Effect of CO2 on C4 grass biomass and fires

[James King]

Hi all,

I have done some runs with CLM5 in land-only mode and have a result I'm not sure how to explain. The runs are at 0.5deg resolution over a regional domain covering sub-Saharan Africa over the period 2015-2100. We have used SSP3 anomaly forcings and done runs with CO2 increase only, CO2 + climate, CO2 + climate + land use change, and CO2 + climate + land use change but with population density fixed at 2015 ('no pop'). One result is confusing - in southern Africa, the CO2 only run shows increasing C4 grass biomass (TOTVEGC) but decreasing C4 grass fire emissions (FireEmis_TOT). In all the other runs, both variables decrease. We were expecting fire emissions to increase if biomass increases when no other inputs are changed besides CO2 emissions. Is there some feedback here I am missing?

Thanks,

James

 

[wwieder]

Hi James,

How are you handling N deposition in these experiments? C4 grasses in CLM5 are very sensitive to N availability. I wondered if the declines in TOTVEGC (and fire fluxes) that you're seeing mirror trends in N deposition in the region (either intentionally or by mistake).

 

[James King]

Hi Will,

Thanks for the tip - we haven't looked at that possibility yet. I think we fix N deposition at 2015 levels in all runs apart from those where we change the land use - but I will need to double check this in my case directories when the current Cheyenne maintenance is completed.