"The world is in a race against time," U.N. Secretary-General Ban Ki-moon said in his opening speech.

The historic agreement on climate change on Friday marked a milestone, with a record 175 countries, including India, signing it. But world leaders made ot clear that more action is needed, and quickly, to fight a relentless rise in global temperatures.

With the planet heating up to record levels, sea levels rising and glaciers melting, the pressure to have the Paris Agreement enter into force and to have every country turn its words into deeds was palpable at the U.N. signing ceremony.

“The world is in a race against time,” U.N. Secretary-General Ban Ki-moon said in his opening speech. “The era of consumption without consequences is over. Today you are signing a new covenant with the future. This covenant must amount to more than promises,” he said.

The >agreement will come into force once 55 countries representing at least 55 per cent of global emissions formally join it, a process initially expected to take until 2020.

But following a host of announcements at the signing event, observers now think it could happen later this year.

China, the world’s top carbon emitter, announced it would “finalise domestic procedures” to ratify the agreement before the G20 summit in China in September. The United States, the world’s second-largest emitter, reiterated its intention to ratify this year, as did Canadian Prime Minister Justin Trudeau and the leaders of Mexico and Australia.

Maros Sefcovic, the energy chief o another top emitter, the 28-nation European Union, also said the EU waned to be in the “first wave” of ratifying countries.

Congo’s President Joseph Kabila, speaking on behalf of the world’s 48 least-developed countries, said all were committed to “to move in one irreversible direction to secure a safer climate”. Even though small emitters, he said they would take the steps required to ratify the agreement “as soon as possible”, a reflection of the wide reach of the agreement.

The Washington-based World Resources Institute said that at least 25 countries representing 45 per cent of global emissions had either joined the agreement on Friday or committed to joining it early.

French President Francois Hollande, the first to sign in recognition of his key role in achieving the December agreement, said he would ask parliament to ratify it by this summer.

“There is no turning back now,” Mr. Hollande told the gathering, adding that a key to success in combating climate change will be to get governments, companies, and people all over the world to work together to move from fossil fuels to renewable energy.

U.S. Secretary of State Kerry said the signing of the agreement had to be followed by a recommitment by world leaders to actually win the “war” against carbon emissions that are making the world hotter every year.

Putting the deal into economic terms, he said, “the power of this agreement is what it is going to do to unleash the private sector” to define the new energy of the future and set the global economy on a new path to growth and development that preserves the environment.

Academy Award-winning actor Leonardo Dicaprio, a U.N. messenger of peace and climate activist, captured the feelings of many when he said: “We can congratulate each other today, but it will mean absolutely nothing if the world’s leaders gathered here go home and do nothing.”

“No more talk, no more excuses, no more 10-year studies,” he told the VIPs. “The world is now watching. You will either be lauded by future generations or vilified by them.”

After he spoke, leaders and diplomats from the 175 countries were called to the front of the chamber to sign the agreement. Mr. Kerry carried his granddaughter in his arms, a symbol of the future generations the agreement is aimed at protecting.

The signing set a record for international diplomacy — Never have so many countries signed an agreement on the first available day. States that didn’t sign on Friday have a year to do so.

The ceremony, held on Earth Day, brought together a wide range of states that might sharply disagree on other issues.

North Korea’s Foreign Minister Ri Su Yong made a rare U.N. appearance to sign and Zimbabwe’s President Robert Mugabe brought applause when he declared, “Life itself is at stake in this combat. We have the power to win it.”

Prime Minister Enele Sosene Sopoaga of Tuvalu, which has seen four of its small islands disappear into the Pacific Ocean since 2000, said the agreement can change the world but islands on the frontline of climate change urgently need better access to financing to protect themselves against rising oceans. He urged international support for an insurance program for Pacific island nations.

Tuvalu was one of 15 nations that not only signed but ratified the agreement on Friday.

Those that haven’t indicated they will sign include some of the world’s largest oil producers Saudi Arabia, Iraq, Nigeria and Kazakhstan, according to the World Resources Institute.

The Paris Agreement was a major breakthrough in U.N. climate negotiations, which for years were slowed by disputes between rich and poor countries over who should do what.

Under the agreement, countries set their own targets for reducing emissions of carbon dioxide and other greenhouse gases. The targets are not legally binding, but countries must update them every five years.

Already, states face pressure to do more. Scientific analyses show the initial set of targets that countries pledged before Paris don’t match the agreement’s long-term goal to keep global warming below 2°C (3.6°F), compared with pre-industrial times. Global average temperatures have already climbed by almost 1°C. Last year was the >hottest on record.

The latest analysis by the Climate Interactive research group shows the Paris pledges put the world on track for 3.5°C of warming. A separate analysis by Climate Action Tracker, a European group, projected warming of 2.7°C.

“We have a once-in-history opportunity to create a new, shared, inspiring and sustainable world,” Professor Nicholas Stern, who heads the climate change institute at the London School of Economics, told a luncheon hosted by the secretary-general. “If we delay, it will be gone.”

“If we do get it right, we will launch a new wave of dynamic innovation and growth in the medium-term,” Prof. Stern said. “The consequences of getting it wrong are unthinkable.”