A new study led by the University of Bristol has looked at the role that ice sheets play in Earth’s carbon cycle. Based on research involving hundreds of scientists over the last three decades, the team found that understanding the dynamics of these frozen regions is critical for carbon and climate predictions.
The study highlights the fact that ice sheets can no longer be overlooked in models of the carbon cycle and under scenarios of climate change.
Study lead author Professor Jemma Wadham explained how a unique set of conditions present beneath ice sheets make them important reactors in the Earth’s carbon cycle.
“Here, grinding of rock by moving ice is high, liquid water is abundant and microbes thrive in melt zones despite inhospitable conditions – the ice sheets erode their bedrock, cold-adapted microbes process the ground rock and boost nutrient release and glacial meltwaters export this nutrient to the oceans, also stimulating the upwelling of further nutrient from depth at glacier marine margins.”
“All this nutrient supports fisheries and stimulates drawdown of carbon dioxide (CO2) from the atmosphere,” said Professor Wadham.
“Ice sheets are also very effective at storing vast amounts of carbon as they grow over marine sediments, soils and vegetation,” said study co-author Professor Rob Spencer.
“The Antarctic Ice Sheet alone potentially stores up to 20,000 billion tons of organic carbon – ten times more than that estimated for Northern Hemisphere permafrost.”
“Some of this carbon is released in meltwaters and fuels marine food webs. The carbon that is left behind in deep parts of ice sheets is converted to methane gas by microbial and/or geothermal activity, which has the potential to be stored as solid methane hydrate under low temperatures and high pressure conditions.”
“We have no idea how stable potential methane hydrate will be in a warming climate if ice sheets thin. There is evidence from past phases of ice sheet wastage in Europe that sub-ice sheet methane hydrate has existed and can be released rapidly if ice thins.”
The researchers also analyzed the last transition from glacial conditions to the warmer conditions of the present day. In ocean cores obtained near Antarctica, the team looked for clues that may link ice sheet nutrient iron export through Antarctic icebergs to productivity in the Southern Ocean, which is a major global carbon sink.
“One important way that the Southern Ocean takes carbon out of the atmosphere is by growth of phytoplankton in its surface waters,” explained study co-author Dr. Jon Hawkings.
“However, these tiny ocean dwelling plants are limited by the availability of iron. We have long thought that atmospheric dust was important as a supplier of iron to these waters, but we now know that icebergs host iron-rich sediments which also fertilise the ocean waters as the bergs melt.”
The sub-Antarctic ocean cores showed that, as the climate warmed at the end of the last glacial period, iron transport to the Southern Ocean decreased while CO2 increased.
“While there are many possible causes for the CO2 rise, the data tantalizingly suggests that falling iron supply to the Southern Ocean via icebergs could have been a contributing factor,” said Professor Karen Kohfeld.
“Ice sheets are highly sensitive parts of our planet – we change temperatures in the air and ocean waters around them and thinning and retreat are inevitable,” added Professor Wadham. “The evidence we present here suggests that ice sheets may have important feedbacks to the carbon cycle which require further study as the uncertainty is still huge.”
“Gaining access to some of the most inaccessible and challenging parts of ice sheet beds, for example via deep drilling, alongside building numerical models which can represent biogeochemical processes in ice sheets will be key to future progress in this field.”
The study is published in the journal Nature Communications.
Image Credit: Dr Stefan Hofer