The continued release of greenhouse gases into Earth’s atmosphere could increase the longevity of space junk in low Earth orbit, a new paper reveals.

Under a high-emissions scenario, this could mean a sharp reduction in the amount of satellites that can safely operate in low Earth orbit by 2100 – putting significant limitations on what humanity can put up there, spinning around our world, lest we set off a runaway Kessler cascade of ever-increasing collisions.

“Climate change and orbital debris accumulation are two pressing issues of inextricable global concern requiring unified action,” writes a team led by aeronautical engineer William Parker of the Massachusetts Institute of Technology (MIT).

“Understanding and respecting the influence that the natural environment has on our collective ability to operate in low Earth orbit is critical to preventing the exploitation of this regime and protecting it for future generations.”

Space is pretty large, but the amount of it around Earth that we can use for satellite operations is rather more finite. There’s only so many objects we can put up there, between altitudes of around 200 to 1,000 kilometers (125 to 620 miles) before conditions become unsafe.

At a certain capacity, low Earth space will become so crowded that collisions are inevitable; those collisions will break and shatter the colliding objects, filling the region with more pieces of space junk, on uncontrollable and unpredictable trajectories that, in turn, will make low Earth orbit more and more unstable. This scenario is known as Kessler syndrome.

One thing we know about low Earth space is that its capacity can change depending on factors that affect Earth’s atmosphere.

For example, during solar maximum, when the Sun is rampant with flare and eruption activity, satellites have a harder time staying in the sky. This is because the increase in energy from the Sun puffs the atmosphere up, increasing the drag experienced by satellites in low-Earth orbit.

The current state of Earth’s climate has no recorded precedent. The consequences of the sheer volume of anthropogenic greenhouse emissions on low Earth orbit has not been explored in detail. We know that these gases cause the shrinking of the thermosphere – the layer of atmosphere between altitudes of around 85 and 600 kilometers.

Parker and his colleagues set out to close this knowledge gap. They used atmospheric modeling to determine the satellite load that low Earth orbit could support in the year 2100 under different emissions scenarios.

They found that decreasing the density of the thermosphere, which occurs as greenhouse emissions increase, also decreases the drag experienced by satellites; that is, the subtle braking effect applied to an object moving through a fluid.

Drag exerts a constant force on satellites that causes their altitude to gradually drop. For a functional satellite, this means altitude corrections are required; so less drag is a good thing. On the other hand, drag on space junk causes it to slowly fall toward Earth.

Once a satellite has outlived its usefulness, engineers might plan for drag to bring it down into Earth’s atmosphere, where it will burn up harmlessly on reentry, thus disposing of it safely. When drag decreases, it’s going to take a lot longer for that process to play out, leaving the defunct satellite in low-Earth orbit for longer, posing a hazard to other objects.

Under their model, moderate to high carbon dioxide emissions will dramatically decrease the number of satellites deorbiting, keeping low-Earth orbit full of junk and limiting how many new satellites we can send up there.

In the worst-case emissions scenario, the altitude range between 400 and 1,000 kilometers sees a 60 percent capacity reduction during solar maximum, and an 82 percent capacity reduction during solar minimum, by the year 2100.

We’re not anywhere near Kessler capacity yet. The team’s calculations show that millions of satellites could safely operate in the lowest altitude shells without triggering Kessler instability; at time of writing, 11,901 satellites operate in Earth orbit, and an estimated 20,000 pieces of space debris are hanging out up there.

But with current efforts to pack Earth’s sky with satellite swarms, we’d do well to think about this problem before it happens, the researchers say.

“Considering the recent, rapid expansion in the number of satellites in low Earth orbit,” they write, “understanding environmental variability and its impact on sustainable operations is necessary to prevent over-exploitation of the region.”

The research has been published in Nature Sustainability.