I'm uncomfortable with this real-life test of geoengineering: http://www.grist.org/climate-change/2011-08-31-is-planet-cooling-balloon-full-of-hot-air
Maybe they could have started with some modeling?
Showing posts with label clouds. Show all posts
Showing posts with label clouds. Show all posts
2011-09-01
2011-08-31
CLOUD experiments
The first results from the CLOUD experiments have been getting a lot of media attention. The focus of the attention is the Nature paper that was published this week: [news][paper].
The goal of this project is to determine whether cosmic rays have a significant impact on clouds.
Let's boil this down a little. This project is a laboratory experiment at CERN. It is a cloud chamber, basically an isolated volume of air that is precisely controlled for temperature and pressure. They put very pure air into the chamber, add a little background water, and some gases like ozone, sulfuric acid, and ammonia. The chamber is heavily instrumented to look for nucleation, which just means that they try to keep track of particle formation that occurs as the vapors interact and possibly start condensing. They can do this in neutral conditions (like a classical cloud chamber), or they can shine a pion beam into the chamber. That beam is adjusted to mimic cosmic ray bombardment. The goal is to see if cosmic rays produce ions that enhance the formation of particles, which could then go on to become the seeds for cloud droplets.
The answer seems to be that shining that beam into the chamber does produce more particles. This actually isn't a surprise, as far as I can tell. One important point is that nucleation rates, that is the rate of particle formation, are smaller than observed rates unless the temperature is quite low. This means that it is unlikely that cosmic rays ionizing gases near the Earth's surface is a major source of particle formation. Certainly there is particle formation, but it is likely to be a small source of the total number of particles. This result may change when they start adding in organic molecules, but that is future work.
There is better coverage on RealClimate: link.
There is hubbub about this result because there is a crack-pot theory that galactic cosmic rays are a major control of climate because of their impact on cloud formation. There are major flaws with this theory. My own take is that cosmic rays probably do produce some of the particles in the atmosphere that go on to become cloud condensation nuclei, but there are many paths to becoming cloud condensation nuclei, and there are lots and lots of these particles around. In fact, I seriously doubt that cloud formation is frequently affected by the limitation of these aerosol particles. I've been thinking about this in terms of observed cloud properties. The number of cloud droplets is connected to the number of aerosol particles available: over land where there are lots more aerosol particles, there tends to be more, smaller droplets in clouds, while over the remote ocean the clouds are made of fewer, larger droplets. In very polluted conditions, we can observe changes in the cloud properties that follow that same trend. I think the downfall of the cosmic ray theory of cloud formation comes from the fact that out in the middle of the ocean there are still tons of aerosol particles. While many of those particles may come from cosmic ray influenced nucleated vapors, there is no evidence that there is a shortage of other sources of aerosol, so if the intensity of cosmic ray bombardment were to change, it seems unlikely that other sources of aerosol wouldn't fill whatever tiny void that change would make.
Besides this basic criticism (which amounts to the originators of the theory simply having a bit of a myopic view of cloud formation), there is also a clear lack of evidence for cosmic ray intensity modulating cloud/climate. The RealClimate piece covers that. Finally, there is the link to climate change, for which there is absolutely no evidence.
So my summary would be something like: This research presents experimental results that suggest that ionization by cosmic ray-like effects can impact nucleation rates in conditions similar to the Earth's atmosphere. The role of such nucleation enhancement in the Earth's atmosphere remains unclear, especially given that the impact seems most pronounced in conditions that are outside the atmospheric boundary layer. This is a nice contribution to basic aerosol research, which should help to constrain models of aerosol formation. The impacts on cloud formation and the Earth's climate can not be assessed with the data collected so far.
The authors are only slightly overselling their results, which is typical for authors of Nature papers. The lead author's comments can be heard in the embedded YouTube clip. The media coverage, and especially the climate change denier blogosphere, is lighting up like this experiment proves something controversial. It does not.
The goal of this project is to determine whether cosmic rays have a significant impact on clouds.
