Delegates from almost 200 countries have begun a two-week international climate conference in Madrid that seeks to step up efforts to stop global warming.
U.N. Secretary-General Antonio Guterres is warning that the efforts so far are insufficient to overcome the "point of no return" in climate change.
"What is lacking is political will," Guterres told reporters on the eve of the Conference of the Parties (COP25) meeting, which opened Monday.
The summit, which moved to the Spanish capital after Chile had to pull out amid anti-government protests, aims to put the finishing touches to the rules governing the 2015 Paris accord.
That involves creating a functioning international emissions-trading system and compensating poor countries for losses they suffer from rising sea levels and other consequences of climate change.
The two-week COP25 meeting will begin with the passing of the presidency to Carolina Schmidt, Chile's environment minister.
Schmidt has called the meeting "the COP of implementation," as delegates will try to resolve sticking points of last year's conference in Poland's Katowice.
Those include how to create functioning international emissions trading systems and compensate poor countries for climate-related losses and damage.
Guterres warned on Sunday that cuts in emissions of greenhouse gases that have been agreed so far under the 2015 Paris agreement were "utterly inadequate" to limit temperature rises to a goal of between 1.5 and 2 degrees Celsius above pre-industrial levels.
Delegates are also under pressure from mounting global calls for climate action, spearheaded by Swedish teen climate activist Greta Thunberg.
On Friday, thousands of protesters gathered in cities from Australia to India and Europe, citing the "climate emergency" declared by EU lawmakers a day earlier and growing scientific evidence of the dire consequences of global heating.
COP25 is seen as a stepping stone to next year's even more crucial summit when countries must ratchet up their commitments to tackle the climate crisis under the Paris accord.