In an open letter published in Chicago Tribune, Swedish fast-food chain Max urges its American peers, like McDonald’s and Burger King, to sell more vegetarian burgers.

Why? Because beef production is one of the world’s largest source of greenhouse gas emissions and global warming.

With the letter, the Swedish chain aims to challenge the industry to begin its climate adaptation, and shows that doing so is also good for the bottom line.

“If we are part of the problem, we have a responsibility to be part of the solution” says Richard Bergfors, CEO of Max Hamburger Restaurants. “As a hamburger chain we want to offer our guests the opportunity to make climate-smart choices.”

In January, Max launched five new green menu items, one of which is completely vegan. Now, every fourth burger they sell is green. In some restaurants, it’s even every second.

The publication of the letter comes as the world’s hamburger tycoons meet in Chicago for the international fast food fair, NRA Show 2016.

Founded in northern Sweden in 1968 by Curt Bergfors and Britta Andersson, Max is the most profitable restaurant chain in Sweden, outperforming both McDonald’s and Burger King, it said in a statement. It has some 120 restaurants across the region.