Let's boil this down a little. This project is a laboratory experiment at CERN. It is a cloud chamber, basically an isolated volume of air that is precisely controlled for temperature and pressure. They put very pure air into the chamber, add a little background water, and some gases like ozone, sulfuric acid, and ammonia. The chamber is heavily instrumented to look for nucleation, which just means that they try to keep track of particle formation that occurs as the vapors interact and possibly start condensing. They can do this in neutral conditions (like a classical cloud chamber), or they can shine a pion beam into the chamber. That beam is adjusted to mimic cosmic ray bombardment. The goal is to see if cosmic rays produce ions that enhance the formation of particles, which could then go on to become the seeds for cloud droplets.
The answer seems to be that shining that beam into the chamber does produce more particles. This actually isn't a surprise, as far as I can tell. One important point is that nucleation rates, that is the rate of particle formation, are smaller than observed rates unless the temperature is quite low. This means that it is unlikely that cosmic rays ionizing gases near the Earth's surface is a major source of particle formation. Certainly there is particle formation, but it is likely to be a small source of the total number of particles. This result may change when they start adding in organic molecules, but that is future work.
There is better coverage on RealClimate: link.
There is hubbub about this result because there is a crack-pot theory that galactic cosmic rays are a major control of climate because of their impact on cloud formation. There are major flaws with this theory. My own take is that cosmic rays probably do produce some of the particles in the atmosphere that go on to become cloud condensation nuclei, but there are many paths to becoming cloud condensation nuclei, and there are lots and lots of these particles around. In fact, I seriously doubt that cloud formation is frequently affected by the limitation of these aerosol particles. I've been thinking about this in terms of observed cloud properties. The number of cloud droplets is connected to the number of aerosol particles available: over land where there are lots more aerosol particles, there tends to be more, smaller droplets in clouds, while over the remote ocean the clouds are made of fewer, larger droplets. In very polluted conditions, we can observe changes in the cloud properties that follow that same trend. I think the downfall of the cosmic ray theory of cloud formation comes from the fact that out in the middle of the ocean there are still tons of aerosol particles. While many of those particles may come from cosmic ray influenced nucleated vapors, there is no evidence that there is a shortage of other sources of aerosol, so if the intensity of cosmic ray bombardment were to change, it seems unlikely that other sources of aerosol wouldn't fill whatever tiny void that change would make.
Besides this basic criticism (which amounts to the originators of the theory simply having a bit of a myopic view of cloud formation), there is also a clear lack of evidence for cosmic ray intensity modulating cloud/climate. The RealClimate piece covers that. Finally, there is the link to climate change, for which there is absolutely no evidence.
So my summary would be something like: This research presents experimental results that suggest that ionization by cosmic ray-like effects can impact nucleation rates in conditions similar to the Earth's atmosphere. The role of such nucleation enhancement in the Earth's atmosphere remains unclear, especially given that the impact seems most pronounced in conditions that are outside the atmospheric boundary layer. This is a nice contribution to basic aerosol research, which should help to constrain models of aerosol formation. The impacts on cloud formation and the Earth's climate can not be assessed with the data collected so far.
The authors are only slightly overselling their results, which is typical for authors of Nature papers. The lead author's comments can be heard in the embedded YouTube clip. The media coverage, and especially the climate change denier blogosphere, is lighting up like this experiment proves something controversial. It does not.
2011-03-27
African biomass burning (part 1?)
The amount of area and material that is burned each year in tropical Africa is staggering. The series of maps shown here is from NASA, showing burning across Africa during 2005 (note they aren't monthly, just 10 day composites ranging from January to August). Globally, biomass burning is estimated to consume somewhere around 8700 Tg of dry matter and release nearly 4 Tg of carbon to the atmosphere. Much of that carbon is returned to the biosphere, though, because the majority of the burning isn't to clear forest, but to clear cropland in savannah regions. People seasonally set the fires to keep harmful plants and pests out of their farmland. The chemical consequences of this huge efflux of gas and aerosol each season across the tropical belt is still only crudely understood. The global impact is poorly understood, as there are processes that are indirectly related to the burning, such as the planetary albedo, cloud and precipitation effects, and chemical effects in both the troposphere and stratosphere. See here for a little more description, especially on the chemical side.
