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PEEB8
Project Earth Energy Balance
Purpose: To develop a quantitative understanding of the temperature of the
Earth, the warming effect of the atmosphere, the anthropogenic impact on the
atmosphere and Earth-Sun energy balance, and how Earth’s temperature can be
controlled.
The radiative forcing potential of the stratospheric aerosol option
The Earth’s near-surface environment is warming rapidly (IPCC 2007). Arctic sea
ice is disappearing at rates greater than previously observed or predicted (Kerr 2007)
and the southern part of the Greenland ice sheet may be at risk of collapse
(Christoffersen & Hambrey 2006). The oceans are acidifying (Caldeira & Wickett 2003)
and coral reefs and other chemically sensitive marine organisms are at risk (HoeghGuldberg et al. 2007).
Mitigation refers to activities that reduce anthropogenic emissions of greenhouse
gases (particularly CO2). The realization that existing mitigation efforts are proving
wholly ineffectual at the global scale, as evidenced by post-2000 trends in anthropogenic
CO2 emissions (Canadell et al., 2007), has fueled a recent resurgence of interest in
geoengineering (Crutzen, 2006), with a growing number of proposals being aired in the
scientific literature (Boyd, 2008).
Geoengineering is defined as the large-scale engineering of our environment in
order to combat or counteract the effects of changes, either natural or anthropogenic, in
atmospheric chemistry (NAS, 1992). The type of geoengineering we currently practice
is, to put it bluntly, one of mindless convenience sustained by deliberate misinformation,
denial of science, and/or a head-in-the-sand attitude toward the consequences. Some
choose to believe that reductions in greenhouse gases emissions will be sufficient, but
again, CO2 mitigation is simply not being achieved on the scale required. Emission of
CO2 into the atmosphere is increasing more rapidly than foreseen in any of the IPCC
marker scenarios (Raupach et al. 2007), with each release of CO2 producing a warming
that persists for many centuries (Matthews & Caldeira 2008; PEEB3). Atmospheric CO2
content is increasing more rapidly than previously anticipated (Canadell et al. 2007). A
continuation of historical trends in carbon dioxide emission presents, at a minimum, a
risk of significant damage to human systems and/or the near-surface environment of
Earth.
We are now, or soon will be, confronting issues of whether, when, and how to
geoengineer a climate that is more to our liking. It has been suggested that purposeful
climate engineering has the potential to diminish this downside risk (Crutzen 2006;
Wigley 2006). If a decision is made to move ahead with purposeful geoengineering,
those engineering the hardware for a specific climate engineering proposal will be asked
to produce a cost/benefit analysis associated with achieving a certain climate forcing
objective. This requires an understanding of how the climate system would respond to
different kinds of climate forcing.
Among various climate engineering strategies being put forward, one in
particular, stratospheric aerosol injections to diminish incoming solar radiation, or
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insolation, by itself appears capable of countering CO2 radiative forcing. Simulations
have found that deflection of approximately 1.8 per cent of sunlight can offset the global
mean temperature effects of a doubling of atmospheric carbon dioxide content
(Govindasamy & Caldeira 2000; Govindasamy et al. 2002, 2003). From a practical
standpoint, climate change is manifesting most strongly in the Arctic so it has been
made an early target for geoengineering.
Simulation results (Caldeira and Wood, 2008) indicate that insolation reduction
can substantially counter the effects of greenhouse gas warming over a broad range of
measures considering both temperature and water. The model simulations exercise a
standard configuration of the National Center for Atmospheric Research (NCAR)
Community Atmosphere Model, v. 3.1, which includes a finite-volume dynamical core, a
grid that is 28 in latitude by 2.58 in longitude, 26 vertical levels, an interactive land
surface, and a thermodynamic sea ice model (Collins et al. 2006). The land surface
model computes fluxes of energy and water based on plant type and stomatal apertures
adjusted to balance carbon assimilation by photosynthesis and water loss through
evaporation. The sea ice model computes the local thermodynamic balances between
heat fluxes and ice formation and melting, but does not include the movement of sea ice.
All simulations were run for 70 elapsed model years, with the first 40 years being
discarded and the last 30 years being used to compute climate statistics.
For a globally uniform 1.84% reduction in solar insolation, the simulations predict
a reversal of approximately 95% of the global warming (and 127% of the global increase
in precipitation):
Simulated temperature profiles assuming (left) a twice pre-industrial CO2 level of
560 ppm without geoengineering intervention, and (right) a geoengineered
globally uniform 1.84% reduction in solar insolation. This idealized climate
engineering simulation suggests that relatively simple climate engineering can
diminish temperature changes in most of the world.
