One month out from the COP26 climate summit, world leaders are under unprecedented pressure to decarbonise their economies and chart humanity's path away from catastrophic global warming.

But in the midst of a pandemic still raging in parts of the globe, and with countries already battered by climate-driven calamities pleading for help -- and money -- the negotiations in Glasgow are likely to be fraught.

The summit, already delayed a year by Covid-19, comes as the gap between what science says is needed to avert disaster and what governments are doing is larger than ever.

Addressing around 50 ministers on Thursday at the start of a pre-COP gathering in Milan, UN Secretary-General Antonio Guterres laid out the choice facing delegates in Glasgow: "We can either save our world or condemn humanity to a hellish future."

COP26 host Britain says the summit's main aim is to keep in play the 1.5 degree Celsius temperature goal enshrined in the 2015 Paris Agreement.

In August, the UN's Intergovernmental Panel on Climate Change dropped a bombshell report warning that the 1.5C threshold -- by far the most ambitious target of the Paris deal -- would be reached as soon as 2030.

By 2050, Earth will be 1.5C hotter than pre-industrial times, no matter what is done about planet-warming carbon emissions in the meantime, it said.

With a little over 1C of warming so far, the two years since the last UN climate summit have seen record-shattering wildfires in Australia and the US, tarmac-melting heatwaves in North America and Siberia, and massive flooding in Southeast Asia, Africa and Northern Europe.

The Paris deal requires nations to renew their plans to cut domestic emissions -- known as national determined contributions, or NDCs -- every five years.

Far from limiting warming to 1.5C, the UN says countries' latest submissions over the last year put Earth on course to heat a "catastrophic" 2.7C this century.

Britain's Boris Johnson summed up his hopes for Glasgow as: "coal, cars, cash and trees" -- meaning deals for global phaseouts of coal power and internal combustion engines, funding for climate-vulnerable nations, and mass tree planting.

But the actual to-do list for delegates at COP26 is not quite so concise.

For starters, six years after the Paris agreement was struck, countries still have not finalised the deal's "rulebook" that specifies how its goals are reached and progress measured.

Long-festering disputes include those over how carbon markets are governed, and a common timeframe for an interim "stock take" to see how each country's action stacks up.

Number of people vulnerable and exposed to climate change in the world as a consequence of temperature increases Number of people vulnerable and exposed to climate change in the world as a consequence of temperature increases Photo: AFP / Simon MALFATTO

Poorer nations, meanwhile, are demanding that richer ones finally make good during COP26 on a decade-old promise to provide $100 billion each year to help them decarbonise their grids and adapt to climate change.

Tasneem Essop, head of the Climate Action Network representing some 1,500 environmental groups, said that Glasgow was taking place after a harrowing few years for vulnerable populations.

"This COP is happening, unlike other COPs, at a time where all this burdens and suffering is sharply felt by the developing countries and in this context we have experienced rich nations who were unwilling to stand in solidarity with poor nations to supply the vaccine," she told AFP.

Essop said there was a huge "trust deficit" between nations already battling climate change and the historic emitters that helped to cause it.

The spectre of vaccine inequity is likely to loom large in Glasgow, with many representatives of poorer nations unable to afford a trip that would include expensive hotel quarantines.

Sonam Wangi, chair of the Least Developed Countries negotiating bloc, said as much this week, tweeting that he was "still concerned about the possibility of getting our delegates to #COP26".

COP26 President Alok Sharma this week sought to allay such fears by saying there had been a "very healthy registration" in participants and that more than 100 world leaders had already confirmed they would attend.

Observers say there are some positive signs, with the US announcing a doubling of overseas climate aid and China saying it will cease new coal production abroad, both in recent weeks.

But for Alden Meyer, a veteran of UN climate talks and a senior analyst at the EG3 think tank, in terms of emissions cuts, "everyone is waiting to see what China will do".

President Xi Jinping announced last year his country's aim for carbon neutrality by 2060 and for domestic emissions to peak "around 2030".

The nation responsible for more than a quarter of manmade emissions has yet to submit a renewed NDC, although one is expected before Glasgow.

A G20 summit in Rome days before COP26, during which Italian Prime Minister Mario Draghi said he would push for members to commit to 1.5C, could also prove influential.

"The hopeful scenario would be that the G20 adds some momentum going into Glasgow," said Meyer.

"The less hopeful scenario would be gridlock and stalemate in Rome and then go from there to the world leaders' summit in Glasgow without real unity."