The Arctic environment is changing rapidly. While Arctic climate change is mostly driven by increases in greenhouse gases produced by human activity, other types of atmospheric emissions also influence the climate system. A recent paper in Reviews of Geophysics explores the relationships between atmospheric aerosol and climate change in the Arctic. Here, the authors of the paper give an overview of aerosol sources, formation processes, variability and trends, and suggest where further research is needed.
What is atmospheric aerosol and why does it matter in the Arctic?
Atmospheric aerosols are tiny solid or liquid particles suspended in the atmosphere. These particles impact Earth’s climate by changing the amount of solar energy that reaches Earth’s surface; either by scattering sunlight, or by participating in the formation of clouds. To understand past and future changes in Arctic climate, we need to understand the processes that drive aerosol formation, transformation and eventual loss from the atmosphere.
Do both human activities or natural processes contribute to aerosol in the Arctic?
Human activities and natural processes both within and outside the Arctic region can impact aerosol.
Natural processes, like the formation of sea spray from oceans or wind-blown dust from soils, can be important within the Arctic, but this natural aerosol can also be transported over long distances into Arctic areas.
Other natural processes are driven more by chemistry taking place in the atmosphere. For example, gases emitted from the oceans can undergo chemical reactions that can cause particle formation or growth of existing particles. Gases emitted from other natural sources, like volcanos or migratory seabirds, also impact Arctic aerosol.
Emissions from human activities at lower latitudes can be transported into Arctic regions, and can have significant impacts on Arctic climate. While few people live in the Arctic, the human presence is increasing, with activities such as shipping, tourism and resource extraction all affecting aerosol.
Does aerosol in the Arctic vary at different times of year?
The balance between aerosol from natural processes and human activities depends strongly on the season. Near the surface, Arctic aerosol is impacted largely by long range transport of pollution from lower latitudes in winter and spring. In the summer and early autumn, Arctic aerosol is impacted more by processes occurring within the Arctic, particularly natural processes such as those associated with open ocean.
This seasonality is driven by a combination of factors. First, the large scale movement of air in winter and spring favors long range transport of pollution from populated areas, while in summer the Arctic is more isolated from lower latitudes. Second, how thoroughly aerosol can be removed at lower latitudes and in the Arctic has a huge impact on Arctic aerosol. As temperatures increase from winter to summer, aerosol is removed from the atmosphere efficiently by drizzle and rain, compared to less efficient aerosol removal at cold temperatures in Arctic winter. Third, sea ice coverage is lowest in summer and autumn while biological activity in the ocean peaks, leading to increased importance of marine processes that form aerosol.
What have been the long-term trends of aerosol in the Arctic and why does it matter?
Significant decreases in sulfur dioxide emissions – a potent precursor for aerosol formation – in Europe, North America and the former Soviet Union since the 1980s have resulted in a large decrease in Arctic aerosol during the more polluted winter and spring. This change has had significant impact on Arctic climate and on sea ice extent.
The dominant effect of pollution aerosol transported to the Arctic is cooling so, as aerosol concentrations have decreased, this has resulted in a warming effect. Concentrations of soot (black carbon) aerosol have also decreased at Arctic stations.
Trends in other types of aerosol, from both natural processes and human activities, are less clear from our current data.
Why does climate change and loss of sea ice in the Arctic impact aerosol?
Warming and the loss of sea ice are impacting Arctic aerosol in several ways. First, increasing temperatures and decreased sea ice extent in summer promote increased emissions of both sea spray particles and gases from the ocean that act as precursors to aerosol.
Both of these aerosol-forming processes can have a significant impact on Arctic cloud properties in summer, when aerosol concentrations can be very low and cloud properties are very sensitive to available aerosol.
Second, loss of sea ice is opening new shipping routes, like the Northern Sea Route and the Northwest Passage, which decrease shipping distances but also introduce new sources of pollution to Arctic regions. Resource extraction activities are also increasing as Arctic sea ice retreats, with additional impacts on Arctic aerosol.
Third, emissions from wildfires can have a strong impact on Arctic aerosol, and increased frequency of fires across the globe is expected with increasing temperatures.
Finally, increasing global temperatures could impact atmospheric transport patterns, with implications for Arctic aerosol transported from lower latitudes.
What are some of the unresolved questions where additional research is needed?
Our community continues to work toward a quantitative understanding of how Arctic aerosol will be impacted by sea ice retreat, considering both natural processes and human-influenced emissions.
Natural aerosol emitted from ice- and snow-covered regions is also an area of uncertainty. This includes sea spray produced from cracks in sea ice and the possibility that aerosol could be produced from wind-blown snow.
The amount of aerosol removed in Arctic regions compared to lower latitudes is important for understanding Arctic aerosol but is poorly quantified. Further, a quantitative understanding of how Arctic aerosol impacts cloud properties is developing, but further research is needed.
These questions continue to require existing and new measurements at long term monitoring stations.
In addition, we have a much poorer understanding of how aerosol changes vertically in the Arctic compared to the long term record we have from surface measurements.
Measurements are also needed during autumn and winter, but frequent airborne observations are challenging to execute.
—Megan Willis (email: [email protected]), University of Toronto, Canada, now at Lawrence Berkeley National Laboratory, USA; Richard Leaitch, Environment Canada; and Jonathan Abbatt, University of Toronto