text: Around 155 countries are expected to formally sign the deal at the UN, setting in motion events that could see the treaty operational within a year.
  The UN says the expected record turnout for the signing shows overwhelming global support for tackling rising temperatures.
  But some environmentalists have dismissed the event as a "distraction".
  Despite the absence of President Obama, around 60 world leaders are expected here at UN headquarters, including French President Francois Hollande and Prime Minister Trudeau from Canada.
  But their signatures alone will not be enough to make the Paris agreement operational. The legal requirements  mean that each country will have to go through a process of ratification. For some this will require nothing more than the assent of the political leader as in the example of the United States.
  Others though, such as India and Japan, will have to take the document to their parliaments; some may need new laws. The European Union is expected to lag behind on this issue as it has not yet agreed with the 28 member states on how emissions cuts will be shared out. Each member state will also have to ratify the deal individually.
  Some countries, including the Marshall Islands, Palau, Fiji and Switzerland, have already completed this step and will be able to formally join the agreement on April 22.
  To become operational, the treaty needs at least 55 countries representing at least 55% of global emissions to complete all the steps.
  While this is a tough threshold to reach an unusual coalition of interests is making it possible.
  Firstly President Obama is keen to ensure the deal is operational before his successor takes office next January. If the next President wants to take the US out of an established treaty they will have to wait for four years - by which time they may no longer be in charge.
  Many of the least developed countries are pushing forward as well because a clerical error in the drafting of the new agreement means it becomes operational as soon at it hits the 55/55 mark, and not in 2020 as many people had supposed.
  Poorer countries fear that if the threshold is reached they could be left out in the cold if they haven't ratified, meaning they would not be able to influence the rules and organisation of the new deal.
  "There was a little buzz a few weeks back with someone suggesting that some of the smaller counties should refrain from signing, in order to get a better deal," explained Reid Detchon from the United Nations Foundation.
  "The fact that there is this large number of developing [countries] that are coming to the table here says that argument hasn't taken root and they really saw how deeply their own national self interest was bound up in success here."
  Scientists and analysts are also keen on a speedy implementation of the agreement for different reasons. According to a new study by researchers at Chatham House, leaving any increase in the level of carbon cutting ambition until 2025 as detailed in the Paris deal would make it nigh on impossible to keep temperatures below 2 degrees C, never mind 1.5.
  "For the Paris agreement to have any credibility we can't afford to wait ten years in order to increase ambition," said Shane Tomlinson, a senior research fellow at Chatham House.
  "Estimates suggest that the gap between where emissions will be and where they need to be, will be around 11 gigatonnes of carbon dioxide in 2025. That's more than the annual emissions of China," he added.
  Reid Detchon agreed on the need for urgency and increased ambition. But he was worried that global leaders didn't quite understand that getting agreement in Paris was, in some ways, the easy bit.
  "My biggest concern really is that we are only on the first step of a ladder of increased ambition. It is going to become clear to the world over the next 3 -5 years how much more we need to do to stabilise the climate."
  Some environmentalists and indigenous leaders believe the whole process is not worth the paper it is written on. According to the International Alliance of Frontline Communities, the Paris Agreement is a "dangerous distraction" from the real issues.
  "I started attending the UN climate meetings in 1999. Over the last 17 years I've witnessed corporate, Wall Street and other financial influence gut any real solutions coming out of the negotiations," said Tom Goldtooth, of the Indigenous Environmental Network, in a statement.
  "As a result, the Paris Agreement goal of stopping global temperature rise by 1.5 degrees C is not real because the pledges each country is making will allow emission levels that will increase global temperature 3 - 4 degrees.  This will be catastrophic to the ecosystem of the world."
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target: The first significant step to putting the Paris Climate Agreement into practice will take place on Friday.