Sunday, February 24, 2013

Sea ice reduction disturbs Arctic greenhouse gas balance

The diminishing Arctic sea-ice extent is resulting in changes in how greenhouse gases interact between the land, ocean and atmosphere according to a new study.

"Changes in the balance of greenhouse gases can have major consequences because, globally, plants and the oceans absorb around half of the carbon dioxide that humans release into the air through the use of fossil fuels. If the Arctic component of this buffer changes, so will the amount of greenhouse gases in the atmosphere", says Dr Frans-Jan Parmentier, a researcher at Lund University, Sweden, and lead author of the study..

Reduced white sea ice and expanded dark ocean surface results in a change to the surface albedo feedback or reflectivity, resulting in increased light absorption and ocean warming which then warms the atmosphere affecting the rate of interchange of CO2.

The decline of summer sea ice has been dramatic with the Arctic Summer Sea Ice Extent Minimum record smashed in 2012.

Figure 2: Trends in sea ice, SWI and normalized difference vegetation index (NDVI).
Time series for the Northern Hemisphere from 1982 to 2008 of sea ice concentration (blue, with inverted y-axis), SWI (red), the highest summer value of NDVI, which represents biomass production and peak photosynthetic activity (dark green), and the unitless time-integrated NDVI, which is the sum of biweekly NDVI values above 0.05 from May to September (light green). Detrended time series of sea-ice concentration, SWI and TI-NDVI correlate with each other at a 95% level.

On land, rising temperatures results in more vigorous growth by vegetation - a positive effect, but this is more than offset by more carbon dioxide and methane released from the soil in processes such as permafrost thaw, which has a strong negative impact on the climate.

The exchange of green house gases between the atmosphere and ocean is also changed but there are many more uncertainties surrounding the effects of the melting sea ice on these processes say the researchers.

"We know very little about how the shrinking sea ice cover disturbs the balance of greenhouse gases in the sea in the long term", said Dr Parmentier.

Figure 3: Simplified representation of Arctic carbon fluxes that are possibly influenced by sea ice retreat.
On land, plants take up carbon while microorganisms in the soil produce methane and respire CO2. In the ocean, methane is released from thawing subsea permafrost, while CO2 is absorbed due to an undersaturation of CO2 in the water compared with the atmosphere. CO2 is then photosynthesized into organic carbon, or converted into CaCO3 by the biological and solubility pumps, maintaining low pCO2 levels. Near the ice edge, cooling induces sinking of the surface water, and with it CO2. Above the ice, the air-ice exchange depends on the carbonate chemistry and temperature of the ice. CO2 can also be discharged into brine channels in the ice, where transport is downwards due to the density difference. In polynyas and leads, CO2 can be taken up from the atmosphere, but methane can also be produced in surface waters. Transport into the deeper and interior waters of the Arctic Ocean ensures storage of carbon. Current best estimates of sink/source strengths are given in Tg C yr-1, where available. The uncertainty ranges of the terrestrial fluxes are shown in brackets. The arrows in this figure do not represent the strength of each flux.

The study was carried out by lead author Frans-Jan W. Parmentier from Lund University in Sweden, along with colleagues from Denmark, Greenland, Canada and the USA. The research was published in Nature Climate Change - The impact of lower sea-ice extent on Arctic greenhouse-gas exchange. The abstract for the paper:

In September 2012, Arctic sea-ice extent plummeted to a new record low: two times lower than the 1979-2000 average. Often, record lows in sea-ice cover are hailed as an example of climate change impacts in the Arctic. Less apparent, however, are the implications of reduced sea-ice cover in the Arctic Ocean for marine-atmosphere CO2 exchange. Sea-ice decline has been connected to increasing air temperatures at high latitudes. Temperature is a key controlling factor in the terrestrial exchange of CO2 and methane, and therefore the greenhouse-gas balance of the Arctic. Despite the large potential for feedbacks, many studies do not connect the diminishing sea-ice extent with changes in the interaction of the marine and terrestrial Arctic with the atmosphere. In this Review, we assess how current understanding of the Arctic Ocean and high-latitude ecosystems can be used to predict the impact of a lower sea-ice cover on Arctic greenhouse-gas exchange.

  • Lund University, 18 February 2013 - Reduced sea ice disturbs balance of greenhouse gases
  • Frans-Jan W. Parmentier, Torben R. Christensen, Lise Lotte Sørensen, Søren Rysgaard, A. David McGuire, Paul A. Miller & Donald A. Walker, Nature Climate Change (2013) doi:10.1038/nclimate1784 The impact of lower sea-ice extent on Arctic greenhouse-gas exchange (abstract)
  • Images from Parmentier et al (2013)