"Right now we are facing a man-made disaster of global scale, our greatest threat in thousands of years, climate change", he told attendees at the 24th Conference of the Parties.
Renowned nature broadcaster Sir David Attenborough has told world leaders that climate change could lead to collapse of civilizations, and much of the natural world.
But Buhari, while speaking in Poland, where he is attending a climate change summit, said he would soon celebrate his 76th birthday, adding that "a lot of people hoped that I died during my ill health". 2018 is the year the countries committed to putting a plan in place for carrying out the agreement, so time is running out.
However, US President Donald Trump's decision to withdraw from the Paris accord has dented trust among vulnerable nations, who fear there is not enough cash available to help them adapt to our heating planet.
"Developed nations led by the U.S. will want to ignore their historic responsibilities and will say the world has changed", said Meena Ramam, from the Third World Network advocacy group. "If we don't take action the collapse of our civilisations and the extinction of much of our world is on the horizon".
To maximise the chances of success in Poland, technical talks began on Sunday, a day early, with delegates from almost 200 nations debating how to meet the Paris target of limiting global warming to less than 2 degrees Celsius and, ideally, aim for a safer 1.5 deg C, which would limit the damage from weather extremes and rising sea levels.
President Trump last week said he "doesn't believe" his own administration's report on the economic impact of climate change, which projected a hit in the hundreds of billions of dollars thanks to climate change.
Patti Lynn, executive director of the Corporate Accountability campaign group, called for nations to agree to the global greenhouse gas draw-down upon which the fate of the Paris agreement now rests.
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The Katowice talks are billed as the most important United Nations conference since Paris, coming ahead of an end-of-year deadline to agree a "rule book" on enforcing action.
"We need to act urgently, and with audacity".
"The world is at a crossroads and decisive action in the next two years will be crucial to tackle these urgent threats", they said in the joint statement.
He said governments should embrace the opportunities rather than cling to fossil fuels such as coal, which are blamed for a significant share of human-caused greenhouse gas emissions.
Governments around the world are taking aim at combatting climate change. "For many people, regions and even countries, this is already a matter of life or death".
Guterres called for a "huge increase in ambitions" during the two weeks of negotiations in Poland, adding "we can not afford to fail in Katowice".
The World Bank on Monday announced $200 billion in climate action investment for 2021-25 - a major shot in the arm for green initiatives but one which needs bolstering by state-provided funding.
He claimed that using "efficient" coal technology did not necessarily run counter to climate action.