of natural gas (without accompanying carbon capture and

storage) could lead to unabated GHG emissions for many

decades, given the typically multidecadal lifetime of energy

infrastructure, thereby greatly complicating climate change

mitigation e ﬀ orts.

GHG Emissions. We calculate that world nuclear power

generation prevented an average of 64 gigatonnes of CO

2

-

equivalent (GtCO

2

-eq), or 17 GtC-eq, cumulative emissions

from 1971 to 2009 (Figure 3a; see full range therein), with an

average of 2.6 GtCO

2

-eq/year prevented annual emissions from

2000 to 2009 (range 2.4 − 2.8 GtCO

2

/year). Regional results are

also shown in Figure 3a. Our global results are 7 − 14% lower

than prev ious esti mate s

8,9

that , amo ng ot her di ﬀ e re nces,

assumed all historical nuclear power would have been replaced

only by coal, and 34% higher than in another study

10

in which

the methodology is not explained clearly enough to infer the

basis for the di ﬀ erences. Given that cumulative and annual

global fossil fuel CO

2

emissions during the above periods were

840 GtCO

2

and 27 GtCO

2

/year, respectively,

11

our mean

estimate for cumulative prevented emissions may not appear

substantial; however, it is instructive to lo ok at other

quantitative comparisons.

For instance, 64 GtCO

2

-eq amounts to the cumulative CO

2

emissions from coal burning over approximately the past 35

years in the United States, 17 years in China, or 7 years in the

top ﬁ ve CO

2

emitters.

11

Also, since a 500 MW coal- ﬁ red power

plant typically emits 3 MtCO

2

/year,

26

64 GtCO

2

-eq is

equivalent to the cumulative lifetime emissions from almost

430 such plants, assuming an average plant lifetime of 50 years.

It is therefore evident that, without global nuclear power

generation in recent decades ,n e a r - t e r mm i t i g a t i o no f

anthropogenic climate change would pose a much greater

challenge.

For the projection period 2010 − 2050, in the all coal case, an

average of 150 and 240 GtCO

2

-eq cumulative global emissions

are prevented by nuclear power for the low-end and high-end

projections of IAEA,

6

respectively. In the all gas case, an average

of 80 and 130 GtCO

2

-eq emissions are prevented (see Figure

3b,c for full ranges). Regional results are also shown in Figure

3b,c. These results also di ﬀ er substantially from previous

studies,

9,10

largely due to di ﬀ erences in nuclear power

projections (see the Supporting Information).

To put our calculated overall mean estimate (80 − 240

GtCO

2

-eq) of potentially prevented future emissions in

perspective, note that, to achieve a 350 ppm CO

2

target near

the end of this century, cumulative “ allowable ” fossil CO

2

emissions from 2012 to 2050 are at most ∼ 500 GtCO

2

(ref 3).

Thus, projected nuclear power could reduce the climate-change

mitigation burden by 16 − 48% over the next few decades

(derived by dividing 80 and 240 by 500).

Uncertainties. Our results contain various uncertainties,

primarily stemming from our impact factors (Table 1) and our

assumed replacement scenarios for nuclear power. In reality,

the impact factors are not likely to remain static, as we

(implicitly) assumed; for instance, emission factors depend

heavily on the particular mix of energy sources. Because our

impact factors neglect ongoing or projected climate impacts as

well as the signi ﬁ cant disparity in pollution between developed

and developing countries,

16

our results for both avoided GHG

emissions and avoided mortality could be substantial under-

estimates. For example, in China, where coal burning accounts

for over 75% of electricity generation in recent decades (ref 14;

Figure S3, Supporting Information), some coal- ﬁ red power

plants that meet domestic environmental standards have a

mortality factor almost 3 times higher than the mean global

value (Table 1). These di ﬀ erences related to development

status will become increasingly important as fossil fuel use and

GHG emissions of developing countries continue to outpace

those of developed countries.

11

On the other hand, if coal would not have been as dominant

a replacement for nuclear as assumed in our baseline historical

scenari o, then our avoided historical impacts could be

overestimates, since coal causes much larger impacts than gas

(Table 1). However, there are several reasons this is unlikely.

Key characteristics of coal plants (e.g., plant capacity, capacity

factor, and total production costs) are historically much more

similar to nuclear plants than are those of natural gas plants.

13

Also, the vast majority of existing nuclear plants were built

before 1990, but advanced gas plants that would be suitable

replacements for base-load nuclear plants (i.e., combined-cycle

gas turbines) have only become available since the early

1990s.

13

Furthermore, coal resources are highly abundant and

widespread,

24,25

and coal fuel and total production costs have

long bee n relati vely low, un like hi sto rica lly ava ilable gas

resources and production costs.

13

Thus, it is not surprising

that coal has been by far the dominant source of global

electricity thus far (Figure 1). We therefore assess that our

baseline historical replacement scenario is plausible and that it

is not as signi ﬁ cant an uncertainty source as the impact factors;

that is, our av oided historical impacts are more likely

underestimates, as discussed in the above paragraph.

Implications. More broadly, our results underscore the

importance of avoiding a false and counterproductive

dichotomy between reducing air pollution and stabilizing the

climate, as elaborated by others.

27 − 29

If near-term air pollution

abatement trumps the goal of long-term climate protection,

governments might decide to carry out future electric fuel

switching in even more climate-impacting ways than we have

examined here. For instance, they might start large-scale

production and use of gas derived from coal ( “ syngas ” ), as coal

is by far the most abundant of the three conventional fossil

fuels.

24,25

While this could reduce the very high pollution-

related deaths from coal power (Figure 2), the GHG emissions

factor for syngas is substantially higher (between ∼ 5% and

90%) than for coal,

30

thereby entailing even higher electricity

sector GHG emissions in the long term.

In conclusion, it is clear that nuclear power has provided a

large contribution to the reduction of global mortality and

GHG emissions due to fossil fuel use. If the role of nuclear

power signi ﬁ cantly declines in the next few decades, the

International Energy Agency asserts that achieving a target

atmospheric GHG level of 450 ppm CO

2

-eq would require

“ heroic achievements in the deployment of emerging low-

carbon technologies, which have yet to be proven. Countries

that rely heavily on nuclear power would ﬁ nd it particularly

challenging and signi ﬁ cantly more costly to meet their targeted

levels of emissions. ”

2

Our analysis herein and a prior one

7

strongly support this conclusion. Indeed, on the basis of

combined evidence from paleoclimate data, observed ongoing

climate impacts, and the measured planetary energy imbalance,

it appears increasingly clear that the commonly discussed

targets of 450 ppm and 2 ° C global temperature rise (above

preindustrial levels) are insu ﬃ cient to avoid devastating climate

impacts; we have suggested elsewhere that more appropriate

targets are less than 350 ppm and 1 ° C (refs 3 and 31 − 33).

Aiming for these targets emphasizes the importance of retaining

Environmental Science & Technology Article