Photo taken on April 22, 2016 shows the signing ceremony of Paris climate deal at the United Nations headquarters in New York. Photo/Xinhua

The signing is the first step toward ensuring that the agreement reached in Paris last December goes into force. The next step will be for each nation to unilaterally ratify the agreement, which in many cases will involve passage by national legislatures.

"We will work with the rest of the international community for early accession to the agreement and to be sure it's early entry into force," said Zhang.

With his granddaughter on his lap, US Secretary of State John Kerry signed the agreement. "The United States looks forward to formally joining this agreement this year," he said. "We call on all of our international partners to do so."

It reaffirmed the joint commitment to tackle climate change by China and the US, which account for 38 percent of the world's greenhouse gas emissions.

"China has set the target of achieving the peak of CO2 emissions around 2030, making the best effort to peak earlier," said Zhang.

China plans to decrease its CO2 emissions by 18 percent in the second half of the decade, according to the 13th Five-Year Plan. The government will control total energy consumption and carbon intensity, pushing for near-zero emission demonstration projects and establishing a nationwide carbon market.

"We will deepen the South-South cooperation on climate change," said Zhang. China will launch new projects under the new South-South Cooperation Climate Fund this year to help other developing countries.

Ban has always praised such effort. "We must support developing countries in making this transition. The poor and most vulnerable must not suffer further from a problem they did not create," he said during his speech.