http://www.essc.psu.edu/essc_web/seminars/fall2006/WigleySci06.pdf

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 CO2

concentrations (7) addresses both problems; but presents

considerable economic and technological challenges (8, 9).

Geoengineering (10–17) 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

should therefore 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

[MAGICC (2, 25, 26)] with a chosen climate sensitivity of

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

SRES (27) 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

feedbacks 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–

9).

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

reductions.

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

investment (33, 34) corresponding 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

mitigation alone.

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

rates (15). 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

chemistry.

References and Notes

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engineering (12–15), and carbon cycle engineering [e.g.,

through modifying ocean alkalinity; (16, 17)]. Here, the

word geoengineering refers specifically to climate

engineering, i.e., direct and intentional management of the

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28. Avoiding dangerous anthropogenic interference with the

climate system is one of the primary guidelines for climate

policy espoused in Article 2 of the UN Framework

Convention on Climate Change.

29. In all cases there is a residual warming tendency arising

from the emissions of non- CO2 gases (CH4, N2O,

halocarbons, tropospheric aerosols). Emissions from these

sources are assumed simply to follow A1B to 2100 and

then remain constant, leading to a slow but long-term

increase in radiative forcing.

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The Scientific Basis, J. T. Houghton et al., Eds.

(Cambridge University Press, Cambridge, UK, 2001), pp.

639–694.

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L05704, doi:10.1029/2004GL021238 (2005).

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33. Teller et al. (34) point out that sulfate aerosols are

“grossly non-optimized” as scatterers of short-wave

radiation, and that metallic or resonant scatterers offer

large mass savings. Although emplacement costs for such

scatterers would be higher, they estimate net costs (for

metals) to be “as much as five times less” than for sulfate

aerosols.

34. E. Teller, R. Hyde, L. Wood, Active Climate

Stabilization: Practical Physics-Based Approaches to

Prevention of Climate Change, Preprint UCRL-JC-

148012, LLNL, Livermore, CA (2002).

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37. S. J. Smith, H. Pitcher, T. M. L. Wigley, Global and

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38. At steady-state, an injection of 5 TgS/yr into the

stratosphere would increase the flux into the troposphere

by the same amount, with larger fluxes in high latitudes.

Given current emissions, the consequences of this extra

flux are likely to be minor, but this aspect warrants further

attention.

39. B. Govindasamy, K. Caldeira, Geophys. Res. Lett. 17,

2151 (2000).

40. NCAR is supported by the U.S. National Science

Foundation.

26 June 2006; accepted 6 September 2006

Published online 14 September 2006;

10.1126/science.1131728

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, 21)], and

eruptions are assumed to occur every four years (top curve),

every two years, or every year. The results shown are annualmean

values plotted year by year. In the two- and (especially)

four-year cases the forcing varies considerably from year to

year, leading to noticeable interannual temperature variations.

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 for 2xCO2 is

assumed.

Fig. 2. Radiative forcing scenarios for the three

geoengineering cases considered. The HIGH GEO case

corresponds approximately to the steady-state forcing that

would result from eruptions of Pinatubo every two years.

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” set

(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 geoengineering

cases shown in Fig. 2. A climate sensitivity of 3°C

equilibrium warming for 2xCO2 is assumed. CO2 emissions

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results depend on the climate sensitivity because of climate

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 ppm (WRE450),

and three scenarios combining both mitigation and

geoengineering. The latter employ the overshoot (reduced

mitigation) scenario (“Overshoot” in Fig. 3) and increasingly

strong geoengineering cases from Fig. 2 (LOW GEO, MID

GEO and HIGH GEO). A climate sensitivity of 3°C

equilibrium warming for 2xCO2 is assumed.