The US East Coast is sweltering in a record-hot summer but it could endure twice as many sweaty days by mid-century without action on climate change, an environmental group said Wednesday.

The US capital, Washington, is on track for 50 days in 2010 in which the mercury crosses 90 degrees Fahrenheit (32 Celsius), a number that could rise as high as 100 by 2050, the National Wildlife Federation said in a study.

The group, which supports action against climate change, said that the number of hot days in 2050 could be kept to 55 if polluters reduce carbon emissions.

The US National Oceanic and Atmospheric Administration has calculated that global temperatures between January and June 2010 were the warmest on record since 1880 when reliable readings began.

July was also abnormally hot throughout the eastern and southeastern United States.

The National Wildlife Federation said the temperatures should be a wake-up call for the US Congress, which last month put on hold legislation that would set up the first nationwide plan to curb carbon emissions.

"The message is that there is a price that we pay for not taking action on global warming," said Tony Iallonardo, a spokesman for the group.

"There's a price in terms of lives and in terms of the structure we're going to have to put in place to prepare better for global warming, including getting seniors and at-risk populations ready for the health risks," he said.

Climate skeptics often voice concern about the costs of transitioning to a low-carbon economy and point out that hot temperatures are not consistent.

The US East Coast was hit by record snowfall last winter. Parts of the western United States have had a cooler than usual summer.

"It's not that the entire planet is warmer simultaneously," Iallonardo said. "Some places will have heat waves at times and others will not, but certainly as a whole the data is showing us that the planet is warming."

The UN's climate panel has warned that without steep cuts in greenhouse gas emissions, the global thermometer could rise by 6.0 degrees Celsius (10.8 degrees Fahrenheit) compared with pre-industrial levels by the end of the century, making large swathes of the planet unlivable.

However, some scientists say it is difficult to judge climate shifts in time-frames shorter than centuries.