Paris - A hard-fought pact to stave off worst-case-scenario global warming enters into force Friday after record-fast ratification by nations reassembling next week for a fresh round of UN climate talks.Dubbed the Paris Agreement, it is the first-ever pact binding all the world's nations, rich and poor, to a commitment to cap average global warming by curbing planet-warming greenhouse gases from burning coal, oil and gas."Humanity will look back on November 4 2016, as the day that countries of the world shut the door on inevitable climate disaster," UN climate chief Patricia Espinosa said.While cause for celebration, "it is also a moment to look ahead with sober assessment and renewed will over the task ahead," she said.This meant drastically cutting emissions in the short term, "certainly in the next 15 years," Espinosa pointed out a day after a UN report said current trends were steering the world towards climate "tragedy".By 2030, said the UN Environment Programme, annual greenhouse gas emissions will be 12 to 14 billion tons of carbon dioxide equivalent (CO2e) higher than the desired level of 42 billion tonnes.The 2014 level was about 52.7 billion tons.2016 is on track to become the hottest year on record, and carbon dioxide levels in the atmosphere passed an ominous milestone in 2015.On Friday, the Eiffel Tower in Paris as well as government and public buildings in Marrakesh, New Delhi, Sao Paulo and Adelaide, among others, will be lit up in green to mark the entry into force of the historic pact.After years of complex and divisive negotiations, a deal was finally endorsed in the French capital last December.It had to be ratified by 55 parties to the UN's climate convention (UNFCCC), representing 55% of global greenhouse gas emissions to take effect. It passed the threshold last month, and now has ratifications from 94 of the 197 UNFCCC parties.'Amazing' speed "For veterans of UN climate talks, who for years saw little or no progress on tackling climate change, the way in which Paris has supercharged action is just astonishing," said Richard Black of the Energy and Climate Intelligence Unit, an advocacy group in London.A major doubt looms over the process, however. US Republican nominee Donald Trump, has threatened to "cancel" Washington's participation in the agreement if he is elected president on November 8."Amid all the chaos going on around the world, this agreement shows that on climate change we actually are witnessing an era of global cooperation and consensus," said Mohamed Adow of Christian Aid, which represents poor people's interests in the climate forum.Yet, with or without the US, the agreement "will not deliver the safe world we need" without being drastically strengthened, he said.Next week, negotiators will gather in Marrakesh for a follow-up to the Paris meeting, a chance to start putting political undertakings into practice."The timetable is pressing because globally, greenhouse gas emissions which drive climate change and its impacts are not falling," said Espinosa.The pact undertakes to hold global warming to "well below" two degrees Celsius over pre-Industrial Revolution levels, and to strive for 1.5°C.Countries submitted voluntary, non-binding carbon-cutting goals towards this goal.