Just add aerosols Lorraine A. Remer Science 344, 1089 (2014); DOI: 10.1126/science.1255398
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Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
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This is close to the value observed by direct satellite measurements of radiative flux changes due to enhanced aerosol over pristine ocean caused by a venting volcano (4). Koren et al. also use a numerical simulation to show that additional aerosols allow more water mass to condense in the initial stages of cloud development. The polluted cloud can then push this mass higher before the onset of mass-induced drag forces impedes cloud growth. Without aerosol particles there would be very few clouds. A world with fewer clouds would reflect much less solar radiation back to space. Koren et al. suggest that the bulk of the aerosolcloud forcing caused by human activities occurred at the beginning of the industrial era, when the world changed from pristine to slightly polluted. However, the world has always had clouds, because the atmosphere has always contained aerosols. Natural processes such as wind erosion, breakClouds in clean air. Koren et al. show how the addition of small amounts of ing waves, volcanoes, and aerosol changes the properties of pristine clouds, such as those shown here biological excretions put above the Cape Verde Islands off the western coast of Africa. particles into the air. In fact, the particles that Koren et al. high, and especially where human activities analyzed in their study over the clean oceans are responsible for these high aerosol loadwere likely natural and not created by huings. If aerosol-cloud climate forcing were man activities. monotonically and linearly linked to the Implicit in the estimates of aerosol climate concentrations of aerosol particles, then the forcing is the assumption that the natural strongest changes to clouds should occur in processes and natural particles are constant biomass-burning smoke plumes and downin time and that century-long changes to the wind from highly polluted urban-industrial global aerosol burden are due to the overlay regions. However, this is not the case. Rather, of human activity on the natural condition. aerosol-cloud climate forcing, through the We have no means to determine the actual cloud condensation nuclei process, is most preindustrial aerosol distribution to verify pronounced at low concentrations of aerosol this assumption, though on decadal time particles and saturates at moderate to high scales we know that the background aeroconcentrations (3). Adding more particles sol state is not constant. Volcanic eruptions to a location with limited water vapor availare intermittent and weather patterns creability cannot increase the number of cloud ate droughts that change soil moisture and droplets indefinitely. Thus, quantifying aerodust aerosol production (5). By focusing on sol-cloud climate forcing requires data from pristine oceans, Koren et al. emphasize the locations with very low aerosol concentranecessity of characterizing background aerotions, far from human activities. sol conditions and even small perturbations Koren et al. now quantify aerosol-cloud from those conditions, to quantify the full climate forcing with data from the cleanaerosol forcing of climate change. ■ est regions of Earth’s atmosphere. They find REFERENCES that the cloud fraction doubles under mod1. D. Rosenfeld, S. Sherwood, R. Wood, L. Donner, Science est increase of aerosol and that this increase 343, 379 (2014). of bright cloud against the dark background 2. I. Koren, G. Dagan, O. Altaratz, Science 344, 1143 (2014). 3. I. Koren et al., Science 321, 946 (2008). of the ocean increases the amount of sun4. T. Yuan et al., Atmos. Chem. Phys. 11, 7119 (2011). light scattered back to space. This allows less 5. M. Chin et al., Atmos. Chem. Phys. 14, 3657 (2014). solar energy to be absorbed by the Earth sys−2 10.1126/science.1255398 tem, creating a negative forcing of −15 W m .
Just add aerosols
Data from clean regions of the atmosphere show how little aerosol is needed to change clouds By Lorraine A. Remer
CREDIT: SCIENCE SOURCE
T
he 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. Some aerosols act as cloud condensation nuclei, providing the seed that allows water vapor in supersaturated conditions to condense to form a cloud droplet and begin the cloud-creation process. Changing the amount of aerosol particles available to form cloud droplets directly affects the number and size of the cloud droplets in that cloud, which in turn will affect the subsequent cloud development and its climatic effect. Clouds with more or more broadly distributed cloud droplets are brighter and reflect more sunlight back to space, leading to cooling. Taller clouds have colder tops and emit less infrared radiation, leading to warming. Observational and modeling studies have provided insights into the associations between changing aerosol concentrations and cloud properties. However, the processes are complex, spanning many orders of magnitudes from microscopic particles to large weather systems. Because of this complexity, observations tend to be uncertain and may be misinterpreted, and models can only parameterize some of the most important processes. Part of the problem is a tendency to look for associations between clouds and aerosols in regions where aerosol concentrations are Joint Center for Earth Systems Technology, University of Maryland Baltimore County, 5523 Research Park Drive, Baltimore, MD 21228, USA. E-mail: [email protected] SCIENCE sciencemag.org
6 JUNE 2014 • VOL 344 ISSUE 6188
Published by AAAS
1089
This copy is for your personal, non-commercial use only.
Permission to republish or repurpose articles or portions of articles can be obtained by following the guidelines here. The following resources related to this article are available online at www.sciencemag.org (this information is current as of June 5, 2014 ): Updated information and services, including high-resolution figures, can be found in the online version of this article at: http://www.sciencemag.org/content/344/6188/1089.full.html A list of selected additional articles on the Science Web sites related to this article can be found at: http://www.sciencemag.org/content/344/6188/1089.full.html#related This article cites 5 articles, 3 of which can be accessed free: http://www.sciencemag.org/content/344/6188/1089.full.html#ref-list-1
Science (print ISSN 0036-8075; online ISSN 1095-9203) is published weekly, except the last week in December, by the American Association for the Advancement of Science, 1200 New York Avenue NW, Washington, DC 20005. Copyright 2014 by the American Association for the Advancement of Science; all rights reserved. The title Science is a registered trademark of AAAS.
