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Transcript
Radiative Forcing of
Climate Change:
Expanding the Concept and
Addressing Uncertainties
Report from the NRC Committee
on Radiative Forcing of Climate
released December 2004
Committee on Radiative Forcing of Climate
DANIEL J. JACOB (Chair), Harvard University
RONI AVISSAR, Duke University,
GERARD C. BOND, Lamont-Doherty Earth Observatory
STUART GAFFIN, Columbia University
JEFFREY T. KIEHL, National Center for Atmospheric Research
JUDITH L. LEAN, Naval Research Laboratory
ULRIKE LOHMANN, Dalhousie University
MICHAEL E. MANN, University of Virginia
ROGER A. PIELKE, SR., Colorado State University
VEERABHADRAN RAMANATHAN, Scripps Institution of
Oceanography
LYNN M. RUSSELL, Scripps Institution of Oceanography
Conceptual framework of climate forcing,
response, and feedbacks
NATURAL
PROCESSES
Sun, orbit, volcanoes
HUMAN
ACTIVITIES
Fuel, industry,
agriculture…
Societal
Impacts
CLIMATE FORCING AGENTS
 Emissions of greenhouse gases and
precursors, aerosols and precursors, and
biogeochemically active gases
 Solar irradiance and insolation changes
 Land-cover changes
Non-radiative
CHANGE IN CLIMATE
Forcing
SYSTEM COMPONENTS
 Atm. lapse rate
Direct
 Atm. composition
Radiative
 Evapotranspiration
Forcing
Indirect
Radiative
Forcing
Feedbacks
CLIMATE RESPONSE
Temperature, precipitation, vegetation, etc.
Climate change research and policy has relied on
global top-of-atmosphere (TOA) radiative forcing
TOA forcing from 1750
to present [IPCC, 2001]
Strengths and limitations
of TOA radiative forcing concept
Strengths
 Linearly related to equilibrium
change in global mean surface
temperaure in GCMs
 Simple, physical, robust, easy to
compute
 Enables comparison of different
forcing agents
 Enables comparison of different
models
 Has established use in policy
analysis
 Directly observable from space
 Inferable from observed changes
in ocean heat content
Limitations
 Does not account for vertical
structure of forcing
 Does not characterize regional
response
 Does not characterize
hydrological response
 Does not accommodate
nonlinear response from large
perturbations
The TOA radiative forcing concept should be retained and expanded
Account for vertical structure of radiative forcing




The relationship between TOA radiative forcing and surface temperature is
affected by the vertical distribution of forcing within the atmosphere,
particularly for absorbing aerosols and for land-use driven changes in
evapotranspiration.
Aerosol radiative forcing
Priority recommendations:
Test ability of climate models to
reproduce observed vertical
structure of forcing;
Characterize dependence
of climate response on the
vertical structure of radiative
forcing;.
Report global mean radiative
forcing at both the surface and
the TOA in climate change
assessments;
.Develop practical tools for
incorporating surface radiative
forcing in policy analyses and
integrated assessment models.
Determine the importance of
regional variation in radiative forcing
Regional variations in radiative forcing may have important regional and global
climatic implications that are not resolved by the concept of global mean radiative
forcing.
Priority recommendations:
 Quantify and compare climate
responses from regional radiative
forcings in different climate models,
and report results in climate change
assessments;
 Use climate records to investigate
relationships between regional
radiative forcing and climate response.
JJA forcing by tropospheric ozone
Determine the importance of non-radiative forcings
Several types of forcings—most notably
aerosols, land-use and land-cover change,
and modifications to biogeochemistry—
impact the climate system in non-radiative
ways, in particular by modifying the
hydrological cycle and vegetation dynamics.
Historical changes in land cover
1700
(a)
Priority recommendations:
 Improve understanding and
parameterizations of aerosol-cloud
thermodynamic interactions and
land-atmosphere interactions in
climate models;
(b)
1900
 Develop improved land-use and
land-cover classifications at high
resolution for the past and present,
as well as scenarios for the future.
(c)
1990
Addressing Key Uncertainties
1. Conduct accurate long-term monitoring of radiative forcing variables
Spectrally resolved radiances from space, ocean heat content…
2. Advance the attribution of decadal to centennial climate change
Mine climate forcing and trend records for past 1000 years
3, Reduce uncertainties associated with indirect aerosol radiative forcing
Improve parameterizations used in models
4, Better quantify the direct radiative effects of aerosols
Improve understanding of aerosol sources, mixing states, sinks
5. Better quantify radiative forcing by ozone
Improve understanding of strat-trop exchange, lightning NOx
6, Integrate climate forcing criteria in environmental policy analysis
Examine climate impacts of policies directed at air quality and land use,
develop GWPs for short-lived agents, determine additivities of forcings,,,