Researchers are beginning to understand aerosols and clouds better. The result is to lower estimates of how much they cool the climate - Oct. 05, 2013
CLOUDS and aerosols have long been two of the more
mysterious forces in the climate. They sometimes warm and sometimes cool the
Earth. The net effect, it was thought, was that they offset part of the overall
warming trend, which would have been greater had it not been for their
influence. But the details were obscure.
They still are, but much less so. Scientists’ improved
understanding of clouds and aerosols is, says Piers Forster of the University
of Leeds, “the most interesting development since AR4”. That acronym refers to
the fourth assessment report of the Intergovernmental Panel on Climate Change
(IPCC), which was published in 2007. The first parts of its successor, the
fifth assessment, were published on September 27th and 30th. And the upshot of
its assessment of clouds and aerosols is that their cooling effect looks less
strong than it did.
In this section
Clouds of (slightly less) unknowing
Science’s Sokal moment
Swans and Falcons
Time is not money
Environmental problems and protection
Nature and the environment
The mystery about aerosols starts with their size and
variety. A cubic centimetre of air typically contains thousands of suspended
liquid or solid particles. These are aerosols. They vary in size from being a
few nanometres to several microns across—that is from billionths of a metre to
millionths of one. Often, aerosols such as sulphates and soot are man-made.
Others, such as sea spray, are natural. If suspended in the lower atmosphere
they hang around for only a few days. In the stratosphere they can stay up for
more than a year.
This variety makes it hard to calculate their effect
on the climate. Some reflect sunlight back into space, cooling the Earth.
Others warm it by absorbing that light. The cooling effect dominates. But the
variety generates uncertainty.
Better measurements have begun to pierce this
uncertainty. Laboratory techniques such as photoacoustic measurements (a
biomedical-imaging method used to investigate cancers) make it possible to look
at how individual aerosol particles behave. This matters, because it turns out
that the ability of an aerosol to, for example, absorb water (which affects its
influence on clouds) depends on its size and composition. An aerosol’s property
of scattering or absorbing heat also depends partly on its size. So, argues
Ilona Riipinen of Stockholm University, scientists can now be more precise
about which aerosols do what.
They have also gained a better understanding of how
aerosols evolve. Such particles do not just float around unchanged. Sulphates,
for example, grow from vapours into clusters of a few molecules and then into
nanometre-sized particles. The rate at which these clusters emerge depends on
the presence of compounds such as ammonia and amines.
Armed with greater detail about what aerosols look like
and how they behave, researchers have been able to paint a more accurate
picture of how they affect clouds, and hence the climate. Clouds grow on
aerosols: that is, water droplets or ice crystals form on aerosol particles.
The more particles there are, the more numerous the water droplets and the more
reflective the cloud. That connection is well established and is the main
reasons for the link between aerosols, clouds and global cooling.
But there was thought to be a secondary connection
which added to the cooling. If more particles grow from a given amount of
vapour, each has to be smaller. That means the droplets which form on them are
smaller too, and that means a cloud is less likely to produce rain or snow and
more likely to reflect even more radiation. It turns out, though, that this
secondary influence is not as strong as was once thought—ie, there is not as
much extra cooling. But one particular aerosol is more influential than had
previously been calculated. This is soot, which absorbs heat, as black things
do. There is more of it around than previously thought. It is also blacker than
Putting all these influences together, the new
assessment reckons the net effect of aerosols is minus 0.82 watts per square
metre of the Earth’s surface (ie, a cooling of that amount). In the 2007
assessment, the IPCC reckoned the cooling was larger: minus 1.2 watts per
New research into the impact of clouds goes further.
Like aerosols, clouds vary greatly, as any landscape painter knows. A rule of
thumb for their impact on the climate is that low clouds reflect sunlight and
also let through heat radiated back from the Earth’s surface, thus cooling
things down. High clouds let sunlight through and also trap outgoing heat, thus
warming things up. So—as with aerosols—the climate science is all about which
clouds matter and where they are.
Some of the more important turn out to be those in the
temperate zones of both hemispheres (ie, in latitudes that embrace America and
Europe in the northern hemisphere, and the southern cone of Latin America and
southern Australia in the southern). These are referred to as “mid-latitude
Over the past 30 years, storm-track clouds have moved
a couple of degrees north or south, towards the poles. This movement has had
three effects, all tending towards greater warming. Because there is less
sunlight near the poles, less is reflected away by low clouds (though there is
slightly less outgoing heat to be trapped, too). And more sunlight is streaming
through at those lower latitudes which are being vacated by the storm-track
clouds (for example, at the southern edge of the northern track, meaning the
Mediterranean and Texas). More sunlight, more heat. Lastly, it seems that at the
higher latitudes where the storm tracks are treading, there are more high
clouds and fewer low ones. Again, that implies more warming.
Intriguingly, argues Frida Bender of Stockholm
University, there may be a connection between the poleward movement of clouds
and climate sensitivity, which measures how much the global climate would warm
up if it settled on a new equilibrium in response to a doubling of
carbon-dioxide concentrations. Climate models which predicted the shift towards
the poles also had higher estimates for climate sensitivity. This presumably
reflects the importance of clouds and aerosols to the climate. However, in its
new report the IPCC has lowered its estimate of climate sensitivity, partly
because of a 15-year pause in the rising trend of surface temperatures. It
remains to be seen how this lowering of sensitivity fits with an improved
understanding of clouds. The picture may be slightly clearer than it was. But
Oct. 05, 2013