PRINCETON, New Jersey — The agreement on climate change reached at Heiligendamm by the Group of Eight leaders merely sets the stage for the real debate to come: How will we divide up the diminishing capacity of the atmosphere to absorb our greenhouse gases?

The G8 leaders agreed to seek "substantial" cuts in greenhouse gas emissions and to give "serious consideration" to the goal of halving such emissions by 2050 — an outcome hailed as a triumph by German Chancellor Angela Merkel and British Prime Minister Tony Blair. Yet the agreement commits no one to any specific targets, least of all the United States, whose president, George W. Bush, will no longer be in office in 2009 when the tough decisions have to be made.

One could reasonably ask why anyone thinks such a vague agreement is any kind of advance at all. At the United Nations Conference on Environment and Development in Rio de Janeiro in 1992, 189 countries, including the U.S., China, India and all the European nations, signed the U.N. Framework Convention on Climate Change, thereby agreeing to stabilize greenhouse gases "at a low enough level to prevent dangerous anthropogenic interference with the climate system."