Millennial timescale carbon cycle and climate change in an efficient Earth system model

A new Earth system model, GENIE-1, is presented which comprises a 3-D frictional geostrophic ocean, phosphate-restoring marine biogeochemistry, dynamic and thermodynamic sea-ice, land surface physics and carbon cycling, and a seasonal 2-D energy-moisture balance atmosphere. Three sets of model climate parameters are used to explore the robustness of the results and for traceability to earlier work. The model versions have climate sensitivity of 2.8-3.3◦C and predict atmospheric CO2 close to present observations. Six idealized total fossil fuel CO2 emissions scenarios, are used to explore a range of 1,100-15,000 GtC total emissions and the effect of rate of emissions. Atmospheric CO2 approaches equilibrium in year 3000 at 420-6000 ppmv, giving 1.5-10.5◦C global warming. The ocean is a robust carbon sink of up to 9 GtC yr−1. Under ‘business as usual’, the land becomes a carbon source around 2100 which peaks at up to 2 GtC yr−1. Soil carbon is lost globally, boreal vegetation generally increases, whilst under extreme forcing, dieback of some tropical and sub-tropical vegetation occurs. Average ocean surface pH drops by up to 1.15 units. A Greenland ice sheet melt threshold of 2.6◦C local warming is just avoided if total emissions are limited to 1,100 GtC, whilst 15,000 GtC emissions cause complete Greenland melt by year 3000, contributing 7m to sea level rise. Total sea-level rise, including thermal expansion, is 0.4-10m in year 3000 and ongoing. The Atlantic meridional overturning circulation shuts down in 2 out of 3 model versions, but only under extreme emissions including exotic fossil fuel resources. ∗Tyndall Centre, UK and School of Environmental Sciences, University of East Anglia, Norwich, UK. E-mail: [email protected] Tel: +44-1603-591414 Fax: +44-1603-507719 †Earth Sciences, Open University, Milton Keynes, UK ‡Tyndall Centre, UK and National Oceanography Centre, Southampton, UK §Southampton e-Science Centre, University of Southampton, Southampton, UK ¶Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, Canada ‖www.genie.ac.uk/people