Insolation reduction in a geographically targeted region has also been simulated.
A comparable global temperature reversal is indicated for the case where insolation
reduction is constrained to arctic latitudes, with a commensurate boosting of the local
insolaton reduction factor:
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Simulated temperature profiles assuming a twice pre-industrial CO2 level of 560
ppm with insolation reduction constrained to latitudes 61° N – 90° N, with
insolation reduction factors of (left) 10% and (right) 50%. The global average
insolation reduction corresponding to the right hand plot is 1.84%, the same as in
the right hand plot in the previous figure.
At high latitudes there is less sunlight deflected per unit albedo change, but
climate system feedbacks operate more powerfully there. These two effects largely
cancel each other, making the global mean temperature response per unit top-ofatmosphere albedo change relatively insensitive to latitude. In this simulation each righthand plot in the above figures corresponds to a total top-of-atmosphere insolation
reduction of 1.84% (or 3.2 petawatts). A linear regression on these results suggests that
restoring September (annual minimum) sea ice extent to its pre-industrial value in a
twice CO2 atmosphere would require reduction of insolation by approximately 21 per
cent over the 2.7 per cent of the Earth that lies north of 71° N.
Change in global annual mean temperature as a function of
percentage of reduction in the top-of-atmosphere insolation (0.73%
or 1.84%). Despite large differences in the spatial extent of the
insolation reduction (global, or arctic above latitude 61° or 71° N),
the global mean temperature response is similar.
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Simulated arctic sea ice extent corresponding to (left-to-right) pre-industrial CO2
(280ppm), twice pre-industrial CO2 (580 ppm), twice CO2 with a global insolation
reduction of 1.84%, and twice CO2 with 50% insolation reduction above 61°.
In the simulations, insolation reduction is accomplished by modifying the code to
allow for different spectrally neutral reductions in incoming solar radiation in different
latitude bands. In practice, small reflective particles appear to be the most effective form
for climate engineering. The particles of greatest interest have dimensions of the order of
the wavelength of the optical radiation to be scattered. Emplacement of sub-microscopic
particles in the stratosphere, for example in the polar stratosphere, has the practical
advantage of residence times exceeding a year – long enough to allow economically
efficient climate engineering while short enough to provide reversibility should
unintended consequences prove greater than anticipated.
In terms of material, resonant scattering materials are superior to metals, which
in turn are advantaged over dielectrics for engineered scatterers (Teller et al. 1997).
However, the relative photochemical inertness of selected dielectric materials,
repeatedly demonstrated at scale in major volcanic eruptions involving extensive
particulate mass insertion into the stratosphere, motivates their initial selection in order
to minimize first-time risks of unwanted side effects (Teller et al. 1997, 1999, 2004; Hyde
et al. 2003), including significant interactions with stratospheric ozone.
The near-ultraviolet and near-infrared spectral bands contain roughly half of the
total insolation in energetic terms. These wavelengths may be largely superfluous (or
actually deleterious, in the case of the shorter wavelength ultraviolet) for biospheric
purposes, and thus portions of these spectra may be attractive candidates for being
scattered back into space by an engineered scattering system (which can be designed
to have considerable spectral selectivity). For example, the use of Rayleigh scattering to
preferentially scatter back into space an appropriate fraction of the deeper ultraviolet
portion of insolation appears to be a relatively appealing approach, since a usefully large
portion of total insolation is available for attenuation and this solar spectral band’s
radiation appears to be net damaging to the biosphere: exposure to UV-B and UV-C
insolation is deleterious to both plants and animals, primarily due to photodamage of
their DNA. Indeed, the World Health Organization estimates approximately 60,000
human deaths occur annually due to sunlight-engendered skin cancer, which is
generally believed to be due rather exclusively to the UV-B spectral component of
insolation (WHO press statement of 26 July 2006 issued by Dr Maria Neira, WHO Public
Health and Environment Director; http:// www.who.int/uv). Associated direct economic
losses may significantly exceed $10 billion per year, and economic impacts of crop
damage may be of comparable scale. Thus, there is potential for geoengineering to
diminish risks to both climate and cancer, as well as to avoid substantial direct economic
costs in sectors ranging from agriculture to public health. The sky may become
discernibly bluer, or redder, by design.
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Among dielectrics, many alternatives have been proposed (e.g. NAS 1992) and
all appear to be fundamentally workable. Liquid SO2 (or perhaps SO3) appears to be
optimized for mass efficiency, transport convenience and relative non-interference with
all known processes of substantial biospheric significance, although fluidized forms of
MgO, Al2O3 or SiO2 (e.g. as hydroxides in water) seem competitive in most respects.
