This is the first universal deal on climate reached by the rich and poor.  The historic, legally binding climate deal that aims to hold global temperatures to a maximum rise of 1.5C above pre-industrial levels, staving off the worst effects of catastrophic global warming, has been secured. The most important thing for the global community to do is to keep things below that 2-degree mark, the conference delegates agreed, to ward off the most severe effect of global warming.

The culmination of more than 20 years of fraught UN climate talks has seen all countries agree to reduce emissions, promise to raise $100bn a year by 2020 to help poor countries adapt their economies, and accept a new goal of zero net emissions by later this century.

Formally adopted in Paris by 195 countries, the first universal climate deal will see an accelerated phase-out of fossil fuels, the growth of renewable energy streams and powerful new carbon markets to enable countries to trade emissions and protect forests.

The agreement admits "with concern" that current national plans are not enough.

2C is the threshold at which politicians hope mankind can avoid the worst climate change impacts: dangerous storms, drought, sea-level rise, water wars, mass migration and the spread of diseases.

As a result, it has built in a number of checks to try and keep the fast-closing 2C window ajar.

Scattered over different sections of the 31-page document, the measures collectively make up what has become known as a "ratcheting up" mechanism.

Now comes the hard part. Experts are under no illusion that celebrations and high-flown rhetoric are enough when it comes to rolling back greenhouse-gas emissions.

If anything, they say, the divisions that beleaguered the nearly two-week haggle have underscored the political and economic obstacles that now lie ahead.

The deal finally struck on Saturday, a day into extra time, enshrines the goal to cap global warming at two degrees Celsius (3.6 degrees Fahrenheit) over pre-industrial levels - and at an even more ambitious 1.5C if possible.

But the bad news is that humanity may already have used up almost 1C of that allocation, the UN's World Meteorological Organization warned in November.

And the emissions-curbing pledges submitted by 185 countries to give the agreement substance, even if fully honoured, set the stage for a 3C warmer world.

The only hope lies in hard-fought provisions in the pact to encourage nations to ramp up their actions over time, and thus keep a 2C goal in focus.

According to the Climate Action Tracker (CAT), a tool developed by four climate research institutes, most country pledges are "inadequate" and "nearly all" governments need to enhance their 2025 or 2030 contributions.

The first step will be a stock-taking in 2018, two years before the agreement enters into force, of the overall impact of countries' progress in abandoning fossil fuels like oil, coal and gas in favour of renewable sources like solar and wind.

The findings must inform the next round of country pledges to replace those that will enter into force with the agreement, in 2020.

Some countries had set 10-year targets for 2025, others 15-year ones until 2030.

Once the agreement takes effect, the collective impact of countries' efforts will be reviewed at five-year intervals from 2023.

The outcome of these reviews will "inform" countries in "updating and enhancing" their pledges every five years starting in 2025.

Many had hoped for more a more onerous obligation on countries to ramp up targets.

But this was always going to be a tall order. There were objectors among both developed and developing nation groups -- albeit for different reasons.

The United States, for example, wants pledges to be purely voluntary to avoid being obliged to take the accord to a hostile Congress for ratification.

Instead of a top-down approach of apportioning emissions targets, it opened the way to a bottom-up approach: nations would set their own emissions-cutting targets and timelines.

The UN's climate science panel says greenhouse-gas emissions have to drop 40-70 percent between 2010 and 2050, and to zero by 2100.

And many hope the battle lines will fade as new low-carbon technologies are developed, costs come down and a hoped-for global price on CO2 - a vital pollution-cutting incentive - emerges.

Indian climate negotiator Ajay Mathur told a news agency this week that the relatively higher cost of green energy competed with the imperative of uplifting millions of people from poverty in developing nations like his one.

"The key challenge, it has to be affordable," he said.

Word changed to strike the deal
It took almost two weeks for negotiators from 195 countries to finally pass the landmark climate accord this weekend after several espresso-fueled all-nighters and long, passionate debates over the meaning of a single word, such as “shall.”

As the final text of the agreement was released, the French president, François Hollande, said: “This is a major leap for mankind. The agreement will not be perfect for everyone, if everyone reads it with only their own interests in mind. We will not be judged on a clause in a sentence, but on the text as a whole. We will not be judged on a word, but on an act.”

World reactions
British prime minister David Cameron welcomed the deal, praising those involved for showing what ambition and perseverance could do. “We’ve secured our planet for many, many generations to come – and there is nothing more important than that,” he said.

US president Barack Obama said in a seven-minute address from the White House that the deal “shows what is possible when the world stands as one”, and added: “This agreement represents the best chance we have to save the one planet that we’ve got.”

But he went on to say the agreement “was not perfect. The problem’s not solved because of this accord”.

Kumi Naidoo, Greenpeace international director, added: “The deal puts the fossil fuel industry on the wrong side of history. But emission targets are not big enough. The nations that cause this problem have promised too little help to those people who are already losing their lives and livelihoods.”

Jennifer Morgan of the Washington-based World Resources Institute said the long-term goal was “transformational” and sent signals to the heart of global financial markets.

Oxfam said the deal would be a landmark step but was not enough to ensure that a rise of 3C could be avoided or that sufficient climate funding had been secured for vulnerable communities.

Phil Jennings, the secretary general of the UNI global union, which has 20 million affiliated members in 140 countries, said: “The world now has a shared vision. Governments and investors around the world now understand that fossil fuels are no longer a safe option for the global economy and look forward to a future powered by renewables.”

Economist Lord Stern added: “This is a historic moment, not just for us but for our children, our grandchildren and future generations. The Paris agreement is a turning point in the world’s fight against unmanaged climate change which threatens prosperity. It creates enormous opportunities as countries begin to accelerate along the path towards low-carbon economic growth.”