While not exactly analogous to other forms of anthropogenic changes to the climate system, this is an obvious and large perturbation that is mostly human induced. Understanding the consequences, both locally and remotely, may help us understand impacts of climate change globally.
Another interesting aspect of the African seasonal biomass burning is that it could represent an interaction between the climate system and human culture. The burning is seasonal, as I mentioned, but the extent and severity of the burning and resulting smoke depends on when and where the fires are started. That in turn depends on the previous rainy season. Where the smoke goes depends on the atmospheric circulation, and where the material ends up may determine the remote impact. For example, in some circulation patterns, the central African smoke is transported toward the Indian Ocean and is mixed into the westerlies, which will disperse the plume rather quickly. In other situations, the plume is transported over the southeast Atlantic, which is an are of large-scale sinking motion, so the smoke is contained within a layer and slowly moved over the ocean. This can affect radiation budget and the clouds in the area, which then have potential effects on other aspects of climate variability (through changes in the ocean surface temperature, for example, which may feed back on Atlantic Nino activity). These links are only tenuously understood, and are worth a good think if you have the time.
 |
| MODIS burning product, see LINK |
2008-10-12
A geoengineering teaser
So, just read the "News and Views" piece in Nature Geoscience (Vol.1, (644), 2008; doi:10.1038/ngeo326) called "Climate change: Cool spray" by Heike Langenberg. I can't spend the time to really get into it, but I certainly will in the short term. The article simply reports some ideas presented by John Latham in two papers in the Philosophical Transactions of the Royal Society (Phil. Trans. R. Soc. A doi:10.1098/rsta.2008.0137; 2008, and Phil. Trans. R. Soc. A doi:10.1098/rsta.2008.0136; 2008). According to Langenberg, Latham proposes as a low-cost geoengineering fix to anthropogenic global warming the injection of sea salt into the marine boundary layer. This takes advantage of the Twomey effect, whereby additional condensation sites lead to smaller cloud droplets which reflect a higher fraction of incident light (a cooling effect).
Okay, well, I haven't read the papers yet, but I will. And since this is one of those areas where I know something, I should be able to address some of the issues that this plan raises. As has been pointed out regarding other geoengineering ideas, this one is fundamentally a shortwave effect, while the global warming is a longwave one. That means that the crux of the plan relies on reducing the total energy in the climate system, probably by using thin ribbons of clouds over subtropical oceans. Meanwhile, outside of those regions, the same sunshine comes in, and the same CO2 is sitting in the atmosphere, and presumably there's still enhanced water vapor. The effect of those clouds is to change the surface temperature beneath them, creating temperature gradients in the surface air temperature and sea surface temperature. This perturbs the low-level circulation. The low level circulation moves some energy around, but the majority of the north-south energy transport in the climate system is accomplished through storms grabbing energy from the low latitudes and moving it northward. So depending on the placement and extent of this cloud shield, the effects on both the low-level wind field and the indirect effect on the storm tracks will significantly alter the naive expectation that reflecting more light back to space will offset human-induced warming.
But more on the details in a future post.
Okay, well, I haven't read the papers yet, but I will. And since this is one of those areas where I know something, I should be able to address some of the issues that this plan raises. As has been pointed out regarding other geoengineering ideas, this one is fundamentally a shortwave effect, while the global warming is a longwave one. That means that the crux of the plan relies on reducing the total energy in the climate system, probably by using thin ribbons of clouds over subtropical oceans. Meanwhile, outside of those regions, the same sunshine comes in, and the same CO2 is sitting in the atmosphere, and presumably there's still enhanced water vapor. The effect of those clouds is to change the surface temperature beneath them, creating temperature gradients in the surface air temperature and sea surface temperature. This perturbs the low-level circulation. The low level circulation moves some energy around, but the majority of the north-south energy transport in the climate system is accomplished through storms grabbing energy from the low latitudes and moving it northward. So depending on the placement and extent of this cloud shield, the effects on both the low-level wind field and the indirect effect on the storm tracks will significantly alter the naive expectation that reflecting more light back to space will offset human-induced warming.
