With the world’s energy needs growing rapidly, can
zero-carbon energy options be scaled up enough to make a significant
difference? How much of a dent can these alternatives make in the world’s total
energy usage over the next half-century? As the Massachusetts Institute of
Technology (MIT) Energy Initiative approaches its fifth anniversary next month,
this five-part series takes a broad view of the likely scalable energy
At any given moment, the world is consuming about 14 TW of
energy—everything from the fuel for our cars and trucks, to wood burned to cook
dinner, to coal burned to provide the electricity for our lights, air
conditioners, and gadgets.
put those 14,000,000,000,000 W in perspective, an average person working at
manual labor eight hours a day can expend energy at a sustained rate of about
100 W. But the average American consumes energy at a rate of about 600 times
that much. “So our lifestyle is equivalent to having 600 servants, in terms of
direct energy consumption,” says Robert Jaffe, the Otto (1939) and Jane
Morningstar Professor of Physics at MIT.
that 14 TW, about 85% comes from fossil fuels. But since world energy use is
expected to double by 2050, just maintaining carbon emissions at their present
rate would require coming up with about 14 TW of new, non-carbon sources over
the next few decades. Reducing emissions—which many climate scientists consider
essential to averting catastrophic changes—would require even more.
to Ernest J. Moniz, the Cecil and Ida Green Distinguished Professor of Physics
and Engineering Systems and director of the MIT Energy Initiative, a 2004 paper
in Science introduced the concept of “wedges” that might contribute to
carbon-emissions reduction. The term refers to a graph projecting energy use
between now and 2050: Wedges are energy-use reductions that could slice away at
the triangle between a steadily rising line on this graph—representing a
scenario in which no measures are taken to curb energy use—and a horizontal
line reflecting a continuation of present levels of energy usage, without
authors of the 2004 Science paper proposed a series of wedges, each
representing about two terawatts of energy savings. (Others have since refined this
model, now referring to anything that can save at least one terawatt as a
course, even eliminating the triangle altogether by holding energy usage at
current levels would not reduce the greenhouse gas emissions that have been
steadily heating up the planet; it would simply stabilize emissions at present
levels, slowing the rate of further growth. But most analyses, such as those by
MIT’s Joint Program on the Science and Policy of Global Change, indicate that
merely stabilizing emissions still presents a better-than-even chance of
triggering a rise in global temperatures of at least 2.3 C by 2100, an amount
that could lead to devastating changes in sea level, as well as increased
patterns of both flooding and droughts. Preventing such serious consequences,
most analysts say, would require not just stabilizing emissions but drastically
curtailing them—in other words, finding additional wedges to implement.
the Science paper, authors Stephen Pacala and Robert Socolow of Princeton University listed 15 possible wedges:
energy-saving technologies to chip away at the triangle. (The paper was recently updated by Socolow, in the Bulletin of the
Atomic Scientists, to reflect the years that have passed since the initial
publication and the lack of any net reductions so far). While there are indeed
technologies that can contribute to reductions on the order of terawatts, Moniz
says Pacala and Socolow’s analysis is “not necessarily very realistic,” and “they made it sound like implementing one of these wedges is too easy.” In
fact, every one of the options has its own difficulties, Moniz says.
some aspects of bringing about such a drastic reduction are not controversial. “The number one thing is demand reduction, that’s clear,” Moniz says. “Most
[scientists] think you need to get more than one wedge” from demand reduction—another
way of saying increased efficiency—”because if you don’t, then we’d need a
miracle” to achieve the needed reductions in emissions through other means, he
fact, efficiency gains may yield several wedges, corresponding to multiple
terawatts saved. That’s not so surprising when you consider that of all the energy
consumed in the United
States from all sources, some 58% is simply
lost—that is, not actually used to do anything useful—says Jaffe, who
co-teaches an MIT class called “The Physics of Energy.” For example, the
typical automobile wastes more than two-thirds of the energy contained in the
gasoline it burns, dumping it into the environment as heat.
transportation, on average, is about 20% efficient,” Jaffe says. “That’s
scandalous. There are tremendous savings to be gained,” he says, such as by
continuing to raise the requirements for fuel efficiency of vehicles.
after picking the relatively low-hanging fruit of efficiency, potential
solutions for reducing emissions become more complex and less potent. Most of
the technologies that are widely discussed as low- or zero-carbon alternatives
are limited in their potential impact, at least within the next few decades.
example, many people talk about a “nuclear renaissance” that could provide
electricity with very little greenhouse gas impact. But to get even one
terawatt of power from new nuclear plants “ain’t so simple,” Moniz says. The
operating costs of new nuclear-plant designs, for example, will have to be
proven through years of operating experience before financial markets will be
willing to fund such systems on a large scale.
the longer run, such technologies may be crucial to meeting the world’s growing
energy demands. By the end of this century, global energy needs could be more
than triple those of today, says Ron Prinn, the TEPCO Professor of Atmospheric
Science and co-director of MIT’s Joint Program on the Science and Policy of
Global Change. “Most of that will be driven by the industrialization of China, India
he explains, as these countries evolve from agrarian to industrialized
Moniz suggests, a non-carbon energy future will likely consist largely of some
combination of nuclear power, renewable energy sources, and carbon-capture
systems that allow fossil fuels to be used with little or no emissions of
greenhouse gases. Which of these will dominate in a given area comes down to
costs and local conditions.
one technology is going to get us into a sustainable energy future,” Jaffe
says. Rather, he says, it’s going to take a carefully considered combination of
many different approaches, technologies, and policies.