NEW YORK (NYTIMES) - The future is on the line.

As presidents and prime ministers arrive in Glasgow, Scotland, this week for a pivotal climate summit, the outcome will determine, to a large extent, how the world's seven billion people will survive on a hotter planet and whether far worse levels of warming can be averted for future generations.

Already, the failure to slow rising temperatures - brought on by the burning of oil, gas and coal - has led to deadly floods, fires, heat and drought around the world.

It has exposed a gaping chasm between the scientific consensus, which says humanity must rapidly reduce the emissions of planet-warming greenhouse gases to avert climate catastrophe, and what political leaders and many corporate executives have been willing to do.

"That we are now so perilously close to the edge for a number of countries is perhaps the tragedy of our times," said Prime Minister Mia Mottley of Barbados.

Tensions loom over the 12-day summit. Some poor countries hard hit by climate disasters are holding out for money promised, and yet to be delivered, by the industrialised nations that fuelled the crisis.

Polluting countries are pressing each other to cut their emissions while jockeying for advantage and wrestling with the impacts on their own economies.

Complicating matters, the need for collective action to tackle such an urgent, existential global threat comes at a time of rising nationalism. This makes the talks in Glasgow a test of whether global cooperation is even possible to confront a crisis that does not recognise national borders.

"I don't think you can solve the climate crisis on your own as a nationalist leader," said Ms Rachel Kyte, a former United Nations official and now dean of the Fletcher School at Tufts University.

"You depend on the actions of others." The science is clear on what needs to be done. Emissions of carbon dioxide, methane and other greenhouse gases driving up global temperatures need to be cut by nearly half by 2030, less than a decade away.

In fact, they are continuing to grow. The World Meteorological Organisation warned last week that the amount of heat-trapping greenhouse gases in the atmosphere had reached a record high in 2020 despite the pandemic and is rising again this year.

As a result, the average global temperature has risen by more than 1 deg C since the Industrial Revolution.

The scientific consensus says that if it rises by 1.5 deg C ,it will significantly increase the likelihood of far worse climate catastrophes that could exacerbate hunger, disease and conflict.

Limiting temperature rise to within the 1.5 deg C threshold has become something of a rallying cry for many powerful countries, including the United States.

That is not within reach: Even if all countries achieve the targets they set for themselves at the 2015 Paris Agreement, average global temperatures are on track to rise by 2.7 deg C by the end of the century.

The US climate envoy John Kerry, who had recently described the summit as "the last best hope" last week, tried to manage expectations.

"Glasgow was never, ever going to get every country joining up in Glasgow or this year necessarily," he said Thursday (Oct 28). "It was going to galvanise the raising of ambition on a global basis."

The goals of the summit are to have countries nudge each other to rein in their emissions, commit financial support to low-income countries to deal with the impacts, and iron out some of the rules of the Paris Agreement.

The agreement stipulated that countries come together every five years to update their climate action plans and nudge each other to do more. The five-year mark was missed because of the pandemic. The climate summit was postponed. Climate disasters piled on.

The pandemic is important in another sense. It offers a grim lesson on the prospects for collective action. Countries turned inward to protect their own citizens, and sometimes their own pharmaceutical industries, resulting in a starkly inequitable distribution of vaccines.

Half the world's population remains unvaccinated, mainly in countries of the global south.

"We've just experienced the worst part of humanity's response to a global crisis," said executive director of Climate Action Network Tasneem Essop.

"And if this is going to be the track record for addressing the global climate crisis, then we are in trouble. I'm hoping this is a moment of reflection and inflection."

Meanwhile, anger is mounting against official inaction. The streets of Glasgow are expected to fill with tens of thousands of protesters.

The main battle lines shaping up at the Glasgow talks, known as the 26th session of the Conference of Parties, or COP26, have to do with who is responsible for the warming of the planet that is already under way, who should do what to keep it from getting worse and how to live with the damage already done.

The venue is itself a reminder. In the mid-19th century, Glasgow was a centre of heavy industry and shipbuilding. Its power and wealth rose as Britain conquered nations across Asia and Africa, extracting their riches and becoming the world's leading industrial power, until the US took the mantle.

