Haze, Clouds and Climate Change
The effect of haze on climate is the largest source of uncertainty in determining how humans are affecting the climate. A COAS research group studies the properties of clouds and haze around the world using observations of reflected sunlight and emitted infrared radiation obtained from NASA and weather satellites.
The first studies that modeled the effects of haze on climate date back to the 70s and 80s, according to COAS atmospheric sciences professor Jim Coakley. But, these studies were largely forgotten by the early 90s when the concern was primarily for the buildup of greenhouse gases in the atmosphere.
“The interest in the role of haze caused by burning fossil fuels and deforestation was rekindled in the 90s when it became understood that haze could have a substantial effect on climate change,” he said. “While the understanding of how haze affects the climate remains poor, the potential impact could be large, and now, just about every issue of journals like the Journal of Geophysical Research has several papers devoted to the effects of haze on climate.”
Haze particles scatter and absorb sunlight
Haze particles are a byproduct of burning fossil fuels. They reflect sunlight, thereby offsetting to some extent, effects due to the buildup of greenhouse gases in the Earth’s atmosphere which also arises primarily from burning fossil fuels. Haze particles can also absorb sunlight and thus heat the atmosphere, but whether through absorption or reflection, the particles shade the Earth’s surface. The reduction in sunlight at the surface could lead to cooler surface temperatures but it’s more likely to lead to a reduction in evaporation, which means changes in rainfall and snowfall not only in the region that contains the haze, but also the regions downwind.
How much haze alters the sunlight reflected by the Earth, or absorbed by the atmosphere is difficult to predict and measure. It depends on the particle size, shape and chemical composition—all highly variable, even from particle to particle within a haze layer, even when the layer is generated by a single particle source. Also, unlike the greenhouse gases, which reside in the atmosphere for tens to hundreds of years, and are thus uniformly distributed around the world, haze particles are removed from the atmosphere by rain. They last in the atmosphere for a few days to a week. Their concentration is highly variable, depending on the sources of particles, whether from industry, trucks and automobiles, or forest fires and the local weather. All of these factors make predicting and measuring concentrations of haze particles and their properties extremely complex.
In addition to the effect of the particles on sunlight, which is referred to as the aerosol direct effect on climate, when they enter clouds, the particles change the numbers and sizes of cloud droplets and ice crystals, thereby altering how much sunlight clouds reflect. The effect of haze on clouds is referred to as the aerosol indirect effect, and because models of cloud properties are highly uncertain, the indirect effect of aerosols gives rise to even greater uncertainties in predictions of climate change.
How cloud droplets form
Left to themselves, water molecules have a difficult time coming together, as they must to form a liquid. It’s easier for them to join when they’re on the surfaces of particles, which are called cloud condensation nuclei. The nucleation of water droplets by particles is the corollary of the nucleation of bubbles in a glass of beer. In beer the liquid is supersaturated with carbon dioxide. The bubbles that form in beer don’t arise randomly from all parts of the liquid. Instead, they seem to arise from certain locations, places where there are small imperfections in the glass. At these locations the bubbles have a place to nucleate and grow. Once it reaches a critical size, the bubble breaks loose and the next bubble nucleates. The same thing happens in the atmosphere. Tiny particles serve as the imperfections on which water condenses.
Pollution affects the amount of sunlight reflected by clouds
Every cloud droplet starts with a tiny particle, the condensation nucleus. When haze increases the number of particles in the cloud environment, it leads to clouds with more, but smaller droplets. There are more droplets because there are more particles on which the water can condense. The droplets are smaller because there is only so much water in a given location available for condensation. The effect is the same as crushing large salt crystals to make table salt. Large salt crystals seem rather dark when compared with the whiteness of the same salt when finely ground.
Clouds behave the same way. Clouds with larger numbers of droplets, even though the droplets are smaller, reflect more sunlight and help to moderate the effects of the buildup of the greenhouse gases. The interactions between the particles and droplets are extremely complex, and anything can happen depending on the amount of water vapor available and the chemical and physical properties of the particles. “Right now we cannot predict with certainty what will happen as clouds become more and more polluted,” Coakley said “That’s what we’re trying to find out.”
Ship tracks
One of the strategies for learning about the response of clouds to haze pollution is to watch the marine stratus off the West Coast respond to the pollution injected by underlying ships. COAS graduate student Matt Segrin is compiling observations for thousands of such instances from observations made by two NASA satellites. Particles in the ship stack plumes get into the clouds, water condenses on the particles and droplets in the clouds affected by the plume become smaller.
