Customers are getting more than cash when they visit hundreds of Citigroup ATMs across the city. Vandals have defaced the machines with stickers produced by the environmental organization Rainforest Action Network, part of a nationwide campaign to protest the bank’s financing of new coal-fired power plants.

The square stickers, which bear the slogan “we fund global warming,” depict black soot pouring from the two i’s in Citi. They are among many guerilla techniques employed by a growing contingent of activists who say they’re tapping into an upsurge in anxiety over climate change.

“There’s a vibrant, sizable movement right now,” a grassroots organizer for RAN, Scott Parkin, said. The group, which is also targeting Bank of America, said participation has surged since Hurricane Katrina three years ago. The damage from the storm “really kind of woke people up about the issue,” Mr. Parkin said.

RAN launched its campaign against the banks this fall because the two are “the top financiers of new coal-fired power plants,” the director of the group’s Campaign for Global Finance, Rebecca Tarbotton, said. According to its Web site, more than 150 new coal-fired power plants are in development in America and, if built, would emit an estimated 600 million tons of greenhouse gases a year.

Aiming to pressure the banks to refuse to finance future coal-fueled plants, the group has used a variety of methods to raise awareness of the issue. In addition to handing out flyers, protests, and speaking at the banks’ shareholder meetings, the group has engaged in “counter-branding,” subtly altering the Citigroup and Bank of America logos to impart an environmental message.

The group printed thousands of stickers and has sent hundreds of them to RAN affiliates and other groups across the country, Ms. Tarbotton said. The stickers and signs are easy to copy, so it’s unclear how many of them are now circulating. “I often see our stickers in places and I have no idea how they got there,” she said.

The banks will likely have to respond to the campaign, if only to lessen any chances it will turn into a widespread boycott, a professor of finance at Northeastern University, Harlan Platt, said. “Thirty years ago, boards of directors were simply looking at the bottom line,” he said. “In today’s world, with YouTube and Facebook, companies have to be on their toes and be concerned about social responsibility.” A Citigroup spokesman, Luis Rosero, said the bank announced in May 2007 that it plans to invest $50 billion in alternative energy and clean technology over 10 years.

“Citi sees the importance of developing the critical building blocks that are needed to help transition the energy infrastructure of entire economies and address what is a complicated and society-wide challenge,” he said.

A Bank of America spokesman, John Yiannacopoulos, said the bank will continue to finance coal and other utility companies. “However, we are working with these companies to help them develop and use cleaner and renewable energy sources,” he said.