Methane Emissions from the Arctic OCean to the Atmosphere: Present and Future Climate Effects

Methane (CH4) is a greenhouse gas with a 100-year global warming potential ~32 times that of CO2, and contributes significantly to recent increases in global warming. Since 2007, atmospheric CH4 is on rise again with its cause still not clearly understood. Methane hydrates (MH) in ocean seabed sediments are a potential source of CH4 to the atmosphere, and scientific results show diversity in the flux of CH4 that actually reaches the atmosphere. MH are potentially susceptible to ocean warming, which could trigger a positive feedback resulting in rapid climate warming. The results and outcome of the MOCA project improve our understanding and knowledge of impact of CH4 from Arctic Ocean now, and over the next 100 years. Progress has been made on most of the driving questions. The overall objective of MOCA was to ?Quantify the present atmospheric effects of CH4 from gas hydrates at the seabed, and future potential climate impacts on decadal to centennial timescales.? To meet these objectives, MOCA used a combination of observations in the ocean and in the atmosphere, integrated with global and regional modelling for both ocean and atmosphere to interpret the observations, and to simulate future changes and climate impacts over this century. Within MOCA we were able to identify low fluxes from potential Arctic emissions areas in the ocean to the atmosphere, and we were able to provide upper estimates of the emissions using models constrained by the observations. The focus area for the field measurements and detailed campaigns in MOCA were located west of Prince Karls Forland, but also areas south and north of Svalbard and south west Barents Sea were studied. The low fluxes of CH4 from subsea sources reaching the atmosphere now does not impact climate significantly, and will not over the next 100 years, under realistic climate projections. We concluded that this important statement is valid also taking the larger Arctic region into account. Furthermore, we identified other regions as highly interesting and susceptible for temperature change in the ocean, both north of Svalbard and in the south-west Barents Sea, with potential emissions of CH4. We detected high concentrations of CH4 above the seabed, but little of this reaches the surface layer of the ocean and the atmosphere. There is large uncertainty in the variations and fate of CH4 in the ocean column. This needs further focus, both regarding annual variations, impact of sea ice and sea ice coverage, wind and sea depth. To reduce uncertainty of the CH4 radiative forcing, we have been involved in the revision of CH4 forcing estimates. This resulted in 25% stronger forcing of CH4 than given in last IPCC report, increasing the relative importance of CH4 as a greenhouse gas and making future climate change more sensitive to changes in CH4 emissions. Finally, MOCA has led to large improvement in the understanding of the ethane budget, including oceanic and geologic sources, and provided new estimates of these. We show that natural geologic emissions and 2-3 times higher anthropogenic fossil fuel emissions are required to reproduce observations of preindustrial and present concentrations.

Methane hydrates (MH) in ocean seabed sediments are a potential source of methane (CH4) to the atmosphere, where CH4 acts as a powerful greenhouse gas. MHs are potentially susceptible to ocean warming, which could trigger a positive feedback resulting in rapid climate warming. MOCA will use advanced measurements to quantify the present atmospheric impact of CH4 from MHs, and simulate the future potential climate effects focused on 2050 and 2100. MOCA is an interdisciplinary project employing measurement c ampaigns and powerful modeling tools involving international collaborations. The project is expected to generate new knowledge about the climate using Svalbard as an experimental test bed to study central polar processes that have potential to impact the entire Earth system and change climate In work package (WP) 1 will MOCA combine land-, ship- and aircraft-based measurements in coordinated campaigns to improve the description of the current environmental state of the Svalbard region. MOCA will quantify the present-day CH4 emissions from the seabed west of Prince Carl Forland and identify main influences on the proportion that enters the atmosphere. The extensive measurements in MOCA will improve the knowledge on the overall activity of the marine seep s ites offshore Svalbard. The activity will generate CH4 emission estimates related to MH in the Arctic Ocean. Coordinated measurement effort is particular useful around Svalbard where current scientific results show a diversity in whether CH4 from MH reach the atmosphere. The detailed observation and process-based studies in WP1 will be up-scaled spatially over the Arctic region and for future time periods in WP2. In WP3, the radiative forcing (direct and indirect effects) and climate impact from MH-based CH4 emissions will be calculated under present-day atmospheric composition and realistic future temperature scenarios. The potential of the biogeochemical cycle as a strong feedback mechanism will be investigated.