Last year's Paris Climate Agreement at COP21 marked a paradigm shift in the international response to climate change. Skeptics argue the targets aren't ambitious enough, but few can deny that COP21, thanks in part to Europe's leadership, achieved the first multilateral climate agreement since the Kyoto Protocol.

In April 2016, with Earth Day as a backdrop, 175 parties accounting for 94% of global emissions reconvened to sign the Paris Climate Agreement. But only 15 countries--mainly small island nations like Maldives, Fiji, Samoa, Palau and Tuvalu--formally ratified. They represent a minuscule 0.4% of global carbon emissions.

Ratification represents a serious hurdle to climate treaties. The Kyoto Protocol required more than seven years to come into force. For the Paris Agreement to come into effect, at least 55 countries representing at least 55% of global greenhouse gas emissions need to ratify.

However, the fate of the Paris Climate Agreement largely rests in the hands of the big four emitters--China, US, EU and Russia. According to the World Resource Institute's Paris Agreement Tracker , in order to reach the 55% emissions thresholds, ratification by at least one of these four is essential. Without it, the Paris Agreement just doesn't mathematically work.

Which of these four big emitters can we count on to ratify? In the COP21 afterglow, ratification felt like a sure thing, but now cracks are beginning to show. For the US, the hope is to ratify by not defining the Agreement as legally-binding, but this robs the accord of its potency. Regardless, a partisan Congress that's already obstructing President Obama's Supreme Court nomination will be an obstacle. And if Donald Trump --who now polls ahead of Hillary Clinton and has disavowed support for climate change--wins the presidency, it could mean the US tries to renegotiate the Paris Agreement for more favourable terms.

China, who accounts for 20% of global emissions and has already signalled its intent to ratify before COP22, would clearly raise expectations for an early start to the Paris Agreement. But it would also be out of character for China who hasn't played a leadership role in climate negotiations and has tended to protect its economic interests over environmental concerns. Russia has never been a first mover on climate change issues.

That leaves the European Union. Given its climate change leadership and heavy investment into a low-carbon economy, the EU is a natural cornerstone of the Paris Agreement. There's no lack in ambition from the EU either. A recent communications release highlights the European Commission's urgency to ratify. Indeed, Maros Sefcovic, the European Commission Vice-President for Energy , has indicated the EU's desire "to be in the first wave of ratifying countries."

But can the EU still be part of the first wave? Because it represents 28 different member states in the emissions reduction plan submitted at COP21, the EU faces the enormous task of dividing the burden among states while trying to keep them unified. At the same time, economic and political circumstances are driving some EU states to pursue their own interests or defer to their own national approval process.

The EU's balancing act has traditionally been an economic one. It's had to weigh each state's unique energy production mix against longer-term, EU-wide climate change ambitions. For example, where nuclear power accounts for 75% of energy in France , coal comprises nearly 85% of Polish power generation . Poland has typically backed domestic labour interests--the coal industry provides roughly 100,000 jobs--over EU climate policies, even vetoing the EU's Low Carbon 2050 Roadmap and the Doha Amendment under its new president, Andrzej Duda.

But while there are solutions to economic problems, the political climate is now the bigger worry for the EU. In June, the UK votes on Brexit, a near-existential referendum that will determine its membership in the EU. Considering the UK accounts for 13% share of EU emissions and is set to hold the EU presidency next year, the electoral result carries significant implications. "Referendums like Brexit expose the EU to the risk of becoming the next Australia on climate change, who ended up doing a hard reverse on its climate commitments and carbon tax," said Cary Krosinsky, Lecturer at Yale College.

Less obvious is how the EU reconciles its COP21 climate pledge with the domestic parliamentary decision-making process. France, for instance, advocates for expedited EU ratification with national-level discussions to follow, but Italy wants national parliamentary discussions before agreeing to support EU ratification. The Belgian legislative process, as Ian Duncan, MP to the Scottish Parliament , comments, requires an arduous approval process by seven independent national and regional parliaments. And many EU states have just begun developing their own national energy and climate plans . So, it's reasonable to assume that some states hold off on EU ratification until they understand exactly what it means for them.

The EU needs to move quickly before ratification devolves into a conflict of national interests. According to Artur Runge-Metzger , climate change head for the European Commission, the Commission is even exploring ways to go around Poland if necessary. "As one member state is not ready, there is a discussion that we need to have at the political level in terms of how to take the whole element. In reality, I think that in the past we have seen pieces of legislation where not all member states did their own ratification."

If these intra-EU issues weaken efforts to ratify the Paris Agreement, some states will proceed on their own but at the loss of a unified Europe. Of course, there will be early adopters. France was the first EU state to ratify on May 17, and others including the UK are adopting stronger national climate policies like carbon price floors to compensate for shortfalls at the EU level. But there will also be climate laggards who offsetting this progress.

If laggards like Poland do hamstring the process, the EU needs to consider forcing through ratification on a top-down basis so that it continues to push the climate change policy agenda forward. Critics insist this top-down approach widens the democratic deficit. But, with the EU focused on the refugee crisis and the economy, allowing national parliaments to delay or undermine ratification will jeopardize the EU's role. It may even downgrade the EU to observer status at COP22 in November, depriving the summit of a negotiating leader and large-bloc signatory.

Of the four large emitters, the EU represents the best opportunity to support the 55 countries, 55% of global emissions milestone. If the EU is to preserve its influence and policy coherence, it needs to overcome its internal conflicts and remain the cornerstone of the Paris Climate Agreement.