In 2003 we began a new project,
GCAP,
(Global Change and Air Pollution), funded by the EPA,
in which we examine the effect of a changing climate on air quality.
We already know that meteorological variables such as boundary layer height and wind speed
strongly influence air quality.
We are investigating how these and other variables change as greenhouse gas concentrations
rise, and what the consequences will be for both surface ozone and particulate matter.
In a pilot study, we included in the GISS general circulation model (GCM) two tracers of anthropogenic pollution, combustion carbon monoxide (COt) and black carbon (BCt). We held the sources of both tracers and the sink of COt constant in time, and let the wet deposition of BCt respond to the changing climate. Our results show that the severity and duration of summertime regional pollution episodes in the midwestern and northeastern United States increase significantly in the future relative to present. Pollutant concentrations in these episodes increase by 5-10% and the mean episode duration increases from 2 to 3-4 days. The increases appear to be driven by a decline in the frequency of mid-latitude cyclones crossing southeastern Canada. The cold fronts associated with these cyclones are known to provide the main mechanism ventilating the midwestern and northeastern United States.
The plot to the right shows the cumulative frequency distributions of daytime carbon monoxide tracer at the surface over the northeastern U.S. in July-August. Each point represents the spatial average for a particular day. Concentrations at the high end of the distributions are significantly greater in the future relative to the present-day. You can read our paper on this research by clicking here: [Full text (pdf)]. You can also check out our press release or powerpoint presentation. More info, including media coverage, is here.
Since 2004, the GCAP project has really taken off.
We have so far explored the response of future aerosol and ozone concentrations
to 2000-2050 climate change.
We have also begun investigating the recent past, particularly 1950-2000 trends
in cyclone frequency and the relationship between cyclone frequency and observed ozone episodes.
In addition, we have plans to study the impact of climate change on mercury deposition in
U.S. ecosystems.
Finally, we have an ongoing project in which we examine the relationship between future climate
change and forest fires, and the effect of changing fire frequency on U.S. air quality.
For more information, please email me or visit the
GCAP webpage.
Earlier at Harvard, I worked with Prashant Murti to implement a detailed
on-line chemistry scheme into the GISS GCM.
I found that the global mean radiative forcing from anthropogenic ozone is
0.44 Wm-2, or about one-fourth the forcing since preindustrial times of CO2.
Over large areas in Northern Hemisphere summer, the forcing is greater than 1.0 Wm-2.
Large forcings, greater than 1.0 Wm-2, appear over the Arctic in summer due to
shortwave forcing over snow and ice. To learn more, go to
[Abstract].
I also investigated the discrepancies between observed and calculated preindustrial ozone
levels at the surface.
Most models, including ours, overestimate preindustrial ozone abundances at the surface by 50-100%.
(Measuring surface ozone was a worldwide endeavor in the late 1800s.)
I found that by varying the natural emissions of ozone precursors within the range of uncertainties,
I could reduce the calculated ozone at the surface and better match the observations.
The resulting radiative forcing from anthropogenic ozone, 0.80 Wm-2, is nearly double the
forcing we previously had calculated. To learn more, go to
[Abstract]
[full text (pdf)].
I have examined the characteristics of the climate response to the change in tropospheric ozone since preindustrial times. Again using the GCM, I carried out a pair of equilibrium climate simulations with present-day and preindustrial ozone distributions. I found that the globally averaged surface temperature increased by 0.3o C, with the change in the Northern Hemisphere about double that in the Southern Hemishere. The addition of tropospheric ozone cooled the Arctic lower stratosphere in winter, with temperatures decreasing by about 1.0o C, with implications for the recovery of polar stratospheric ozone. I also investigated the climate response to (1) a uniformly applied ozone increase and (2) a CO2 forcing equivalent in magnitude to the ozone forcing. To learn more, go to [Abstract] [Full Text (pdf)] [Powerpoint Presentation].
The last two projects were part of
CACTUS
(Chemistry, Aerosol, Climate: Tropospheric Unified Simulation), funded by NASA.
The CACTUS team is continuing its investigation, with a new component focusing on the indirect
effect of aerosols on climate.
For Loretta's cv, click here.
For a slightly less technical powerpoint presentation on climate change, click here.
Back to Loretta's homepage.