Could geoengineering research help answer one of the biggest questions in climate science?

Abstract

Anthropogenic aerosol impacts on clouds constitute the largest source of uncertainty in quantifying the radiative forcing of climate, and hinders our ability to determine Earth's climate sensitivity to greenhouse gas increases. Representation of aerosol–cloud interactions in global models is particularly challenging because these interactions occur on typically unresolved scales. Observational studies show influences of aerosol on clouds, but correlations between aerosol and clouds are insufficient to constrain aerosol forcing because of the difficulty in separating aerosol and meteorological impacts. In this commentary, we argue that this current impasse may be overcome with the development of approaches to conduct control experiments whereby aerosol particle perturbations can be introduced into patches of marine low clouds in a systematic manner. Such cloud perturbation experiments constitute a fresh approach to climate science and would provide unprecedented data to untangle the effects of aerosol particles on cloud microphysics and the resulting reflection of solar radiation by clouds. The control experiments would provide a critical test of high‐resolution models that are used to develop an improved representation aerosol–cloud interactions needed to better constrain aerosol forcing in global climate models.

Substantial Cloud Brightening From Shipping in Subtropical Low Clouds

“…There are also practical implications of our detectability findings for the study of aerosol‐cloud interactions generally, and in particular for possible field tests of marine cloud brightening as a geoengineering response to anthropogenic climate change (Latham et al, ; Wood & Ackerman, ; Wood et al, ). The first paper documenting ship tracks in the 1960s remarked that it may be possible to cool the climate via cloud‐seeding over the oceans (Conover, ).…”

“…However, the confounding of aerosol effects and other meteorological variations that can significantly influence cloud properties remains a serious challenge for disentangling the magnitude of the aerosol effects alone (Adebiyi et al, ; Gryspeerdt et al, ; Stevens & Feingold, ). For this reason, “natural experiments” in which there is a clear aerosol perturbation independent of meteorological influence, such as volcanic eruptions (Gryspeerdt, Goren et al, ; Malavelle et al, ; McCoy & Hartmann, ; Toll et al, , ) and ship tracks (Chen et al, ; Gryspeerdt, Goren et al, ; Toll et al, , ), may represent our best opportunity for constraining ACI absent controlled experiments (Wood et al, ).…”

The Radiative Forcing of Aerosol–Cloud Interactions in Liquid Clouds: Wrestling and Embracing Uncertainty

“…An added benefit is that they provide a wealth of data to quantify aerosol and meteorological covariability with both observations and observationally validated process model output. Proposed field experiments to evaluate solar radiation management [157], in conjunction with modeling, would provide valuable data to quantify the aerosol-cloud radiative effect in well-characterized settings.…”

Mission-driven research for stratospheric aerosol geoengineering

“…This is a result of the timescales involved (or more precisely, the fact that the aerosol lifetime in the stratosphere is much longer than the transport timescales). For MCB, a single experiment conducted over a small area but at deployment-scale radiative forcing can in principle simultaneously resolve many of the relevant cloud-aerosol uncertainties (50)(51)(52). The same is not true for SAs.…”