Efforts to help developing nations adapt to the impacts of climate change have been called "woefully inadequate" by a UN-commissioned report.
Rich countries have focused on ways to reduce carbon emissions but have largely ignored helping poor nations cope with the consequences, it says.
The findings appear in the UNDP's Human Development Report 2006.
The authors say farmers whose crops are reliant on rainfall are already having to cope with unpredictable weather.
The report, called Beyond Scarcity: Power, Poverty and the Global Water Crisis, says climate change "now poses what may be an unparalleled threat to human development".
Lead author Kevin Watkins said people living in vulnerable conditions were already having to adapt.
"There is a lot of evidence that the droughts in the Horn of Africa this year are connected to climate change," he told reporters. "This is not an issue for 50 years down the road, it is an issue for today."
Mr Watkins added that the worst affected areas were regions with very limited water infrastructures, such as Sub-Saharan Africa.
"It is not a region that has the irrigation capacity or the water harvesting capacity to store water in ways that can smooth out irregularities in supply," he observed.
"More than 90% of people living in rural Sub-Saharan Africa are dependent on rain-fed agriculture, so what happens to rain and moisture content in the soil has very profound and immediate implications for poverty."
He warned that crops yields could fall by a third or more in some regions.
While the outcomes may vary from country-to-country, the report said some "broad consequences" could be predicted:
- agriculture and rural development will bear the brunt of climate risk
- extreme poverty and malnutrition will increase as water insecurity increases
- more extreme weather patterns will increase the risk of floods and droughts
- shrinking glaciers and rising sea levels will reduce access to fresh water
Because industrialised nations have focused their climate change initiatives on reducing the amount of greenhouse gases being pumped into the atmosphere, support for adaptation in developing countries has been "piecemeal and fragmented", the report says.
It calls the international response "woefully inadequate", because of the lack of serious investment by nations in adaptation projects.
"The adaptation agenda is somewhere between embryonic and heavily under-developed," Mr Watkins said.
"Funding... under the Kyoto Protocol currently amounts to $20m annually; so this is something that, as part of the multilateral negotiations, has not had any weight attached to it."
He also said that adaptation funding through the Global Environment Facility (GEF) would be about $50m over the next three years.
"What we are facing is one of the potentially biggest set-backs to human development in Africa in the past 100 years or more, and the response from the international community to date has been $70m," Mr Watkins said.
The latest round of international negotiations on tackling climate change is currently underway in Nairobi, and the issue of adaptation is expected to be high on the agenda.
At the start of the global gathering in the Kenyan capital, the United Nations Framework Convention on Climate Change (UNFCCC) published a report that described global warming as a serious threat to Africa.
It listed a series of reasons why measures to help African countries "climate-proof" their societies, economies and infrastructure was widely seen as vital.
Next week sees the start of the high-level segment of the conference, where any new agreement on adaptation would be reached.
Kevin Watkins hoped the talks would deliver the funding and strategies needed for people living in vulnerable rural areas: "Their future critically depends upon the international community getting serious about adaptation."