92nd American Meteorological Society Annual Meeting (January 22-26, 2012)

Wednesday, 25 January 2012: 5:15 PM
The radiative forcing of fires; Global model estimates for past, present and future
Room 350/351 (New Orleans Convention Center )
Daniel Ward, Cornell University, Ithaca, NY; and S. Kloster, N. Mahowald, B. Rogers, J. Randerson, and P. Hess

Emissions from fires affect the composition and chemistry of the atmosphere, thereby changing the transfer of radiation through the atmosphere. Radiative forcing (RF) from the emissions of both natural and human-caused fires impacts the Earth's climate to a potentially important, yet largely unquantified degree. To investigate the role of fire emissions in the global climate, fires are simulated with a modified version of the Community Land Model (CLM) from a pre-industrial environment to the year 2100. Future fires were simulated using ensembles of future climate predictions from the Community Atmosphere Model (CAM) and ECHAM. Emissions are computed from the amount of carbon lost due to fire and speciated according to biome-specific estimates from the literature.

The emissions of various species from the model years 1845-1855, 1996-2006, and 2090-2100 were used to force emissions from fires in a set of atmosphere simulations using the Community Atmosphere Model (CAM) version 4.9 with MOZART chemistry. The aim of this experiment is to isolate the contribution of fires to global concentrations of greenhouse gases ozone, methane and carbon dioxide, and the resulting RF from each. The suite of simulations includes two-year runs for past, present and the two future ensembles, all started from a spun-up atmosphere. For each time period the model was run without fire emissions and then with the fire emissions computed from CLM. An additional simulation was carried out with fire emissions from the Global Fire Emission Database (GFEDv2.1) for comparison. The last 12 months of each two-year run was analyzed, leaving the first year for spinup of the model chemistry. All simulations were run with an identical climate, using year 2000 atmospheric forcing and no feedbacks to the climate from atmospheric chemistry or radiation.

Emissions from fires include NOx and volatile organic compounds (VOCs) which are ozone precursor gases. Ozone in the troposphere acts as a greenhouse gas, absorbing longwave radiation emitted from the Earth's surface. To isolate the global effects of ozone produced from fires, the CAM radiation code is run offline using the atmospheric chemistry from the experiment simulations. The radiation code is run with ozone excluded from the model atmosphere and compared to the radiative transfer when ozone is included to diagnose the contribution to total RF from ozone. Then the ozone RFs for the fire and no-fire simulations are compared, to extract the role of fires. Results show that the RF from fire-produced ozone is important regionally (up to 0.2 W/m2) but minor on a global scale (0.01 to 0.05 W/m2). It was significantly more important in the pre-industrial simulations than in the present day or future runs. This could be because of the lower background concentrations of NOx in the past.

Fires emit methane directly but also impact methane concentrations indirectly by altering the atmosphere's capacity for oxidation. Smoke includes high levels of CO and non-methane VOCs that are oxidized by the hydroxl radical, OH, which is removed from the atmosphere in the reaction. Therefore, fires often act as a sink for OH. Since oxidation by OH is the main sink for methane in the atmosphere, the present day and future simulations show an increase in methane lifetime when fire emissions were included. The greater lifetime leads to an increase in the RF from methane due to fires (between 0.03 to 0.09 W/m2). In contrast, the pre-industrial fires caused a decrease in the RF due to methane. The large amounts of ozone produce in the pre-industrial fire plumes acted as an additional source of OH, enhancing methane destruction enough to cause a decrease in its concentration. In the preindustrial simulations the negative RF due to changes in methane concentration from fires almost exactly counteracts the positive RF due to ozone.

Estimating the RF due to changes in carbon dioxide concentration from fires is complicated by the carbon-sink potential of post-fire ecosystems. To compute the RF of carbon dioxide from fires a separate set of CLM transient simulations was carried out that excluded changes in the storage of carbon in the biosphere due to fire. The changes in carbon storage due to fires could then be assessed between pre-industrial, present day and future time periods. A fraction of the released carbon is assumed to remain in the atmosphere as carbon dioxide and the RF can then be estimated. RF from carbon dioxide emitted by fires is considered to be greater than the RF from methane and ozone. On the whole, the impact of fires on greenhouse gas concentrations in the atmosphere is now better quantified. The RF from these impacts was found to be positive and slightly underestimated using the CLM fire emissions when compared to GFED for the present day climate. For a more comprehensive examination of fire emissions climate impacts, a similar experiment will be constructed to look at contributions from the direct and indirect effects of fire aerosols and also the deposition of aerosols on snow-covered surfaces.

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