The world's big cities must collectively cut their carbon footprint by nearly half within a decade if global climate goals are to be met, according to an analysis unveiled Thursday.

Without aggressive measures by cities, the 196-nation Paris Agreement to curtail global warming "cannot be realistically delivered," according to Mark Watts, executive director of C40, a global network of large cities.

"The next four years will determine whether or not the world's megacities can deliver their part," he said.

The report was released as mayors from 84 megacities gathered in Mexico City for a C40 urban summit, to mull long-term commitments for slashing carbon pollution.

The agreement, inked in the French capital last December, calls for keeping average global warming below two degrees Celsius (3.6 degrees Fahrenheit) above pre-industrial levels, and at 1.5 C if possible.

To underpin the pact, countries have made carbon-cutting pledges which, scientists calculate, place the world on track for 3 C of warming -- a sure-fire recipe for climate catastrophe.

With only 1 C of warming so far, the world has already seen an upsurge in extreme weather, including droughts, superstorms, heat waves and coastal flooding boosted by rising seas.

- Sharp, immediate cuts -

"If we are going to avoid the worst impacts of global warming, cities are going to have to play an even bigger role in the months and years ahead," former New York Mayor Michael Bloomberg, a UN special envoy for climate change and C40 board president, told a telephone briefing ahead of the report's release.

The 90 C40 cities account for a quarter of the global economy and are home to more than 650 million people -- almost 10 percent of the world's population.

The 100-page report compiled for the Mexico meeting, outlines four roadmaps to a low-carbon future, tailored to different levels of wealth and CO2 emissions.

Cities with high per capita levels of both -- such as Toronto, New York and Melbourne -- should immediately and sharply cut carbon pollution.

Poorer cities with high CO2 output -- such as Cape Town in South Africa -- would be given more leeway, with emissions allowed to briefly increase before dropping.

Rich cities such as Stockholm or Seoul that have already lowered emissions, should manage a steady decline.

Finally, poor cities with low carbon pollution -- Quito or Caracas, for example -- should have a longer grace period before per capita CO2 output must drop.

- 70% of greenhouse gases -

To meet the 2 C target, city emissions would have to decline from an average of five tonnes of CO2 per person per year to under three tonnes within a decade, said the report.

By 2050, that number would need to be one tonne, a hugely ambitious goal.

All the scenarios assume that scientists will have figured out by mid-century how to suck massive amounts of carbon out of the air.

Some 70 percent of global greenhouse gas emissions come from cities, which house just over half the world's population.

Members of C40 account for about seven percent of city emissions.

Meanwhile, a grouping of 62 state, provincial and regional governments said Thursday its members were on course to more than halve their combined global greenhouse gas contribution to 1.2 billion tonnes of CO2 equivalent (GtCO2e) by 2050.

Members of the Compact of States and Regions represent about 17 percent of the global economy and more than three billion tonnes in annual greenhouse gases -- more than the total emissions of India and Canada, said the group's latest report.

The world as a whole emits some 48 GtCO2e per year -- a number climate scientists say must be slashed by 40-70 percent by 2050 and to near zero by 2100 for a chance to stay under the 2 C ceiling.

Six major signatory regions -- Scotland (UK), Catalonia (Spain), Lombardy (Italy), Carinthia (Austria), Connecticut (US) and Provence-Alpes-Cote d'Azur (France) -- have already met or exceeded their 2020 targets, said the Compact report.

Beyond 2020, however, much tougher action will have to be taken.

"Only around half of the governments included in the analysis have a 2050 (emissions-reduction) target," it said.

"The lack of long-term targets translates into emissions reduction levels that are not sufficient to stay below the 2 C scenario post 2030."