Science Briefs

Projecting Climate Change From Short-Lived Emissions Due To Human Activities

The largest single factor contributing to modern climate change is the increase of carbon dioxide in the atmosphere. However, molecules and particles with much shorted lifetimes also play an important role in driving climate change, though they are more familiar for their role as harmful air pollutants.

In a new study in support of the US Climate Change Science Program, we use the climate models from the NASA Goddard Institute for Space Studies, NOAA Geophysical Fluid Dynamics Laboratory, and NSF/DOE-sponsored National Center for Atmospheric Research to study the climate impact of the future evolution of short-lived species that influence climate (tropospheric ozone and aerosols). The models used the Intergovernmental Panel on Climate Change's "A1B" mid-range projected emission scenarios for ozone and aerosol precursors, independently calculated the resulting composition change, and then performed transient simulations to 2050 examining the response to projected changes in the short-lived species and to changes in both long-lived and short-lived species together.

Two line graphs of global mean temperature response to species

Figure 1, above: Global mean annual average temperature in the simulations with time-varying long-lived species only (top) and due to short-lived species (bottom). GFDL and GISS results are ensemble means. Click for large GIF or PDF of figure.

By 2050, two models show that the global mean annual average warming due to long-lived GHGs is enhanced by 20-25% due to the short-lived species (Figure 1). One model shows virtually no effect from short-lived species. Differences between models are largely related to differences in emissions projections for short-lived species, which are substantial even for a particular socio-economic storyline (Figure 2). This currently limits our ability to provide definitive statements on their contribution to future climate change.

Two line graphs of sulfur dioxide and black carbon emissions trends Figure 2, at right: Emissions trends used in the three models for sulfur dioxide (SO2; top) and black carbon (BC; bottom) following the A1B mid-range socio-economic storyline. Note that in the NCAR model, the present day black carbon distribution was scaled in the future rather than calculating future amounts from BC emissions. Scaling followed the global sulfur-dioxide emissions, a 40% increase over 2000 at 2030 and 10% at 2050. The NCAR 2000 black carbon global emission is set at the average of the GISS and GFDL 2000 values, and follows this scaling in the future, for illustrative purposes. Click for large GIF or PDF of figure.

For aerosols, these emission uncertainties are usually dominant, although for sulfate (an important reflective aerosol) uncertainties in aerosol physics are also substantial. Natural aerosols such as dust and sea salt also play an important role in climate and their emissions and interactions differed significantly among the models, with consequences to the role of short-lived pollutants. For tropospheric ozone, uncertainties in physical processes are more important than uncertainties in precursor emissions. Differences in future atmospheric burdens and radiative forcing for aerosols are dominated by divergent assumptions about emissions from South and East Asia.

In all three models, the spatial distribution of radiative forcing is less important than that of climate sensitivity in predicting climate impact. Both short-lived and long-lived species appear to cause enhanced climate responses in the same regions of high sensitivity rather than short-lived species having an enhanced effect primarily near polluted areas. Since short-lived species can significantly influence climate, regional air quality emission control strategies for short-lived pollutants will thus substantially impact climate over large scales, such as the Northern Hemisphere. Hence climate and air quality policies should take into account the important linkages between these two environmental issues.

Reference:

Shindell, D.T., H. Levy, II, M.D. Schwarzkopf, L.W. Horowitz, J.-F. Lamarque, and G. Faluvegi, 2008: Multimodel projections of climate change from short-lived emissions due to human activities. J. Geophys. Res., 113, D11109, doi:10.1029/2007JD009152.