From Prof Jeffrey D. Sachs.

Sir, With reference to "Hopes increase for a credible climate deal" (December 7): the rich world is once again urging the poor world to buy a pig in a poke. As we come down to the wire in Copenhagen, the poor countries are being asked to sign on to vague financing commitments for mitigation and adaptation, starting with a meagre down-payment of $10bn a year during 2010-12. Yes, the rich countries acknowledge, the sums should somehow grow tenfold during the decade, to $100bn a year by 2020, but without any commitments as to how this will happen.

We've been here before, in fact, repeatedly. The rich world preens itself about its commitment to development assistance, but has been unwilling even to honour a clear and repeated "Gleneagles Commitment" to increase development aid to Africa by $30bn between 2005 and 2010. Soon after the Gleneagles Commitment was made in 2005, I attended a high-level UK government meeting in London about the follow up. I asked the simple question: "Where's the spreadsheet explaining how the commitment will be fulfilled?" I was told: "The US has insisted on no spreadsheets." The rich world abjures accountability and transparency.

Pushing the developing countries into a climate accord based on tiny sums today and vague financial promises for the future is a guarantee of future suffering by the poor due to unattended climate change caused mainly by the rich. We should think about these financial flows, in any event, as compensation for damages caused mainly by the rich world. The poorest countries stand to suffer the most but have contributed the least to the crisis.

The US, it is reported, is setting aside $1.4bn in next year's budget for climate change in the poor countries. This, we can note, is about what the US will spend in Iraq and Afghanistan roughly every two-and-a-half days, and will be perhaps a 20th or less of this year's Wall Street bonuses.

Jeffrey D. Sachs,

Director of the Earth Institute,

Columbia University,