D. Koch1, G. Schmidt2, I. Alienov2, D. Shindell2, G. Faluvegi2, R. Ruedy3, S. Menon4, J.
McConnell5, S. Bauer2, R. Miller2
1Columbia University, New York, USA; 2NASA Goddard Institute for Space Studies, New York, USA; 3Sigma Space Partners, NASA Goddard Institute for Space Studies; New York, USA; 4Lawrence Berkeley Laboratory, Berkeley, California, USA; 5Hydrologic Sciences Division, Desert Research Institute, Reno, Nevada, USA
We modeled the climate effects of BC deposition on snow in the GISS model during the 20th century. The climate effects were based on both equilibrium climate experiments with a Qflux ocean for the years 2000 vs 1890 (Koch et al., 2009), and on transient climate experiments for the 20th century from 1880 through 2000 coupled to a model with deep ocean. In both cases the aerosols are online and fully coupled to climate, and the model includes direct, indirect-cloud as well as BC-albedo effects. For both studies, multiple simulations were performed allowing us to distinguish among direct effects, indirect effects and BC-albedo effects on climate. BC emissions in the transient simulation are from Bond et al. (2007). Biomass burning emissions are from GFED where we have scaled these by ½ for tropical burning in 1880 and assumed linear increase to the year 2000; extra-tropical burning is assumed constant throughout the century.
Since we are modeling BC effects, mostly in the Arctic, we have taken some care to compare the model with arctic BC measurements. Model BC surface concentrations and absorption aerosol optical depths are smaller than observed/retrieved in the Arctic, although the GISS model has more BC than most other global models. On the other hand, the GISS model compares fairly well with tropospheric aircraft measurements in the Arctic (Koch et al., 2009b) and with BC deposition measurements (Koch et al., 2009). The BC deposition was tuned to agree with these measurements, by adjusting the fraction of BC removed by frozen precipitation relative to that removed by liquid rain.
The BC albedo scheme in the GISS model (Koch et al., 2009) is based on Warren and Wiscombe’s (1985) dependence of snow albedo on BC concentrations in snow. Since this depends on snow grain size, we parameterize snow grain size using surface air temperature and snow age as given by Marshall (1989).
We have also compared the model BC 20th century trends with available measurements.
Compared with Greenland ice core records (McConnell et al., 2008), the model BC peaks at the right time (1910s-1920s) but does not peak as high as observed, and does not drop to values as low in one of the cores (ACT2). Model BC deposition in northern Europe peaks in the 1950s, in qualitative agreement with Svalbard BC measurements of Hicks and Isaksson (2006). Compared with BC lake core sediments in the Adirondaks of NY for 1880–2000 (Husain et al., 2008), the model peaks in 1920 as observed and drops to lowest values in recent decades, however the model does not have as much BC as observed from 1950s-1990s. Compared with BC in lake cores in China (Yongming Han, personal communication) the model trends are qualitatively correct in the north but do not rise as much as observed in recent decades on the Tibetan plateau.
Over the century, the GISS model has relatively small BC-albedo forcing, 0.01 Wm-2, however the impact on snow albedo and surface air temperatures (SAT) are fairly large
71 compared with previous studies (Koch et al., 2009). The mean arctic BC-albedo forcing was largest in the 1950s, so the change from 1880s to 1950s is 0.05 Wm-2; however this forcing decreased from the 1950s to the 1990s, -0.04 Wm-2, as arctic BC has declined during these years. Nevertheless, the 1990s BC-albedo forcing is quite substantial, 0.18 Wm-2 in the Arctic, suggesting that there is substantial opportunity to reduce BC effects further in the Arctic. The model indicates that 80% of arctic BC is from fossil fuel and biofuel, and 20%
from biomass burning.
We have considered the model’s ability to simulate climate (SAT) change during the 20th century by dividing the century into three periods, a strongly warming period from 1890s to 1940s, a stable climate period from 1940s to 1970s and another strong warming period from the 1970s to the 1990s. The GISS model with interactive species does a reasonable job of simulating SAT changes up to the 1970s but does not warm as much as observed during 1970s-1990s. This deficiency may be due to excessive indirect effects at the end of the century, connected to insufficient sulfate decline (in emissions).
The model indicates that the BC-albedo effect contributed 20% of arctic climate warming over the century, 25% during the 1890s-1940s. From the 1970s-1990s the BC-albedo effect decrease actually cooled the Arctic due to decreased BC especially from Europe.
According to the model, the BC-albedo effect caused 20% of Arctic and 35% of global ice/snow cover loss over the century. This effect is strongest early in the century, from the 1890s-1940s it caused 40% of Arctic and nearly all of the global ice/snow loss. From the 1940s-1970s the BC-albedo effect caused all arctic snow/ice loss. From the 1970s-1990s the BC-albedo effect caused increased snow/ice cover by an amount comparable to the net modeled loss due presumably to greenhouse gas warming.
References
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Historical emissions of black and Organic carbon aerosol from energy-related combustion, 1850–2000, Global Biogeochemical Cycles, 21, GB2018, doi:10.1029/2006GB002840, 2007.
Hicks, S., and E. Isaksson, Assessing source areas of pollutants from studies of fly ash, charcoal, and pollen from Svalbard snow and ice, J. Geophys. Res., 111, D02113, doi:10.1029/2005JD006167, 2006.
Husain, L., A. J. Khan, T. Ahmed, K. Swami, A. Bari, J. S. Webber, and J. Li. Trends in atmospheric elemental carbon concentrations from 1835 to 2005. J. Geophys. Res., v 113, D1312, doi:10.1029/2007JD009398, 2008.
Koch, D., S. Menon, A. Del Genio, R. Ruedy, I. Aleinov, and G.A. Schmidt. Distinguishing aerosol impacts on climate over the past century. J. Climate, 22, 2659–2677, doi:10.1175/2008JCLI2573.1., 2009a.
Koch, D. Evaluation of Black carbon estimations in global aerosol models. Atmos. Chem. Phys. Disc., 2009b.
Marshall, S. E. A physical parameterization of snow albedo for use in climate models. Ph.D. thesis, University of Washington, NCAR, and University of Colorado, 161 pp., 1989.
McConnell, J. R., R. Edwards, G. L. Kok, M. G. Flanner, C. S. Zender, E. S. Saltzman, J. R. Banta, D. R.
Pasteris, M. M. Carter, J. D. W. Kahl, 20th-Century industrial Black carbon emissions altered arctic climate forcing, Science 7, 1381–1384, DOI: 10.1126/science.1144856, 2007.
Warren, S. G., and W. J. Wiscombe, Dirty snow after nuclear war. Nature, 313, 467–470, 1985.
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