Scientific Colloquium
February 13, 2009
STEPHEN
SCHWARTZ
BROOKHAVEN NATIONAL LABORATORY
"Consider a Spherical
Earth: Heat Capacity, Time Constant and sensitivity of Earth's Climate
System"
Despite decades of intense
research the equilibrium sensitivity of Earth's climate system, the
amount by which the global mean surface temperature would change in
response to a sustained change in a radiative flux component, remains
uncertain to more than a factor of 2. Earth's climate system is a
balance between incoming shortwave (solar) radiation and outgoing
longwave (thermal infrared) radiation. Consequently any changes in
Earth's temperature or heat content must be due to an imbalance between
these two energy fluxes. From energy balance considerations the
equilibrium sensitivity of such an isolated system is equal to the
quotient of the relaxation time constant of the system and the
pertinent heat capacity. These considerations are applied to Earth's
climate system to provide an independent empirical determination of
Earth's climate sensitivity. Observational data are used to determine
the heat capacity of the global ocean from regression of ocean heat
content vs. global mean surface temperature, GMST, is 14 ± 6 W
yr m-2 K-1, equivalent to 110 m of ocean water; other sinks raise the
effective planetary heat capacity to 17 ± 7 W yr m-2 K-1 (all
uncertainties are 1-sigma estimates). The time constant pertinent to
changes in GMST is determined from autocorrelation of that quantity
over 1880-2004 to be 8.5 ± 2.5 yr. The resultant equilibrium
climate sensitivity, 0.51 ± 0.26 K/(W m-2), corresponds to an
equilibrium temperature increase for doubled CO2 of 1.9 ± 1.0 K,
somewhat lower than the central estimate of the sensitivity given in
the 2007 Assessment Report of the Intergovernmental Panel on Climate
Change, but consistent within the uncertainties of both estimates. The
short climate system time constant implies that global mean surface
temperature is in near equilibrium with the applied forcing. Forcing
over the twentieth century other than that due to greenhouse gases,
ascribed mainly to tropospheric aerosols, is estimated as -1.1 ±
0.7 W m-2.