Add this to the distant-doomsdays file. Earth's continental crust, which forms the land we live on, has been slimming down, according to a new estimate.
If the slimming rate holds, the continents might disappear into the sea within a couple of billion years.
But if we survive all that, the slow flattening of continents could end our world as we know it. As the continental crust erodes, land would disappear into the oceans – even without climate induced sea level rises.
There are already movies about what the most extreme case might look like, jokes Bruno Dhuime of the University of Bristol, UK, who led the study. "I'm thinking Waterworld."
Written in the stones
To reconstruct the history of continental crust, Dhuime's team collated measurements of 13,000 rock samples from around the world. They found that in rocks from the Andes and Central America – where continental crust is forming – the amount of silicon and the relative concentrations of rubidium and strontium isotopes are related to the thickness of the crust that the rocks had come from.
This relationship can allow us to estimate the thickness of crust at different times in the past. Dhuime's team did just that, inferring the rise and fall of continents over the Earth's history.
Continents today are about 35 kilometres thick, on average, with the buoyant rock bobbing next to the 7-kilometre-thick, denser oceanic crust, which rides lower.
But before about 3 billion years ago, Dhuime thinks, the continents were slimmer. Having less volume made them less buoyant, meaning they couldn't float up above sea level.
Once plate tectonics began on Earth in earnest, the continents spent the next 2 billion years beefing up when plates collided, pushing the crust up. Continental crust peaked in thickness about a billion years ago – around the time Earth's mightiest continents banded together to form the supercontinent Rodinia.
The mountains raised by that event have been eroding ever since, and not enough new crust is forming to offset the losses.
"If it continues for the next 2 billion years, then the crust will again reach that state where the continents are submerged beneath the ocean," Dhuime says.
Rise and fall
So just how worried should we be? Dhuime does say that his record of continental thickness is an inference from isotope concentrations.
"There are a lot of assumptions and models in here," says Clark Johnson of the University of Wisconsin-Madison. For example, he says, for the continents to get thinner in the long term, erosion would have to also outpace magma that attaches to the base of the crust – not just the build-up of crust at plate convergences that Dhuime's team considered.
Some support comes from suggestions that continental crust may have shrunk over the last few hundred million years, an idea that is based on looking at subduction zones, where two plates come together and one slides beneath another.
The first bloom of life on Earth happened at about the same time as the first rise of continental crust, when shallow seas could host algae, about 2.5 billion years ago. According to Dhuime's model, land was just starting to peek out of the water and now it might be slowly headed back in that direction.
Will it be the final doomsday? If the crust does go and we somehow manage to survive in the brave new water world, we will still face the sun's expansion in 5 billion years or so that will swallow up the planet. There's also the heat death of the universe to prepare for.