N o one knows when millions of monarch butterflies began criss-crossing North America, spending their winters clustered on the same hillsides in central Mexico, a blaze of orange wings in the green forest.

But over the centuries, they were mythologised by generations of humans. Locals believed they carried the souls of their ancestors. Scientists saw the migration as the proof of insect evolution – a brittle organism that could fly 6,000 miles a year to avoid severe weather.

Now, the monarch has morphed into a different kind of symbol. One of the world’s oldest, most resilient species could be destroyed – and soon – by climate change.

The butterflies are among the world’s experts in climate adaptation. They spend their summers in the northern United States and Canada; they breed in the southern United States during the fall and spring; and most spend their winters in central Mexico, in a few giant clusters.

Their life cycles are driven by a search for optimal conditions: temperatures ideally between 12C and 22C when they migrate, some rain during their winters and plenty of milkweed when they mate. The criteria are narrow – and dependent on relatively consistent weather patterns.

That consistency no longer exists. Now summer temperatures in the midwest are soaring. The milkweed in Texas is drying up. Winter storms, once rare, are snaking through central Mexico regularly as air warms over the Pacific Ocean and blows across the region.

“The question we’re asking is: can one of the world’s most adaptive insects adapt to climate change?” says Karen Oberhauser, who studies the species at the University of Wisconsin. “We are changing the conditions and just waiting to see.”

If you’re talking 20, 30, 40 years out, we’re not going to be talking about monarchs any more Chip Taylor, University of Kansas

To specialise in monarch butterflies in 2019 is to wonder constantly if your work is about to go extinct. While almost all species could eventually be consumed by a changing climate and habitat loss, few are as likely to vanish as abruptly as the monarch.

Between 1990 and 2015, the US Fish and Wildlife Service says, a billion butterflies vanished. Because more than 95 per cent of the population migrates en masse to a few patches of Mexican forest, each smaller than half a football field, a single storm or heat stroke could effectively kill off the population. (A smaller percentage of the butterflies winter in southern California or Florida, where they face their own challenges.)

That nearly happened in 2002, when a winter storm killed about 75 per cent of monarchs. And again in 2012, when a heat wave in the midwest killed tens of thousands. “At every stage in their migration, they are threatened by climate change,” says Eduardo Rendón, the monarch butterfly coordinator for the World Wildlife Fund in Mexico.

Millions of monarch butterflies arrive each year to breed at the oyamel firs forest in Michoacan state, Mexico (AFP via Getty)

Each time, they rebounded – but barely. Last year, the population grew when a cold front in north Texas forced butterflies to stay in south Texas for longer than usual, increasing the length of their breeding season.

But this year brought another scare, as temperatures in Oklahoma and Kansas soared as monarchs passed through. It was the second hottest September in 125 years in those states. The impact on the population has not yet been calculated. “If you’re talking 20, 30, 40 years out, we’re not going to be talking about monarchs any more,” says Chip Taylor, the founder of Monarch Watch and a professor at the University of Kansas.

One way to chronicle the toll of climate change is simply to list, one by one, the species that it eliminates. By one measure, that list started this year. In February, the government of Australia recognised the first mammal to go extinct due to climate change: the Bramble Cay melomys, a small rodent that lived on an island in the Great Barrier Reef.

