A year ago today, nearly 200 nations came to agreement on one of the world's most sweeping and important environmental and economic pacts. The Paris Climate Agreement commits, for the first time, all nations to a collective fight against climate change.

For the thousands of delegates who had gathered for two weeks at an airfield on the edge of Paris, the deal was the culmination of nearly 20 years of often bitter diplomacy. Though not perfect and with doubts about whether it is strong enough to avoid humanity cooking the planet, the agreement has set the world on a better path for retooling industries, power plants and transport to make them cleaner and more efficient.

It remains unclear whether a Donald Trump administration will honour the pact. The United States President-elect famously tweeted in 2012: "The concept of global warming was created by and for the Chinese in order to make US manufacturing non-competitive."

Before the US election, he vowed to cancel the Paris Agreement, but last month told The New York Times he had an open mind on US involvement in the pact. Last week, he met climate change activist Al Gore and then picked a climate sceptic and fossil fuel industry ally to head the Environmental Protection Agency. No one really knows what Mr Trump will do on climate change next.

If he does try to pull America out of the Paris pact, he will face fierce resistance internationally and domestically. A number of US states are forging ahead with their own low-carbon energy plans and will fight Mr Trump in the courts if he interferes.

This is a pact that nations want to be a part of and implement because each nation is allowed to fight climate change in its own way, just as long as it is transparent and can be measured and monitored - and strengthened over time.

At the annual UN climate talks last month in Morocco, Mr Trump got a taste of the international solidarity behind the Paris Agreement. Nations appealed to him to take climate change seriously. China issued a call for the US to stay inside the Paris Agreement. Some diplomats in Morocco also talked of financial penalties for the US.

It's a sign that impatience, particularly from poorer, more vulnerable nations, is growing. That is because climate change and its impacts, from extreme heat, storms, floods and melting ice caps, are accelerating. In the race to keep global warming below 2 deg C, nations need to lift their game in cutting emissions from industry, agriculture and transport and do so in a way that ensures no back-sliding.

For the moment, there's no sign of rowing back.

The years 2014 and 2015 were the hottest on record and 2016 looks set to maintain this trend.

The Arctic Ocean, meant to be slipping back into the icy clutches of winter this time of the year, was instead baking under temperatures up to 20 deg C above normal late last month. Record rains hit Louisiana in August, while large parts of the northern Great Barrier Reef have died off or are badly damaged by bleaching caused by abnormally warm waters.

The list goes on.

But there is plenty of positive news, too. The Paris Agreement went into effect last month, far faster than many expected. To date, 115 nations have ratified it.

Last year, clean-energy investment rose to a record US$329 billion (S$470 billion), according to Bloomberg New Energy Finance, up 4 per cent from 2014, despite low prices for coal, oil and gas. Much of the money went into wind and solar power.

Two other agreements this year have also lifted the mood. In October, nations agreed on a deal to phase out hydrofluorocarbons (HFCs), greenhouse gases used in appliances such as refrigerators and air-conditioners. HFCs are powerful greenhouse gases that are many times more effective at trapping heat than carbon dioxide.

The same month, a global deal under the UN was struck to curb the growth of emissions from aviation, one of the fastest-growing sources of greenhouse gases. Criticised by green groups as being too weak, the deal under the International Civil Aviation Organisation is at least a start after years of talks.

Indonesia has also made progress in its efforts to halt the conversion of carbon-rich peatlands - a huge source of greenhouse gases and thick smoke when burned. A new peatland restoration agency began work in January and this month, President Joko Widodo signed a strengthened regulation to prevent further damage to peatlands from agriculture and mining.

But the Paris deal still faces major challenges and it remains to be seen whether the global goodwill can be sustained as the costs and political pressure on cutting emissions grow.

Signatories last month agreed to a 2018 deadline to work out a rule book for implementing the Paris Agreement. This is needed so that countries know how to report and monitor their national pledges to curb emissions.

Under the Paris deal, there will be a mechanism to regularly review national pledges and strengthen them over time based on what science is telling policymakers about the pace of climate change. The first evaluation of pledges comes in 2023. The United Nations says current pledges are not enough to limit warming below 2 deg C, so deeper cuts are needed.

The ultimate test is what happens to the trajectory of greenhouse gas emissions. Scientists say global emissions must peak soon and then fall quickly if the world is to avoid dangerous climate-change impacts.

"Over the next five to 10 years, if we succeed in bending the present upward curve of emissions and ramp up climate action - meaning that by 2025 emissions are well and truly on a downward trajectory - then we will be able to say the agreement is working," says climate scientist Bill Hare, currently a visiting scientist at the Potsdam Institute for Climate Impact Research.

That means a significant drop in emissions from burning fossil fuels and a big jump in energy from renewable sources - doable but politically tough.

The Paris deal also faces weaknesses in its lack of penalties for nations that aren't a part of it or choose to bail out. Yes, there's the stigma of becoming an international pariah but is that a strong enough threat?

"Ratification means few legal obligations for participating countries. Paris (deal) entering into force has more symbolic than legal strength," says Mr Luke Kemp, a lecturer in international relations and environmental policy at the Australian National University in Canberra.

In a recent commentary, he said that "aside from social pressure, the Paris Agreement is extremely weak against countries who choose not to join, or opt to withdraw".

The hope for now is that the Trump administration won't try to wreck the pact and that the strong collective will that gave birth to it will remain - and that recalcitrant nations will be shamed into action.

The Paris Agreement is the last and probably best chance the world has to quickly address climate change. At a time of deep political upheaval, the deal stands as a hopeful sign of global unity when faced with a growing crisis.