Starbucks will give customers who bring a reusable tumbler to participating stores a free brewed coffee on Thursday, part of its effort to recruit customers to help reduce their shared environmental impact. For years, the company has offered a 10-cent discount on drinks for customers who bring their own mugs and tumblers.

Its goal is for all its cups to be reusable or recyclable by 2015. Those are tricky terms, because Starbucks' paper and plastic cups are recycled and composted in some parts of the country, but the company will not call them "recyclable" unless they can be recycled in most of the cities where it does business. It's working on that with governments, business partners, non-profits and other experts, who will take part in its second "Cup Summit" in Boston on April 22 and 23. (The first cup summit was in Seattle last May.)

Reusable cups -- porcelain cups provided by Starbucks and mugs and tumblers that customers bring themselves -- are part of the equation. The company said more than 26 million drinks were served in reusable cups in its company-owned stores in the U.S., Canada and the U.K. last year, saving about 1.2 million pounds of paper from going to landfills.

Still, Starbucks goes through 3 billion paper cups and 1 billion plastic cups a year. That quantity, along with Starbucks' bottled and canned drinks, motivated a shareholder proposal for more recycling that won 11 percent of the vote last month.

"While our cup has become an integral part of the coffeehouse experience over the years, it has also become an environmental concern," Ben Packard, Starbucks' vice president of global responsibility, said in a release.

Starbucks encourages customers to think about reusable cups the way they do reusable grocery bags, he said. The company's web site includes an "impact calculator" by the Environmental Defense Fund that figures out how many trees you can save by switching from paper cups to your own tumbler. Someone who drinks two cups a day would save 228 acres of trees. The site invites customers to pledge to use tumblers every day and encourage others to do the same through social tools like Facebook (where Starbucks now has nearly 6.6 million fans).