By Megan Rowling

POZNAN, Poland, Dec 8 (Reuters) - The impact of climate change could uproot around six million people each year, half of them because of weather disasters like floods and storms, a top U.N. official said on Monday.

The U.N. refugee agency (UNHCR) was making plans based on conservative estimates that global warming would force between 200 million and 250 million people from their homes by mid-century, said L. Craig Johnstone, the U.N. Deputy High Commissioner for Refugees.

"That means a displacement of something like six million people a year -- that's a staggering number," he told Reuters on the sidelines of the Dec. 1-12 U.N. climate talks in Poland.

"Our operating assumption is to cover the minimum ... but we're not anywhere near being able to cover that right now," he said.

Johnstone said relief agencies would need to aid almost three million people a year displaced by sudden disasters.

Another three million would likely migrate due to gradual changes like rising sea levels, and be more able to plan.

Most would stay in their countries rather than crossing national borders, Johnstone said.

UNHCR statistics show 67 million people were uprooted around the world at the end of 2007, 25 million of them because of natural disasters.

Johnstone said steps to limit greenhouse gas emissions and adapt to climate change would not be enough to prevent rising disasters or conflict over resources, which would hit the poorest people hardest.

"You can expect that as you have droughts, as you have scarcity of resources ... it will increase tensions and it will increase conflict," he said.

But plans could be made to deal with disasters because experts understood which areas of the world would likely be affected, he said.

Predicted global warming impacts include more intense storms hitting coastal areas in Asia and the Caribbean, and more frequent floods and droughts in Africa, Asia and Latin America.

Johnstone said aid agencies would need to boost the relief supplies they keep in stock for emergencies by 10 to 20 times.

"(Climate change) adds a substantial additional burden to humanity," he said. "(UNHCR's) presence in the world corresponds almost perfectly with the hotspots ... So we will be called on to help and we need to be prepared for that."

He added that a new global climate pact due to be agreed by the end of 2009 in Copenhagen should include funding to prepare for disasters, because it would save money in the long run. (Reporting by Megan Rowling, editing by Diana Abdallah)

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