Daniel Dudek has visited China more than 200 times for various environmental projects. [Photo provided to China Daily]

Two things have made US environmental expert Daniel Dudek very happy in the past few weeks.

The first was when UN Secretary-General Ban Ki-moon announced on Oct 5 that the Paris Agreement on climate change is set to enter into force on Nov 4. The second: Member states of the UN civil aviation agency agreed on a new standard to control greenhouse gas emissions from international airline flights through a market-based plan.

As vice-president of the New York-based nonprofit Environmental Defense Fund, the 69-year-old Dudek believes the market can be used to solve environmental problems.

The joint effort of the US and China on environmental protection, he adds, is important to the world.

The leaders of the world's two biggest greenhouse gas-emitting countries, President Xi Jinping and President Barack Obama, together announced their plans to ratify the Paris Agreement on the eve of the G20 summit in Hangzhou last month－"which pushed other countries to also make their commitment", Dudek says.

"Without that, I think, there would be no agreement."