Opponents of climate change action have long relied on two main rhetorical weapons. The first one, which has been in use since the 1980s, is to argue the the science linking human activities to global warming is uncertain or flawed. But with polls showing the American public unpersuaded by this argument, many opponents have focused more on the assumption that tackling climate change will be too costly.

The U.S. Chamber of Commerce, for example, has opposed the Obama administration's climate change initiatives, arguing that they'll be job killers.

See also: Top 5 Takeaways From the United Nations Climate Summit

Until the past two weeks, that economic argument had gone virtually unchallenged by senior Obama administration officials and other leaders.

That is no longer the case.

For example, in an unusually blunt policy address on Thursday, EPA Administrator Gina McCarthy called opponents of climate change action "sad," and portrayed restrictions on greenhouse gas emissions as a net economic winner. In June, the EPA put forward far-reaching proposed regulations to limit emissions from existing power plants, known as the "Clean Power Plan."

That plan is deeply unpopular in states dependent on coal-mining for jobs and revenue, such as Kentucky and West Virginia.

In a speech to the environmental economics group Resources for the Future in Washington, D.C., McCarthy said avoiding action on global warming will be far more expensive in the long run, compared to reducing emissions and hardening infrastructure to cope with climate impacts.

"... When it comes to climate change, the most expensive thing we could do, is to do nothing," she said. "The bottom line is: We don't act despite the economy, we act because of it."

Global annual average temperature (as measured over both land and oceans). Red bars show temperatures above the long-term average, and blue bars indicate temperatures below the long-term average. The black line shows atmospheric carbon dioxide (CO2) concentration in parts per million (ppm). Image: National Climate Assessment

McCarthy lambasted climate science skeptics, who doubt the science showing that manmade emissions of greenhouse gases are causing climate change, as well as those who say reducing emissions will be too costly to pursue.

"... It's sad to see a small but vocal group of critics hide behind the word "economy" to protect their own special interests; when the truth is, climate action is in everyone's best interest. It's worrisome when we hear those critics say, quote "I'm not a scientist, but climate action is going to ruin the economy," she said.

"Simply put: the economy isn't a reason to fear action, it's a reason to take it."

McCarthy's speech came two days after a landmark U.N. Climate Summit, and more than a week after an international assessment about the economics of climate change showed that taking action sooner rather than later could be an economic boon. That report found that continuing down a high carbon emissions path could put economic growth at risk due to increasingly severe climate change impacts, from sea level rise swallowing parts of coastal megacities to heat waves and droughts spurring massive migration within and between countries.

Another report, this one by a bipartisan group of American business leaders, warned that if greenhouse gas emissions continue unabated, by 2050, between $66 to $106 billion of existing coastal property in the U.S. will be below sea level. This would grow to up to $507 billion by 2100.

Other Obama administration officials have also made the case that the economic benefits of reducing emissions outweigh the costs, including Shaun Donovan, who is the director of the White House Office of Management and Budget (OMB).

Companies seek ways to price global warming risks

At the Climate Summit, the World Bank revealed a list of more than 1,000 companies that support putting a price on carbon, which would incentivize less carbon intensive investments. In addition, a separate report from the Carbon Dislosure Project found that major companies including Delta Air Lines, Google, and Disney are already using an internal "shadow" price on carbon to take it into account when making infrastructure investments and other high-level decisions.

In an interview with Mashable at the Social Good Summit in New York on Sept. 22, World Bank Group President Jim Yong Kim said countries and major corporations are beginning to perceive climate change as a serious economic risk that must be minimized.

Kim said the discussions about carbon pricing have always happened with the government and companies meeting separately, but now there is increased interest in having the two communities come together.

“In one way or another, we will get to the point of actually putting a price on something that actually has a cost,” he said of greenhouse gas emissions.