‘An exuberant rite of spring” is how the New York Times described 22 April, 1970. In Manhattan, and across America, “huge, light-hearted throngs ambled down autoless streets.” Earth Day had been born, an outburst of protest – and revelry – that involved everyone from save-the-whales activists to opponents of new freeways. Denis Hayes, now 72, was the man tasked with organising it. “What we did was pull together an event that told all of those people, ‘You know you’ve really got something in common and this should be one big movement where we’re supportive of one another’.”

It sparked, he tells me, the most profound change in American society since the New Deal. “We now have different kinds of buildings, different kinds of automobiles, different planes, different lighting, different land use. People are choosing to have diets for environmental reasons, choosing to have one child for environmental reasons.” And all that, he says, “didn’t come from political leadership at the top, it came from a bunch of demands down at the grassroots”.

Positive action … Denis Hayes, the founder of Earth Day. Photograph: Brendan Hoffman/Getty Images

In 1990, Earth Day went international, reaching 200 million people in 141 countries. Now, according to its website, more than a billion participate in celebrations, teach-ins and protests around the world. This year, the Earth Day Network has teamed up with scientists for a rally on the National Mall in Washington DC. Volunteers around the world are convening their own versions of this “March for Science”, to champion “evidence-based policies in the public interest”.

Vanessa Furey, one of the organisers of the London march, says she was motivated by the “rise in fake news and ‘alternative facts’” during the past year, “and that quote from Michael Gove [during the referendum campaign] that ‘people have had enough of experts’”. The experts are fighting back. “Scientists aren’t the natural group to go out marching – and I know there are some that still have reservations about doing it. But it felt like a proactive, positive move for [them] to come out and say, let’s get the public involved.”

But is there any point in these big days out when the world seems to be hurtling ever faster towards disaster? Last year was the warmest on record, yet progress on environmental issues seems to be going into reverse. Donald Trump is still mulling over whether to abandon the US’s commitment to the Paris agreement on limiting global warming. Scott Pruitt, the man he appointed to lead the Environmental Protection Agency (EPA), the creation of which was the original Earth Day’s greatest achievement, doesn’t believe the scientific consensus on climate change. The UK government, once a global leader in the campaign to reduce carbon emissions, will be absorbed by Brexit for years. This week it privatised the Green Investment Bank, with no guarantee its mission will be maintained over the long term.

“It’s pretty tough to look at what’s going on today with clear eyes and not get fairly depressed,” says Hayes. The challenges facing activists have certainly changed in the 47 years since he became involved, when “you couldn’t see 100 yards in front of your nose” in cities such as Pittsburgh, Los Angeles or Gary, Indiana. “The easy stuff environmentally is when something poses an immediate threat to you, to your children, to your neighbourhood. When that’s happening, it is not that hard to organise people and to get them sufficiently incensed to demand change.”

Hayes says that it’s tougher with the big global issues such as climate change, threats to the world’s oceans and migratory species – issues that cross national boundaries and can’t be solved by any one country acting alone. “Those sort of things are all in much worse shape in 2017 than they were back in 1970,” he says.

But despair, appropriate as it might seem, isn’t just bad for the soul. A growing movement of “Earth Optimists” believes that it could actually make things worse. In an editorial for Science magazine’s Earth Day issue, professors Andrew Balmford and Nancy Knowlton argue that “presenting people with huge problems without solutions leads to disengagement”. They continue: “Without examples of conservation succeeding, hopelessness could itself emerge as a driver of extinction.” And there are successes: in Brazil, for example, the rate of Amazon deforestation is down to one-third of its 2004 peak. Chinese investment in renewable energy is set to exceed $300bn by 2020.

It’s those kinds of triumphs, and myriad smaller ones, that have inspired the Earth Optimism summit, also taking place in Washington DC this weekend. In the UK, Earth Optimism Cambridge will carry the torch. Speakers including Jane Goodall, David Attenborough and Steven Pinker will relay “stories of hope”, and there’s a “solutions fair” with stalls telling people how to consume less and get involved in campaigns. Dr Rosie Trevelyan, director of the Tropical Biology Association, is one of the brains behind the event. She argues that if people dwell on the negative, they won’t learn what actually works, and what they can do to help.

Examples come from near and far. “We’ve got someone from the Wildlife Trust, which has restored fenland around here,” she says. “It’s an incredibly important habitat and it had practically disappeared. I went to a fenland on Sunday and saw a bittern. Bitterns had almost gone extinct in this country, and they’re now breeding out there.” One of Trevelyan’s students from Ghana will talk about how he and his colleagues managed to save the Togo slippery frog from extinction. When they were selecting positive stories, she says, “we had to cut loads of them, we had so many. It’s about making people aware of the fact that we don’t need to just stand by and watch these things disappear, we can actually do something.”

Earth Day has its detractors, particularly in the US, where environmentalism is seen through the distorting lens of the culture wars. “Earth Day is a yearly reminder that humanity must be controlled, manipulated and even destroyed for the good of the planet,” wrote rightwing pundit Glenn Beck last year. He claimed that the EPA’s ban on the insecticide DDT in 1972 led to millions of avoidable deaths from malaria as other countries followed suit – despite the fact that the US wasn’t the first to outlaw the chemical and continued to allow its use for public health reasons.

For Ron Bailey, Reason magazine’s science correspondent, the mixing of politics and ecology has turned off many who might otherwise find common cause with the protesters. “When it was started back in 1970,” he says, “there were obvious problems with pollution and so forth that needed to be dealt with. But it wasn’t an all-out assault on capitalism and market economies, which it has more or less turned into. It’s very alienating for those who are actually anxious to address specific problems that are occurring in the environment both locally and globally. If the only solution is destroying the economy, you’re not going to get a lot of people on board.”

Global action … Indian girls on the eve of World Earth Day at a school in Patiala in 2015. Photograph: STR/AFP/Getty Images

No fan of Trump, Bailey nevertheless thinks the scientists marching in Washington today are making a mistake. “Certainly we should celebrate science and its objectivity, and what this has brought to modernity. But by tying it to Earth Day, they are undermining their message. It automatically tinges them with politics.” That, he reckons, could jeopardise funding for scientific research, which in the US has tended to be bipartisan.

If not a day, though, then what? “If you’re interested in these kind of things, then go to your local city council, go to your local legislature, write letters to your congressman, testify at hearings, that kind of thing, about the issues that particularly concern you,” says Bailey.

It is not a colourful solution. For Trevelyan, it’s not a question of either/or. And while she doesn’t strike me as particularly political, she believes the reaction to Trump and the new spirit of protest are something to be welcomed. “I think it’s mobilised more people to worry about things,” she says. “I think we should watch this space. The March for Science has been catalysed by seeing [him] as president. People are no longer being complacent.”

Hayes addresses what he sees as the self-harming democratic choices we have made more directly. “If you happen to be an American or a Briton or a French person or a Turk, it’s difficult not to be a little bit ashamed of your country. That’s just a reason not to get despondent, but to get energised and angry and committed.”

“In 1970,” he says, “we called it Earth Day, but it was really a focus on individual cities in the US and schools and neighbourhoods. As we move forward, I think it genuinely is becoming ‘Earth’ Day.” The stakes are higher than they’ve ever been. “You know, this is not one of those casual things that if you win or lose, you don’t mind; or you say, ‘Jeez, sorry, we lost that one.’ We’re talking now about impoverishing the human prospect for the next 100 generations. We can’t lose on this.”