WASHINGTON – The increase in global temperatures has slowed over the past 15 years, a noted climatologist told a Senate panel last month, but the impact of climate change caused by man-made and natural forces is nonetheless having a dire impact on the planet.

Heidi Cullen, chief climatologist at Climate Central, an independent research organization, testified before the Senate Environment and Public Works Committee that climate change is manifesting itself in the U.S. through extreme conditions like heat waves, wildfires, heavy downpours, floods, droughts and rising sea levels.

These conditions, Cullen said, apparently are brought on by a change in the temperature balance between the arctic, an area warming twice as fast as the rest of the northern hemisphere, and the middle latitudes, thus changing the course of the jet stream, which is responsible for moving weather from west to east.

“Weather systems are moving more slowly, increasing the chances for long duration extreme events, like droughts, floods, and heat waves,” Cullen said. “The tendency for weather to get stuck in one pattern is going to favor extreme weather conditions.”

Cullen warned that “global warming has not stopped” and that “human impact on our climate system is significant.” Studies, she said, are showing that the effects are “largely irreversible,” creating a need for action before conditions worsen.

Cullen was one of several scientists and economists called before the committee to discuss the impact of climate change. President Obama has discussed reducing greenhouse gases and the Environmental Protection Agency is in the process of developing new regulations regarding emissions from coal-fired power plants, the country’s primary producer of carbon dioxide.

While most there agreed that problems are looming, there was testimony that concerns over global climate change are overblown and that the greenhouse gas reduction proposals championed by the Obama administration carry a hefty price tag.

Diana Furchtgott-Roth, a senior fellow at the Manhattan Institute, a conservative think tank, told committee members that since 2003 global temperatures appear to have plateaued despite rising greenhouse gas emissions from Asia and other emerging economies.

Greenhouse gases, like carbon dioxide, are supposed to have an adverse effect on the climate, Furchtgott-Roth noted. The fact that they have not led to a continued skyrocketing of global temperatures “is a puzzle.”

What’s more, she said, a study of the data over the past 100 years finds that there has been no steady increase in the number of hurricanes or floods despite the substantial rise in greenhouse gas emissions.

“Polls show that many believe protecting the environment is less important to Americans than economic growth,” Furchtgott-Roth said. “With the slowdown in many measures of global warming over the past decade, climate change is playing second fiddle to jobs.”

Even if rising greenhouse gas emissions are affecting the climate, she added, any actions taken by the U.S. would prove futile in the absence of changes by nations like China and India who are increasingly leaning on coal, the world’s most significant producer of carbon dioxide, as an energy source. The U.S. global share of greenhouse gases is 17 percent.

“Americans know that no reduction in global warming will occur if America reduces greenhouse gases without similar action by China and India, and these countries have not agreed to comparable steps,” Furchtgott-Roth said.

Republicans on the committee, all of whom have either expressed doubts about either global warming or the human impact on the phenomenon, were quick to embrace the Furchtgott-Roth testimony. Sen. David Vitter (R-La.), the ranking member, noted that Lisa Jackson, the former EPA administrator, similarly told the panel in a prior appearance that unilateral action by the U.S. on reducing greenhouse gases would have little impact.

“Have things changed so drastically in the last four years or so that now U.S. action alone can control the climate?” he asked.