PEEB7
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.
Geoengineering can be defined as large-scale engineering of our environment in
order to combat or counteract the effects of changes in atmospheric chemistry, in particular
to mediate the effects of elevated greenhouse gas concentrations, especially C02. In
contrast, mitigation refers to activities that reduce anthropogenic emissions of greenhouse
gases. The realization that existing mitigation efforts are proving wholly ineffectual on the
global scale, as evidenced by post-2000 trends in anthropogenic C02 emissions, has fuelled
a resurgence of interest in geoengineering, with a growing number of proposals being aired
in the scientific literature. The proposed geoengineering remedies vary from counteracting
the radiative forcing due to anthropogenic greenhouse gas emissions, to enhancing the
capture and reabosprption of greenhouse gasses.
Already, people living in the Maldives and Marshall Islands (capital cities: about three
feet above sea level) view human control of air and ocean temperature as a matter of
survival.* People not directly threatened by rising sea level are not necessarily sympathetic
to the plight of coastal dwellers. Either way, accurate quantifications of potential climate
cooling solutions is crucial. Yet there is a shortage of these, especially for carbon cycle
geoengineering proposals, where most efforts stop at quantifying an effect on atmospheric
C02, if indeed they get that far.
Your task is to quantify, using the common currency of Wm-2, the climate
cooling potential of the geoengineering proposals discussed in a paper by Lenton and
Vaughn (see my website). Temperature is not the only criterion against which these
proposals should be judged, but that is our primary focus here. Some highlights and two
figures from this paper are summarized below. Rank the potential effectiveness of the
various approaches, and on what time horizon, specifically with regard to temperature
control. What is your informed opinion as to which, either alone or in combination, is
capable of countering a projected radiative forcing of 3.7 Wm-2 (estimated for a
doubling of CO2 to 560 ppm). Which, if any, are potentially capable of saving the
Maldives? Again, the capital city of the Maldives is about three feet above sea level, and
sea level is currently rising about 3.5 mm per year. Which if any, are practical?
The warming surface temperature of Earth** results from the imbalance of incoming
solar (shortwave) radiation vs. outgoing terrestrial (longwave) radiation. The aim of the
geoengineering approaches described in Lenton and Vaughn is to induce a negative
radiative forcing to counter this warming trend. Temperature geoengineering is simply the
deliberate rebalancing of the two-way the photon traffic, by decreasing the shortwave
radiation absorption by Earth, and/or by increasing the longwave radiation escaping Earth.
These options can be further subdivided:
A. Shortwave radiation options
1. Increased reflection of solar radiation
a. Above-atmosphere sunshades to reflect a fraction of incoming solar
radiation using mirrors at the inner Lagrange point (L1) (Angel, 2006), or in an Earth orbit
(NAS, 1992; Pearson et al., 2006).
b. Increased reflection of solar radiation (albedo) within the atmosphere using
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sulphates (Crutzen, 2006), soot, or manufactured particles (Teller et al., 1997).
c. Increased reflectivity of low level marine stratiform clouds by mechanical
(Latham, 1990) or biological (Wingenter et al., 2007) generation of cloud condensation
nuclei.
d. Increased reflection of solar radiation (albedo) at the surface, including
albedo modification of deserts (Gaskill, 2004), grasslands (Hamwey, 2007), croplands
(Ridgwell et al., 2009), human settlements (Hamwey, 2007), and urban areas (Akbari et al.,
2009).
B. Longwave radiation options
1. Removal of C02, from the atmosphere via land carbon sinks
2. Removal of C02, from the atmosphere via ocean carbon sinks
Feel free to use the above outline as a template for your reflections. Even
though adding sulphate aerosols to the stratosphere (NAS, 1992) has been criticized due to
negative impacts on human health, include it in your comparison.
Schematic overview of climate geoengineering proposals. The arrowheads indicate:
Black – shortwave radiation
White – enhanced natural flows of carbon
Grey – engineered flow of carbon
Grey upward – engineered flow of water
Dotted vertical – sources of cloud condensation nuclei
Dashed boxes – carbon stores
Lenton and Vaughn (2009)
C. Brucker
Summary of estimates for the radiative forcing potential of climate geoengineering options.
Note the logarithmic horizontal scale. The potential of longwave (CO2 removal) options is
given on three different time horizons, assuming a baseline strong mitigation scenario: CO 2
450 ppm in 2050 and 500ppm in 2100. The rightward pointing arrows, which refer to mirrors
in space, stratospheric aerosols, and air capture and storage on the year 3000 timescale,
indicate that their potential could be greater than suggested by the diamonds (radiative
-2
forcing to be counteracted: 3.71 Wm due to 2CO2 = 556 ppm for the shortwave options,
-2
and 1.43 Wm due to 363 ppm CO2 in the year 3000 under the strong mitigation scenario.
Lenton and Vaughn (2009)
Footnotes
*
Extra credit opportunity: Establish a Skype connection between a high school
science or math classroom in the Maldives and your own classroom. The Maldives are 13
hours ahead of California time: 9:00pm Friday in Maldives is 8:00am Friday in California; or,
9am Friday in Maldives is 8pm Thursday in California. Purpose: to hear first hand how
these young people view the future of their nation. Note: The Maldivian President,
Mohamed Nasheed, is buying land in Australia in anticipation of a climate-forced mass
migration.
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**
For perspective, I note that when I was your age the concern was global cooling.
Alarm bells had rung because the average ground temperature had fallen by 0.5ºF from 1945
to 1968, taking the planet about a sixth of the way toward the last Ice Age average.
Scientists proposed radical warming solutions such as melting the artic ice cap by covering it
with black soot. The cooling is now thought to be the result of radiative forcing by
atmospheric pollutants from post-World War II industrialization, which was countered by
cleaning up smokestacks in the western world, primarily for reasons of human health.
Sulfate particles, a large component of the polluting emissions, are white and reflecting.
Surprisingly, black soot particles injected into the stratosphere can have an overall effect
opposite to that of soot particles on the ground - even though some solar radiation is
absorbed, the enhanced escape of infrared photons from that altitude can have a net cooling
effect. This is the origin of the “nuclear winter” postulated to be one consequence of a
thermonuclear war.
References
Angel, R.: Feasibility of cooling the earth with a cloud of small
spacecraft near the inner Lagrange point (l1), P. Natl. Acad. Sci.
USA, 103, 17184–17189, 2006.
Lenton, T. M. and Vaughan, N. E., The Radiative Forcing Potential of Different Climate
Geoengineering Options, Atmos. Chem. Phys., 9, 5539–5561, 2009.
C. Brucker