Good news—we have extra time before the Sun ends life on Earth - Ars Technica
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It’s a bit worrying when a scientific paper begins, “How long will life on Earth survive?” But in this case—a study by Jacob Haqq‐Misra of Blue Marble Space and Eric Wolf at the University of Colorado Boulder—the billion-plus-year timeline under consideration shouldn’t cause you too much existential panic.
The context for this question is that we understand the Sun will brighten as it eventually matures into a red giant that swallows the Earth in a solar furnace. So, where along that 5 billion-year path will life on Earth, in fact, be cooked?
Weathering and the weather
This isn’t just a question of incoming radiation. Among the thermostat-like stabilizing feedback loops in Earth’s climate, the cycling of CO2 through the solid Earth is a major factor over timescales this long. The weathering of silicate rocks at the surface converts atmospheric CO2 into carbonate that ends up on the seafloor, where it can be subducted into the mantle with tectonic plates. (And eventually, it can cycle back out to the atmosphere through volcanoes.)
The weathering of bedrock depends, in part, on temperature. Warmer temperatures and a more active hydrologic cycle mean an increased rate of weathering, which pulls more CO2 out of the atmosphere. That slows rising temperatures. But in this scenario, it could also lead CO2 to fall to extremely low levels—and photosynthesis requires CO2.
This far-future puzzle has been the focus of many model simulations over the past few decades. With a steadily brightening Sun, when does the Earth either get too hot or too CO2-starved for the base of the food chain to survive?
Some of those models have been relatively simple equations. Others have been more complex one-dimensional layer models, representing an ocean and an atmosphere separately in the math, for example. This new study brings a 3D model to the party and uses a pair of scenarios that mark opposite ends of a spectrum.
The difference between the scenarios is based on extreme views of the temperature/carbon relationship described above. That was done in part because the idea that CO2 would eventually fall to very low levels has been challenged recently, based on some evidence of a much weaker relationship between bedrock weathering and global temperature. So in one scenario, the researchers held the planet’s temperature constant (equal to today’s) and let CO2 drop to compensate exactly for the brighter Sun. This is a world where strong weathering acts as a perfect thermostat. In the other scenario, CO2 is instead held constant at a modern value while temperature increases, representing a very weak weathering thermostat.
Too hot to handle
With weak weathering, the world is around 21° C (38° F) warmer 1.5 billion years from now, and it jumps an additional 40° C (72° F) between then and 2 billion years. Even with CO2 remaining at 400 parts per million, those temperatures would wipe out land plants on Earth.
Specifically, the physiological limits of most land plants are crossed by 1.68 billion years, and the rest are toast at 1.87 billion. (Boiling off the oceans and losing our water to space wouldn’t be far behind.)
In the strong weathering scenario, the temperature doesn’t change. But after 1 billion years, CO2 drops to about 34 parts per million, and after 2 billion years it falls to less than 1 part per million. The limit for most land plants is around 150 parts per million, while the much less common C4 plants could survive down to 3–10 parts per million. The latter limit gets hit between 1.35 and 1.64 billion years in.
A few plants, like cacti as well as some marine life, can cheat by using bicarbonate in the water if dissolved CO2 is low. They can probably make it down to 1 part per million. That would buy them a little more time, and they’d make it to about 1.84 billion years.
Model timelines for the weak and strong weathering scenarios. Temperature in the top plot is in kelvins—a change of 1 kelvin is equivalent to 1 °C. Carbon dioxide in the bottom plot is on a logarithmic scale. (101 is 10 parts per million, 102 is 100 parts per million, etc.)
Credit:<br>Haqq-Misra and Wolf/JGR Atmospheres
Model timelines for the weak and strong weathering scenarios. Temperature in the top plot is in kelvins—a change of 1 kelvin is equivalent to 1 °C. Carbon dioxide in the bottom plot is on a logarithmic scale. (101 is 10 parts per million, 102 is 100 parts per million, etc.)
Credit:
Haqq-Misra and Wolf/JGR...