The climate is driven by a complicated interaction of
factors. While greenhouse gases contribute to warming, other influences
produce a cooling that may mask the full effects of greenhouse gases. We
knew that aerosolized particulates produce a cooling effect, but it has
been difficult to figure out how large this is, given the ongoing
warming.
Research presented in Nature Geoscience now tries
to put a number on the aerosol effect. Additional work on the same issue
shows that recent changes to emissions regulations may have strongly
accelerated the warming happening in the Arctic.
Aerosolized particles in the atmosphere scatter light and
cool the planet by reflecting some of the Sun’s light back out into
space. Additionally, these particulates may act as catalysts for the
formation of clouds, allowing water droplets to accumulate around them.
Clouds have a mixed impact on the climate, reflecting incoming sunlight
while also insulating the planet to a degree. This particular
interaction, between aerosols and clouds, remains a source of
uncertainty in climate science.
Storelvmo et. al. use observational climate data from three
data sets that span 1964 to 2010 to calculate how global temperature
would have evolved if carbon dioxide emissions had remained at 1964
values. Their analysis shows that the cooling effects of aerosols masked
approximately one-third of the global warming that is attributable to
increased greenhouse gas concentrations; this masking effect is
strongest before 1990.
Though global climate change is affecting the whole planet,
the Arctic has seen accelerated warming compared to the rest of the
planet. The reasons for this amplified Arctic warming are still not
completely clear. A paper published by Navarro et. al. in the same issue
of Nature Geoscience suggests that it, too, comes down to
aerosols. The paper uses climate models to show that European emissions
regulations, which reduced the output of aerosol particulates starting
in 1980, may have inadvertently contributed considerably to Arctic
warming by reducing the aerosol cooling effect.
Navarro et. al. simulate the global climate using the
Norwegian Earth System Model to study this phenomenon. Their
investigations incorporate sulfate-based aerosols by including the real
emissions reductions that occurred in Europe between 1980 and 2005,
comparing that to a scenario where those emissions reductions had not
occurred (it fixes emissions at the 1980 levels).
The results show that an average of half the Arctic warming
during this period may be explained by the reduction in emissions from
Europe, which reduced aerosolized sulfates and their cooling effects. In
other words, an action taken to protect the environment may have
actually ended up causing some harm. The results are surprising because
aerosolized sulfur particles typically do not last long in the
atmosphere, so it was expected that the effects of these particles would
be primarily localized to the area producing the emissions.
Taken together, these two articles demonstrate the profound
effects of human emissions on the environment. The degree to
which aerosol particulates can mask greenhouse warming has been unclear
and may have been underestimated due to uncertainty about the effect of
clouds. These papers suggest the impact has been significant and that,
as we clean up aerosols, we may also be enhancing future warming.
Our aerosol emissions are blocking a third of the climate warming
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