U.N. Secretary-General Antonio Guterres warned a climate summit of world leaders on Wednesday there is not much time left to avert an environmental catastrophe.

"We must make up for time lost to foot-dragging, arm-twisting and the naked greed of entrenched interests raking in billions from fossil fuels," Guterres told world leaders at the start of the daylong General Assembly symposium at United Nations headquarters in New York.

After Guterres' opening remarks, heads of state representing 34 nations were set to speak on the importance of sustainability, including Brazil, Pakistan, South Africa, Canada, the European Union and Tuvalu, a Polynesian island nation imperiled by rising sea levels. Brazilian President Luis Inacio "Lula" da Silva withdrew after falling ill. His environment minister was expected to speak in his place.

The two largest economies that also are the biggest polluters - the United States and China - were noticeably left off the speakers' list. Only nations who planned to increase their pledges to slash emissions were invited to the podium. U.S. Special Envoy on Climate John Kerry was in attendance.

Guterres said that the global shift from fossil fuels to renewables is underway, but that progress is decades overdue. The harms of climate change, he said, are hitting the developing world the hardest, and the Global North is mostly to blame.

"Many of the poorest nations have every right to be angry, angry that they are suffering from a climate crisis they did nothing to create, angry that promised finance has not been materialized, angry that their borrowing costs are sky-high," the U.N. chief said.

Guterres said he is optimistic that the climate summit will help persuade some of the richest countries and corporations to meet the U.N.'s worldwide target of net-zero emissions by 2050. He said he is hopeful the powers that be take sharp action and invest more in the future of renewable energy.

But the focus was not solely on curbing pollution in the wealthiest echelons of the Global North.

Kenyan President William Ruto urged countries in the Global South to pool together their trillions of dollars in collective resources to independently finance climate initiatives.

"Neither Africa nor the developing world stands in need of charity from developed countries," he said.

Ruto floated another progressive idea in his address: a universal tax on the sale of fossil fuels.

The climate summit also featured executives from Allianz, the travel insurer, as well as numerous key global lenders, such as the World Bank and the International Monetary Fund. Additionally, the mayor of London and the governor of California were set to speak.

A U.N. report released earlier this month noted global temperatures are on track to rise 1.5 degrees Celsius above the pre-industrial average in the next decade, an increase widely recognized as a tipping point in the battle to reverse climate change.

Some information for this story was provided by Reuters.