The global population is 6 billion and our collective use of fossil fuels — coal, oil, and natural gas — releases to the atmosphere about 6 billion tons of carbon in the form of carbon dioxide every year. Under "business as usual," emissions will continue to rise, perhaps tripling by the end of this century. That trajectory would very likely cause major climate disruption with a high probability of serious ecological damage. The resulting strains on our infrastructure would be huge, with grave risks to our economic and political systems. Conversely, to limit the atmospheric carbon dioxide level to no higher than 450 parts per million, we would have to cut our emissions to about one third of today's levels by century's end.
To move from "business as usual" to this more sustainable trajectory will require a portfolio of responses. Four major ingredients are needed: (1) Strong shift to wind, solar, and other nonfossil forms of energy supply; (2) Major improvements in energy efficiency throughout our economies; (3) Management of carbon dioxide as waste product so that we no longer treat the atmosphere as an open sewer; and (4) Development of a strong ethic of "sufficiency," the idea that our quality of life is not directly tied to the quantity of material-intensive goods and services that we produce and consume.
To make the necessary transformation will require strong contributions from many sectors of society. Because energy is so important for human well-being and because our current system is so heavily dependent on unsustainable fossil energy, we need to make a transformation that is unprecedented in scope and duration. The Copenhagen meetings provide an important opportunity for international leaders to demonstrate constructive and cooperative leadership that is an essential ingredient on our journey toward a sustainable future.