No Ironing out of CO2
Thursday, May 7, 2009 4:19:28 PM
“In the cold waters of the Southern Ocean (surrounding Antactica) iron is biolimiting, and it has for some years been suggested that fertilising it with iron could slow global warming by enhanced phytoplankton photosynthesis that would pull large amounts of carbon dioxide from the atmosphere, helping to counteract the buildup of this greenhouse gas.”
The story goes back to the oceanographer John Martin, who died in 1993, and who in the 1980s proposed that iron added to regions of the ocean that are otherwise rich in nutrients but poor in iron can stimulate the growth of phytoplankton, a scientific hypothesis that has since been proven correct. John
Martin went further, however, when he suggested that artificial iron fertilisation of the oceans could change the climate. “Give me half a tankerful of iron and I’ll give you an Ice Age,” he boasted in 1988. This now seems NOT to be correct.
The crucial question is not whether plankton blooms can be induced by iron fertilisation, but whether the carbon they capture is removed to the deep sea, where they end up in sediments. New research reveals that plankton blooms do not send atmospheric carbon to the deep ocean – based on data that deep-diving Carbon Explorer floats collected around the clock for well over a year in the Southern Ocean at 66° south. Strong sedimentation – the sinking of large numbers of carbon particles to the deep ocean – was never observed.
A lot of the carbon tied up in plankton blooms appears not to sink very fast or very far. This may among other things be due to the seasonal feeding behavior of planktonic animal life, and specifically to the effects of the dark Antarctic winter on plant and animal growth and the mixing of surface and deep waters by winter storms. Phytoplankton blooms in the spring may indicate that much of the zooplankton (animal) population essential for carbon sedimentation has starved during the winter.
Apparently Iron is not the only factor that determines phytoplankton growth in polar regions where iron is biolimiting. Light, mixing, and hungry zooplankton are fundamentally as important as iron.
The main message seems to be that global warming can NOT be slowed or even reversed by fertilising plankton with iron in regions that are iron-poor but rich in other nutrients like nitrogen, silicon, and phosphorus.
“Year-round observations of carbon biomass and flux variability in the Southern Ocean,” by James K. B. Bishop and Todd J. Wood, will appear in Global Biogeochemical Cycles. Preprints are available online to subscribers at http://www.agu.org/journals/gb/papersinpress.shtml , using doi:10.1029/2008GB003206.