As the sun evolves, it is getting brighter. Our star is currently producing about a third more energy than it did at the dawn of the solar system 4.5 billion years ago. And it will continue to get hotter until it eventually dies in about 5 billion years.

Scientists have wondered for decades how long life on Earth will manage to cling to existence as the sun brightens. In 1982, James Lovelock and colleagues estimated that Earth’s photosynthetic biosphere — which includes all plants and forms the basis for most of the planet’s biology — would end about 100 million years from now. Successive studies have pushed back the deadline for the death of all life on Earth.

“We were trying to show that life on Earth — complex vegetation — could survive longer into the future than previous studies had shown,” study co-author Jacob Haqq-Misra, an astrobiologist at space exploration charity Blue Marble Space, told Live Science.

The boundaries of life

Life on Earth relies on photosynthesis, the process used by plants, algae and some bacteria to turn sunlight into energy. The mechanism chemically converts carbon dioxide and water into sugars and oxygen. It requires both CO₂ and sunlight.

But at certain temperatures, plants’ photosynthetic machinery shuts down. Eventually, the sun will warm Earth to the point that plants are no longer able to photosynthesize, which would in turn cause entire food webs to collapse and all life to perish.

Another issue is that as the sun dies and it gets brighter, there will be less carbon dioxide in the atmosphere, effectively starving plants.

“The Earth has stayed pretty hospitable in terms of surface temperature for most of the last 4 billion years because it has a built-in thermostat” by storing CO₂ in rocks and releasing it during volcanic eruptions, Robert Graham, a planetary science researcher at the University of Chicago, who was not involved in the research, told Live science.

When it is hotter, the planet pulls more carbon dioxide out of the atmosphere and stores it in rocks underground, Graham said. This offsets the warming to keep the temperature stable but means that the carbon dioxide isn’t accessible to plants.

Climate models and extreme plants

In the new study, Haqq-Misra and colleague Eric Wolf, a research scientist at Blue Marble Space, used 29 climate models to estimate what would happen to Earth’s vegetative biosphere under different scenarios. They used the two extreme cases as limits — when Earth is too hot for life but the CO₂ was stable; and when there is not enough CO_2, but the temperature was stable. They then looked at the range of CO₂ and sunlight conditions in between those extremes. This enabled them to include situations in which Earth was very efficient at pulling carbon from the atmosphere when temperatures started rising.

They also included information about a variety of plants. Some plants can survive on a much lower ratio of atmospheric CO₂ than others. The study included plants that have a special photosynthetic process (known as crassulacean acid metabolism), such as succulents and orchids. These plants can sustain themselves on relatively tiny amounts of CO₂. The same is true of some marine plants, which can dissolve and access carbon in the ocean system.

Other experts were impressed by the findings.

“Haqq-Misra and Wolf have used a sophisticated 3D climate model to show that Earth’s climate may remain hospitable to plant life significantly longer into the future than predicted” by simpler models, said Graham, who authored one of those earlier studies. “It’s an advance over previous work and suggests that complex biospheres like that of Earth are more resilient to environmental change from stellar brightening than previously suggested.”

Looking to the future

Andrew Rushby, an astrobiologist at Birkbeck University of London who was not involved in the research, told Live Science that the paper updated the concept of the lifetime of the biosphere. However, he cautioned that the results remained “broad estimates.”

“It is not possible for us to predict or know the possible evolutionary adaptations that the photosynthetic biosphere may undergo in response to increasing solar output and lower [atmospheric CO₂], especially over billions of years,” he said.

In their paper, the authors wrote that “limits posed by thermal stress or starvation may only reflect our observations of the biosphere today rather than hard limits on how the biosphere may evolve.” There is also no way of knowing how life might adapt to new circumstances.

Haqq-Misra said that he found the results comforting. “Earth’s system is resilient, and we are part of something that could have a much, much longer future,” he said.

The results could also help scientists figure out what the thresholds could be on other planets. “Part of the challenge is starting with these Earth-based models, and then generalizing the physics as much as possible to be able to simulate a wider range of atmospheres,” he said.