The first time I heard there was a $150 billion market in carbon I thought I’d misheard the letter “b.” That was about a year ago. I quickly discovered that, indeed, it was “billion,” not million. But how could you spend $150 billion on something that barely anyone—in the U.S. anyway—understands?

The number is the value of carbon offsets that have been traded on the international carbon market. The market was created in 2005 by the cap-and-trade system established in countries that signed the Kyoto Protocol (every developed country has ratified the treaty except the United States). In other words, “cap,” meaning government-mandated emission limits, and “trade,” meaning you can buy your way out of those limits through investments that reduce emissions elsewhere.

Roll forward five years, and carbon is now the fastest-growing commodity in the world, with companies buying and selling carbon credits much like pork bellies or silver. Once the U.S. establishes its own set of emission limits—and offsets to meet them—the market for carbon is expected to explode into a multitrillion-dollar business.

Finding out what lies behind those numbers led the Center for Investigative Reporting and FRONTLINE/World to launch our Carbon Watch series. Our first step in this ongoing investigation took us far from negotiations in Washington deep into the forests of Brazil. Three major U.S. companies, each with their own huge carbon footprint, invested in 50,000 acres of forestland on Brazil’s southeast coast. By agreeing not to cut down the trees, and thus sequestering the carbon dioxide rather than releasing it into the atmosphere, they’re hoping to obtain offset credits.

It’s something we’re likely to see more of: Every major bill now being considered by the U.S. Congress includes the potential for American companies to invest in forests like this one and use them as credits to either offset their own emissions or sell on the market.

While most Americans are not aware of what this market is, people in Brazil certainly are. While we visited the carbon reserve in Parana state, my colleague Andres Cediel, the producer of this segment, filmed a high school class in the port town of Antonina, on the edge of Brazil’s Atlantic Forest, one of the most bio-diverse regions on earth. The teacher turned the blackboard over to her star student and asked her to present the basic principles behind how a Brazilian forest could be turned into an offset.

It was a scene I would not forget: You can watch it for yourself in the clip above. If a student in Brazil could explain to her peers how the forests in her backyard are being turned into an offset by American companies thousands of miles away, then we should start trying to figure this out. The resulting FRONTLINE/World video and companion print story in the November/December issue of Mother Jones begin the first chapter.

We’ll be headed to Washington shortly and next month to Copenhagen where all the key political and financial players will be hammering out a new global climate deal. Stay with us as we investigate the financial, geopolitical and environmental forces at work in this new economy of global warming.

Mark Schapiro specializes in international and environmental stories. His award-winning work appears in all media: in publications such as Harpers, The Atlantic, Mother Jones and Yale 360; on television, including PBS FRONTLINE/World and KQED; on public radio including Marketplace; and on the web. He is currently writing a book for Wiley & Co. investigating the backstory to our carbon footprints. His previous book, "EXPOSED: The Toxic Chemistry of Everyday Products and What’s at Stake for American Power," reveals the health and economic implications of the tightening of environmental standards by the European Union.