A warmer Mars could support plants long before people could breathe its air

A forest in a Martian crater is an arresting possibility, but this study offers something narrower than a prediction that forests will grow there. It maps where temperature, pressure and oxygen might meet selected organisms’ requirements—assuming artificial warming and atmospheric changes that nobody has yet shown can be delivered at planetary scale.

In an arXiv preprint posted on November 9, Ashwin S. Braude and colleagues report what they describe as the first three-dimensional global climate simulations of an oxygen-rich, terraformed Mars. Their central distinction is between making the climate less hostile and supplying enough oxygen for a complex biosphere. Those are different problems, with different obstacles.

What the simulations assume

The team used MarsWRF, a climate model that calculates atmospheric circulation, radiation and exchanges of heat with the ground. They ran 24 simulations combining six atmospheric pressures with four levels of artificial greenhouse warming. The model used a horizontal grid of 60 by 36 cells and 52 vertical layers.

Pressures ranged from 18.3 to 183 hectopascals, roughly three to thirty times present-day Mars pressure. These values refer to Mars’ reference elevation, called the datum: local pressure is higher in deep basins and lower on mountains.

Every simulation retained 18 hectopascals of carbon dioxide, with additional pressure supplied by oxygen. This deliberately separates the effects of atmospheric thickening from changes in carbon dioxide abundance; it is not a simulation of a self-consistent biological oxygenation process.

Artificial warming came from an idealised “grey gas”: a stand-in that lets sunlight through but absorbs outgoing infrared radiation. It represents an imposed warming effect, not a tested substance or deployment system.

Warmer nights, still difficult seasons

Adding greenhouse warming strongly raised average temperatures. Thickening the atmosphere had a smaller effect on the global average, but helped soften the difference between day and night. Each doubling of pressure reduced that difference by about 5 degrees Celsius, mainly through warmer nights.

The seasons were harder to tame. Mars’ elongated orbit places southern summer near its closest approach to the Sun, intensifying southern seasonal extremes. Even the highest-pressure simulation without artificial greenhouse warming retained average seasonal temperature differences exceeding 45 degrees, reaching about 70 degrees in Hellas Basin.

Hellas nevertheless stands out as an early potential refuge. Its deep floor provides higher local pressure and oxygen availability. Higher pressure also raises water’s boiling point, widening the temperature interval in which liquid water could exist. Later, equatorial and northern lowlands could offer more moderate seasonal conditions.

Too much warming can work against life. Under thin air, daytime temperatures can exceed water’s low boiling point. A warmer annual average therefore does not necessarily mean a larger habitable area.

A conditional route from microbes to trees

The researchers compared modelled conditions with survival and growth requirements for selected microbes, lichens, mosses, plants and humans. Surviving a freeze is not the same as growing: their criteria generally require a sustained period suitable for biological activity, with organism-specific exceptions.

Around 30 hectopascals at the reference elevation, some locations could meet the assessed requirements for lichens and mosses. Around 60 hectopascals, suitable conditions could begin to include trees in Hellas, assuming appropriate artificial warming. These are conditional habitat estimates, not demonstrated ecosystems or guaranteed planting thresholds.

Human breathing demands more. Crucially, the simulations retain carbon dioxide far above the paper’s stated human tolerance limit. Their oxygen-based breathing thresholds therefore require carbon dioxide removal too; the simulated mixtures themselves are not safe outdoor air. Radiation adds another constraint: a supplementary calculation using a 5-millisievert annual cosmic-ray dose limit restricts human settlement to reference pressures above roughly 180 hectopascals.

Oxygen is the harder accounting problem

The study’s optimistic productivity estimates suggest that increasing pressure from roughly three to five times today’s value could take 500–7,000 Earth years. That is an early atmospheric step, not the time needed to create breathable air. The estimate assumes Earth-derived biological production rates and complete recycling of atmospheric carbon dioxide.

Photosynthesis releases oxygen from water while incorporating carbon dioxide into organic matter. Known accessible Martian carbon dioxide reserves would support only about 20 hectopascals of oxygen without recycling—insufficient for unaided human breathing. Recycling carbon through reactions involving iron or sulphur might allow more production, but the resulting materials must remain separated from oxygen to prevent its consumption. The feasibility is unresolved.

The climate calculations also omit dust and water-cycle effects, while the main habitat maps omit water shortages, hostile soil chemistry, nutrient constraints and radiation. Some biological limits are estimates or drawn from related species. These omissions make the mapped habitats upper bounds. The preprint identifies conditions worth investigating, not evidence that a self-sustaining Martian biosphere is achievable.