Ferek et al. (1998) determined that cloud condensation nuclei in the airborne effluents of ships off the west coast of the United States were responsible for producing ship tracks, i.e., brighter and more persistent streaks, in the overlying layer of natural and less-reflective cloud, both of which alterations create a cooling influence during daylight hours.  Based on what is known of the properties of the aerosols responsible for jet aircraft contrails, Meerkotter et al. (1999) came to a similar conclusion, although aircraft-induced increases in high-level cirrus clouds are typically thought to elevate near-surface air temperature (Boucher, 1999), as the warming effect of their greenhouse properties is believed to predominate over the cooling effect of their solar radiation-reflecting properties.  Nakanishi et al. (2001), for example, suggest that aircraft-induced increases in high-cloud amount in the interior of central Alaska may be largely responsible for the region's recent warming trend, lifting some of the burden typically laid on the shoulders of CO2 in such instances.

Although various of man's impacts on clouds can thus both heat and cool the planet, Charlson et al. (2001) note that the net effect of all anthropogenic-produced aerosols averaged over the entire world is one of cooling.  Furthermore, they conclude that its estimated strength - which they say is generally believed to be equivalent to the strength of the warming effect of all anthropogenic-produced greenhouse gases - may be too conservative.  With respect to the first of these phenomena, for example, they report that recent studies indicate "both the forcing and its magnitude may be even larger than anticipated."  What is more, they say current IPCC estimates of future climate change "do not include the combined influences of some recently identified chemical factors, each of which leads to additional negative forcing (cooling) on top of that currently estimated," as we note in our Editorial of 1 August 2001.  Consequently, any new findings in this field of research are of great significance, as they may hold the key to determining whether warming or cooling will ultimately result from the sum total of human activities.

In pursuit of this objective, i.e., a proper understanding of man's many impacts on earth's climate, Facchini et al. (1999) studied the effects of atmospheric solutes collected from cloud water in the Po Valley of Italy, finding that water vapor was more likely to form on its organic-solute-affected aerosols of lower surface tension - as opposed to the less-organic-solute-affected aerosols of the natural environment with their higher surface tension - creating more and smaller (and, therefore, more-highly-reflective) cloud droplets, which, of course, tend to cool the local environment.  They also observed that the organic fractions and concentrations of the aerosols they studied were similar to those found in air downwind of other large agricultural/industrial regions, hinting at the likely widespread occurrence of this human-induced cooling influence.

In studying this phenomenon several years earlier, Kulmala et al. (1993) additionally noted "it is likely that the smaller droplet size will decrease precipitation so that the clouds will have a longer lifetime."  In addition, their observation that "cloud formation can take place at smaller saturation ratios of water vapor" in the presence of organic-solute-affected aerosols suggests that clouds will be able to form at earlier times and in places where they would not otherwise form.  In response to this particular type of anthropogenic effluent, therefore, cloud lifetimes expand at both ends of their existence spectrum - they are born earlier and die later - and, in imitation of the starship Enterprise, they are able to grow where no clouds have grown before.

How significant are these phenomena?  Approximately one decade ago, Leaitch et al. (1992) concluded that the increased radiative cooling power due to just the increase in cloud albedo that results from pollution-induced increases in cloud droplet concentration averages about 2 Wm^-2 over North America, which is about half the radiative warming power that is typically predicted to accompany a nominal doubling of the air's CO2 content.  Nowadays, and adding the impact of increased cloud cover, the overall effect would likely be considerably greater.

In another study of the climatic implications of anthropogenic-produced aerosols, Satheesh and Ramanathan (2000) measured the clear-sky radiative consequences of the December-to-April northeastern low-level monsoonal flow of air that transports sulphates, nitrates, organics, soot and fly ash (among other anthropogenically-produced substances) from the Indian sub-continent and southern Asia thousands of kilometers over the entire north Indian Ocean and as far south as 10°S latitude.  They found that the "mean clear-sky solar radiative heating for the winters of 1998 and 1999 decreased at the ocean surface by 12 to 30 Wm^-2," which Schwartz and Buseck (2000) indicate is "three to seven times as great as global average longwave (infrared) radiative forcing by increases in greenhouse gases over the industrial period … but opposite in sign."  This finding, however, was somewhat tempered by the study of Ackerman et al. (2000), who suggested the large cooling effect was likely counterbalanced by a simultaneous reduction in cloud cover (see our Editorial of 1 June 2000).  But the very next year, a long-term study of real-world data (Norris, 2001) proved this suggestion to be wrong, thereby reaffirming the overall implications of the results of Satheesh and Ramanathan.  Norris reasoned that if the conclusion of Ackerman et al. was correct, the great increase in anthropogenic aerosol emissions from southern and southeast Asia over the last half-century should have significantly decreased the low-level cloud cover over the northern Indian Ocean over this period.  A test of this idea with data from the Extended Edited Cloud Report Archive, however, revealed that daytime low-level cloud cover over this part of the world not only did not decrease over the last half-century, it increased ... and it did so in both the Northern and Southern Hemispheric regions of the study area and at essentially all hours of the day.

