Global — Smoke from northern forest fires may help cool the Arctic for several weeks to months at a time, according to a detailed analysis by researchers at NOAA and the University of Colorado in the US. Smoke in the atmosphere temporarily reduces the amount of solar radiation reaching the Earth’s surface an effect that could partly offset some of the warming caused by greenhouse gases and other pollutants.
The amount of solar energy “blocked” depends on how thick the smoke is, how far the Sun is above the horizon and the brightness of the Earth’s surface, says team leader Robert Stone.
Stone’s team calculated the effects of numerous wildfires that occurred in Alaska and western Canada in 2004 on short-term climate in the Arctic. Data was collected at Barrow in Alaska as part of ongoing efforts by NOAA to monitor geophysical variables critical to understanding climate change in the region. Other measurements, such as LIDAR, were made at a nearby US Department of Energy site.
“Specifically, we use one-minute resolved solar radiation data to quantify the impact of boreal smoke on the surface radiation budget in terms of a quantity called ‘aerosol optical depth’ (AOD), explained Stone. This is a measure of the turbidity, or opacity, of the atmosphere because of wildfire smoke.”
The researchers also simulated the smoke using radiative transfer models the results compare well with empirical results obtained. They used the model to study different scenarios and conditions that cannot be measured, such as smoke over open water and effects from smoke in the upper atmosphere. Stone says that the aerosols in the smoke tend to warm the layers in which they reside, while cooling the surface at the same time.
Despite helping to cool the Arctic, Stone stresses that smoke should not be exploited in “geoengineering” experiments to cool the planet until more information is available. However, he does point out that the new study supports the idea that injections of aerosols into the stratosphere “could provide the kind of cooling effect that may someday be needed to offset greenhouse warming”. The aerosols would need to be smaller in size and persist for very long periods to make this economically feasible, and the effects of cloud-aerosol interactions must be taken into account as the aerosols eventually fall through the lower atmosphere.
The work is a first step towards Arctic-wide assessments of how aerosols transported to the Arctic from lower latitudes impact the region’s climate. It could help improve Arctic climate models that are currently prone to large uncertainties, said Stone.
Using the same methodology, the team is now analysing the impact of Arctic haze (mostly from industrial pollutants) that accumulates in the Arctic each winter and spring. The researchers will also study flow or circulation patterns that determine how aerosols are distributed in the Arctic. “We will work with scientists who are quantifying all these processes and attempt to project scenarios of how forcing by aerosols will change in a warming climate,” added Stone.
The effects of dust from Asian deserts, which has nearly twice the cooling effect of smoke and is transported to the Arctic all year round, also need to be studied further, says Stone. How this dust interacts with clouds and affects the radiation budget at the surface is not well understood — or taken into account when modelling the climate.