What can make significant reductions in greenhouse gas emissions?

MIT News
Oct 24, 2011

At any given moment, the world is consuming about 14 terawatts (trillions of watts) 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.

To put those 14,000,000,000,000 watts in perspective, an average person working at manual labor eight hours a day can expend energy at a sustained rate of about 100 watts. But the average American consumes energy (in all forms) 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.

Of that 14 terawatts (TW), about 85 percent 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.

According to Ernest J. Moniz, the Cecil and Ida Green Distinguished Professor of Physics and Engineering Systems and director of the MIT Energy Initiative, a widely cited 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 increases.

The 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 wedge).

INFOGRAPHIC: 'Wedges' to reduce emissions

Of 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 degrees Celsius 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.
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