These are worrying times for anyone who cares about the future of the planet. Carbon emissions are hitting record levels, and without urgent action the world risks locking itself into a fossil fuelled future which will condemn us to catastrophic levels of warming. This message comes not just from green groups, but from sober bodies such as the International Energy Agency, the World Bank, Price Waterhouse Coopers, the World Economic Forum and the UN Environment Programme.
Bafflingly, the alarm bells appear to have been falling on deaf ears among politicians. Yet it is hard to see how the world can respond adequately to the scale and urgency of the climate change threat without clear action by our political leaders.
So president Obama's recent inaugural speech can only be seen as good news - not least after an election campaign in which climate change was almost invisible. The president pledged "to respond to the threat of climate change, knowing that the failure to do so would betray our children and future generations." Moreover, he said, America must lead the path towards sustainable energy sources, seizing the promise of new jobs and industries.
In another positive move, president Obama has nominated Senator John Kerry as secretary of state. Senator Kerry has a strong track record on climate change. Last week he made clear that he sees tackling climate change internationally as one of his top priorities, and spoke strongly of the economic benefits of investing in clean energy.
The president's new commitment has been welcomed by environmentalists and green businesses - though few are getting carried away. We have heard fine words before, not least in Obama's first inaugural speech four years ago. Congress remains gridlocked on climate change and the national debate in the US remains highly polarised, thanks in large part to the toxic activities of climate change sceptics and the well-funded activities of the fossil fuel lobby.
Obama also has some tough decisions in his in-tray. Will he approve or reject the highly controversial Keystone XL pipeline, which would permit the export of huge quantities of oil from Canada's dirty tar sands onto the world market? Will he get tough on oil companies' risky plans to drill for oil in the fragile Arctic? Another key priority, given the difficulty of getting legislation through the US Congress, is to introduce tough emission standards for new and existing power plants and to accelerate the shift to renewable energy.
What role will Obama play on the international stage? For far too long, the US has been a major barrier to progress in the UN climate talks and elsewhere. Many hope that the President's new commitment might inject momentum into the painful negotiations towards a comprehensive international climate agreement in 2015. But it is hard to see this happening without an emerging coalition of leaders to work with Obama, to help build the mutual confidence needed to make real progress.
On the face of it, David Cameron might be expected to be first in line - building on the UK's fabled 'special relationship' with the US. The UK prime minister has himself promised leadership on climate change. After his iconic trip to the Arctic with WWF in 2006, he helped secure the UK's world-leading Climate Change Act. Famously, he promised to lead the "greenest government ever."
Sadly, David Cameron's promised leadership on climate change has evaporated. The coalition government is racked by internal divisions on energy and climate policy, and Chancellor George Osborne is undermining targets for emissions and clean energy in the mistaken belief that these will hinder, rather than help, economic recovery.
Internationally, the prime minister has been all but silent on climate change. This summer he will host the G8 summit in Northern Ireland - but he has failed to put climate change or wider environmental concerns on the agenda. In last week's speech on EU reform and a future referendum, Cameron did acknowledge the importance of a joined-up European approach on climate change and energy - but then then suggested that Europe has "gone too far" on environmental legislation. Meanwhile, the EU's flagship policy to tackle climate change, its emissions trading scheme, is on the point of collapse - yet political leaders are failing to invest time and effort to fix the problem.
Barack Obama's renewed desire to tackle climate change creates an opportunity for David Cameron to rediscover his own promises of leadership. He could reach out to the president to create the kernel of a "coalition of the willing" to help shift the global politics on climate change. He could amend the G8 agenda to include climate change, perhaps addressing the need to phase out fossil fuel subsidies. He could work with the president to overcome US hostility to a credible international agreement to regulate international aviation emissions. And of course, David Cameron could reassert the UK's own leadership role by supporting a target to for a carbon-free power sector in the energy bill and fighting for stronger European action on climate change and renewable energy.
Political leadership is a strange and fickle creature. It is easy to lead when times are good. But perhaps the prime minister should reflect that real leadership is at its most valuable when times are at their toughest.