UNITED NATIONS, Sept. 25 (Xinhua) -- UN Secretary-General Antonio Guterres on Tuesday asked for global action to tackle the "direct existential threat" of climate change.

"We have reached a pivotal moment. If we do not change course in the next two years, we risk runaway climate change," Guterres told world leaders who gathered here for this year's high-level General Debate of the General Assembly.

"Climate change is moving faster than we are, and its speed has provoked a sonic boom SOS across our world."

According to the World Meteorological Organization, the past two decades included 18 of the warmest years since record-keeping began in 1850. A few weeks ago, the thick permanent sea ice north of Greenland began to break up. The concentration of carbon dioxide in the atmosphere is the highest in 3 million years and rising, he noted.

He said there is a need for greater ambition and a greater sense of urgency.

Governments need to be courageous and smart, he said. "That means ending trillions of dollars in subsidies for fossil fuels. It means establishing an adequate price for carbon. It means stopping investments in unsustainable infrastructure that lock in bad practices for decades to come."

"Our future is at stake. Nothing is immune -- climate change affects everything and everything can be undermined. Keeping our planet's warming to well below 2 degrees is essential for global prosperity and the security of nations," said the UN chief.