A prominent climate change skeptic told Congress on Monday he no longer doubts that global warming is real and caused by humans, and joined other scientists in urging action to stop it.

Physicist Richard Muller, director of the Berkeley Earth Surface Temperature Project, whose two-year research was funded in part by a foundation formed by the conservative billionaire Koch brothers, said he could find no bias in other studies.

"We confirm that over the last 50 years, temperature has risen 0.9 degrees Celsius, or 1.6 degrees Fahrenheit. This is the same number that the IPCC (UN Intergovernmental Panel on Climate Change) says."

Muller told the House Committee on Natural Resources that while he remains cautious about the extent to which humans have played a role, he now hopes other climate skeptics will come on board with his findings.

"As they read and study our papers, I am hoping that many of them will reflect my belief that they are open-minded and come to agree that yes, climate change temperature increase certainly has happened," he said.

"The amount that is due to humans is still open and there are very big uncertainties in that," Muller added, urging continued study of the matter.

"In my mind, humans have contributed to climate change. The real issue is how much?"

Muller's appearance on Capitol Hill was his first since his research was released last month, and comes just weeks after the Department of Energy reported a six percent increase last year in carbon output worldwide, the biggest jump ever.

Ranking committee Democrat Ed Markey lamented the United States' failure to act and applauded Australia's recent approval of a carbon tax to force its coal-fired power stations and other major emitters to "pay to pollute."

"Other countries are taking the threat seriously. Australia just passed a set of bills designed to reduce carbon pollution and positioned their country to compete in the global clean energy race," Markey said.

"The attacks on climate science have been a colossal distraction from the debate we should be having Congress on what actions should be taken to reduce pollution, create jobs, reclaim our lead in the clean energy race."

His colleague, Democrat Henry Waxman, said the Republican-controlled Congress had voted 21 times to block actions that would have addressed climate change.

"History will look back on this science denial with profound regret," Waxman said.

"When a prominent skeptic publishes a study determining that global warming is real, that is information Congress needs to hear."

Also on the panel was Ben Santer, research scientist at Lawrence Livermore National Laboratory, who told lawmakers that international scientists have considered a host of different factors in their research, including rainfall, sea level pressure, continental runoff, surface humidity, and atmospheric moisture.

"The bottom line message in each case is, natural causation alone cannot explain the changes we see," Santer said.

"People sometimes incorrectly say, 'You climate scientists never consider alternate hypotheses, alternate plausible explanations of the observations.' That is not true," he added.

"We routinely consider such alternate hypotheses and try and determine whether they fit the available observations. They do not."

William Chameides, dean of Duke University's Nicholas School of the Environment in North Carolina and vice chair of the National Academies' Committee on America's Climate Choices, said there is a "pressing need for substantial actions."

Chameides helped produce a report earlier this year that urged the US government to reduce greenhouse gas emissions through a carbon pricing system and invest in research and development to mitigate its harmful effects.

"We know we are facing a risk. We know that the longer we take to act, the worse that risk is," he told lawmakers.

"With each ton of greenhouse gasses that we put into the atmosphere, we are increasing the risks of dangerous impacts of climate change, and those risks will be with us for many, many years. Perhaps 1,000 years," he added.

"Twenty percent of the C02 that we emit today when we all drive home and I fly home tonight, will be in the atmosphere, warming the atmosphere in 1,000 years," he said.

"It is something to think about very carefully when we think about our legacy for future generations."