“Hope is essential – despair is just another form of denial,” Al Gore said last week, in an interview to promote the sequel to his 2006 climate change documentary An Inconvenient Truth. As well as the very bad news of Donald Trump’s science-denying presidency, An Inconvenient Sequel: Truth to Power, which opens in the UK today, brings good news: the plummeting cost of renewable electricity and the 2015 Paris climate agreement.

In 2017, denial of the facts of climate change – and myriad linked dangers including air and ocean pollution, famine and a refugee crisis the likes of which we can hardly imagine – is in retreat, with the Trump administration the malignant exception. Virtually all governments know that climate change is happening, and polls show most people do too – with those living in Latin America and sub-Saharan Africa particularly worried. The question is not whether global warming is happening, but what we are going to do about it. There are, and need to be, many answers to this. Gore believes the solutions to climate change are within reach, if people can only find the political will to enact them. Even if how to whip up sufficient zeal to make this happen remains a puzzle, his essential message is one of optimism.

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Others are less sanguine. A widely shared article by David Wallace-Wells in New York magazine last month sketching out some worst-case scenarios included an interview with pioneering climate scientist Wally Broecker, now 84, who no longer believes even the most drastic reductions in carbon emissions are sufficient to avert disaster. Instead, he puts his hopes in carbon capture and geoengineering. Others oppose anything that smacks of a techno-fix, believing the very idea that human ingenuity can get us out of this mess is yet another form of denial.

The human reaction – or lack of one – to climate change is a subject of interest in itself. The novelist Amitav Ghosh wrote The Great Derangement, a book about why fiction writers mostly ignore the subject, and argued that the profound alteration of Earth’s climate is difficult to think about. Wallace-Wells, in New York magazine, refers to “an incredible failure of imagination”. Politics, supposed to help us make sense of the world, has sometimes been more hindrance than help: is climate change really an inconvenient truth, because it means we have to give up eating beef and taking long-distance flights, or a too-convenient truth for anti-capitalists who want to bring down the financial system?

Such left-right binarism, and the relentlessly partisan nature of US politics, is surely why Gore now prefers to frame climate change more as a “moral” issue than as a political one. But the clearest and simplest message from his decade of advocacy is the need for action at every level. Such action takes many forms, ranging from protests against the Dakota Access pipeline in the US to anti-fracking demonstrations in Lancashire. This year the Guardian in conjunction with Global Witness is documenting the deaths of people all over the world who are killed while attempting to defend the environment from damage or destruction.

In a similar vein, the Natural History Museum has chosen its revamped central hall to showcase a key moment for environmental activism. When it was first announced that Dippy the dinosaur would be replaced with a blue whale skeleton that had previously hung quietly among the mammals, there were grumbles. But a month after its grand reopening in the presence of royalty and Sir David Attenborough, the revamped museum is a smash hit with more than 115,000 visitors a week.

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Partly this is because the installation of the skeleton brings Alfred Waterhouse’s 1870s terracotta building, with its marvellous moulded monkeys, back to life in the most magnificent way. Whereas visitors once mostly stuck to the ground floor until they joined the procession to the dinosaurs, the aerial position of the whale bones now draws people upstairs. From an overcrowded lobby, Hintze Hall has been raised into a wondrous public space.

But the whale, which died as a result of being stranded off the coast of Ireland in 1891, is more than a 19th-century relic. What the museum has done by giving this vast, dead creature such prominence is to issue a warning and a call to action. And it makes no bones about this: “Rescued from the brink of extinction in the 1960s, the blue whale is a symbol of hope for the future of the natural world,” says the information panel. “Threats such as marine pollution and climate change linger – the blue whale remains a vulnerable and endangered species.”

Like the hole in the ozone layer over Antarctica, which stopped growing after a 1987 treaty phased out chlorofluorocarbons (CFCs), whale conservation is one of the global environmental movement’s greatest success stories. Blue whales were critically endangered, until activists persuaded governments to legislate to save them, and the museum’s new exhibit is called Hope.

Optimism alone won’t halt climate change, or prevent further extinctions. But like Gore, the director of the Natural History Museum, Michael Dixon, and his colleagues understand that the most vital currency of the environmental movement is hope. With the knowledge we now have of climate change’s likely consequences, the alternative is nihilism.

• Susanna Rustin is a Guardian columnist