New paper shows anthropogenic emissions have had a net cooling effect since beginning of industrial revolution

A paper published today in Science claims the transition from "pristine" to "slightly polluted" atmosphere at the beginning of the industrial revolution in the 18th century had a "dramatic aerosol effect [of increasing] clouds" over the oceans. According to the authors, 

"transition from pristine to slightly polluted atmosphere yields estimated negative forcing of ~15 watts per square meter (cooling), suggesting that a substantial part of this anthropogenic forcing over the oceans occurred at the beginning of the industrial era, when the marine atmosphere experienced such transformation."

By way of comparison, the IPCC alleged change in radiative forcing from CO2 [plus alleged positive water vapor feedback] since the beginning of the industrial era is +1.8 watts per square meter at the top of the atmosphere or ~8.3 times less. According to an accompanying editorial to the paper, the authors "show that even small additions of aerosol particles to clouds in the cleanest regions of Earth's atmosphere will have a large effect on those clouds and their contribution to climate forcing."

*Per the IPCC formula: 5.35*ln(395/280) = 1.8 W/m2 at the top of the atmosphere

From aerosol-limited to invigoration of warm convective clouds

1. Ilan Koren1,*, 
2. Guy Dagan1, 
3. Orit Altaratz1

+Author Affiliations

1. ¹Department of Earth and Planetary Sciences, Weizmann Institute, Rehovot 76100, Israel.

1. ↵*Corresponding author. E-mail: ilan.koren@weizmann.ac.il

• ABSTRACT
• EDITOR'S SUMMARY

Among all cloud-aerosol interactions, the invigoration effect is the most elusive. Most of the studies that do suggest this effect link it to deep convective clouds with a warm base and cold top. Here, we provide evidence from observations and numerical modeling of a dramatic aerosol effect on warm clouds. We propose that convective-cloud invigoration by aerosols can be viewed as an extension of the concept of aerosol-limited clouds, where cloud development is limited by the availability of cloud-condensation nuclei. A transition from pristine to slightly polluted atmosphere yields estimated negative forcing of ~15 watts per square meter (cooling), suggesting that a substantial part of this anthropogenic forcing over the oceans occurred at the beginning of the industrial era, when the marine atmosphere experienced such transformation.

Two commentaries on this paper also published today in Science:

ATMOSPHERIC SCIENCE

Just add aerosols

1. Lorraine A. Remer

+Author Affiliations

1. Joint Center for Earth Systems Technology, University of Maryland Baltimore County, 5523 Research Park Drive, Baltimore, MD 21228, USA.

1. E-mail: remer@umbc.edu

The more carbon dioxide and other greenhouse gases in the atmosphere, the stronger the climate warming that results. Likewise, the more aerosol particles suspended in the atmosphere, the greater the ability of these particles either to scatter sunlight back to space and cool the planet or to absorb sunlight in the atmosphere, thereby warming the atmosphere while cooling Earth's surface. However, not all such climate forcing processes depend linearly on the concentrations of their forcing agent. The climatic effects of aerosols are complicated by their interactions with clouds (1). On page 1143 of this issue, Koren et al. (2) show that even small additions of aerosol particles to clouds in the cleanest regions of Earth's atmosphere will have a large effect on those clouds and their contribution to climate forcing.

CLOUD PHYSICS

Invigorating convection in warm clouds

1. H. Jesse Smith

Atmospheric aerosols—tiny airborne particles—affect the way clouds form and how they affect climate. Koren et al. investigated how the formation of warm clouds, such as those that form over the oceans, depends on pollution levels (see the Perspective by Remer). Aerosols affect cloud formation in cleaner air disproportionately more than in more polluted air. Before the widespread air pollution of the industrial era, it seems, warm convective clouds may have covered much less of the oceans than they do today.

Science, this issue p. 1143; see also p. 1089