A Combined Mitigation/Geoengineering Approach to Climate Stabilization
T. M. L. Wigley
National Center for Atmospheric Research, Post Office Box 3000, Boulder, CO 80307–3000, USA.
Projected anthropogenic warming and CO2 concentration
increases present a two-fold threat: both from the climate
changes, and from CO2 directly through increasing
acidity of the oceans. Future climate change may be
reduced through mitigation (greenhouse-gas emissions
reductions) or through geoengineering. Most
geoengineering approaches, however, do not address the
problem of increasing ocean acidity. A combined
mitigation/geoengineering strategy could remove this
deficiency. We consider here the deliberate injection of
sulfate aerosol precursors into the stratosphere. This can
significantly offset future warming and provide additional
time to reduce dependence on fossil fuels and so stabilize
CO2 concentrations cost-effectively at an acceptable level.
In the absence of policies to reduce the magnitude of future
climate change, the globe is expected to warm by
approximately 1–6°C over the 21st century (1, 2). Estimated
CO2 concentrations in 2100 lie in the range from 540 ppm to
970 ppm, sufficient to cause substantial increases in ocean
acidity (3–6). Mitigation directed towards stabilizing
concentrations (7) addresses both problems; but presents
considerable economic and technological challenges (8, 9).
could help reduce future climate
change, but does not address the ocean acidity problem.
Mitigation is therefore necessary, but geoengineering could
provide additional time to address the economic and
technological challenges faced by a mitigation-only approach.
The geoengineering strategy examined here is the injection
of aerosol or aerosol precursors (such as sulfur dioxide, SO2)
into the stratosphere to provide a negative forcing of the
climate system and so offset part of the positive forcing due
to increasing greenhouse-gas concentrations (18). Volcanic
eruptions provide ideal experiments that can be used to assess
the effects of large anthropogenic emissions of SO2 on
stratospheric aerosols and climate. We know, for example,
that an eruption like that of Mt. Pinatubo [June 1991 (19, 20)]
caused detectible short-term cooling (19–21), but did not
seriously disrupt the climate system. Deliberately adding
aerosols or aerosol precursors to the stratosphere so that the
loading is similar to the maximum loading from Pinatubo
present minimal climate risks.
Increased sulfate aerosol loading of the stratosphere may
present other risks, such as through the influence on
stratospheric ozone. This particular risk, however, is likely to
be small. The effect of sulfate aerosols depends on the
chlorine loading (22–24).
With current elevated loadings
there would be enhanced ozone loss. This would delay the
recovery of stratospheric ozone slightly, but only until
anthropogenic chlorine loadings returned to 1980 levels
(expected by the late 2040s).
Figure 1 shows the effect of multiple sequential eruptions
of Pinatubo, every year, every two years and every four years.
The Pinatubo forcing used here has a peak annual-mean value
of –2.97 W/m2 (20, 21).
The climate simulations were carried
out using an upwelling-diffusion, energy balance model
25, 26)] with a chosen climate sensitivity
3°C equilibrium warming for a CO2 doubling. Figure 1
suggests that a sustained stratospheric forcing of around –3
W/m2 (the average asymptotic forcing for the biennial
eruption case) would be sufficient to offset much of the
anthropogenic warming expected over the next century.
Figure 1 also shows how rapidly the aerosol-induced cooling
disappears once the injection of material into the stratosphere
stops, as might become necessary should unexpected
environmental damages arise.
To illustrate possible options for the timing and duration of
aerosol injections, three scenarios are considered. In each
case, the loading of the stratosphere begins in 2010 and ramps
up linearly to –3 W/m2 over 30 years. The scenarios depart
from each other after this date (Fig. 2). These geoengineering
cases are complemented by three future CO2 emissions
scenarios: a central “no-climate-policy” scenario from the
set, viz. the A1B scenario; an ambitious CO2
stabilization scenario stabilizing at 450 ppm (the present level
is about 380 ppm), WRE450 (7); and an overshoot CO2
concentration case rising to 530 ppm in 2080 before declining
to 450 ppm. (Note that even 450 ppm “produces both calcite
and aragonite undersaturation in most of the deep ocean” (4),
so a level even less than this may ultimately be desirable.)
CO2 concentrations and corresponding fossil-fuel
emissions for these three CO2 cases are shown in Fig. 3.
