Hopes of a breakthrough in international climate change ambitions are being downplayed for a landmark meeting in New York. The United Nations climate action summit looks set to disappoint the thousands of campaigners who will take to the city's streets just days earlier.

The summit is arguably the most important moment for climate change since the Paris climate deal was agreed in 2015. A key part of the historic agreement was that by 2020, countries would "ratchet up" the carbon-curbing plans they put forward for Paris, which are insufficient to meet the accord's goals.

UN secretary general António Guterres has called on leaders to come to New York with concrete, realistic plans, rather than "beautiful speeches".

He has set the bar high for heads of state, who are expected to include Narendra Modi of India, Emmanuel Macron of France, Angela Merkel of Germany, and the UK's Boris Johnson.

Guterres has made four specific requests: carbon neutrality plans for 2050, ways to tackle fossil fuel subsidies, taxing carbon and no new coal power beyond 2020.

Three days before the meeting, Greta Thunberg was among the thousands that took to New York's streets for a "global climate strike", with potentially millions more joining worldwide. Two days after the UN summit, scientists will issue a special report on how global warming will affect the planet's oceans and frozen corners.

But despite the spotlight these events will shine on the summit, hopes are relatively low. "I don't think we should expect some huge breakthrough," says Nicholas Stern of the London School of Economics.

A source close to the UK government, who does not want to be named, says: "It is not quite where everybody hoped it would be at this point."

This is partly due to the EU and the US. The leadership role that the latter played ahead of the Paris summit was crucial to unlocking commitments from China, but such leadership has been absent under Donald Trump, who has kickstarted the process of withdrawing the US from the Paris accord in 2020.

The EU's failure this summer to adopt a goal of net zero emissions for 2050 also hurt momentum this year.

Despite the gloom, anywhere between 60 and 100 countries are expected to come with a plan on 23 September. No major economies are expected to announce a stronger nationally-determined contribution (NDC), UN jargon for countries' carbon-curbing plans, but some smaller ones may. Many will come with a commitment to commit later.

"For me it's a really important staging post, an inflection point where at leader level we get a sense of how transformational this can be for economies," says Nick Bridge, the UK's top climate envoy. He believes Guterres has been right to make bold demands.

"A lot of this is getting back to the evidence and the science. Are we meeting what we need to do? No."

The ambition in existing NDCs needs to increase five times for the world to limit temperature rises to 1.5°C and three times for 2°C, the tougher and minimum targets of the Paris deal, says Niklas Hagelberg of the UN Environment Programme.

The expectation is that most countries will submit a new NDC in the first half of next year, ahead of a key UN climate conference in November likely to be co-hosted by the UK and Italy in Glasgow.

If current pledges are delivered, the world will warm by around 3°C by the end of the century, says Niklas Höhne of Climate Action Tracker (CAT). Those pledges, which include China peaking emissions by around 2030, will see emissions continue to rise for the next 10 years.

By contrast, for a 2°C world, over that period they must fall 30 percent, and 50 percent for a 1.5°C world.

"We are not a little bit off, we are really far off," says Höhne. At best, the new NDCs in aggregate might shave something in the order of 0.1°C off future warming, rather than a dramatic change like 0.5°C, he says.

However, he sees reasons for optimism beyond national governments.

An analysis by CAT found that if cities, region and business deliver all the emissions cuts they have promised by 2030, by that point the world could still stay under 2°C, albeit not 1.5°C. "That is encouraging," he says.

Stern doesn't think this month's UN summit will be the point when we see promises materialise that close the gap between 3°C and "well below" 2°C as Paris demands.

"I think the most important thing is the shared recognition of the magnitude of the task ahead," says Stern.

This article originally appeared in New Scientist. It is republished here as part of ScienceAlert's partnership with Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.