But more on the details in a future post.
2007-10-04
Shipping Lanes
I've been sitting on the idea for this post for almost a week, but haven't had a chance to work it up. Since it doesn't look like I'm going to get to do it the way I originally wanted, I'm giving in and just going for the gusto. Maybe (yeah right) I'll come back and round out the rough edges later, but for now I want to get the basic ideas out there.
Ship tracks are the contrails of the sea. Perhaps more accurately, ship tracks are to the marine atmospheric boundary layer what contrails are to the upper troposphere. They are lines of what we will call clouds that form behind a ship. They are the focus of a recent article that I found very interesting. A news summary can be found on the Science (LINK) web site, while the paper appears in GRL. For a good picture of ship tracks, NASA's MODIS is a good resource.
The idea in the paper is to establish the radiative forcing associated with ship tracks on the global scale. This hasn't been done before using observations because ship tracks are very low, very small clouds that cover a tiny amount of Earth's surface area. However, they are common, as the paper points out, in several regions, notably off the coast of Africa and in the North Pacific. These are, somewhat coincidentally (but not really), the same regions where we think about extensive stratocumulus decks.
Schreier et al. use one year of satellite imagery, from the ENVISAT-AATSR, and go through a straight forward but intensive process of identifying ship tracks and then estimating their radiative forcing. The bottom line is that in some regions the radiative impact of ship tracks, lets call it the local radiative effect, can be a non-trivial -0.05 W/m2, but on the global scale the effect is miniscule at -0.4 to -0.6 mW/m2 (plus or minus 40%). Note that the global value is in milliWatts, so is 100 time smaller than the largest regional radiative effect (-0.05 W/m2 = -50 mW/m2). The negative sign arises because ship tracks are very low clouds that are very white (i.e., reflective), so when they appear they provide a more reflective surface for sunlight to bounce off, which to first order reduces the amount of energy in the climate system (because most of the reflected light goes back out to space) and cools the climate. This is familiar if you've been exposed to cloud "feedback" ideas, in which more low cloud cover increases the albedo of Earth and cools it. In fact, this is a terrific example of that effect, but we'll come to that shortly. It is also good to note that the radiative forcing associated with a doubling of atmospheric carbon dioxide is about 4 W/m2, which is itself a small signal in the total radiative budget (with 1365 W/m2 of incoming sunlight at the top of the atmosphere, distributed over a day (divide by 4) and an albedo of about 0.3 you're talking in the neighborhood of a 225 W/m2 of sunlight being absorbed at the surface, and all of global warming comes down to 4W/m2 give or take!).
Okay so before I sign off, leaving you totally confused. I wanted to point out a couple of interesting things about ship tracks that aren't necessarily in the article. First of all, it is helpful to remember why ship tracks form. The ships are steaming ahead, burning fairly dirty fuel to get where they are going, and the exhaust goes right out into the atmosphere. This exhaust contains particulate matter as well as precursors for particles, so the ship is basically making a trail of particles behind it. These particles act as nucleation sites for water, forming small cloud droplets. Because the ships spew out so much stuff, there are enough nucleation sites available to grow lots of droplets and form these linear clouds. Why don't the clouds form anyway if there's that much water in the atmosphere already? Well, a couple of reasons. One is that the relative humidity isn't quite 100% in fair weather conditions, but even if it were, water doesn't like to condense unless there are surfaces (supplied by the particles). At a relative humidity of about 80%, there just aren't enough particles floating around the clean maritime boundary layer to let the water condense into clouds. The ships provide the extra nucleation sites necessary, and make it even easier by supplying the boundary layer with hygroscopic particles, meaning the particles effectively decrease the saturation specific humidity (http://en.wikipedia.org/wiki/Hygroscopic). That just means that the particles are very efficient at turning water vapor into liquid water. So a ship goes by, spews out water-loving particles, water condenses on those particles forming droplets, and a big collection of droplets is a cloud. Fine, what else?