Emissions for the stabilization cases were calculated using an
inverse version of MAGICC, accounting for climate
on the carbon cycle. The WRE450 case is an
archetypal mitigation-only case, stabilizing at a level that
many believe would avoid “dangerous anthropogenic
interference” with the climate system (28). The overshoot
case is introduced here to be considered in conjunction with
the three geoengineering options. The overshoot case allows
much larger CO2 emissions, and a much slower departure
from the A1B no-policy baseline. Although the rate of decline
of emissions in the mid to late 21st century is more rapid than
in WRE450, these reductions begin 15–20 years later
allowing additional time both to phase out existing CO2-
emitting fossil-fuel energy technologies and to develop and
deploy energy sources that have net-zero CO2 emissions (7–
Figure 4 shows global-mean temperature and sea level
projections for (i) the no-policy (A1B) case, (ii) the
mitigation-only (WRE450) case, and (iii) the overshoot CO2
case combined with the three alternative geoengineering
options (labeled “HIGH GEO”, “MID GEO” and “LOW
GEO”). For the early decades after 2010, changes in aerosol
forcing in all three “GEO” scenarios occur more rapidly than
forcing changes for the CO2 scenarios, so the net effect is
cooling. After 2040, the HIGH GEO cooling tends to balance
the warming from the overshoot CO2 stabilization scenario,
eventually leading to a slight cooling that would bring globalmean
temperatures back to near their pre-industrial level. The
MID and LOW GEO cases lead to temperatures roughly
stabilizing at 1°C and 2°C relative to 2000 (29). After 2100,
LOW GEO (where injection into the stratosphere is ramped
down to zero by 2090) closely matches the WRE450
mitigation-only case, but requires less stringent emissions
The sea level results, using models employed in the IPCC
Third Assessment Report (30, 31),
show the much larger
inertia of this part of the climate system. LOW GEO and
WRE450 again are similar, with neither tending towards
stabilization. Even the HIGH GEO case shows a continuing
(but slow) sea level rise at the end of the study period, but the
rate of rise is small even relative to observed changes over the
20th century (30, 32).
A combined mitigation/geoengineering approach to
climate stabilization has a number of advantages over either
employed separately. A relatively modest geoengineering
to the present LOW GEO
case could reduce the economic and technological burden on
mitigation substantially by deferring the need for immediate
or near-future cuts in CO2 emissions. More ambitious
geoengineering, when combined with mitigation, could even
lead to stabilization of global-mean temperature at nearpresent
levels and reduce future sea level rise to a rate much
less than observed over the 20th century – aspects of future
change that are virtually impossible to achieve through
As a guide to the amount of SO2 required, the eruption of
Pinatubo injected about 10 TgS into the stratosphere (35, 36),
and the analysis here suggests that an annual flux of half that
amount would have a significant influence. Smaller aerosols
would have longer lifetimes and require still smaller injection
5 TgS/yr is only about 7% of current SO2
emissions from fossil-fuel combustion (37, 38). Further
analysis is required to assess the technological feasibility of
the suggested injections of SO2 [or of more radiatively
efficient material (34)] into the stratosphere, the economic
costs of this option relative to the reduced costs of mitigation
that an overshoot CO2 stabilization pathway would allow, and
the detailed effects of the proposed SO2 injections and CO2
concentration changes on climate [cf. (39)] and stratospheric
References and Notes
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word geoengineering refers specifically to
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timescale (21) and long-term (26)
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“grossly non-optimized” as scatterers
radiation, and that metallic or resonant scatterers
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metals) to be “as much as five times
less” than for sulfate
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40. NCAR is supported by the U.S. National
26 June 2006; accepted 6 September 2006
Published online 14 September 2006;
Include this information when citing this paper.
Fig. 1. Global-mean temperature response to multiple
volcanic eruptions. The “standard”
eruption used is Mt.
Pinatubo [forcing data from Ammann et al. (20,
eruptions are assumed to occur every four years
every two years, or every year. The results
shown are annualmean
values plotted year by year. In the two- and
four-year cases the forcing varies considerably
from year to
year, leading to noticeable interannual temperature
These appear as bands of values because the
abscissa scale in
the graph is insufficient to resolve these
rapid variations. A
climate sensitivity of 3°C equilibrium warming
Fig. 2. Radiative forcing scenarios for the three
geoengineering cases considered. The HIGH GEO
corresponds approximately to the steady-state
would result from eruptions of Pinatubo every
Fig. 3. (A) CO2 concentration projections used in the analysis
together with (B) corresponding fossil-fuel emissions. “A1B”
is a central scenario from the SRES “no-climate-policy”
(27). “WRE450” is a concentration stabilization case from
ref. (7), used as a mitigation-only example. “Overshoot” is a
case with CO2 concentrations stabilizing at 450 ppm but with
less mitigation (higher emissions) than in
WRE450. This is
the case that is used in conjunction the three
cases shown in Fig. 2. A climate sensitivity
equilibrium warming for 2xCO2 is assumed. CO2 emissions
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results depend on the climate sensitivity because
feedbacks on the carbon cycle.
Fig. 4. Global-mean temperature (A)
and sea level changes
(B) for the baseline “no-climate-policy” scenario (A1B), a
mitigation-only scenario stabilizing at 450
and three scenarios combining both mitigation
geoengineering. The latter employ the overshoot
mitigation) scenario (“Overshoot”
in Fig. 3) and increasingly
strong geoengineering cases from Fig. 2 (LOW
GEO and HIGH GEO). A climate sensitivity of
equilibrium warming for 2xCO2 is assumed.