Amounts presently considered for stratospheric injection are of the order of 1 per cent of
the SO2 annual mass injection into the troposphere by all processes, which are roughly
half each of natural and anthropogenic origin, so that the eventual descent of
stratospheric sulphate particulates into the troposphere will add negligibly to the globally
averaged levels, although somewhat greater fractional increases may be expected at
high latitudes. With respect to prospective impact on the ozone layer, Crutzen (2006)
estimates the probable magnitude of geoengineering-contemplated stratospheric
injections of sulphate particulate to be less than that of the Mt. Pinatubo eruption. Any
scheme would need to take into consideration particle aggregation and interaction with
water and other compounds found in the stratosphere.
For PEEB-8, provide brief answers to these questions.
1. Do you think the Maldives can be saved by mitigation alone, i.e. reducing the positive
radiative forcing of greenhouse gasses by reducing emissions of such gasses? Recall
that Malé, the capital city of the Maldives, is three feet above sea level, and that sea
level is rising at a rapidly accelerating rate, currently 3.5mm per year.
2. If your answer to the above is yes, explain your approach. If no, do you think the
human race should take deliberate control of Earth’s temperature, i.e. embrace designed
geoengineering to counter the unwitting geoengineering we already are engaged in?
3. Critique the SO2 stratospheric aerosol approach to cooling the planet. Can it, by
itself, save the Madives? Relative to current SO2 emissions: Is it polluting? Is it
dangerous to human health? Is it harmful to the ozone layer? How practical (expensive)
is it? Could it be funded by a rich country? A poor country? A wealthy individual?
4. Presuming the United States does not take a leadership role in climate management,
do you think China or some other rogue nation or individual will unilaterally decide to
modify the climate (suppose, e.g., water stops flowing out of the Himalayas, or droughtinduced famine stirs unrest in Africa or the Middle East.)
In the above, consider the likelihood that jumping the chasm is more attuned to human
nature than putting on the brakes – just consider your reaction when your parent nags
you to turn off the lights. Isn’t the better solution to install renewable energy solar
panels, even though it costs money in the short term? You might also review the
reaction of the Greek people to austerity measures in the face of their financial crisis.
In short, is climate change better solved by cooperative application of modern
technologies than by international measures focused on prohibition?
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References
Boyd, P. W.: Ranking geo-engineering schemes, Nat. Geosci., 1, 722–724, 2008.
Caldeira, K. & Wickett, M. E. 2003 Anthropogenic carbon and ocean pH. Nature 425, 365.
(doi:10. 1038/425365a).
Caldeira, K. and Wood, L.: Global and Arctic climate engineering: numerical model studies,
Philosop. T. R. Soc. A, 366, 4039– 4056, 2008.
Canadell, J. G. et al. 2007 Contributions to accelerating atmospheric CO2 growth from economic
activity, carbon intensity, and efficiency of natural sinks. Proc. Natl Acad. Sci. USA 104, 10 288–
10 293. (doi:10.1073/pnas.0702737104) 0702737104v1.
Christoffersen, P. & Hambrey, M. J. 2006 Is the Greenland Ice Sheet in a state of collapse? Geol.
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Collins, W. D. et al. 2006 The formulation and atmospheric simulation of the Community
Atmosphere Model version 3 (CAM3). J. Clim. 19, 2144–2161. (doi:10.1175/JCLI3760.1).
Crutzen, P. 2006 Albedo enhancement by stratospheric sulfur injections: a contribution to resolve
a policy dilemma?. Clim. Change 77, 211–219. (doi:10.1007/s10584-006-9101-y).
Govindasamy, B., Thompson, S., Duffy, P., Caldeira, K. & Delire, C. 2002 Impact of
geoengineering schemes on the terrestrial biosphere. Geophys. Res. Lett. 29, 2061. (doi:10.
1029/2002GL015911).
Govindasamy, B., Caldeira, K. & Duffy, P. B. 2003 Geoengineering Earth’s radiation balance to
mitigate climate change from a quadrupling of CO2. Global Planet. Change 37, 157–168. (doi:10.
1016/S0921-8181(02)00195-9).
Hoegh-Guldberg, O. et al. 2007 Coral reefs under rapid climate change and ocean acidification.
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Raupach, M. R., Marland, G., Ciais, P., Le Quere, C., Canadell, J. G., Klepper, G. & Field, C. B.
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