So okay, the ships go by and make lines of clouds, but we now know (or strongly suspect) based on Schrier et al. that the global effect of these cloud is negligible and the local effect is also pretty small. Can we be done with it then? Not quite. These clouds are a great example of the Twomey effect, which is an old idea now and just says that by increasing the number of particles in the air, the size of cloud droplets gets smaller, and when clouds are made up of small droplets they are brighter (i.e., more reflective). Coakley et al. (1987) presented ship tracks as such an example, showing with satellite data that the reflectivity of ship tracks is higher than the surrounding low-level cloud cover. This is exactly what leads to the radiative forcing that has now been estimated by Schreier et al. The important thing to recognize here is that the Coakley et al. study is essentially a proof of concept, showing that pollution can impact atmospheric radiative transfer. They definitely did not say ships were impacting global climate.
There is a related effect, sometimes called the Albrecht effect, which takes into account the change in cloud fraction associated with changes in particles in the atmosphere. It is presented by Albrecht (1989), and is also a pretty simple idea. When extra particles are put into the atmospheric boundary layer, they form droplets and brighter clouds, as discussed above. Smaller droplets can also change the formation of raindrops, or more precisely in the case of shallow maritime clouds, drizzle drops. The change is to reduce the precipitation efficiency, which increases the liquid water in the cloud layer, and can lead to an increase in the fractional cloudiness. The important point here is that not only could increased particle concentration in the marine atmospheric boundary layer make brighter clouds, but could actually increase the overall cloudiness. This would amplify the effects discussed by Coakley et al. because there would now be a larger area covered by brighter clouds. The Albrecht study makes use of ship tracks only in the sense of the Coakley et al. study, and only suggests that changes in precipitation could account for the sustained difference in ship tracks from the stratiform cloud in which they are embedded. This is supported to some extent by aircraft observations.
And finally, since we're covering so many bases, there's another effect that should be mentioned. Pincus and Baker (1994) present a study that extends the Albrecht study in that it accounts for the change in the thickness of clouds in the presence of varying particle concentration. They use a model of a cloudy boundary layer and account for changes in absorption and precipitation with cloud thickness and droplet number, respectively. This effect is not quite as "obvious" as the other indirect effects, but the bottom line is that more droplets can make thicker clouds with a higher albedo, which is thus another facet of this negative feedback associated with changes in atmospheric aerosol (particles). They note, however, that you'd expect to see ship tracks extend higher than surrounding clouds, which at that time was not observed. I'm not sure where this effect really stands, but it is interesting to consider.
So these are the indirect effects of aerosol on climate. We came a long way in this post, from a recent study showing that the globally averaged radiative forcing due to ship tracks is small all the way through aerosol effects on cloud albedo, precipitation processes, and horizontal and vertical cloud distribution. Well done. There are a lot more details that could have been added, and tons more studies. These will be left for future posts, though. I've included some references below for those of you who want to follow up.
