential geoengineering solutions. Substantial progress made over the past 40 years in observing, understanding, and modelling aerosol processes helped quantify aerosol radiative forcing, but uncertainties remain large.In spring 2018, under the auspices of the World Climate Research Programme's Grand Science Challenge on Clouds, Circulation and Climate Sensitivity, thirty-six experts gathered to take a fresh and comprehensive look at present understanding of aerosol radiative forcing and identify prospects for progress on some of the most pressing open questions. The outcome of that meeting is a review paper, Bellouin et al. (2019), accepted for publication in Reviews of Geophysics. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable and arguable lines of evidence, including modelling approaches, theoretical considerations, and observations. A substantial achievement is to focus on lines of evidence rather than a survey of past results or expert judgement, and to make the open questions much more specific.This talk will present the key messages and arguments of the review and identify work that show promise for improving the quantification of aerosol radiative forcing.

Download Full-text

Origins of a relatively tight lower bound on anthropogenic aerosol radiative forcing from Bayesian analysis of historical observations

Journal of Climate ◽

10.1175/jcli-d-21-0167.1 ◽

2021 ◽

pp. 1-51

Author(s):

Anna Lea Albright ◽

Cristian Proistosescu ◽

Peter Huybers

Keyword(s):

Lower Bound ◽

Climate Sensitivity ◽

Radiative Forcing ◽

Credible Interval ◽

Ocean Heat Uptake ◽

Aerosol Radiative Forcing ◽

Joint Inference ◽

Aerosol Forcing ◽

Wide Range ◽

Aerosol Radiative

AbstractA variety of empirical estimates have been published for the lower bounds on aerosol radiative forcing, clustered around -1.0 Wm−2 or -2.0 Wm−2. The reasons for obtaining such different constraints are not well understood. In this study, we explore bounds on aerosol radiative forcing using a Bayesian model of aerosol forcing and Earth’s multi-timescale temperature response to radiative forcing. We first demonstrate the ability of a simple aerosol model to emulate aerosol radiative forcing simulated by ten general circulation models. A joint inference of climate sensitivity and effective aerosol forcing from historical surface temperatures is then made over 1850–2019. We obtain a maximum likelihood estimate of aerosol radiative forcing of -0.85 Wm−2 [5-95% credible interval -1.3 to -0.50 Wm−2] for 2010–2019 relative to 1750 and an equilibrium climate sensitivity of 3.4°C [5-95% credible interval 1.8 to 6.1°C]. The wide range of climate sensitivity reflects difficulty in empirically constraining long-term responses using historical temperatures, as noted elsewhere. A relatively tight bound on aerosol forcing is nonetheless obtained from the structure of temperature and aerosol precursor emissions and, particularly, from the rapid growth in emissions between 1950–1980. Obtaining a fifth-percentile lower bound on aerosol forcing around -2.0 Wm−2 requires prescribing internal climate variance that is a factor of five larger than the CMIP6 mean and assuming large, correlated errors in global temperature observations. Ocean heat uptake observations may further constrain aerosol radiative forcing but require a better understanding of the relationship between time-variable radiative feedbacks and radiative forcing.

Download Full-text

Rethinking the Lower Bound on Aerosol Radiative Forcing

Journal of Climate ◽

10.1175/jcli-d-14-00656.1 ◽

2015 ◽

Vol 28 (12) ◽

pp. 4794-4819 ◽

Cited By ~ 111

Author(s):

Bjorn Stevens

Keyword(s):

Lower Bound ◽

Radiative Forcing ◽

Time History ◽

Historical Record ◽

Natural State ◽

Model Parameters ◽

Aerosol Radiative Forcing ◽

Aerosol Forcing ◽

The Individual ◽

Aerosol Radiative

Abstract Based on research showing that in the case of a strong aerosol forcing, this forcing establishes itself early in the historical record, a simple model is constructed to explore the implications of a strongly negative aerosol forcing on the early (pre-1950) part of the instrumental record. This model, which contains terms representing both aerosol–radiation and aerosol–cloud interactions, well represents the known time history of aerosol radiative forcing as well as the effect of the natural state on the strength of aerosol forcing. Model parameters, randomly drawn to represent uncertainty in understanding, demonstrate that a forcing more negative than −1.0 W m−2 is implausible, as it implies that none of the approximately 0.3-K temperature rise between 1850 and 1950 can be attributed to Northern Hemisphere forcing. The individual terms of the model are interpreted in light of comprehensive modeling, constraints from observations, and physical understanding to provide further support for the less negative (−1.0 W m−2) lower bound. These findings suggest that aerosol radiative forcing is less negative and more certain than is commonly believed.

Download Full-text