Maximum estimated from PMIP 2 simulations : 2 climate , vegetation and geographic controls

It is an open question to what extent wetlands contributed 4 to the interglacial-glacial decrease in atmospheric methane concentration. 5 Here we estimate methane emissions from glacial wetlands, using newly avail-6 able PMIP2 simulations of the Last Glacial Maximum (LGM) climate from 7 coupled atmosphere-ocean and atmosphere-ocean-vegetation models. These 8 simulations apply improved boundary conditions resulting in better agree-9 ment with paleoclimatic data than earlier PMIP1 simulations. Emissions are 10 computed from the dominant controls of water table depth, soil temperature 11 and plant productivity and we analyse the relative role of each factor in the 12 glacial decline. It is found that latitudinal changes in soil moisture, in com-13 bination with ice-sheet expansion, cause boreal wetlands to shift southward 14 in all simulations. This southward migration is instrumental in maintaining 15 the boreal wetland source at a significant level. The mean emission temper-16 ature over boreal wetlands drops by only a few degrees, despite the strong 17 overall cooling. The temperature effect on the glacial decline in the methane 18 flux is therefore moderate, while reduced plant productivity contributes equally 19 to the total reduction. Model results indicate a relatively small boreal and 20 large tropical source during the LGM, with wetlands on the exposed con-21 tinental shelves mainly contributing to the tropical source. This distribution 22 in emissions is consistent with the low interpolar difference in glacial methane 23 concentrations derived from ice-core data.