References
Schreier, Mathias; Mannstein, Hermann; Eyring, Veronika; Bovensmann, Heinrich
Global ship track distribution and radiative forcing from 1 year of AATSR data
Geophys. Res. Lett., Vol. 34, No. 17, L17814
10.1029/2007GL030664 (LINK)
JAMES A. COAKLEY JR., ROBERT L. BERNSTEIN, and PHILIP A. DURKEE
Effect of Ship-Stack Effluents on Cloud Reflectivity
Science 28 August 1987: Vol. 237. no. 4818, pp. 1020 - 1022 DOI: 10.1126/science.237.4818.1020
BRUCE A. ALBRECHT
Aerosols, Cloud Microphysics, and Fractional Cloudiness
Science 15 September 1989: Vol. 245. no. 4923, pp. 1227 - 1230 DOI: 10.1126/science.245.4923.1227
ROBERT PINCUS & MARCIA B. BAKER
Effect of precipitation on the albedo susceptibility of clouds in the marine boundary layer
Nature 372, 250 - 252 (17 November 2002); doi:10.1038/372250a0
Ship tracks are the contrails of the sea. Perhaps more accurately, ship tracks are to the marine atmospheric boundary layer what contrails are to the upper troposphere. They are lines of what we will call clouds that form behind a ship. They are the focus of a recent article that I found very interesting. A news summary can be found on the Science (LINK) web site, while the paper appears in GRL. For a good picture of ship tracks, NASA's MODIS is a good resource.
The idea in the paper is to establish the radiative forcing associated with ship tracks on the global scale. This hasn't been done before using observations because ship tracks are very low, very small clouds that cover a tiny amount of Earth's surface area. However, they are common, as the paper points out, in several regions, notably off the coast of Africa and in the North Pacific. These are, somewhat coincidentally (but not really), the same regions where we think about extensive stratocumulus decks.
Schreier et al. use one year of satellite imagery, from the ENVISAT-AATSR, and go through a straight forward but intensive process of identifying ship tracks and then estimating their radiative forcing. The bottom line is that in some regions the radiative impact of ship tracks, lets call it the local radiative effect, can be a non-trivial -0.05 W/m2, but on the global scale the effect is miniscule at -0.4 to -0.6 mW/m2 (plus or minus 40%). Note that the global value is in milliWatts, so is 100 time smaller than the largest regional radiative effect (-0.05 W/m2 = -50 mW/m2). The negative sign arises because ship tracks are very low clouds that are very white (i.e., reflective), so when they appear they provide a more reflective surface for sunlight to bounce off, which to first order reduces the amount of energy in the climate system (because most of the reflected light goes back out to space) and cools the climate. This is familiar if you've been exposed to cloud "feedback" ideas, in which more low cloud cover increases the albedo of Earth and cools it. In fact, this is a terrific example of that effect, but we'll come to that shortly. It is also good to note that the radiative forcing associated with a doubling of atmospheric carbon dioxide is about 4 W/m2, which is itself a small signal in the total radiative budget (with 1365 W/m2 of incoming sunlight at the top of the atmosphere, distributed over a day (divide by 4) and an albedo of about 0.3 you're talking in the neighborhood of a 225 W/m2 of sunlight being absorbed at the surface, and all of global warming comes down to 4W/m2 give or take!).
Okay so before I sign off, leaving you totally confused. I wanted to point out a couple of interesting things about ship tracks that aren't necessarily in the article. First of all, it is helpful to remember why ship tracks form. The ships are steaming ahead, burning fairly dirty fuel to get where they are going, and the exhaust goes right out into the atmosphere. This exhaust contains particulate matter as well as precursors for particles, so the ship is basically making a trail of particles behind it. These particles act as nucleation sites for water, forming small cloud droplets. Because the ships spew out so much stuff, there are enough nucleation sites available to grow lots of droplets and form these linear clouds. Why don't the clouds form anyway if there's that much water in the atmosphere already? Well, a couple of reasons. One is that the relative humidity isn't quite 100% in fair weather conditions, but even if it were, water doesn't like to condense unless there are surfaces (supplied by the particles). At a relative humidity of about 80%, there just aren't enough particles floating around the clean maritime boundary layer to let the water condense into clouds. The ships provide the extra nucleation sites necessary, and make it even easier by supplying the boundary layer with hygroscopic particles, meaning the particles effectively decrease the saturation specific humidity (http://en.wikipedia.org/wiki/Hygroscopic). That just means that the particles are very efficient at turning water vapor into liquid water. So a ship goes by, spews out water-loving particles, water condenses on those particles forming droplets, and a big collection of droplets is a cloud. Fine, what else?
