Decline of reflective snow cover likely to speed overall warming
By Summit Voice
SUMMIT COUNTY — Heat-trapping greenhouse gases aren’t the only reason the spring snow cover across the northern hemisphere has been declining steeply the past few decades.
By tweaking a sophisticated set of climate models, researchers found that black carbon and dust — both generated by human activities — are at least part of the reason that spring snow cover in Eurasia is declining faster than across North America.
Declining spring snow cover has a feedback effect of intensifying warming because snow-covered ground reflects incoming radiation. Once the snow is melted, the heat is absorbed.
Some aerosols reflect incoming solar energy, potentially cooling underlying surfaces, but black carbon tends to warm surfaces by absorbing incoming solar energy. Particulates that fall to the surface also reduce snow’s reflective qualities, causing even more radiation to be absorbed.
Generated by human activity, dust storms, and forest fires, Asia produces high levels of both types of aerosols, which blow across the Eurasian land mass and affect the surface and nearby atmosphere in a variety of ways.
In the Northern Hemisphere, spring-time snow cover is unique because of its widespread distribution, and because intense incoming solar radiation during that season amplifies atmospheric aerosols’ effects.
Because higher concentrations of organic matter and black carbon are typical in the atmosphere and on the snow-covered surfaces in Eurasia, Flanner and his colleagues hypothesize that those aerosols might account for regional snow-cover differences. By including black carbon and organic matter aerosols in climate models, the researchers hypothesized that the models might more effectively match spring-time observations.
To test their hypothesis, the team first ran a number of modeling scenarios to see if the inconsistency might relate to ocean-based effects. If oceans proved to have a leading role, the aerosol hypothesis would likely be incorrect. However, after constraining the oceans’ effects, the models continued under-predicting land-surface temperature trends. The findings indicated that a land effect had to account for the discrepancy between observations and models showing warming and melting trends.
Having eliminated ocean effects, the researchers enhanced the models with snow-darkening characteristics, mimicking the impact of dark materials deposited on top of pristine snow. With this adjustment, the models correctly indicated increased springtime warming in Eurasia.
Next, the researchers incorporated human-produced carbon dioxide, or CO2, into the models. The scientists found that over North America, CO2 had more of an impact on springtime snow cover than black carbon and organic materials, but in Eurasia, as hypothesized, the particulates were far more influential, having as much of an effect as CO2.
“While this research does not fully explain why springtime land temperatures and snow cover are changing so much faster over Eurasia than North America, it does suggest that snow darkening from black carbon, a process lacking in most climate models, is playing a role,” Flanner said.
Ultimately, Flanner said, the magnitude of Earth’s climate response to CO2 and other human-generated products depends on feedbacks. Changes in snow cover amplify initial climate changes and constitute one of the most powerful feedbacks. Because snow covers much of the Northern Hemisphere during spring, Flanner and his colleagues expect to see some of the strongest climate change signals in northerly regions during local spring.