Tundra carbon cycling in relation to sea-ice decline in a warming Arctic

Abstract: We set out to investigate whether arctic tundra acted as a sink or source of carbon dioxide (CO2) between 1980-2014 and to relate changes in the net carbon balance to Arctic sea-ice decline. We want to understand how climate change and arctic warming influence arctic carbon cycling. Identifying the ways higher temperatures affect tundra carbon cycling, one of them being the decline of sea ice, can deepen the understanding we have of climate change. Trends in arctic tundra Net Ecosystem Exchange (NEE) over 1980-2014 were investigated and related to changes in sea-ice area and temperature. We analyzed the changes in modelled NEE in relation to sea-ice decline and increasing temperatures by looking at trends in simulated Gross Primary Production (GPP), autotrophic respiration (RA) and heterotrophic respiration (RH). These were correlated with sea-ice area and temperature data. Carbon fluxes were modelled by a customized arctic version of the LPJ-GUESS dynamic global vegetation model. NEE became more negative over 1980-2014, indicating an increase in the carbon sink of tundra (GPP). Also, auto- and heterotrophic respiration increased with warming temperatures and sea-ice decline, agreeing with previous work that a warming Arctic facilitates tundra productivity. The increases in GPP, RA and RH were found to be significantly related to the decreases in sea-ice area and temperature increases. Seasonal investigations revealed that GPP was affected the least by temperature in autumn and winter. This causes the typical variations in NEE over the year, varying from negative (sink of carbon) to positive (source of carbon) values. The CO2 flux of GPP increased between the first and last 10 years investigated (1980-1989 and 2005-2014), by an average of 762 TgC y-1. CO2 from ecosystem respiration increased by an average of 605 TgCyr-1 and NEE decreased by 157 TgCyr-1. Our finding of an increasingly negative NEE from 1980 to 2014 shows that the increased carbon losses through RA and RH were compensated by an increasing carbon sink through GPP and that this sink increased with sea-ice decline and arctic warming.