Climate change: Decade's defining issue in pictures Show all 20 1 /20 Climate change: Decade's defining issue in pictures Climate change: Decade's defining issue in pictures California In this decade, humans have become ever more aware of climate change. Calls for leaders to act echo around the globe as the signs of a changing climate become ever more difficult to ignore Getty Climate change: Decade's defining issue in pictures Athens, Greece Fierce wildfires have flared up in numerous countries. The damage being caused is unprecedented: 103 people were killed in wildfires last year in California, one of the places best prepared, best equipped to fight such blazes in the world AFP/Getty Climate change: Decade's defining issue in pictures Redding, California Entire towns have been razed. The towns of Redding and Paradise in California were all but eliminated in the 2018 season AP Climate change: Decade's defining issue in pictures Athens, Greece While wildfires in Greece (pictured), Australia, Indonesia and many other countries have wrought chaos to infrastructure, economies and cost lives AFP/Getty Climate change: Decade's defining issue in pictures Carlisle, England In Britain, flooding has become commonplace. Extreme downpours in Carlisle in the winter of 2015 saw the previous record flood level being eclipsed by two feet AFP/Getty Climate change: Decade's defining issue in pictures Hebden Bridge, England Hebden Bridge in West Yorkshire has flooded repeatedly in the past decade, with the worst coming on Christmas Day 2015. Toby Smith of Climate Visuals, an organisation focused on improving how climate change is depicted in the media, says: "Extreme weather and flooding, has and will become more frequent due to climate change. An increase in the severity and distribution of press images, reports and media coverage across the nation has localised the issue. It has raised our emotions, perception and personalised the effects and hazards of climate change." Getty Climate change: Decade's defining issue in pictures Somerset, England Out west in Somerset, floods in 2013 led to entire villages being cut off and isolated for weeks Getty Climate change: Decade's defining issue in pictures Dumfries, Scotland "In summer 2012, intense rain flooded over 8000 properties. In 2013, storms and coastal surges combined catastrophically with elevated sea levels whilst December 2015, was the wettest month ever recorded. Major flooding events continued through the decade with the UK government declaring flooding as one of the nation's major threats in 2017," says Mr Smith of Climate Visuals Getty Climate change: Decade's defining issue in pictures London, England Weather has been more extreme in Britain in recent years. The 'Beast from the East' which arrived in February 2018 brought extraordinarily cold temperatures and high snowfall. Central London (pictured), where the city bustle tends to mean that snow doesn't even settle, was covered in inches of snow for day PA Climate change: Decade's defining issue in pictures London, England Months after the cold snap, a heatwave struck Britain, rendering the normally plush green of England's parks in Summer a parched brown for weeks AFP/Getty Climate change: Decade's defining issue in pictures New South Wales, Australia Worsening droughts in many countries have been disastrous for crop yields and have threatened livestock. In Australia, where a brutal drought persisted for months last year, farmers have suffered from mental health problems because of the threat to their livelihood Reuters Climate change: Decade's defining issue in pictures Tonle Sap, Cambodia Even dedicated climate skeptic Jeremy Clarkson has come to recognise the threat of climate change after visiting the Tonle Sap lake system in Cambodia. Over a million people rely on the water of Tonle Sap for work and sustinence but, as Mr Clarkson witnessed, a drought has severley depleted the water level Carlo Frem/Amazon Climate change: Decade's defining issue in pictures Addis Ababa, Ethiopia In reaction to these harbingers of climate obliteration, some humans have taken measures to counter the impending disaster. Ethiopia recently planted a reported 350 million trees in a single day AFP/Getty Climate change: Decade's defining issue in pictures Morocco Morocco has undertaken the most ambitious solar power scheme in the world, recently completing a solar plant the size of San Francisco AFP/Getty Climate change: Decade's defining issue in pictures London, England Electric cars are taking off as a viable alternative to fossil fuel burning vehicles and major cities across the world are adding charging points to accomodate AFP/Getty Climate change: Decade's defining issue in pictures Purmerend, The Netherlands Cities around the world are embracing cycling too, as a clean (and healthy) mode of transport. The Netherlands continues to lead the way with bikes far outnumbering people Jeroen Much/Andras Schuh Climate change: Decade's defining issue in pictures Xiamen, China Cycling infrastructure is taking over cities the world over, in the hope of reducing society's dependency on polluting vehicles Ma Weiwei Climate change: Decade's defining issue in pictures Chennai, India Despite positive steps being taken, humans continue to have a wildly adverse effect on the climate. There have been numerous major oil spills this decade, the most notable being the BP oil spill in the Gulf of Mexico in 2010 AFP/Getty Climate change: Decade's defining issue in pictures Amazon rainforest, Brazil More recently, large swathes of the Amazon rainforest were set alight by people to clear land for agriculture AFP/Getty Climate change: Decade's defining issue in pictures California This decade may have seen horrors but it has led to an understanding that the next decade must see change if human life is to continue Getty

But the melomys, with its glassy eyes and tiny rat-like mouth, was – to the human imagination, at least – as far from the majestic monarch as a species can get. Sympathy for animals is not democratic. There’s nothing logical about our love for the monarch, nothing that makes its possible extinction any more tragic than that of the melomys.

But! To see the monarch migration arrive in central Mexico is to be transported into a waking dream. Tree branches droop under the weight of thousands of butterflies. Each morning, when the sun hits, they scatter in a glittering cloud of orange wings. In parts of the forest, looking up from the ground, visitors see more butterflies than sky, the only sound the quiet flapping of wings against the ground and leaves.

“The monarch’s overwintering colonies in Mexico rank as one of the great biological wonders of the world,” wrote Lincoln Brower, the world’s pre-eminent expert on the species, who died last year

A group of monarch butterflies in the snow at the Ocampo community, Michoacan state (AFP via Getty)

The United Nations reported this year that 1 million species of plants and animals face possible extinction “within decades” – due in large part to climate change. The monarch is among them. American scientists spent more than a century trying to find the population in Mexico. A National Geographic headline celebrated their discovery by the Canadian zoologist Fred Urquhart in August 1976: “Discovered: The Monarch’s Mexican Haven.”

Yet the people of Rosario had seen the butterflies land here for centuries. It was their own private spectacle, remote enough that no Americans had located it. Pre-Hispanic pottery has been found emblazoned with images of butterflies. Across the region, soccer teams, schools and businesses are named after the monarchs. “Our grandparents told us, ‘Take care of them. They’re angels,’” said Homero Gomez Gonzalez, the former commissioner of Rosario.

He remembers the mid-1970s, when the American scientists started to arrive. And then, soon after, the American tourists. “We came to believe that the tourism could be good for us,” he says.

Scientists still aren’t entirely sure why the monarchs concentrate on this same hillside and a few others. The high altitude has generally provided a temperate climate. The tree cover and access to water is also crucial. But there are other hillsides in the region that seem to offer similar conditions. “There’s a lot we still don’t understand about why they end up there,” Oberhauser says.

Gonzalez and other local leaders tried to restore the forest that had been badly damaged by illegal logging, which in turn displaced clusters of butterflies. That restoration has largely been successful. But now climate change poses a greater threat.

Some humans are making small efforts to help the monarchs prepare climate change. In Rosario, the World Wildlife Fund is planting drought-resistant oyamel trees, which should be able to provide shelter to the monarchs even as temperatures rise. But experts say the effort isn’t likely to change much.

“The migration will disappear unless we solve climate change,” Taylor says.