It could cause a mass movement of refugees “unprecedented in human history,” according to Barack Obama. It is a global crisis that may already be “passed the point of no return” and which might just bring human civilization to an end within a lifetime.

But anyone would think this pressing concern – global warming – simply did not exist, judging by the general election campaign, the Green Party has claimed.

Its co-leader, Caroline Lucas, said: “It beggars belief that this election has been almost environment-free when we face an air pollution crisis, a climate denier in the White House, the threat of an extreme Brexit and accelerating climate change.

“Environmental protection isn’t an optional extra – it must be part of any long-term vision for this country’s future.

“But shamefully it’s an issue that has been conspicuous by its absence so far in this election campaign – and this has got to change.

“With almost no mention in the leaders’ debates and glossed over in the manifestos, it’s hard to believe any other parties are committed to stopping climate targets slipping further out of reach, cleaning up our air or ending the free-fall in biodiversity.

“We have the chance to make this general election a turning point in our history, when the UK steps to become a world leader in environmental protection. But to do that we need to put the environment back on the political agenda.”

There are fears among environmentalists that regulations designed to reduce carbon emissions and protect the natural world will be watered down or scrapped completely after Brexit – partly because of calls by right-wing Conservatives to do just that.

Ms Lucas took a sign with a giant green question mark to Downing Street and to Labour party headquarters to ask Theresa May and Jeremy Corbyn “where’s the environment?”

The Tory manifesto said the party would ensure that the UK will “lead the world in environmental protection” and reiterated an earlier pledge to “leave the environment in better condition than we inherited it”.

The Greens described it as an “absolute car crash for the environment”, but Friends of the Earth praised its rejection of Donald Trump-style climate science denial.

10 photographs to show to anyone who doesn't believe in climate change Show all 10 1 /10 10 photographs to show to anyone who doesn't believe in climate change 10 photographs to show to anyone who doesn't believe in climate change A group of emperor penguins face a crack in the sea ice, near McMurdo Station, Antarctica Kira Morris 10 photographs to show to anyone who doesn't believe in climate change Floods destroyed eight bridges and ruined crops such as wheat, maize and peas in the Karimabad valley in northern Pakistan, a mountainous region with many glaciers. In many parts of the world, glaciers have been in retreat, creating dangerously large lakes that can cause devastating flooding when the banks break. Climate change can also increase rainfall in some areas, while bringing drought to others. Hira Ali 10 photographs to show to anyone who doesn't believe in climate change Smoke – filled with the carbon that is driving climate change – drifts across a field in Colombia. Sandra Rondon 10 photographs to show to anyone who doesn't believe in climate change Amid a flood in Islampur, Jamalpur, Bangladesh, a woman on a raft searches for somewhere dry to take shelter. Bangladesh is one of the most vulnerable places in the world to sea level rise, which is expected to make tens of millions of people homeless by 2050. Probal Rashid 10 photographs to show to anyone who doesn't believe in climate change Sindh province in Pakistan has experienced a grim mix of two consequences of climate change. “Because of climate change either we have floods or not enough water to irrigate our crop and feed our animals,” says the photographer. “Picture clearly indicates that the extreme drought makes wide cracks in clay. Crops are very difficult to grow.” Rizwan Dharejo 10 photographs to show to anyone who doesn't believe in climate change Hanna Petursdottir examines a cave inside the Svinafellsjokull glacier in Iceland, which she said had been growing rapidly. Since 2000, the size of glaciers on Iceland has reduced by 12 per cent. Tom Schifanella 10 photographs to show to anyone who doesn't believe in climate change A river once flowed along the depression in the dry earth of this part of Bangladesh, but it has disappeared amid rising temperatures. Abrar Hossain 10 photographs to show to anyone who doesn't believe in climate change A shepherd moves his herd as he looks for green pasture near the village of Sirohi in Rajasthan, northern India. The region has been badly affected by heatwaves and drought, making local people nervous about further predicted increases in temperature. Riddhima Singh Bhati 10 photographs to show to anyone who doesn't believe in climate change A factory in China is shrouded by a haze of air pollution. The World Health Organisation has warned such pollution, much of which is from the fossil fuels that cause climate change, is a “public health emergency”. Leung Ka Wa 10 photographs to show to anyone who doesn't believe in climate change Water levels in reservoirs, like this one in Gers, France, have been getting perilously low in areas across the world affected by drought, forcing authorities to introduce water restrictions. Mahtuf Ikhsan

Labour’s manifesto said it would ban fracking “because it would lock us into an energy infrastructure based on fossil fuels long after the point in 2030 when the Committee on Climate Change says gas in the UK must sharply decline”.

The UK is currently not going to meet targets to reduce greenhouse gas emissions in the late 2020s and early 2030s, but Labour said it would “put us back on track” to meet commitments under the UK Climate Change Act and the Paris Agreement.

The Labour Party dismissed the premise of the Greens’ question as “nonsense”.

“Labour are committed to protecting our environment. In our manifesto we explicitly pledge to transition to a low-carbon economy and meet climate targets, introduce a new Clean Air Act and to defend and extend environmental protections,” it said in a statement.