“To find out how clouds respond to the pollution,” Coakley said, “we combine the satellite observations with physical models to infer the size of the droplets and the amounts of liquid water in the polluted clouds, indicated by the tracks, and compare these with what we find in the uncontaminated clouds on either side of the tracks. Even though the tracks do not appear in the reflected sunlight at visible wavelengths, as shown by the relative lack of tracks in the color image, careful analysis of the reflected light for the ship tracks and the nearby uncontaminated clouds reveals that indeed, the reflectivities of the polluted clouds are significantly higher than those of their nearby uncontaminated neighboring clouds.
When we calculate the liquid water that must be in the clouds, based on the amount of sunlight being reflected and the size of the cloud droplets, we find that the polluted clouds, on average, actually have less liquid water than the surrounding uncontaminated clouds. This loss of liquid water for the polluted clouds is contrary to climate model simulations which suggest that in clouds with smaller droplets, drizzle is suppressed. The amount of water in polluted clouds should therefore be greater and the polluted clouds last longer, causing polluted clouds to have substantially higher reflectivities and last longer than their unpolluted counterparts. The loss of water found in the ship tracks, however, calls to question these climate model simulations.”
To see how ship tracks, which are point sources of pollution, might compare with the effects of continental scale pollution, COAS graduate student Mark Matheson is looking at air pollution, coming off western Europe over the northeastern Atlantic during summer months. “If you’ve ever been to Europe during the summer,” Coakley said, “you’ve probably noticed the low-level marine stratus that invariably sits off the European coast. When the winds are right, the European plume goes right into these clouds, and we see the droplet sizes getting smaller where the pollution is entering the clouds compared with areas where similar stratus clouds are in relatively unpolluted air. Clearly, the haze coming from Europe is affecting the clouds.”
In common with the ship tracks, as the aerosol concentrations in the northeastern Atlantic go up, the droplet sizes get smaller. “We also find that the polluted clouds reflect more sunlight. But when we calculate from the satellite observations the amount of liquid water in the clouds, we find that the clouds well out to sea are retaining their water so that the polluted clouds end up with more water than the unpolluted clouds, consistent with climate model simulations of the effects of pollution on clouds.
On the other hand, when the clouds are near the continents, as off the coast of the Iberian Peninsula, the polluted clouds have slightly higher reflectivities but like the ship tracks, they’re losing liquid water. We suspect that the pollution is coming with dry continental air. The larger droplet numbers in the polluted clouds give rise to more turbulence at the cloud tops which mixes the dry air into the cloud. With the enhanced mixing of dry air, the polluted clouds dry out in comparison with uncontaminated clouds.”
Why the concern over pollution in clouds?
Because of buildup of greenhouse gases since the start of the Industrial Revolution, the Earth is a little bit more than a degree Fahrenheit warmer than before the Industrial Revolution. Some types of aerosols absorb sunlight. Most aerosol particles seem to have some piece of black carbon in them. The particles form around black carbon, which is typically produced by burning fossil fuels, as well as organic carbons such as burning biomass.
“For each aerosol type, we try to estimate how much sunlight is being reflected and absorbed,” Coakley said. “Because we think we know the sensitivity of the Earth’s temperature to changes in reflected sunlight, we can estimate how much climate change we can expect based on the net effect of the particles on the reflected sunlight.”
Satellite observations affect understanding
Segrin uses NASA satellites Terra and Aqua to log positions of thousands of ship tracks. Marine stratus is typical off the West Coast during the early summer. Sometimes ship tracks break out of extensive cloud banks and leave a line of cloud in otherwise clear or partly cloudy skies. Clearly, in such instances the ships are seeding the clouds.
“It’s my bet that the models are vastly overestimating the effects of aerosols on clouds,” Coakley said. “The type of changes in the clouds that the models are predicting occur only in places where there is sufficient moisture and pollution is getting into the clouds. How many places in the world does that happen? Clearly, the effect of haze particles on climate gives rise to a sizable amount of uncertainty in climate prediction. The effect of the particles on the clouds may prove to be about as large as their direct effect, through scattering and absorption on reflected sunlight. Actually, Matheson is finding that the effect of the particles on the clouds is slightly larger than their direct effect on reflected sunlight. But that’s just for the summertime northeastern Atlantic. We have to analyze data for many more years and more locations to see whether we can generalize these findings to the rest of the Earth.
“We are seeing the effects of pollution in the clouds. But what we’re finding is that the enhancement in reflected sunlight for the polluted clouds doesn’t seem to be as large as predicted by models.”
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