Malta signed a landmark climate change agreement this afternoon, during a ceremony at the UN headquarters in New York. 

More than 170 countries have agreed to sign the Paris Agreement on climate change, bringing the deal a step closer towards coming into force years ahead of schedule. 

Never have so many countries signed an agreement on the first available day. The signatures come on Earth Day, the worldwide day in support of environmental protection. 

Prime Minister Joseph Muscat tweeted news of Malta's signature shortly after it occurred. Environment Minister Leo Brincat was also in attendance. 

"The era of consumption without consequences is over," UN secretary-general Ban Ki-moon told the gathering.

Many expect the climate agreement to come into force long before the original deadline of 2020. Some say it could happen this year.

After signing, countries must formally approve the Paris Agreement through their domestic procedures. The United Nations says 15 countries, several of them small island states under threat from rising seas, are set to do that on Friday by depositing their instruments of ratification.

The agreement will enter into force once 55 countries representing at least 55% of global emissions have formally joined it. The United States and China, which together account for nearly 40% of global emissions, have said they intend to join this year.

Maros Sefcovic, the energy chief for another top emitter, the 28-nation European Union, told reporters the EU wants to be in the "first wave" of ratifying countries.

French President Francois Hollande said he will ask parliament to ratify the Paris Agreement on climate change by this summer. "There is no turning back now," Mr Hollande told the gathering.

Countries that had not yet indicated they would sign the agreement on Friday include some of the world's largest oil producers, including Saudi Arabia, Iraq, Nigeria and Kazakhstan, the World Resources Institute said.

The Paris Agreement, the world's response to hotter temperatures, rising seas and other impacts of climate change, was reached in December as a major breakthrough in UN 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 do not match the agreement's long-term goal to keep global warming below 2C, compared with pre-industrial times. Global average temperatures have already climbed by almost 1C. 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 3.5C of warming. A separate analysis by Climate Action Tracker, a European group, projected warming of 2.7C.

Either way, scientists say the consequences could be catastrophic in some places, wiping out crops, flooding coastal areas and melting Arctic sea ice.

The US is a key concern for the Paris Agreement as other countries worry what the next president might do. Analysts say that if the agreement enters into force before Barack Obama leaves office in January, it would be more complicated for his successor to withdraw from the deal, because it would take four years to do so under the agreement's rules.

The Obama administration is expected to treat the deal as an executive agreement, which needs only the president's approval.

China said it will "finalise domestic procedures" to ratify the Paris Agreement before the G20 summit in China in September.

The world's top carbon emitter had already said it would ratify the agreement this year, but the announcement of a deadline is new.

Mr Ban welcomed the pledge.

Leonardo DeCaprio urged world leaders to leave fossil fuels "in the ground where they belong", telling them they are the "last best hope" for saving the planet from the disastrous effects of global warming.

The actor, who is a UN messenger of peace with a special focus on climate change, spoke shortly before the leaders began signing the Paris Agreement.

"We can congratulate each other today, but it will mean absolutely nothing" if you return to your countries and do not take action to implement the deal, DiCaprio said.