The world's pre-eminent climate scientists produced a blunt assessment of the impact of global warming on the US yesterday, warning of droughts that could reduce the American south-west to a wasteland and heatwaves that could make life impossible even in northern cities.

In an update on the latest science on climate change, the US Congress was told that melting snow pack could lead to severe drought from California to Oklahoma. In the midwest, diminishing rains and shrinking rivers were lowering water levels in the Great Lakes, even to the extent where it could affect shipping.

"With severe drought from California to Oklahoma, a broad swath of the south-west is basically robbed of having a sustainable lifestyle," said Christopher Field, of the Carnegie Institution for Science. He went on to warn of scorching temperatures in an array of cities. Sacramento in California, for example, could face heatwaves for up to 100 days a year.

"We are close to a threshold in a very large number of American cities where uncomfortable heatwaves make cities uninhabitable," Field told the Senate's environment and public works committee.

The warnings were the first time Congress had been directly confronted with the growing evidence that the impact of climate change will be far more severe than revealed even in the UN's most recent report, in 2007.

The hearing was also the first time senators had been permitted to hear testimony about the dangers to human health from climate change. In 2007, the Bush administration censored testimony from the Centres for Disease Control and Prevention (CDC) on the rise in asthma and other respiratory illnesses, as well as the increasing occurrence of "tropical" parasites.

"The CDC considers climate change a serious public health concern," said Howard Frumkin, the director of the centre for environmental health at the CDC.

Yesterday's gathering of climate scientists, led by the head of the UN's Intergovernmental Panel on Climate Change, RK Pachauri, was designed to give momentum to efforts by the Democratic leadership to press ahead on energy reform.

"If we don't do it people are going to die. They are going to get sick and they are going to die," said Barbara Boxer, who as chair of the Senate environment and public works committee is key to securing the passage of climate change legislation.

But even with the new administration and the Democratic leadership in Congress now united on the urgency of acting on climate change, there were still signs of battles ahead.

The hearing saw a steady stream of bickering between Boxer and her Republican counterpart, James Inhofe, renowned as a climate change sceptic.

Republicans argued that Barack Obama's proposed carbon cap legislation would be costly. "I will certainly oppose raising energy costs on suffering families and workers during an economic crisis when the science says our actions [to combat climate change] will be futile," said Kit Bond, a Republican senator from Missouri.

The Republican minority on the committee also invited testimony from Professor William Happer, a physicist at Princeton University, who is a well-known climate change sceptic. "It's still not as warm as it was when the Vikings settled England," said Happer.