New research reveals the importance of winter sea ice in the year-to-year variability of the amount of atmospheric CO2 absorbed by a region of the Southern Ocean.
In years when sea ice lasts longer in winter, the ocean will overall absorb 20% more CO2 from the atmosphere than in years when sea ice forms late or disappears early. This is because sea ice protects the ocean from strong winter winds that drive mixing between the surface of the ocean and its deeper, carbon-rich layers.
The findings, based on data collected in a coastal system along the west Antarctic Peninsula, show that what happens in winter is crucial in explaining this variability in CO2 uptake.
The study was led by scientists at the University of East Anglia (UEA), in collaboration with colleagues from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI, Germany), British Antarctic Survey (BAS, UK) and Institute of Marine Research (IMR, Norway). It is published today in the journal Communications Earth & Environment.
The global ocean takes up about a quarter of all CO2 that humans emit into the atmosphere. The Southern Ocean is responsible for about 40% of this and the researchers wanted to know why it varies so much from year to year.
Lead author Dr Elise Droste, of UEA's School of Environmental Sciences, said: "Our picture of the Southern Ocean's carbon cycle is incomplete, and so we cannot predict whether its atmospheric CO2 uptake – and therefore its contribution to climate change mitigation – will increase, decrease, or remain the same in the future.
"Whatever it does, it will affect what our climate will look like and how fast it will change. To improve predictions, our work suggests that we need to look at how sea ice affects the exchange of carbon between the deep and shallow parts of the ocean. To do this, we need more wintertime observations in the Southern Ocean."
Much of the Southern Ocean surrounding the west Antarctic Peninsula is covered by sea ice in winter, which disappears in spring and summer. In spring and summer, phytoplankton growth and melt water lead to low CO2 concentrations at the ocean surface. This allows the Southern Ocean to absorb large amounts of atmospheric CO2, significantly reducing the global impact of anthropogenic emissions.
In winter, as sea ice forms, the ocean underneath mixes with deeper waters that contain lots of 'natural' carbon that has been in the ocean for centuries. This can cause CO2 at the ocean surface to increase to the point where it can be released into the atmosphere.
Sea ice blocks a large amount of this CO2 'outgassing'. However, it is part of the natural seasonal cycle that some CO2 does escape the ocean. This seasonal balance means that the total amount of CO2 absorbed by the Southern Ocean within one year often depends on how much CO2 is absorbed in summer and how much is released in winter.
"We don't have a good grasp on what is driving this year-to-year variability, which is making it difficult to fully understand the system and to improve the predictability of how the ocean's CO2 uptake will change in the future," said Dr Droste. "One major reason is because we have relatively little data on the Southern Ocean, particularly in the wintertime.
"It is extremely challenging to collect observations in the harsh weather and sea conditions of the Southern Ocean, not to mention sea ice cover making much of it inaccessible, even for the strongest icebreaker. However, this study takes us a step in the right direction."
The study draws on data for 2010-2020, a time series led and maintained by BAS, which collects year-round measurements along the west Antarctic Peninsula. At Rothera, the UK's Antarctic research station, ocean scientists measured physical aspects of the seawater in Ryder Bay and collected samples for nutrient and CO2 analysis, carried out at both Rothera and UEA.
Using other physical and chemical data collected at the same time, the team was able to study why years with long sea ice duration differed from those with short sea ice duration.
Dr Hugh Venables, from BAS, said: "A series of ocean scientists have wintered at Rothera on the Antarctic Peninsula to collect these and other samples, from either a small boat or a sea ice sledge, to build a unique time series of year-round oceanographic conditions for the last 25 years.
"This important result shows the importance of this winter sampling and will hopefully lead to more year-round sampling in the Southern Ocean, both by humans and autonomous technology."
Prof Dorothee Bakker, Professor in Marine Biogeochemistry at UEA, added: "The fact that this data has been collected throughout the year at the same location allows us to investigate which mechanisms are important to explain the year-to-year variability of CO2 uptake by the ocean at this particular location, but we can also use these insights to better understand how the rest of the Southern Ocean works."
The study also involved scientists from the National Institute of Oceanography and Applied Geophysics (Italy) and University of Gothenburg (Sweden). It was supported by funding from the UK's Natural Environment Research Council and European Union's Horizon 2020 research and innovation programme.
'Sea ice controls net ocean uptake of carbon dioxide by regulating wintertime stratification' , Elise Droste et al, is published in Communications Earth & Environment on June 18.
