In one of the largest single donations made yet to the fight against climate change, New York City mayor Michael Bloomberg has announced he will donate $50 million of his own money to help the Sierra Club shut down America’s coal-fired power plants and replace them with green energy.

As a brutal heat wave broke hundreds of temperature records across the United States, Bloomberg stood in 100-degree heat on Thursday morning outside the GenOn power plant in Alexandria, Virginia. Joined by Sierra Club executive director Michael Brune, Bloomberg said he hoped his philanthropic contribution would improve public health both at the local level—by reducing emissions of the mercury, dioxin and other pollutants that are released when coal is burned—and at the global level, by limiting the severity of climate change.

“If we are going to get serious about reducing our carbon footprint in the United States, we have to get serious about coal,” said Bloomberg, according to his prepared remarks. “Coal is a self-inflicted public health risk, polluting the air we breathe, adding mercury to our water, and [is] the leading cause of climate disruption.”

Bloomberg’s gift will help the Sierra Club build on the biggest victory against climate change scored to date in the United States: the de facto moratorium on new coal plants achieved in recent years by local activists working under the banner of the club’s “Beyond Coal” campaign. “Since we began targeting coal in 2003, we have blocked 153 planned coal plants from being built,” Brune told The Nation. “Since January 2010, we’ve shut down ninety-one existing plants. Closing those ninety-one existing plants alone will prevent 114 million tons of carbon dioxide equivalent from entering the atmosphere, which is about the same as taking 20 million cars off the road.”

With Bloomberg’s help, the Beyond Coal campaign now hopes to “revoke the social license for burning coal in this country, just like the social license for smoking cigarettes was revoked,” said Brune. Although many progressives have criticisms of Bloomberg—in fact, Brune notes, the local Sierra Club chapter did not endorse him when he ran for a third term as mayor two years ago—Bloomberg makes a logical partner for such an effort. As mayor, he has championed high-profile initiatives to ban cigarette smoking and slash greenhouse gas emissions. “Some things a modern society can move beyond,” Brune argued. “We don’t cut down old-growth redwood trees anymore to make decking material, and we don’t need to burn coal to keep our iPhones charged.”

The only fossil fuel more dangerous than coal to public health and climate stability is tar sands, and they too are being newly targeted by renewed grassroots opposition. Author and activist Bill McKibben said he has signed up some 1,100 volunteers, including actor Danny Glover, writer Naomi Klein and labor leader Joe Uehlein, to join him in committing civil disobedience in front of the White House beginning on August 20. Their goal is to pressure President Obama to block the proposed Keystone XL pipeline, designed to transport tar sands from Canada to the Gulf Coast. The Canadian province of Alberta has the world’s largest deposits of tar sands, a thick, sticky form of crude oil that requires vast amounts of energy to dislodge from the earth and leaves behind extensive land and water pollution.  Even more environmentally destructive is the burning of tar sands, which have a carbon footprint 82 percent larger than conventional oil, according to the US Environmental Protection Agency.

“The federal government’s most important climate scientist, Jim Hansen of NASA, has said that if we significantly tap into the tar sands, it’s essentially Game Over for the climate,” said McKibben.  “There’s enough carbon embedded in the Alberta tar sands that burning all of it would raise the atmospheric concentration of carbon dioxide by 200 parts per million. [The current concentration is 391 ppm.] By definition, that also means there is enough oil in there to generate trillions of dollars. And trillions of dollars have a way of exerting a lot of pressure.”

Both the tar sands civil disobedience and the redoubled attack against coal represent a departure from the tactics and strategies the environmental movement as a whole has pursued since Barack Obama’s election in 2008. Environmental groups spent over $100 million (some estimates run as high as $300 million) urging Congress and the administration to pass climate legislation and lead international efforts to sign an ambitious agreement limiting greenhouse gas emissions. Most of the effort was focused on inside-the-Beltway lobbying and closed-door meetings rather than on rallying popular support out in the country, and it failed miserably. By contrast, the Beyond Coal and tar sands campaigners believe that only grassroots people power can overcome corporate and right-wing resistance and secure real progress against climate change.

“We learned from [the first phase of the Beyond Coal campaign] that grassroots organizing wins almost every time,” said Brune. “So we’re doubling down [on that] now, putting our faith not in Congress but in the power of local communities to protect their health and planet.”

Most of Bloomberg’s $50 million, which will be dispensed over the next four years, will go to hiring more grassroots organizers. “We will have two hundred organizers waking up every day and working on nothing else but figuring out how to shut down the remaining coal plants in this country and replace them with clean energy,” said Brune. As in the first phase of the Beyond Coal campaign, organizers will work with a wide range of stakeholders—local citizens, elected officials, public health workers, faith and youth groups—“to identify the main sources of pollution in a community and then figure out how that coal plant can be shut down and keep the lights on at the same time.”

Challenging the coal industry’s main defense, Brune argues that shutting coal plants will actually save consumers money. “Clean energy, starting with energy efficiency, is cheaper than dirty energy,” said Brune, “which is why solar and wind are growing dramatically.” In Colorado, he adds, where state law requires 30 percent of electricity to come from renewable sources by 2020, the main utility company, Xcel, recently announced that it will meet that target in 2012—eight years ahead of schedule.

In the case of the tar sands protest, McKibben said his model is the civil disobedience that anti-apartheid activists practiced in the 1980s hundreds of people were arrested outside the South African embassy in Washington. Like that action, the tar sands civil disobedience will not be merely a one-day event; it will continue day after day, from August 20 until at least September 3, so visibility and pressure can build. Also like the anti-apartheid action, the tar sands protest will be dignified in dress and demeanor. “I want to get across to people watching from afar that the radicals on this issue are not us,” McKibben said. “The radicals are the oil companies, the Koch brothers, the Chamber of Commerce. Abbie Hoffman on his most radical day never thought of anything as radical as changing the chemical composition of the atmosphere.”

“When Obama came into office he said, ‘I need you to keep pressing me—in essence, to give [him] room to operate,’” McKibben added. “My hope is that [this action] will demonstrate that there’s not only an overwhelming scientific reason to reject this [pipeline], there’s also strong political support for doing that. And for once Obama is the one who makes the call. Unlike on the climate bill, [Republican congressmen] Fred Upton and James Inhofe don’t get to say a word on this. We often think of the president, incorrectly, as having unlimited power. But in this particular case, Obama actually does, and it will be very revealing to see what he does.”