Barring some unstoppable and unforeseen geo-cataclysm such as a large asteroid impact, scientists who construct models of our planet’s distant future had previously posited a date of about 300 – 600 million years from now when an inexorable loss of water due to an expanding solar radius and consequent “run-away” greenhouse effect robs our planet of all life.
But two new planetary climate models have given this planetary “die out” scenario a new lease on life, so to speak, and have extended our watery planet’s lifespan from a billion to 1.5 billion years — hundreds of millions of years more than previous model predictions.
An Ever-Expanding and Brightening Sun (and a ‘Hothouse’ Earth)
Predictions of the Earth’s demise are based upon a fundamental parameter called solar brightening, or solar luminosity, which is due to the ever-expanding reach of our sun’s heliosphere (as it runs out of nuclear fuel at its core) on its way to becoming a red giant star. This brightening occurs at a rate of about 1% every 100 million years. Along with it comes an increase in solar radiation which naturally heats up the planet’s surface and atmosphere. This fate for Earth is inevitable.
A fairly recent, long-term, planetary simulation conducted by Ravi Kopparapu and colleagues at Pennsylvania State University showed that just a 6% brightening of the sun is enough to trigger the run-away greenhouse effect that will spell doom for life here on Earth by vaporizing nearly all of its water (note: a life-robbing, run-away greenhouse effect can be brought about in other ways, such as what is believed to have occurred on Venus many millions of years ago, and possibly also to Mars billions of years ago).
So, by this metric, the model predicts that Earth has about 600 million years to go. However, the effects of greatly increasing atmospheric warming will be felt far earlier than that; at about 150 million years out, according to this model, the upper reaches of our atmosphere will be sufficiently warmed that water vapor molecules will be able to break through the stratified, gaseous bands of the stratosphere (this is termed evaporative loss). There, exposed to intensified solar radiation, the water molecules will break apart and the hydrogen will be able to escape into space (as most free, atmospheric hydrogen does). This is known as a “moist greenhouse effect”, which sounds rather pleasant, but in fact is characterized by surface conditions far too hot to support complex life as we know it — save for only the most robust of microorganisms (known as extremophiles).
The Newest Models of Our Planet’s Future – More Time to Heat Up
However, a more recent computer simulations challenge this “early demise” scenario on the basis that the model by Kopparapu et al is one dimensional — measuring the change in altitude of water vapor only, and assumes that climate factors such as humidity are constant all over the planet.
To test this previous model, Eric Wolf and Owen Brian Toon of the University of Colorado at Boulder utilized a more “realistic” 3D climate model developed originally at the National Center for Atmospheric Research (NCAR). Like the earlier model, theirs included cloud formation, but also incorporated other climate factors such as regional variations in moisture levels (thus the potential for cloud formation) and 25% higher CO2 levels — starting at 500 parts per million (ppm). This model maintained the higher level of CO2 throughout its simulation (note: this model also makes assumptions, such as the 500 ppm CO2 level remaining steady, indefinitely).
At that point, the U of Colorado researchers turned up the heat. Making the sun 15.5% brighter than it is today, the model produced an average warming of 40°C (note: Earth’s current average is about 15°C) which is pretty darned hot (for most all life forms) but would still permit liquid water to exist on the planet’s surface. In contrast to the previously discussed model, this new one did not show oceans boiling off or greatly increased heating of the stratosphere — precluding a moist greenhouse effect.
This multi-dimensional model assumes a more slowly warming Earth due to the capacity of clouds and and dryer regions to deflect heat back into space. The result of this “3D” simulation — published this month in Geophysical Research Letters — is that our Earth has at least 1.5 billion more years left in which some forms of life would survive in the regions just above or below the polar regions (but probably not humans).
Quoting from the published paper’s abstract:
“Numerical limitations prevent simulation of climates much warmer than this. Nonetheless, our results imply that Earth’s climate may remain safe against both water loss and thermal runaway limits for at least another 1.5 billion years and probably for much longer.”
Buttressing this model’s predictions — though somewhat less generously — is another 3D climate simulation (reported last month in the journal Nature) conducted by astrophysicist Jérémy Leconte and colleagues at the University of Toronto in Canada. While also predicting a run-away greenhouse effect like these other studies, the University o f Toronto study predicts a timeline of at least 1 billion years before Earth becomes a life-barring hothouse. This latter simulation’s different time estimation (as compared to the Wolf/Toon model) is attributed to the different way cloud formation and evaporation is modeled (“clouds have a destabilizing feedback effect on the long-term warming”).
The model also recalculates what’s known as the insolation threshold for the runaway greenhouse state to occur and finds it to be higher (at about 375 W m−2) than previous estimates. This means that it will take more warming per square meter to trigger the greenhouse effect, and hence greater solar luminosity, thus a longer time frame for Earth’s demise.
And, so as not to be geo-centric, the model by Leconte et al was developed for simulating the effects of increasing solar luminosity on all Earth-like planets anywhere in the galaxy.
Quoting from the paper’s abstract:
“…because of wavelength-dependent radiative effects, the stratosphere remains sufficiently cold and dry to hamper the escape of atmospheric water, even at large fluxes. This has strong implications for the possibility of liquid water existing on Venus early in its history, and extends the size of the habitable zone around other stars.”
3D Modeling A Planet’s Future – Assumptions and Possibilities
Despite the greater dimensionality of these “next-gen” 3D climate simulations, they can still underestimate the Earth’s life-supporting capacity. This is due to the fact that all such models are based upon key assumptions (necessary in most respects to set the parameters and starting conditions of the simulation), such as the assumption that CO2 levels will reach a given point (e.g., 500 ppm) and remain more or less steady. Counter-intuitively, at some point in a warmed-up future climate, CO2 levels may actually fall. This is due to the potential for a greater rate of calcareous (calcium carbonate) rock formation, favored by warming temperatures (particularly in the oceans). Such calcium carbonate rocks are one of Nature’s counter-balancing, carbon sequestering strategies and can serve to mitigate some climate warming from CO2, at least in the short to medium term (note: this would be a type of negative climate feedback, but one that would occur over many hundreds of thousands of years).
Further, such models do not currently have the capacity to simulate living systems’ responses to climate change; we simply do not know how various lifeforms will adapt, nor how long they will survive.
Still, these new 3D modeling studies — based upon solar luminosity — may prove most helpful for more precisely defining some neighboring or distant stars’ habitable (“Goldolocks”) zones wherein a rocky, Earth-like exoplanet might possibly possess ample liquid water for life to evolve and thrive. Indeed, the study by Kopparapu et al accurately estimates our sun’s habitable zone (with the Earth falling just inside this zone at 0.97 – 0.99 astronomical units, where 1 AU* is our planet’s average orbit). With these newer climate studies by Wolf /Toon and Leconte et al, this habitable range is slightly expanded (with 0.93 AU and 0.95 AU respectively) — meaning that our galaxy may host up to 6% more water-harboring planets than previously estimated.
* An astronomical unit (AU) is based upon the Earth’s average distance from the sun, at about 92.8 million miles.
Some source material for this post came from the Science NOW article: Earth Won’t Die as Soon as Thought