Royal Society - David Keith

Increasing the albedo of maritime cloud by increasing
the number density of cloud-condensation nuclei would
appear to be capable of achieving a radiative forcing
of ~ -4 W/m 2 . The principal implementation strategy being
considered at present is seeding clouds with micro salt
grains from seawater droplets dispersed from unmanned
sea-going vessels. This approach should be compared with
the costs and timescales of more conventional approaches
using crewed ships or the direct release from aircraft of a
suitable hydrophilic powder. Principal concerns are the
potential impact on rainfall patterns over down-wind land
areas and the possible adverse effects of local cooling on
winds and ocean currents. These issues are currently being
explored via computational simulations although current
AOGCM codes may not be adequate for such relatively
fine-scale effects. However, the approach may be useful
in offering extra protection to particularly vulnerable
regions like the Arctic. Conversely, applying a reduction
in insolation in one hemisphere but not the other would be
expected to shift the seasonal range of the inter-tropical
convergence zone (ITCZ) and so modify monsoons, the
potential consequences of which would need to be
examined with extreme care.
Injection of sulphate aerosols into the stratosphere is the
one area of SRM where experimental evidence (provided
by volcanic eruptions) has shown the magnitude of the
reduction in global temperatures that can be expected.
There is not yet agreement on the best delivery
mechanism, but the approach could, if necessary, be
started on a timescale of a few years. However, even a
preliminary exploration of the potential environmental
impacts might take several years. The issues mainly
concern undesirable side effects of which one is the
impact on the ozone layer, while possible adverse impacts
on precipitation patterns are also of considerable concern.
To this should be added a range of feedback processes
that may become important with a continually renewed
stratospheric sulphate layer (as opposed to the transient
effects from a volcanic eruption). Concerns have been
expressed that deployment of stratospheric sulphates
could lead to increased ‘acid rain’ and exacerbate ocean
acidification. The quantities of sulphates added to the
stratosphere would however be extremely small compared
to both those of natural volcanic releases and the acidifying
effect of CO 2 emissions and would therefore not directly
cause any significant increase in the ocean acidification
process.
While an interesting variety of spaced-based strategies has
been proposed, methods advocating placement of a ‘cloud’
of deflectors (or reflectors) at the L1 point seem the most
plausible (with deflectors carrying some type of positioncontrol
mechanism, both to stop them drifting away and,
in an emergency, to disperse some or all of the cluster).
However, the costs of setting in place such a space-based
armada for the relatively short period that SRM
geoengineering may be considered applicable (decades
rather than centuries) would likely make it uncompetitive
with other SRM approaches. As noted in Section 3.3.4,
however, if the duration of application were to change
to centuries, it seems possible that this may then offer
an approach as cheap as any of the geoengineering
alternatives. Moreover, unlike stratospheric aerosols and
cloud brightening techniques, space-based approaches
avoid releasing artificial materials into the atmosphere and
the Earth’s ecosystems (other than those generated by the
launch and manufacturing activity involved).
3.5 Conclusion
The SRM methods may provide a useful tool for reducing
global temperatures rapidly should the need arise.
However, as greenhouse gas concentrations are not
reduced by these methods, the application of any SRM
method would carry with it the termination problem, and
would not address ocean acidification or other CO 2 effects.
The impact of SRM methods on climate is dependent on
where in the atmosphere they are targeted, and their
geographical location, and it should therefore not be
assumed that a zero net global average radiative forcing
means that there are no regional climate effects.
None of the methods assessed are yet ready for
deployment, and all require significant research including
in some cases, pilot scale trials, to establish their potential
effectiveness and effects on climatic parameters including
temperature and precipitation at both the global and
regional scales.
Of the methods assessed the global techniques appear to
be the safest methods for reducing global average
temperature. The early stage of development of the various
space based methods proposed, and their high R&D costs
relative to the other global SRM methods mean that they
are unlikely to be feasible in the medium term.
Stratospheric aerosols therefore appear to be the most
promising as they could be more rapidly developed and
implemented than the space based methods. However,
significant R&D would be required to identify and evaluate
potential impacts on the hydrological cycle, stratospheric
ozone and on the biosphere prior to deployment.
36 I September 2009 I Geoengineering the Climate The Royal Society