Will the Sun roast Earth’s plants or starve them?
It’s somewhat alarming when a scientific paper opens with, “How long will life on Earth survive?” However, in this instance—a study conducted by Jacob Haqq‑Misra from Blue Marble Space and Eric Wolf at the University of Colorado Boulder—the billion‑plus‑year timeframe being examined shouldn’t provoke excessive existential dread.
The setting for this inquiry is our understanding that the Sun will grow brighter as it gradually evolves into a red giant, ultimately engulfing Earth in a fiery inferno. So, at what point along that 5‑billion‑year journey will life on Earth actually be roasted?
Weathering and the weather
This involves more than just the amount of incoming radiation. Among the stabilizing feedback loops that act like a thermostat in Earth’s climate, the cycling of CO₂ through the solid planet is a key factor over such extended timescales. The weathering of silicate rocks on the surface transforms atmospheric CO₂ into carbonate, which ends up on the ocean floor, where it can be subducted into the mantle via tectonic plates. (And eventually, it can be returned to the atmosphere through volcanic activity.)
The weathering of bedrock is partly influenced by temperature. Warmer conditions and a more vigorous hydrologic cycle lead to an increased weathering rate, which removes more CO₂ from the atmosphere. This helps curb rising temperatures. But in this scenario, it could also cause CO₂ levels to drop to extremely low levels—and photosynthesis requires CO₂.
This distant‑future puzzle has been the subject of numerous model simulations over the past several decades. With a steadily brightening Sun, when does Earth become either too hot or too starved of CO₂ for the foundation of the food chain to endure?
Some of those models have relied on relatively simple equations. Others have employed more complex one‑dimensional layer models, which treat the ocean and atmosphere separately in the calculations, for instance. This new study introduces a 3D model and uses two scenarios that represent opposite ends of a spectrum.
The distinction between the scenarios is based on extreme interpretations of the temperature‑carbon relationship described earlier. This approach was taken partly because the notion that CO₂ would eventually drop to very low levels has recently been questioned, based on some evidence of a much weaker link between bedrock weathering and global temperature. So, in one scenario, the researchers kept the planet’s temperature constant (equal to today’s) and allowed CO₂ to decline precisely to offset the brighter Sun. This represents a world where strong weathering functions as a perfect thermostat. In the other scenario, CO₂ is instead held constant at a modern value while temperature rises, representing a very weak weathering thermostat.
Too hot to handle
With weak weathering, the world is about 21°C (38°F) warmer 1.5 billion years from now, and it surges an additional 40°C (72°F) between then and 2 billion years. Even with CO₂ remaining at 400 parts per million, those temperatures would eliminate land plants on Earth.
Specifically, the physiological limits of most land plants are exceeded by 1.68 billion years, and the remainder are finished at 1.87 billion years. (The boiling away of the oceans and loss of our water to space would not be far behind.)
In the strong weathering scenario, the temperature remains unchanged. But after 1 billion years, CO₂ drops to about 34 parts per million, and after 2 billion years it falls to less than 1 part per million. The limit for most land plants is around 150 parts per million, while the much rarer C4 plants could survive down to 3–10 parts per million. The latter limit is reached between 1.35 and 1.64 billion years in.
A few plants, such as cacti and some marine life, can circumvent this by using bicarbonate in the water if dissolved CO₂ is low. They can likely survive down to 1 part per million. That would buy them a bit more time, and they would last until about 1.84 billion years.
That’s optimistic?
The, uh, positive aspect of these estimates for the end of complex life on Earth is that they are actually somewhat more optimistic than most earlier studies. This is due to the 3D model producing slightly less warming for a brighter Sun, the expectation that CO₂ declines more slowly over time, and a modest expansion of the CO₂ range believed to be survivable by plants. Many previous estimates had placed life’s expiration date at less than 1 billion years from now.
Obviously, there are numerous additional factors that could significantly alter this picture, and the researchers mention a few. If civilization persists long enough to witness some of these changes, geoengineering would certainly be an option—such as dispersing aerosols in the stratosphere to reflect sunlight, for example.
There are even some more outlandish suggestions out there, like shifting Earth’s orbit farther from the Sun or removing some of the Sun’s mass to tame the red giant. (After all, we have a billion years to work on the logistics.)
Less speculatively, evolution could influence the physiological limits of Earth’s plants. Any adaptations that broaden the range of survivability would extend the timeline.
Ultimately, the purpose of modeling this sort of thing is not to make a confident prediction. Beyond simple natural curiosity about what will happen to our world, it is also relevant to pondering the potential for life on other worlds. The window of time during which life on Earth is possible tells us something about where to look outside our Solar System.
Land plants have existed on Earth for nearly 500 million years, and if this new estimate is correct, they could persist for almost 1.9 billion more. As was the case for a few billion years early on, microbial life might again have the planet to themselves for a while after that.
JGR Atmospheres, 2026. DOI: 10.1029/2025JD045586 (About DOIs).