The United States is an exceptional country in so many ways, as visitors can attest. For Brits, a common language at first masks the vast cultural divide between the Old and New Worlds. Americans themselves though are keenly aware of the sense of difference. This uniqueness is not just about culture, but about the country’s position in the world. Having been the world’s largest economy since the 1920s, the willingness to engage more fully in global affairs led to the superpower status that has dominated world politics since the middle of last century, albeit in competition with the Soviet Union for much of that time. This sense of difference, mission and superiority is encapsulated in the concept of American exceptionalism, which has been embedded in the national psyche since independence, although economic power has added another (important) dimension. This has not always made the USA popular, and the country remains out of step on some otherwise-consensual issues, such as refusing to be bound by the International Criminal Court. Now, President Trump has taken a big step in reinforcing this pattern, by pledging to withdraw from the Paris Agreement on climate change. There has been intense criticism of this decision, although it was not entirely unexpected. This is overlaid on a deep dislike of the President from many quarters although, it has to be said, this is countered by equally strong backing from his supporters. The 45th President is certainly both a divisive as well as unconventional head of state, but it’s worth taking a step back now that the dust has settled to analyse just what this latest move means. First, to put it in context, America has form on climate change policy. President Clinton signed the Kyoto Protocol, but never put it to a vote in the Senate, where it would have been heavily defeated. Vice-President Gore has been a high profile supporter of the IPCC orthodoxy for some time, but George W Bush, who defeated him to become 43rd President, attracted criticism for his withdrawal from the Protocol. Some people have wondered how a different result at this election – the tightness of which introduced us to the concept of ‘hanging chads’ in Florida – would have altered the situation. The simple answer is ‘not at all’; a hypothetical President Gore would have retained Bill Clinton’s position, but there was never a chance of the Senate voting to ratify the Protocol. The Obama administration reversed the position again, by supporting action to reduce emissions, via the Clean Air Act and various EPA actions. More recently, President Obama was a vocal supporter of the Paris Agreement and the joint support from the US and Chinese governments was instrumental in getting rapid ratification by the great majority of countries that have signed. Virtually all 196 parties to the UNFCC have signed, with the exception of Nicaragua and Syria. By ensuring that the language of the Agreement was couched in the language necessary to argue that it was not a Treaty, ratification by the Senate was deemed not to be needed, and Barack Obama ratified it by Executive Order. While the debate about the constitutionality of this rumbles on in some quarters, Donald Trump has accepted its legitimacy by announcing the US withdrawal, rather than pronouncing it invalid. The important issue is what his action really means and what the wider implications may be. The first point is that, although the Paris Agreement has been lauded as the first global agreement on climate change policy, it is toothless. It effectively uses peer-group pressure to encourage individual states to keep on the straight and narrow, demanding only that countries produce their individual plans to reduce emissions and commit to setting more ambitious targets at a later date. For emerging economies and less developed countries, its importance lies in the promise of finance from richer countries to smooth their paths towards lowering emissions. The second point is that America has already made very good progress in cutting emissions – without any commitment to international targets – by switching from domestic coal to even-cheaper domestic shale gas to provide much of its energy, along with improvements to energy efficiency. And it seems certain that a number of US States and cities will continue along their own paths to cut local emissions in any case. In other words, the Donald’s decision will make precious little real difference. However, the symbolic importance of this action is huge. While peer pressure may work to an extent, there are really only four main players on a global scale: China, the USA, India and the EU. Between them, they account for more than 50% of global emission. China has been the biggest emitter for some time and, while India’s emissions rank significantly lower, the country will become a much bigger user of energy as the economy grows and the population overtakes that of China before too long. The EU is ideologically committed to its present path of emissions reduction, despite a distinct lack of enthusiasm in some of the newer member states and the high costs of only modest success for Germany’s energiewende. China and India have nothing to lose: their emissions are continuing to grow and their only commitment is to have these peak at some undetermined time. To do this, they will receive a significant subsidy from rich country taxpayers. They can well afford to join with the EU in condemning American action. Having spent decades hammering home the message that global action on emissions reduction is vital, this action by one of the big players may have little practical impact on planned actions, but it does make other countries perhaps consider the once unthinkable: a watering down of stated ambitions and even a withdrawal from international agreements. If known waverers are tempted to follow Donald Trump’s lead and other countries are perhaps spurred into action by economic realities, the culmination of years of negotiation could be fatally compromised. No matter that it may make little or no difference to levels of CO2 in the atmosphere, the loss of support for the Paris Agreement from more than a handful of countries could mark the high water mark of international consensus on climate change policy. The present complex, unwieldy and ineffectual top-down policies may have effectively had their day. For those who believe that slashing emissions is an urgent priority, this should catalyse a search for new, simpler and economically-competitive ways to achieve their goals.