In a somewhat similar study, Croke et al. (1999) determined that the mean cloud cover of three regions of the United States (coastal southwest, coastal northeast and southern plains) rose from 35% to 47% from 1900 to 1987, while global mean air temperature rose by approximately 0.5°C.  Likewise, Chernykh et al. (2001) determined that global cloud cover rose by nearly 6% between1964 and 1998.  These observations suggest that earth's hydrologic cycle does indeed tend to moderate the thermal effects of any impetus for warming and, as noted by the latter authors, is "consistent with the decrease in diurnal temperature range evident over most of the globe," which tends to make for a more stable natural environment.

Another way by which clouds tend to stabilize earth's climate was suggested by Sud et al. (1999).  Based on data from the Tropical Ocean Global Atmosphere - Coupled Ocean-Atmosphere Response Experiment, these investigators found that deep convection in the tropics acts as a thermostat to keep sea surface temperature (SST) vacillating over a rather narrow range.  Starting at the low end of the range, the tropical ocean acts as a net receiver of energy, and it warms.  Soon thereafter, however, the cloud-base airmass is charged with the moist static energy needed for clouds to reach the upper troposphere; and the cloud cover thus formed reduces the amount of solar radiation received at the sea surface, while its cool and dry downdrafts also tend to promote surface cooling.  This "thermostat-like control," as the authors put it, tends to "ventilate the tropical ocean efficiently and help contain the SST between 28-30°C."  Presumably, it would also act to keep any CO2-induced warming below the same upper bound.

Yet another way in which tropical ocean temperatures may be constrained by cloud-mediated phenomena has been described by Lindzen et al. (2001).  [See also our Editorial of 21 March 2001.]  Based on upper-level cloudiness data obtained from the Japanese Geostationary Meteorological Satellite and SST data obtained from the National Centers for Environmental Protection, these researchers determined that the cloudy moist region of the eastern part of the tropical western Pacific "appears to act as an infrared adaptive iris that opens up and closes down the regions free of upper-level clouds, which more effectively permit infrared cooling, in such a manner as to resist changes in tropical surface temperature."  Indeed, the strong inverse relationship they found between upper-level cloud area and mean SST was determined to be sufficient to "more than cancel all the positive feedbacks in the more sensitive current climate models," which, of course, are the ones that are used to predict the consequences of projected increases in the air's CO2 content.

Earth's plant life also plays an important role in stabilizing climate.  The pioneering paper of Charlson et al. (1987), for example, describes how an initial SST increase leads to increased phytoplanktonic productivity, which leads to a greater sea-to-air flux of dimethyl sulfide (DMS), which undergoes a gas-to-particle conversion that leads to greater numbers of cloud condensation nuclei that create more and brighter clouds that reflect more incoming solar radiation back to space, thereby countering the initial impetus for warming.  Ayers and Gillett (2000) recently reviewed what has been learned in subsequent years, concluding that "major links in the feedback chain proposed by Charlson et al. (1987) have a sound physical basis," additionally noting there is "compelling observational evidence to suggest that DMS and its atmospheric products participate significantly in processes of climate regulation and reactive atmospheric chemistry in the remote marine boundary layer of the Southern Hemisphere."  Additional recent support for the powerful negative feedback loop is provided by Simo and Pedros-Alio (1999), who studied the effect of the depth of the surface mixing-layer on DMS production.

Although real-world studies thus continue to elucidate the workings of the planet's complex climate system and improve our understanding of it, there continue to be major problems with computer models that attempt to mimic it.  Groisman et al. (2000), for example, evaluated the ability of a number of climate models to reproduce mean daily cloud-temperature relations at different times of year.  Although most models did a good job in the cold part of the year, the authors note that "large discrepancies between empirical data and some models are found for summer conditions."  In fact, the overall cloud effect on summer near-surface air temperature computed by one of the models was even of the wrong sign!

In another study, Gordon et al. (2000) examined the response of a coupled general circulation model of the atmosphere to quasi-realistic specified marine stratocumulus clouds and compared the results to what they obtained from their model when operating in its normal mode, which fails to adequately express the presence of the clouds and their effects.  And what were the consequences of this failure?  When they removed the low clouds, as occurs in the model's normal application, the sea surface temperature warmed by fully 5.5°C.

Clearly, the current roster of climate models still has a long way to go before being able to accurately predict the ultimate climatic consequences of the vast array of pertinent human activities.  When one tallies up the scorecard of empirical observations, however, the Climatic Coolers are found to be way ahead of the Planetary Warmers.

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