So okay, the ships go by and make lines of clouds, but we now know (or strongly suspect) based on Schrier et al. that the global effect of these cloud is negligible and the local effect is also pretty small. Can we be done with it then? Not quite. These clouds are a great example of the Twomey effect, which is an old idea now and just says that by increasing the number of particles in the air, the size of cloud droplets gets smaller, and when clouds are made up of small droplets they are brighter (i.e., more reflective). Coakley et al. (1987) presented ship tracks as such an example, showing with satellite data that the reflectivity of ship tracks is higher than the surrounding low-level cloud cover. This is exactly what leads to the radiative forcing that has now been estimated by Schreier et al. The important thing to recognize here is that the Coakley et al. study is essentially a proof of concept, showing that pollution can impact atmospheric radiative transfer. They definitely did not say ships were impacting global climate.
There is a related effect, sometimes called the Albrecht effect, which takes into account the change in cloud fraction associated with changes in particles in the atmosphere. It is presented by Albrecht (1989), and is also a pretty simple idea. When extra particles are put into the atmospheric boundary layer, they form droplets and brighter clouds, as discussed above. Smaller droplets can also change the formation of raindrops, or more precisely in the case of shallow maritime clouds, drizzle drops. The change is to reduce the precipitation efficiency, which increases the liquid water in the cloud layer, and can lead to an increase in the fractional cloudiness. The important point here is that not only could increased particle concentration in the marine atmospheric boundary layer make brighter clouds, but could actually increase the overall cloudiness. This would amplify the effects discussed by Coakley et al. because there would now be a larger area covered by brighter clouds. The Albrecht study makes use of ship tracks only in the sense of the Coakley et al. study, and only suggests that changes in precipitation could account for the sustained difference in ship tracks from the stratiform cloud in which they are embedded. This is supported to some extent by aircraft observations.
And finally, since we're covering so many bases, there's another effect that should be mentioned. Pincus and Baker (1994) present a study that extends the Albrecht study in that it accounts for the change in the thickness of clouds in the presence of varying particle concentration. They use a model of a cloudy boundary layer and account for changes in absorption and precipitation with cloud thickness and droplet number, respectively. This effect is not quite as "obvious" as the other indirect effects, but the bottom line is that more droplets can make thicker clouds with a higher albedo, which is thus another facet of this negative feedback associated with changes in atmospheric aerosol (particles). They note, however, that you'd expect to see ship tracks extend higher than surrounding clouds, which at that time was not observed. I'm not sure where this effect really stands, but it is interesting to consider.
So these are the indirect effects of aerosol on climate. We came a long way in this post, from a recent study showing that the globally averaged radiative forcing due to ship tracks is small all the way through aerosol effects on cloud albedo, precipitation processes, and horizontal and vertical cloud distribution. Well done. There are a lot more details that could have been added, and tons more studies. These will be left for future posts, though. I've included some references below for those of you who want to follow up.
References
Schreier, Mathias; Mannstein, Hermann; Eyring, Veronika; Bovensmann, Heinrich
Global ship track distribution and radiative forcing from 1 year of AATSR data
Geophys. Res. Lett., Vol. 34, No. 17, L17814
10.1029/2007GL030664 (LINK)
JAMES A. COAKLEY JR., ROBERT L. BERNSTEIN, and PHILIP A. DURKEE
Effect of Ship-Stack Effluents on Cloud Reflectivity
Science 28 August 1987: Vol. 237. no. 4818, pp. 1020 - 1022 DOI: 10.1126/science.237.4818.1020
BRUCE A. ALBRECHT
Aerosols, Cloud Microphysics, and Fractional Cloudiness
Science 15 September 1989: Vol. 245. no. 4923, pp. 1227 - 1230 DOI: 10.1126/science.245.4923.1227
ROBERT PINCUS & MARCIA B. BAKER
Effect of precipitation on the albedo susceptibility of clouds in the marine boundary layer
Nature 372, 250 - 252 (17 November 2002); doi:10.1038/372250a0
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