Jennifer A. Logan
Senior Research Fellow
School of Engineering and Applied Sciences
Harvard University

HINDCASTS - HOW WELL DO WE UNDERSTAND RECENT TRENDS AND VARIABILITY IN TRACE GASES?

BACKGROUND:

Methane and tropospheric ozone are radiatively important trace gases that impact climate, and recent observations show trends and interannual variability (IAV).  Ozone precursors, NOx and CO, also show trends and IAV.  Models are used to project how these gases will evolve over the coming century.  Hindcasts – model simulations of past behavior – are essential to test the models which incorporate our state of knowledge of processes affecting trace gases and aerosols. We used the GEOS-Chem model to conduct hindcasts, investigating the causes of trends and IAV in CH4, CO, and tropospheric ozone.

Hindcasts use meteorological fields for the appropriate time period, and time dependent emissions.  We developed interannual and seasonally dependent biomass burning emissions using the TOMS Aerosol Index, ATSR and AVHRR fire counts (Duncan et al., 2003 [PDF]) and a climatology for biomass burning developed by J. A. Logan and R. M. Yevich (Lobert et al. 1999 [PDF]) as part of this work.

FINDINGS:

Global budget and trends in CO, 1988-1997 (Duncan et al., 2007)

  • Emissions from fossil fuel and Industry hardly change in 1988-1997, as increases in Asia are offset by decreases in Europe and North America
  • The model captures the 20% decrease at high northern latitudes and the 10% decrease in the North Pacific
  • These trends are largely caused by decreases in European emissions

Analysis of the slowdown and interannual variability in the CH4 growth rate, 1988-1997 (Wang et al., 2004).

  • The slowdown in growth is caused by slower growth in sources and increases in OH
  • The economic downturn in the former Soviet Union and Eastern Europe was a large contributor to the slowdown in emissions.
  • The 1992-93 anomaly was caused by fluctuations in wetland emissions and in OH after the Pinatubo eruption
  • The recent slowdown in growth may be temporary

Tropospheric Ozone Trends in 1970-1995 (Fusco and Logan, 2003).

  • Local increases in NOx emissions have caused most of the increases seen in lower tropospheric ozone over Europe and Japan.
  • Increases in CH4 are responsible for about 1/5th of the anthropogenically induced increase in ozone at northern mid-latitudes.
  • Changes in precursors do not explain the spatial differences in observed ozone trends or the observed decreases in ozone over Canada throughout the troposphere.
  • Ozone depletion in the lowermost stratosphere is likely to have reduced the stratospheric source by as much as 30% from the early 1970s to the mid 1990s.

PUBLICATIONS:

Duncan, B. N., J. A. Logan, I. Bey, I. A. Megretskaia, R. M. Yantosca, P. C. Novelli, N. B. Jones, and C. P. Rinsland, The global budget of CO, 1988-1997: Source estimates and validation with a global model, J. Geophys. Res., 112, D22301, doi:10.1029/2007JD008459, 2007. [PDF]

Duncan, B.N., R.V. Martin, A.C. Staudt, R. Yevich, J.A. Logan, Interannual and Seasonal Variability of Biomass Burning Emissions Constrained by Satellite Observations, J. Geophys. Res., 108 (D2), 4040, doi:10.1029/2002JD002378, 2003. [PDF]

Fusco, A.C. and J.A. Logan, Analysis of 1970-1995 Trends in Tropospheric Ozone at Northern Hemisphere Midlatitudes with the GEOS-CHEM Model, J. Geophys. Res., 108(D15), 4449, doi:10.1029/2002JD002742, 2003. [pdf]

Wang, J.S., J.A. Logan, M.B. McElroy, B.N. Duncan, I.A. Megretskaia, and R.M. Yantosca, A 3-D model analysis of the slowdown and interannual variability in the methane growth rate from 1988 to 1997, Global Biogeochem. Cycles, 18, GB3011, doi:10.1029/2003GB002180, 2004. [PDF]

SUPPORT: NSF, NASA