The UK government won high praise six months ago for taking a leading role in the successful Paris climate change agreement, the first legally binding commitment on curbing carbon emissions by all 195 United Nations countries.
With the vote to leave the EU, the UK’s future participation in that landmark accord is now in doubt.
More importantly, for the rest of the world, the Leave campaign’s victory provides a fillip globally for groups opposed to climate action, and if it causes delays to the Paris accord coming into effect, it could provide an opening for aspiring right-wing leaders - including Donald Trump - to try to unpick the pact.
"There is a risk that this could kick EU ratification of the Paris agreement into the long grass," Jonathan Grant, director of sustainability at PwC, told the Guardian.
That would be a setback to the UN in itself, but also concerns participants because of the US presidential election this year.
Donald Trump has vowed to withdraw from the Paris agreement if elected. Proponents of the agreement are therefore hoping for a quick process of ratification by as many parties as possible, including EU member states, which would bring the agreement into immediate effect and make it much harder for countries to renege upon afterwards.
As an EU member state, the UK negotiated on key issues such as greenhouse gas emissions limits as part of the bloc, and was expected to take on its own tally of emissions reductions based on an EU-wide "burden-sharing" agreement, yet to be worked out. But while the UK is also individually party to the agreement, as a sovereign nation, neither the government nor the EU has yet ratified the accord in law.
This means a future, possibly Eurosceptic, prime minister will face the choice of whether to ratify, unless the current government, led by David Cameron for the next three months, decides to do so as a matter of urgency.
France became the first EU member state to ratify the agreement individually earlier this month, so in theory Britain could follow suit quickly. But this would be an unusual step given the host of pressing issues following from the referendum, and would be likely to prompt an outcry from sections of the pro-Brexit right, prominent members of which are also climate change sceptics.
Amber Rudd, the energy and climate change secretary, who was praised by many other countries for taking a leading role at Paris, has not yet revealed what the plans are likely to be.
The UN’s climate chief, Christiana Figueres, said in advance of the referendum that Brexit would require a "recalibration" of some kind but it is not clear what that might entail.
The UK would have to agree its own contribution to emissions cuts if it stays in the Paris accord. These would most likely be based on the Climate Change Act, which sets out long-term targets on greenhouse gases and five-yearly "carbon budgets" that governments must meet. To renege on the act’s commitments would require its repeal, as favoured by some Brexit campaigners, but this is unlikely in the short term as they lack broad enough support in parliament.
Stephen Cornelius, chief adviser on climate change at WWF-UK, said: "The UK has signed up to the Paris agreement in its own right. Outside the European Union the UK can still play a leading role in fighting climate change. It should ratify the Paris agreement as soon as possible, pass the fifth carbon budget under our domestic Climate Change Act and turn this into an ambitious international pledge to cut emissions."
That will be largely an issue for the UK, which accounts for less than 2% of global emissions. What is of much wider concern is the signal the referendum vote to leave sends to the world.
Grant said: "Today’s outcome is a major setback for the type of collaboration needed to tackle global environmental issues such as climate change. The UK government has been a champion of climate action at home, within the EU, and in Paris. This leadership is at risk, with many supporters of Brexit also opposed to climate policies such as carbon taxes and efficiency standards."
The wider political implications, rather than the mechanics of the accord, will take time to work out, but it is already clear that the Brexit vote will be used as a rallying cry for an agenda that frequently includes climate scepticism among its tenets, alongside curbs to immigration and to government regulation.
Many climate sceptics around the world will have been encouraged by the Brexit vote, as there is so much overlap between the two camps, and environmental and carbon goals under the EU were a key target of the Leave campaigners. For instance, Lord Lawson, one of the leaders of the Leave camp is also founder of the Global Warming Policy Foundation, a climate sceptic thinktank.
Trump hailed the referendum result in visiting the UK. Some of his supporters share his climate scepticism, and the common cause with Brexit campaigners will have given both sides a boost.
The calls from right-wing parties for further breaks from the EU could also endanger climate consensus within the EU, which has been a key driver of actions on climate change within the UN and other international institutions. Without a unified EU, support for those actions could decline.
Green campaigners were quick to call for the UK to maintain its commitments on climate change, irrespective of Brexit.
John Sauven, executive director of Greenpeace UK, told the Guardian: "Britain isn’t leaving the international community, and we’re certainly not leaving the planet. That means the Paris agreement is every bit as vital to our future as it was yesterday. In fact, sticking to our Paris promises is more important than ever: we are about to negotiate new trade deals, and the last thing we can afford to do is break the commitments we made to the world just six months ago.
"Cameron’s successor has a chance to immediately reaffirm Britain’s relationship with the international community by ratifying the Paris deal."
Debbie Stockwell, managing director of Sandbag, a campaigning group focused on the EU’s carbon commitments, said: "The UK has agreed to contribute to the Paris agreement as part of the EU. Now that the country has voted to leave, both the UK and the EU will have to reconsider this arrangement. It is vital that the UK continue to work with the EU to deliver ambitious action to tackle climate change."
Nick Mabey, chief executive of E3G, predicted that the UK would hold true to its promises, and its national interests: "The Brexit vote does not end the climate crisis. The UK will still ratify and implement the Paris climate agreement as this protects Britons from the worst impacts of climate change."
