Global temperatures are likely to rise by 5.4 degrees Fahrenheit by the end of the century, sharply increasing drought and water shortages, Britain's chief scientist said.

The increase, more than five times the rise in global temperatures during the 20th century, will probably occur even if the world puts in place the most ambitious international limits on greenhouse gases currently under discussion.

Sir David King, the government's top adviser on scientific issues, told British Broadcasting Corp. radio that projected greenhouse gas increases are expected to bring increases in sea level and global temperatures "that would be extremely difficult for world populations to manage."

The U.N.-backed Intergovernmental Panel on Climate Change says temperatures rose by about 1 degree during the 20th century. Various computer models have predicted increases of between 2.5 and 10.4 degrees by 2100, depending on how much is done to limit greenhouse gas emissions.

A British government report said a 3-degree increase would cause millions of tons of cereal crops to fail and put between 1.2 billion and 3 billion people at risk of water shortages.

King called for major global efforts to adapt to the effects of climate change.

"We don't have to succumb to a state of despondency where we say there's nothing we can do so let's just carry on as per usual," King said. "It is very important to understand that we can manage the risks to our population."

Prime Minister Tony Blair has called for a new international agreement to limit emissions of the greenhouse gases blamed for global warming. But the United States refuses to cap its emissions, and those of rapidly growing economies like China and India are increasing.

British ministers also have acknowledged that Britain is unlikely to meet its own target of cutting carbon dioxide emissions by 20 percent by 2010.