SCIENCE

www.sciencemag.org

VOL 314, 24 NOVEMBER 2006

EDITORIAL

Carbon Trading

William H. Schlesinger

ENTHUSIASM IS SPREADING FOR CAP-AND-TRADE SYSTEMS TO REGULATE THE

AMOUNT of CO 2 emitted to Earth's atmosphere. In 1990, the U.S.

Environmental Protection Agency set a limit on SO 2 emissions from

obvious point sources and allowed those who emit less than their

quota to trade excess allowances. As a result, regional acid

deposition was dramatically reduced. Can the world do the same for CO

2?

Fundamental differences in the biogeochemistry of SO 2 and CO 2

suggest that establishing a comprehensive, market-based cap-and-trade

system for CO 2 will be difficult. For SO 2 , anthropogenic point

sources (largely coal-fired power plants), which are relatively easy

to control, dominate emissions to the atmosphere. Natural sources,

such as volcanic emanations, are comparatively small, so reductions

of the anthropogenic component can potentially have a great impact,

and chemical reactions ensure a short lifetime of SO 2 in the

atmosphere.

CO 2, in contrast, comes from many distributed sources, some

sensitive to climate, others sensitive to human disturbance such as

cutting forests. It is thus impossible to control all of the

potential sources. Human-derived emissions from fossil fuel

combustion are one of the smaller components of the atmospheric flux

of CO 2 , which is dominated by exchange between forests and the

oceans. During most of the past 10,000 years, the uptake and loss of

CO 2 from forests and the oceans must have been closely balanced,

because atmospheric CO 2 showed little variation until the start of

the Industrial Revolution. CO 2 from coal, oil, and natural gas

combustion now comes from many segments of society, including

electric power generation, industry, home heating, and

transportation. Unbalanced by equivalent anthropogenic sinks for

carbon, fossil fuel emissions account for the vast majority of the

rise of CO 2 in Earth's atmosphere. Caps on emissions, like those

instituted for SO 2 , will be difficult to institute if the burden of

reducing CO 2 is to be borne equally by all emitters.

Because land plants take up CO 2 in photosynthesis and store the

carbon in biomass, forests and soils seem to be attractive venues to

store CO 2 . Market-based schemes propose substantial payments and

credits to those who achieve net carbon storage in forestry and

agri-culture, but these projected gains are often small and dispersed

over large areas. We will need to net any such carbon uptake against

what might have occurred without climate-policy intervention.

Conversely, will Canada and Russia be billed for incremental CO 2

releases that stem from the warming of cold northern soils as a

result of global warming from the use of fossil fuels worldwide?

If credit is given to those who choose not to cut existing forests,

the increasing total demand for forest products will shift

deforestation to other areas. Frequent audits will be needed to

determine current carbon uptake, insurance will be necessary to

protect past carbon credits from destruction by fire or windstorms,

and payments will be necessary if the forest is cut. All these

efforts will be costly to administer, diminishing the value of the

rather modest carbon credits expected from forestry and agriculture.

Many environmental economists recognize that a tax or fee on CO 2

emission from fossil fuel sources is the most efficient system to

reduce emissions and spread the burden equitably across all sources:

industrial and personal. A tax on emissions of fossil fuel carbon

could replace the equivalent revenue from income taxes, so the total

tax bill of consumers would be unchanged. A higher tax on gasoline

would preserve the personal right to drive a larger car or drive long

distances, but it would also motivate decisions to do otherwise. A

tax on emissions from coal-fired power plants, manifest in monthly

electric bills, would motivate the use of alternative energies and

energy-use efficiencies at home and in industry.

The biogeochemistry of carbon suggests that both emissions taxes and

cap-and-trade programs will work best if restricted to sources of

fossil fuel carbon. Other net sources and sinks of carbon in its

global biogeochemical cycle are simply too numerous and usually too

small to include in an efficient trading system. Simple, fair, and

effective must be the hallmarks of policies that will wean us from

the carbon-rich diet of the Industrial Revolution, and we must begin

soon if we are to have any hope of stabilizing our climate.

William H. Schlesinger is

dean of the Nicholas

School of the Environment

and Earth Sciences, Duke

University, Durham, NC.

Published by AAAS

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