The largest share of the emissions that have already heated the planet came mainly from the US and Europe, including Britain, while the largest share of emissions produced right now comes from China, the world's factory.

In some cases, the divisions in Glasgow pit advanced industrialised countries, including the US and Europe, against emerging economies, including China, India and South Africa.

In other cases, they set large emerging polluters, like China and India, against small vulnerable countries, including low-lying island nations in the Pacific and Caribbean, which want more aggressive action against emissions.

Tensions over money are so profound that they threaten to derail cooperation.

In 2010, rich countries had promised to pay US$100 billion (S$135 billion) a year by 2020 to help poor countries address climate change. Some of that money has been paid but the full amount will not materialise until 2023, three years late, according to the latest plan announced by a group of industrialised countries.

Even more fraught is the idea of industrialised countries also paying reparations to vulnerable nations to compensate for the damage already done. Known in diplomatic circles as a fund for loss and damage, discussions about this have been postponed for years because of opposition from countries like the US.

Mr Kerry this week said he was "supportive" of the idea of assisting countries who can't adapt their way out of climate change, but remained concerned about opening the floodgates of liability claims.

Then there are tensions over whether countries are doing their fair share to reduce their emissions.

The Biden administration has pledged that the US will slash emissions by about half by 2030, compared with 2005 levels. But President Joe Biden's ability to reach that target is unclear, as legislation has been watered down and stalled in Congress, partly by a single Democratic lawmaker with ties to the fossil fuel industry.

The US has been leaning hard on China to set more ambitious targets in Glasgow. But so far, Beijing has said only that its emissions will continue to grow and decline before 2030.

China is wary of the US' ability to fulfil its emissions and finance targets, a skepticism only fuelled by Mr Biden's inability so far to get his climate agenda through Congress.

Besides, the two countries are locked in bitter tensions over a host of other issues, from trade to defence to cybersecurity.

While Mr Biden is in Glasgow, President Xi Jinping of China is likely to appear only by video, precluding any face-to-face discussions.

President Jair Bolsonaro of Brazil also plans to stay home. President Vladimir Putin of Russia is not going, either, but may offer remarks remotely.

India is unlikely to commit to phase out its heavy reliance on coal power to meet its growing energy needs, although it is quickly expanding solar power in its energy mix.

The most optimistic diplomats say countries will be forced to come around and cooperate.

"Because of the global nature of this threat," the Danish environment minister, Mr Dan Jorgenson, said, "you will see countries, in their own interest, work with countries they see as their competitor."

No matter what happens at the summit, success in battling climate change will be measured by how quickly the global economy can pivot away from fossil fuels. Coal, oil and gas interests, and their political allies, are fighting that transition. But a transformation is visible.

The global use of fossil fuels, which has been on a steady march upward for 150 years, is projected to peak by the middle of this decade, assuming that countries mostly hew to the promises they've made under the Paris accord, according to projections by the International Energy Agency. Wind and solar have become the cheapest source of electricity in some markets, coal use is set to decline sharply by mid-century, despite an uptick this year driven by increased industrial activity in China, and electric vehicles are projected to drive down global oil demand by the 2030s.

Global temperature rise has also slowed since 2015, when the Paris Agreement was signed.

Some see that as evidence that climate diplomacy is working. Most countries are doing what they signed up to do, which is to set their own climate targets and "egg each other on" to do better, said president of World Resources Institute Ani Dasgupta. The institute is a Washington-based research and advocacy group.

"The ratcheting up of ambition, we do see it happening," he said. "It's not happening fast enough." From her home in Barbados, PM Mottley sees another promising sign: Pressure on leaders of countries in the global north, as the dangers of climate change increasingly afflict their citizens. That includes the floods that killed nearly 200 people in Germany, Europe's richest country, and the fires that scorched homes in California, America's richest state.

"It is the populations of the advanced countries coming to the recognition that this is a serious issue that is causing the needle to move," she said. "It is that kind of domestic political pressure from ordinary people that is going to save the world in my view."