Extra aid for Africa, agreed by the group of eight industrial nations, could be cancelled out by global warming, a senior scientist has warned.
African poverty was set as a priority for Britain's G8 presidency
President of the Royal Society Lord May says the money risks being entirely consumed by the hunger which will be created by the changing climate.
G8 nations agreed in July to boost aid to poorer countries by $50bn (£28.8bn).
Tony Blair made African poverty and climate change twin priorities of Britain's presidency of the G8.
Lord May's letter comes ahead of a conference on climate change - due to be chaired by the prime minister - next week in London.
He is highly critical of what he sees as the lack of concrete progress in agreeing new steps to tackle global warming.
'Give and take'
"As long as greenhouse gas concentrations continue to rise, there is the very real prospect that the increase in aid agreed at Gleneagles [G8 summit venue] will entirely be consumed by the mounting cost of dealing with the added burden of adverse effects of climate change in Africa," he wrote.
"In effect the Gleneagles communiqué gave hope to Africa with one hand ... but took that hope away with the other through its failure to address adequately the threat of climate change," he said.
He called for urgent action to tackle the potential catastrophe.
The world leaders agreed to take a series of measures to tackle global warming, but did not set targets or deadlines.
The conference on 1 November is due to assess progress since the July meeting.