 Drought can devastate human populations and their livestock |
Recent research suggests that by 2050, five times as much land is likely to be under "extreme" drought as now.
Tearfund wants richer states to look at helping poorer ones adjust to drought at next month's UN climate summit.
This week the UK's climate minister said he was confident of reaching an deal on adaptation funds at the talks.
There was an "urgent need" for such measures, Ian Pearson told a parliamentary committee.
 |  It's the extremes of water which are going to provide the biggest threat to the developing world from climate change Sir John Houghton |
The Tearfund report, Feeling the Heat, urges donors to ramp up assistance quickly. Other charities are likely to make similar pleas in the run-up to the Nairobi summit, which begins on 6 November.
Citing research by the Oxford academic Norman Myers, Tearfund suggests there will be as many as 200 million climate refugees by 2050.
Areas where people are already on the move to avoid climate excesses include, the report says: Brazil, where one in five people born in the arid northeast region relocates to avoid drought China, where three provinces are seeing the spread of the Gobi desert Nigeria, where about 2,000 sq km is becoming desert each year
Attributing the movement of people to climate impacts is, however, a difficult issue, with many other factors including economic opportunity behind decisions to relocate.
Level of rhetoric level
One of Britain's leading climate scientists, Sir John Houghton, said the severity of climate change was getting through to world leaders "at a level of rhetoric", but not yet at a level of action.
"There were promises made at the G8 summit and at the last UN meeting in Montreal about money for adaptation," he told the BBC News website, "but I understand that very little of that has come through."
Sir John, who contributed a foreword to the Tearfund report, said water shortages will be the biggest climate threat to developing countries.
|  Dealing with drought |
"It's the extremes of water which are going to provide the biggest threat to the developing world from climate change," he said.
"Without being able to be too specific about exactly where, droughts will tend to be longer, and that's very bad news. Extreme droughts currently cover about 2% of the world's land area, and that is going to spread to about 10% by 2050."
Overall, he said, climate models show a drying out of sub-Saharan Africa, while some other areas of the world will see more severe flooding.
Sir John is a former head of the UK Meteorological Office, former chairman of the Royal Commission on Environmental Pollution, and co-chaired one of the Intergovernmental Panel on Climate Change (IPCC) working groups.
He is now chairman of the John Ray Initiative, whose mission is to "connect environment, science and Christianity".
The positive side of the Tearfund report is that simple measures to "climate-proof" water problems, both drought and flood, have proven to be very effective in some areas.
In Niger, the charity says that building low, stone dykes across contours has helped prevent runoff and get more water into the soil, while in Bihar, northern India, embankments have been built to connect villages during floods, with culverts allowing drainage.
