Implications of Arctic Sea Ice Reduction on Arctic Tropospheric Chemical Change (Invited)

We examine the drastic reduction of Arctic sea ice in this decade and discuss the potential implications on bromine, ozone, and mercury change in the Arctic troposphere. We are witnessing extraordinary change in the Arctic sea ice cover. In the context of a half century change, perennial sea ice, the class of thicker and older ice important to the stability of Arctic sea ice, has been declining precipitously in this decade. Perennial ice extent declines at rate of 0.5 million km2 per decade in the 1970s-1990s while there is no discernable trend in the 1950s-1960s. Abruptly, the rate of decrease has tripled to 1.5 million km2 per decade in the 2000s. A record was set in the reduction of Arctic perennial ice extent in winter 2008. By 1 March 2008, perennial ice extent was reduced by one million km2 compared to that at the same time in 2007, which continued the precipitous declining trend observed in this decade. While the record low of total ice extent in summer 2007 is a historical mark of sea ice loss, the distribution and extent of different sea ice classes in spring (March-May) are critical information to understand the implications of sea ice reduction on photochemical processes, such as bromine explosions, ozone depletion episodes (ODEs), gaseous elementary mercury depletion episodes (MDEs), which occur at the time of polar sunrise. In this regard, the drastic reduction of perennial ice means that the Arctic becomes dominated by seasonal ice consisting of thinner ice, more leads, polynyas, frost flowers, and salty snow (due to seawater spray from open water), representing the overall saltier condition of the Arctic sea ice cover conducive to ice-mediated chemical processes leading to Arctic tropospheric ODEs and MDEs. To date (2009), the extent of perennial sea ice remains low and the extent of the thinner and saltier seasonal ice continues to dominate the Arctic sea ice cover. The shift of the state of Arctic sea ice cover to the dominance domain of seasonal ice can impact photochemical processes, leading to potentially significant implications on Arctic chemical change. Such implications, within the context of Arctic climatic change, are to be investigated in order to assess consequential changes in the Arctic habitat that may affect the health of people and wildlife. Regarding Arctic climatic change, seemingly opposing scenarios of Arctic chemical change have been hypothesized. In the first scenario, if sea ice cover continues to reduce in a warming trend in the 21st century, frost flower growth and bromine explosions might be suppressed and thus there would be less ozone and mercury depletion. Alternatively, in a different scenario, if the extent of seasonal ice during spring time in the Arctic continues to expand together with more cold spells due to temperature extremes exacerbated by climatic change, the abundance of seasonal ice, leads, and frost flowers may lead to more prevalent episodic events of bromine explosion and more intensive tropospheric ozone and mercury depletion in cold episodes. However, fundamental science questions remain to be addressed, and we have formed an international science team to plan for a future interdisciplinary research on those issues.