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.
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
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
BRUCE A. ALBRECHT
Aerosols, Cloud Microphysics, and Fractional Cloudiness
Science 15 September 1989:
Vol. 245. no. 4923, pp. 1227 - 1230
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