Royal Society - David Keith

These model experiments (also discussed above) are
designed such that the reduction in absorbed solar
radiation exactly balances the radiative forcing due to
the increased concentration of greenhouse gases. The
resulting climate is compared with that of a world of preindustrial
CO 2 concentrations and no reduction in sunlight.
It is found that the temperature of the air near the surface
is substantially less affected in the geoengineered world
than in the non-geoengineered case but, nevertheless, is
slightly cooler in tropical regions, where the solar effect
dominates, and warmer at high latitudes, where the
greenhouse trapping is greater. The weaker latitudinal
temperature gradient affects other climate parameters.
For example the amplitude of the seasonal cycle is
reduced, giving warmer winters and cooler summers.
The cooler tropics result in less evaporation and a generally
drier atmosphere with less precipitation. One model study
(Lunt et al. 2008), which included fully coupled ocean
circulations, also shows a decreased intensity of El Niño
events, with concomitant impacts on tropical climate, in
particular tending to enhance overall precipitation over
south-east Asia and India.
Even though the global radiation balance is the same for
both, the pre-industrial and geoengineered simulations
show significant regional and temporal differences.
Nevertheless, these differences are small compared to
those associated with a non-geoengineered future.
There are numerous and considerable uncertainties
involved in most aspects of the proposed space-based
SRM methods and all these would need to be addressed
by detailed research before any method might be deemed
potentially fit for purpose. Apart from the development
of necessary technology, and the solution of problems
concerning its implementation and maintenance in space,
the research would need to investigate carefully the
potential impacts on the climate system. All the spacebased
SRM methods propose to reduce the total amount
of solar energy entering the atmosphere but each affects
the incoming solar beam differently. Reflectors at the L1
point would essentially have the effect of reducing the
solar constant and initial studies of the impact of this,
within in a high CO 2 atmosphere, have already been carried
out, as outlined above. More detailed assessments would
investigate the impact on regional meteorology,
temperature and precipitation patterns, including any
changes in seasonality and variability, and also impacts
on polar ice cover and ocean circulations. Reflectors in low
Earth orbit would redistribute solar radiation in far more
complex ways which would each need to be carefully
determined, even before any model assessment could be
made of their climate impact. While such studies would be
able to give some indication of the potential impacts of the
space-based SRM methods all would be subject to the
caveats expressed in Box 1.2 concerning the limitations
of climate models.
All of the space-based techniques summarised in this
section (see Table 3.5) contain such great uncertainties
in costs, effectiveness (including risks) and timescales
of implementation that they are not realistic potential
contributors to short-term, temporary measures for
avoiding dangerous climate change. This is not to dismiss
them from future consideration, however. If, in the future
it became probable that some form of geoengineering
would be needed for a period approaching a century or
longer, on such a timescale (and with the continual
advance of technical capabilities) it is quite possible that
the best examples of this type may offer a cheaper and
less risky approach to SRM than any of the stratospheric
or near-Earth techniques. With launch costs to near-Earth
orbit approaching a few $ M per tonne (http://www.
thespacereview.com/article/233/1), it may eventually be
possible to place a one Megatonne system into high orbit
at a price of a few $ trillion, of the same order as that for
some of the other proposals considered, and potentially
with a much longer lifetime. Some designs proposed for
the L1-based systems have masses considerably lower
than a Megaton, so even at current launch costs; the cost
of launch could conceivably be smaller than the cost of
other SRM techniques (Keith 2000). However at these
rates, the costs of placing billions of tonnes of material
(eg dust) into orbit would be prohibitive. Desk-based
engineering design studies could advance understanding
of the likely feasibility and costs of such proposals
Table 3.5. Summary evaluation table for space-based methods
Space-based methods
Effectiveness
Affordability
Timeliness
Safety
No inherent limit to effect on global temperatures
SRM method so does nothing to counter ocean acidification
High cost of initial deployment (depends on mass required): plus additional
operational costs (eg maintaining positions): but long lifetime once deployed
Would take several decades (at least) to put reflectors into space
Once in place, reflectors would reduce global temperatures within a few years
Residual regional climate effects, particularly on hydrological cycle
No known direct biochemical effects on environment beyond possible effects of
reduced insolation
High
Very low
to Low
Very low
Medium
The Royal Society
Geoengineering the Climate I September 2009 I 33