World leaders will certainly struggle to reach a new global climate deal in December, but the biggest hurdle - worse than getting big polluters like the US and China to reduce their greenhouse gas emissions – will be how to pay for it. The price tag for the agreement will be a staggering $100 billion a year by 2020, say economists, and some guess it may top out at $1 trillion. That money will help rapidly-developing countries like India and Brazil convert to cleaner technologies as they industrialise, and assist the poorest in coping with the effects of climate change such as droughts and rising seas. The financing is a key part of the final agreement because developing nations must curb the growth of their emissions if the world is to limit rising temperatures. International Energy Agency numbers say that from 2005 to 2030, 75% 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.