I. Earth’s radiation budget
Total solar irradiance: instantaneous radiation received by a
plane perpendicular to Sun’s radiation, at top of Earth’s
atmosphere (~1365 W/m2)
Albedo: fraction of solar radiation reflected by Earth back to
space (~0.3)
Effective radiating temperature: temperature that Earth needs
to attain to re-emit solar radiation back to space according
to Stefan-Boltzmann Law (steady state)
Greenhouse effect: atmospheric trapping of longwave radiation
that causes ERT to be in the upper atmosphere, and
surface temperatures to be much higher
Insolation: solar radiation (W/m2) striking a plane horizontal at
the top of Earth’s atmosphere, averaged over a day; varies
with latitude and season
(NASA - Earth Radiation Budget Experiment)
II. Radiative forcings
Factors that may change on their own (external forcings) or
in response to climate changes (feedbacks) to affect Earth’s
climate
A. Orbital forcing (5.2.1.1)
Slow changes in Earth’s orbit affect the distribution of
insolation
(after Berger and Loutre, 1991)
B. Solar forcing (5.2.1.2)
TSI and SSI vary cyclically
(Yohkoh, NOAA/HAO)
(Solar Dynamics Observatory)
(Steinhilber et al., 2009)
C. Volcanic forcing (5.2.1.3)
stratospheric sulfate increases albedo
(NASA GISS)
(Gao et al., 2008)
D. Greenhouse gases (5.2.2.1-2)
• may be ‘external’ or feedback
• benchmark of 2X CO2 equivalent
• direct radiative impact ~4 W/m2
• equilibrium climate response (sensitivity) depends on other
feedbacks
• most IPCC GCMs: ~3 1C warming
(Lüthi et al., 2008)
(IPCC4)
E. Dust loadings (5.2.2.3)
mineral dust aerosols mainly increase albedo
(Martinez-Garcia et al., 2011)
III. Other climate feedbacks
• water vapor (+) due to enhanced evaporation & GH effect
• low clouds (-?) due to increased albedo
• high clouds (+?) due to IR absorbance
• ice (+) due to melting and decreased albedo
• land vegetation (?) albedo, evapotranspiration