There is quite a bit we already know to be true about global warming. We know that carbon dioxide and other molecules in the atmosphere, such as methane, strongly absorb infrared radiation from the Earth, re-emitting radiation back to the surface and warming the planet. We know that many industrial processes to convert coal and fossil fuels to useful forms of energy release tremendous amounts of these greenhouse gases into the atmosphere, leading to anthropogenic global warming. Recent models are now giving us more and more confidence in predicting how this increased warming will lead to more extreme weather patterns – droughts, storms, etc. We know all this to be true, yet how clouds fit into this picture continues to be the elusive question.
Clouds are important to climate change because they can force the climate in multiple directions. On the one hand, they reflect radiation from the sun, known as shortwave radiation (shortwave because its high energy and thus has a short wavelength), thus cooling the atmosphere. But as clouds grow vertically, their tops emit less and less infrared radiation (longwave radiation), thus keeping the atmosphere warm. How these effects balance across the planet is currently an unknown yet crucial ingredient to better understanding climate dynamics and the future of climate change.
The key factor in cloud formation is the aerosol concentration in the atmosphere. Cloud formation needs a little help getting started – little particulates of matter need to be hanging around in the air for water vapor to latch onto and form clouds. Known as cloud condensation nuclei (CCN), these particles can be dust, soot, carbon, salt from sea water, or other types of molecules.
CCNs start out under a micron, but as they attract more and more water vapor particles, they end up forming water droplets much larger than the original CCN.
Clouds form as a huge conglomeration of these water droplets nucleated around the CCN. So no aerosols (CCN), no clouds, in general. But we don’t know the relationship between the level of aerosols and cloud growth. Is it linear? Nonlinear? Is there a saturation point at which point adding more aerosols does not lead to any more cloud formation? Do you need a considerable amount of aerosols to get cloud growth started? These are vital questions to understanding how our pollution – many aerosols are pollutants from industrial processes – will affect climate forcing. Fortunately, a recent Science article has now given some new insight into how and when cloud formation has the most effect on climate forcing, indicating that the crucial point in cloud growth is a shift from no aerosols to only a small aerosol concentration in a given region.
To try to find the best real world example of clouds in isolation, the researchers from the Weizmann Institute in Israel looked for locations on the planet with almost no cloud cover, and thus very little aerosol concentration. This would limit further effects such as cloud-cloud interactions that would complicate the problem. Specifically, they wanted to look at what the how cloud formation is affected by moving from no aerosols to only small concentrations.Using spectroscopic data (from the Moderate Resoluation Imaging Spectroradiometer (MODIS)), they picked three areas in the Pacific, Atlantic, and Indian Oceans, shown by the boxes in the figure above, with little aerosol concentration (areas in red are high in aerosol concentration). From these areas, they sampled large amounts of data about clouds in these regions – rain rate, cloud top pressure (which indicates its vertical height), cloud top temperature, and cloud fraction (basically, the fraction of the area containing clouds). They plotted these values as a function of aerosol optical depth (AOD), which measures the density of condensation nuclei (0.1 AOD is equal to about 300 CCN/cm^3). Above is the most important data from the study. First, all four graphs demonstrate how an increase in AOD leads to increased cloud formation, meaning that even increasing aerosol concentration a small amount starts a great amount of cloud formation. As reference, many urban areas have an AOD > 0.1, so anything less than that is considered quite clean. We’re talking about very small concentrations of aerosols, yet, as can be seen on the rightmost graph, moving from about AOD=0.06 to AOD=0.1 doubles the cloud fraction, signifying huge amounts of cloud formation. We can also see the nonlinearity of the relationship, as the cloud fraction levels out by about AOD=0.2. On the other hand, cloud top pressure and temperature continue to rise quite quickly. The key result in all this is that only tiny amounts of aerosols are required to spur cloud formation, after which feedback processes likely take over to continue more and more cloud generation. There has been some previous evidence that there is a point at which more added aerosols have little effect on cloud formation (aerosol saturation effect), and this new evidence supports the fact that it is the initial small concentrations that are crucial to consider.
In addition, to understand how the increased vertical height of clouds with aerosols (warming the atmosphere) balances with increased light scattering (cooling), the authors used Clouds and Earth’s Radiant Energy System (CERES) data to examine the shortwave and longwave radiation trends as a function of AOD. They found that increased cloud height does indeed warm the atmosphere by about 12 W/m^2, but the scattering of shortwave radiation cools by about 27 W/m^2, leading to a net cooling of 15 W/m^2. This is more concrete evidence that, although clouds have both cooling and heating trends, they are likely to provide a net cooling effect to the atmosphere.
These results have some interesting implications, one of which is that the preindustrial atmosphere likely looked very different. Although there are natural CCN from volcanic eruptions and other planetary events, we have brought many more pollutants into the atmosphere since the 18th century. It appears, from this study, that during those first few decades of the Industrial Revolution, during which we first moved the atmosphere from almost no aerosols to small amounts, that huge leaps in cloud formation began.
Koren, I., Dagan, G., & Altaratz, O. (2014). From aerosol-limited to invigoration of warm convective clouds Science, 344 (6188), 1143-1146 DOI: 10.1126/science.1252595