One day, far in the future, a leaf will turn brown and crumble to dust, representing the end of all plants on Earth. Without a crystal ball, nobody knows for certain when that day will come.

A number of studies have made valiant attempts to predict how long the vegetative biosphere has left, with some early guesses placing this date as soon as 100 million years from now. Others think plants have a little longer, closer to a billion years.

Jacob Haqq-Misra and Eric Wolf, researchers with the charity Blue Marble Space, argue in their recently published paper that Earth could stay green for nearly 1.9 billion years or more, depending on how the future plays out.

There are a lot of caveats to consider in these kinds of calculations.

Sure, there’s little we can do about the slow expansion of our Sun into a red giant, slowly baking our relatively tiny ball of organic-crusted rock to a crisp in the next few billion years. But future humans – or whatever sentient minds persist on our planet in eons to come – could either hasten the demise of life, or extend it with some kinds of ingenious geohacking solutions that buy us time against our star’s death.

Even if we ignore interventions of any sort, evolution could yet deliver new tools that overcome ecological challenges in ways we can barely imagine.

With that in mind, there are restrictions on what today’s photosynthesizing organisms can achieve that can inform our predictions of future life.

Haqq-Misra and Wolf assess future climates at intervals by taking into account increasing solar radiation and decreasing carbon dioxide, arriving at two trajectories – one where CO2 remains steady as surface temperatures rise, and one where carbon dioxide levels drop before temperatures become a problem.

Modelling temperature rise as the limiting factor, we can expect a slow rise of just over 20 degrees Celsius (a little under 40 degrees Fahrenheit) over the next 1.5 billion years. Over the next half a billion years, the increase will accelerate, adding another 40 degrees Celsius.

The result would be roast vegetables, with the hardiest of plants vanishing somewhere around the 1.87 billion CE. Give or take.

What if changes in weathering accelerate carbon’s scrubbing from the atmosphere?

As we’re tragically discovering, concentrations of carbon in our atmosphere are subject to a variety of balances. Burning fossil fuels is just one factor, with biological and geological processes belching out and soaking up carbon at the whim of tectonic forces, atmospheric conditions, and ocean currents.

In that scenario, CO2 could drop from just over 400 parts per million to just over 30 parts per million in just 1 billion years. Few plants could manage such a scarcity of carbon, which in the most optimistic of cases might see them struggle to survive 1.84 billion years from now.

Either way, salads can anticipate extinction well before the 2 billion year mark, assuming pockets of water remain after the last of our oceans vaporize in about 1.5 billion years.

That’s not to say microbial life couldn’t persist deep underground by then, pushing the dying gasps of Earth’s biosphere forward another billion years.

So smell the roses while you still can.

This research was published in JGR Atmospheres.

Fact-checked by Bronwyn Thompson

Editor's note: This story was originally published on our dedicated Health & Science site Refractor.io a week ago.