Downloaded from www.sciencemag.org on June 5, 2014
If you wish to distribute this article to others, you can order high-quality copies for your colleagues, clients, or customers by clicking here.
ATMOSPHERIC SCIENCE
This is close to the value observed by direct satellite measurements of radiative flux changes due to enhanced aerosol over pristine ocean caused by a venting volcano (4). Koren et al. also use a numerical simulation to show that additional aerosols allow more water mass to condense in the initial stages of cloud development. The polluted cloud can then push this mass higher before the onset of mass-induced drag forces impedes cloud growth. Without aerosol particles there would be very few clouds. A world with fewer clouds would reflect much less solar radiation back to space. Koren et al. suggest that the bulk of the aerosolcloud forcing caused by human activities occurred at the beginning of the industrial era, when the world changed from pristine to slightly polluted. However, the world has always had clouds, because the atmosphere has always contained aerosols. Natural processes such as wind erosion, breakClouds in clean air. Koren et al. show how the addition of small amounts of ing waves, volcanoes, and aerosol changes the properties of pristine clouds, such as those shown here biological excretions put above the Cape Verde Islands off the western coast of Africa. particles into the air. In fact, the particles that Koren et al. high, and especially where human activities analyzed in their study over the clean oceans are responsible for these high aerosol loadwere likely natural and not created by huings. If aerosol-cloud climate forcing were man activities. monotonically and linearly linked to the Implicit in the estimates of aerosol climate concentrations of aerosol particles, then the forcing is the assumption that the natural strongest changes to clouds should occur in processes and natural particles are constant biomass-burning smoke plumes and downin time and that century-long changes to the wind from highly polluted urban-industrial global aerosol burden are due to the overlay regions. However, this is not the case. Rather, of human activity on the natural condition. aerosol-cloud climate forcing, through the We have no means to determine the actual cloud condensation nuclei process, is most preindustrial aerosol distribution to verify pronounced at low concentrations of aerosol this assumption, though on decadal time particles and saturates at moderate to high scales we know that the background aeroconcentrations (3). Adding more particles sol state is not constant. Volcanic eruptions to a location with limited water vapor availare intermittent and weather patterns creability cannot increase the number of cloud ate droughts that change soil moisture and droplets indefinitely. Thus, quantifying aerodust aerosol production (5). By focusing on sol-cloud climate forcing requires data from pristine oceans, Koren et al. emphasize the locations with very low aerosol concentranecessity of characterizing background aerotions, far from human activities. sol conditions and even small perturbations Koren et al. now quantify aerosol-cloud from those conditions, to quantify the full climate forcing with data from the cleanaerosol forcing of climate change. ■ est regions of Earth’s atmosphere. They find REFERENCES that the cloud fraction doubles under mod1. D. Rosenfeld, S. Sherwood, R. Wood, L. Donner, Science est increase of aerosol and that this increase 343, 379 (2014). of bright cloud against the dark background 2. I. Koren, G. Dagan, O. Altaratz, Science 344, 1143 (2014). 3. I. Koren et al., Science 321, 946 (2008). of the ocean increases the amount of sun4. T. Yuan et al., Atmos. Chem. Phys. 11, 7119 (2011). light scattered back to space. This allows less 5. M. Chin et al., Atmos. Chem. Phys. 14, 3657 (2014). solar energy to be absorbed by the Earth sys−2 10.1126/science.1255398 tem, creating a negative forcing of −15 W m .
Just add aerosols
Data from clean regions of the atmosphere show how little aerosol is needed to change clouds By Lorraine A. Remer
CREDIT: SCIENCE SOURCE
T
he 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. Some aerosols act as cloud condensation nuclei, providing the seed that allows water vapor in supersaturated conditions to condense to form a cloud droplet and begin the cloud-creation process. Changing the amount of aerosol particles available to form cloud droplets directly affects the number and size of the cloud droplets in that cloud, which in turn will affect the subsequent cloud development and its climatic effect. Clouds with more or more broadly distributed cloud droplets are brighter and reflect more sunlight back to space, leading to cooling. Taller clouds have colder tops and emit less infrared radiation, leading to warming. Observational and modeling studies have provided insights into the associations between changing aerosol concentrations and cloud properties. However, the processes are complex, spanning many orders of magnitudes from microscopic particles to large weather systems. Because of this complexity, observations tend to be uncertain and may be misinterpreted, and models can only parameterize some of the most important processes. Part of the problem is a tendency to look for associations between clouds and aerosols in regions where aerosol concentrations are Joint Center for Earth Systems Technology, University of Maryland Baltimore County, 5523 Research Park Drive, Baltimore, MD 21228, USA. E-mail: [email protected] SCIENCE sciencemag.org
6 JUNE 2014 • VOL 344 ISSUE 6188
Published by AAAS
1089
