Just two days after US President Barack Obama called China a free rider in an interview with New York Times columnist Thomas Friedman, US Secretary of State John Kerry, in his Asia policy speech at the East West Center in Hawaii last Wednesday, listed climate change a common cause for the two largest greenhouse gas emitters on the planet.

That is indeed true. Although China's per capita carbon emission last year was less than that in Europe and only about one-third the level of the United States, its environmental challenge as evidenced by the worsening pollution in air, water and soil has become ever more serious.

However, as some Americans tout the fact that the US has pumped less carbon dioxide into the atmosphere than years ago, they ignore the fact that a large portion of the greenhouse gases produced in China and other pollutions are a result of China being the manufacturer of the world.

That is to say that much of the pollution and heavy carbon emission industries have been relocated from the US and other developed countries to China in the past decades.

In a study published early this year in the journal of Proceedings of the National Academy of Sciences, researchers, led by Lin Jintai of Peking University in China, found that, in 2006 alone, about one-fifth to one-third of China's air pollution, including sulfur dioxide, nitrogen oxide and carbon monoxide, were associated with export manufacturing. And about one-fifth of the amounts were directly linked to the production of goods for the US market. As China surpassed the US and Germany to become the top trading nation in the world in the past years, China's exports to the US have also shot up more than 50 percent from 2006 to 2013, meaning that US consumers have left an even deeper carbon footprint in China.

If a free rider is one who enjoys services without paying the cost, such pollution relocation is worse because it causes huge damages to the host nations and their people. For example, the Chinese government has to spend an enormous amount of taxpayers' money, estimated at 5 trillion yuan ($806 billion) in the 12th Five-Year Plan (2011-15) according to China's ministry of environmental protection, to tackle the environment challenge, and the 1.4 billion Chinese people are bearing the consequences of living in a deteriorating environment.