The scientific rationale for CALIPSO: a review and update for 2006
Robert J. Charlson, University of Washington
The Stefan-Boltzmann law in physics (SB) dictates that changes in the flux of radiation in/out of
a planet (forcings, W/m2) must cause changes in its so-called emission temperature, Te . In turn,
the average temperature of the Earth’s surface, Tsfc, which is a primary index of climate, is
generally thought to be linearly related to Te. In the case of the Earth, a change of only 1 Wm-2
out of the roughly 240 Wm-2 area-averaged flux (in and out) is calculated by SB to change Tsfc by
0.25 K. Instrumental and proxy records of Tsfc (e.g., the H and O isotope variations in ancient ice
cores and ocean sediments) show that the climate has been stable to within ca. +/- 1 K or less
over the past ca. 104 years. Tsfc has increased measurably (ca. 0.6 K) over the industrial era. .
Questions thus arise about the cause of the long-term stability and the magnitude of forcing
responsible for the recent warming. What portion of the observed industrial-era temperature
increase is outside the range of natural fluctuations? Is the pre-industrial stability the result of a
lack of any significant forcings or is it the result of stabilizing feedbacks such as those that might
be involved in the radiative effects of clouds? In principle, the first of these questions can be
addressed by quantification of all of the changes in energy balance that have occurred over the
past several decades of observed temperature increase. The second question regarding long-term
stability can be partly addressed by quantification of today’s feedbacks. But, inasmuch as there
are no proxy records for feedbacks over the last several millennia (e.g., there are no paleo-records
of clouds), the second question cannot be addressed in toto.
There is no question that the radiative balance of Earth has been modified over the last several
decades by anthropogenic changes in the atmospheric content of so-called greenhouse gases
(notably CO2, CH4, etc.), and there is no doubt that this has imposed a change in energy balance
or climate forcing at the top of the atmosphere of 2.4+/- 0.2 Wm-2. If that were the only change
in energy balance, it would neatly explain the observed warming of ca. 0.6 K.
However, there are other changes also known to be imposed on the energy balance (i.e., forcings)
that must be taken into account. These include:
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Changes in the flux of energy from the sun
Direct forcing due to reflection and absorption of sunlight by natural and anthropogenic
aerosols
Indirect forcing due to the multiple influences of such aerosols on the radiative effects of
clouds
Feedbacks that influence radiative balance such as the effects of changes of Tsfc on water
vapor and clouds (e.g., via enhanced evaporation of water from the Earth’s surface).
The last three of these involve changes in the global average reflectivity of Earth, the planetary
albedo, which has been recently recognized as one of the least-well-known properties of Earth.
The latter two involve clouds which influence long-wave emission by the planet as well as the
albedo. Inasmuch as CALIPSO will have en exquisite ability to detect and map clouds, it will
also allow refinement of studies of long-wave emission.
Quantitative estimates of the latter three items have been obtained primarily from global climate
models which are themselves known to be uncertain. Current estimates of climate forcing by
anthropogenic aerosols (direct plus indirect) approach -1 Wm-2 or more, such that it is not yet
possible to rule out the possibility that the net forcing (i.e., the sum of greenhouse gas plus
aerosol forcings) may be small (implying large climate sensitivity). Among the objectives of
CALIPSO and other satellites in the A-Train is to develop an observational basis for estimating
the aerosol forcings and the albedo-related feedbacks as an alternative to the sole dependence on
models. Satellites offer the only practical means for global observations - surface-based or
airborne observations, for example, cannot feasibly provide the necessary global coverage.
The primary instrument aboard CALIPSO is a two wavelength, polarization-sensitive lidar
designed to sense and quantify aerosols, thin cloud layers and the reflectance of thicker cloud
tops. The jointly launched CloudSat radar will sense and characterize thicker clouds. Together,
CALIPSO and CloudSat will augment the wealth of aerosol, cloud, and energy-balance data from
the A-Train by providing extremely accurate measurements of the vertical location of aerosol and
cloud layers. In addressing the above scientific questions, these new data will be used:
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To develop new and more physically realistic ways to quantitatively separate clear and
cloudy skies.
In clear skies, to test and refine knowledge of aerosol attenuation of the direct solar beam,
quantified as the aerosol optical depth.
To test, refine and map the efficiency of the aerosol, per unit optical depth, at changing
Earth albedo and energy balance.
To test, refine and map the magnitude of total direct forcing by aerosols.
To attribute this observationally based quantification of the direct forcing
to the various components of the aerosol using chemical transport models and
chemical sampling in an in-situ underflight program.
To assess, in regions of partial to complete cloud cover, the indirect influence of aerosols
on clouds, and
To explore for the first time at a global scale (a) the transition between aerosols and
clouds and (b) possible feedbacks involving Tsfc , water vapor, and cloud albedo.
In addition to these issues of climate science, and because of the altitude-resolving capabilities of
the lidar aboard CALIPSO, the vertical-profile information will be used:
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To globally and accurately locate and quantify the relative amounts of low and high
clouds; noting that low clouds contribute primarily to albedo while high clouds contribute
to both albedo and the greenhouse effect, and
To globally quantify the aerosol optical depth above clouds, and
With regard to the aerosol indirect effect on clouds, to ascertain whether aerosol and cloud
layers are located in the same atmospheric strata such that they are subject to the same
thermodynamic and chemical conditions and can interact, and
To determine the height of aloft aerosol layers which is a critical determinant of global
aerosol transport and dispersion.
The above presently pose substantial uncertainties in both climate and air quality models.
Given that the climate forcing by anthropogenic aerosols has been demonstrated to be comparable
in magnitude but opposite in sign to the climate forcing by anthropogenic greenhouse gases,
considerable importance is attached to successfully reducing the uncertainty in the overall effects
of aerosols on albedo. Research using CALIPSO and A-Train measurements has the potential for
greatly improving quantitative knowledge of aerosol climate forcings which, in turn, would
improve understanding of climate sensitivity. Moreover, these same data should help quantify
the role of clouds in amplifying and/or damping forced climate change.