This article was originally published in autumn 2019.

Temperatures could hit 70ºC in 250m years’ time. Oh, and all the continents will be squished into one.

The world is in a state of alarm – and rightly so – at how much damage will be caused by rising temperatures of even a few degrees as a result of climate change. So what’s it going to be around here a little further ahead – okay, a lot further ahead, like a few hundred million years or so?

The bottom line is that in less time than it has taken higher lifeforms to evolve into land creatures, Earth’s biosphere may be changed by the inevitable course of the evolution of our sun. In 300m years or less, it may become very inhospitable for life to continue to exist on the land, and if we leave it alone, evolution may encourage life to return to the sea where the climate will be a bit more moderate. And where will that leave humans?

Since 1967, some astronomers have intensively studied the evolution of stars similar in mass and age to our sun. For example, Prof. Iko Iben at MIT published a ground-breaking paper in 1967 (in the Astrophysical Journal, vol. 147) in which he calculated the changes in temperature, size and luminosity of stars with masses similar to our sun. What he found is that the Sun, now 4.5bn years old, will change its luminosity by a factor of two in the next 5bn years. In the next 1bn years, the amount of solar radiation reaching Earth will increase by 8%. Figure 1 shows a typical timeline for the increase of solar luminosity as it evolves. 

This 8% increase doesn’t sound like much, but if you look at a recent 1994 report by the National Academy of Science, Solar Influence on Global Climate, you will discover that a 0.1% increase in solar radiation causes a climate forcing of 0.24 watts per sq. m, which leads to an increase in the mean global temperature of 0.2 ºC. From this, we can estimate that our 8% increase in solar radiation will cause a 16ºC increase in the mean solar temperature over the 1bn years. Or 5ºC in the next 300m years.

The map in figure 2 shows the average annual temperature of Earth based on satellite data. An 8% increase in solar energy would cause all the annual temperatures to increase by 5ºC, which means a significant expansion of the brown temperate zone into the sub-Arctic latitudes of Canada and an expansion of the equatorial high-temperature zone into the mid-latitudes.

If 16ºC over 1bn years doesn’t seems that bad, just remember that our atmosphere will probably respond gradually to this increase by becoming cloudier as its water vapour content climbs, and will also become richer in carbon dioxide as plant growth is stimulated and the oceans begin to give up some of their dissolved carbon dioxide. The increased greenhouse heating could make the global temperature increase somewhat higher than the 16ºC change due to the sun alone.

Let’s say that the mean winter temperature globally is about 20ºC now. A 5ºC increase in 300m years takes that up to about a 25ºC average winter temperature, with very few places where we could expect to see snow and ice. The average summer temperature would be closer to 30ºC. If we add enhanced greenhouse heating, these estimates might easily be much higher, with the average global temperature in 300m years looking more like 35-40ºC.

The biggest problem facing life on Earth is continental drift. In about 250m years a new supercontinent will have formed as the current continents continue their movements. Called Pangea Ultima, the interior of the new supercontinent will be utterly uninhabitable by life, with daytime temperatures, by some forecasts, exceeding 70ºC (for Americans, that’s 160ºF). This may cause global warming to the degree that a new ‘hothouse Earth’ is established due to water vapour and carbon dioxide build-up in the atmosphere. Although this will be a temporary condition that will subside as the continent breaks apart, the timescale for this change is long enough that any extant humans or land-based life will be under enormous environmental stresses, with inevitable population reductions.

Apart from this temporary change due to Pangea Ultima, a review of long-term climate variations among the inner planets by Michael Rampino and Ken Caldeira (Annual Reviews of Astronomy and Astrophysics, 1994) suggests that an even bleaker outlook may be in store for Earth when you take into account the carbon dioxide in the atmosphere. The various sources and sinks are sensitive to temperature, and in the next 1.5bn years the global mean temperature could well exceed 80ºC. The evaporation of our oceans would be well under way 1bn years from now. We can assume that millions of years before this, Earth will have become uninhabitable. Life more complex than a bacterium has only been around for 600m years, so it looks like we are about halfway through the golden years. To me, this is uncomfortably short, because it suggests that in perhaps as short as a few hundred million years, life could get very unpleasant here.

Where does all this leave humans? Will we be able to adapt as Earth changes, continuing to live on the land while life around us is driven back to evolving under the sea? Or will we decide to leave the planet altogether? Estimates suggest that it would only take about 5m-10m years to colonise the entirety of the Milky Way galaxy, so I think we will have plenty of opportunity to survive as a species – even when Earth has become a second cousin to how Venus is today.