As world leaders struggle to hash out a new global climate deal by December, they face a hurdle perhaps more formidable than getting big polluters like the United States and China to reduce greenhouse gas emissions: how to pay for the new accord.

The price tag for a new climate agreement will be a staggering $100 billion a year by 2020, many economists estimate; some put the cost at closer to $1 trillion. That money is needed to help fast-developing countries like India and Brazil convert to costly but cleaner technologies as they industrialize, as well as to assist the poorest countries in coping with the consequences of climate change, like droughts and rising seas.

This financing is an essential part of any international climate agreement, negotiators and scientists say, because developing nations must curb the growth of their emissions if the world is to limit rising temperatures. Based on calculations by the International Energy Agency for 2005 to 2030, 75 percent of the growth in energy demand will come from the developing world.

Many developing countries have made it clear that they will not sign a treaty unless they get money to help them adapt to a warmer planet. Acknowledging that a new treaty needs unanimity for success, industrialized nations like the United States and those in Europe have agreed in principle to make such payments; they have already been written into the agreed-upon structure of the treaty, to be signed in Copenhagen in December.