The rise of the bush: what happened when scientists warmed the Rocky Mountain wilderness by 2C?

YesIn late summer, people descend on the wildflower capital of Colorado to see meadows dotted with cornflowers, aspen sunflowers and sub-alpine larkspur. In January 1991, a team of scientists led by Professor John Harte set up a unique experiment in these Rocky Mountain pastures. It was one of the first (and longest) to determine how climate change will affect ecosystems.

At the time, it was believed that increased temperatures would lead to longer, lusher grass. But instead of thriving, the grass and wildflowers began to disappear, replaced by sage brush. The experimental pastures became a desert-like wasteland. Even the fungi in the soil were changed by the heat.

The Warming Meadow experiment shed light on the future. These meadows will disappear in the coming decades if warming reaches 2C above pre-industrial levels, according to results published in the Proceedings of the National Academy of Sciences. The findings are alarming, not just for Colorado, but for mountains across the planet as “shrubification” takes over.

Test

The Rocky Mountain Biological Laboratory is located in a Gothic period town, abandoned after the closure of its silver mines. During the winter, the land lies quietly under a bed of snow. At the start of the year, the only way for researchers to reach the test sites – at an altitude of 10,000ft – is by cross-country skiing.

Electric infrared radiators heat five test areas of 30 sq meters all year round. The overhead heaters were on day and night over the pasture, keeping it 2C above normal with an annual electricity bill of $6,000 (£4,450). They heat the top six inches of soil. Animals could come and graze and the natural order was preserved as much as possible.

Over 29 years, researchers have found that shrubs have increased by 150% in warm areas compared to non-warm areas. The soil surface was dried up to 20%, and plants with shallow roots were stressed. Some wildflowers disappeared in warmer climates. “It’s a sign of things to come,” says lead researcher Lara Souza of the University of Oklahoma.

Scientists have also noticed significant changes in the invisible world of fungi and soil microbes. Shrubs and sagebrush are not dependent on fungi in the same way as grass. They found a decrease in fungi that help plants obtain nutrients, and an increase in fungi that destroy organic matter. “This highlights that when you have a big change on the surface, you can have a big change on the ground,” says Souza. “It’s not very easy to go back.”

Alpine pastures are often overlooked in terms of their richness. Europe’s mountain pastures contain 50% of Europe’s vegetation on just 3% of the land area. It is home to many species of plants found nowhere else in the world. “It has been here for thousands of years,” says Dr Patrick Möhl of Lancaster University who studies pristine alpine meadows in Austria and their disappearance due to climate change.

“There’s so much diversity, we’re going to lose a lot of it,” he says.

Möhl has seen tree species – mostly pine – rising as the climate warms. “It’s a big change in the ecosystem – the landscape is changing, from a grassland to a forested environment,” he says.

Arctic ‘shrubification’

This is not only observed in mountainous areas.

The expansion of bush cover is one of the most important ways the Arctic is changing, with polar “green” patterns visible even on satellites. Rising summer temperatures are a key driver. Shrub cover has increased by 2.2% each year in the western Canadian Arctic, according to data recorded between 1984 and 2020.

In cold climates, plants tend to stay small. Larger plants can be damaged by wind and cold, heavy snow, or face problems developing leaves and stem tissue in a short growing season. As the weather cools down and becomes undisturbed, shrub and tree species can enter.

Sarah Dalrymple, an environmentalist at Liverpool John Moores University, who is studying the changes in Iceland, says: “Global warming is removing some of the restrictions on plant growth that were associated with cold conditions in high latitudes and high altitude environments. “There is a change from grassland, or soil, to shrubs, and finally to trees.”

Grasses and soil ecosystems that have been maintained in good condition for thousands of years may become irreversible in the coming decades. Dalrymple says: “The erosion itself is not a problem, but the fact that we are losing the Arctic environment is a problem.

Growth of shrubs recorded in the same area between 1987 (left) and 2019 (right) Qikiqtaruk, Hershel Island, Canada.

Some people embrace shrubs and trees – they provide shelter for wildlife, livestock and people. “But at the global level, deforestation of cold regions is worrying because it is linked to the melting of permafrost and the acceleration of carbon production,” added Dalrymple.

“It’s scary to see this deforestation happening so quickly. The speed of change, and the knock-on effects on things like the carbon cycle, are very worrying. It’s not just about whether each tree is good or bad.

“What is ‘bad’ is our inability to control our carbon. The underestimation is a symptom of this, not the cause, and we should treat it as such.”

The way we manage the planet and where we live depends on the fact that we think this planet will be there forever, and it will not change. But these changes are global, not local to Colorado. “It’s all happening faster than the predictions would have predicted,” says Dalrymple.

Souza is still fascinated by the insect-infested meadows that surround the research center. He has been coming since 2012 and the magic never changes.

He says: “It’s like flowers on steroids. “For me it’s surreal, every time I come.” But this vision is full of sadness about what the future will be. This fragile place – like many people across our planet – is on the brink of a big change.

This article was amended on 26 and 27 March 2026. The headline was changed to correctly reflect the content of the article; and the text itself was amended to include that Prof John Harte was the founder of the Warming Meadow project.

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