Under the Bright Lights of an Aging SunJuly 4, 2014 / Written by: Aaron GronstalVenus can be seen as a black dot eclipsing the Sun in this image from 2012. Venus orbits too close to the Sun to the planet to be habitable for life as we know it. Venus experiences a runaway greenhouse and the average surface temperatures are thought to be around 864ºF. Image Credit: NASA/SDO & the AIA, EVE, and HMI teams; Digital Composition: Peter L. Dove
Astrobiologists supported by the Exobiology and Evolutionary Biology element of the Astrobiology Program have shed new light on the future habitability of Earth. The tools they are using could also tell us about habitability around distant stars.
Life as we know it on Earth is linked to our star, the Sun, which provides our planet with just the right amount of heat and energy for liquid water to be stable in our lakes, rivers and oceans. However, as the Sun ages, it is steadily growing brighter and brighter. Eventually, the sunlight that supports life will become too great, and it will bring an end to habitability on our planet.
The Atmosphere in 3-D
Previous models have predicted that an increase of just 6 percent in the solar constant (a measure of incoming solar electromagnetic radiation) would cause a runaway greenhouse effect on Earth that would render the planet uninhabitable as the oceans boil away to space. Based on this number, Earth’s habitability could come to an end in around 650 million years from now. However, a more recent study has extended the expected lifetime of Earth as a habitable world.
The new research shows that the accuracy of previous studies, which were based on ‘one-dimensional’ models of Earth’s climate, could be improved.
“One-dimensional models treat the atmosphere as a single vertical column. This single column is meant as a representative average of all points on the Earth,” explains Eric Wolf of the Department of Atmospheric and Oceanic Sciences at the University of Colorado Boulder. “While one-dimensional models can treat radiative transfer well (i.e. solar energy and the greenhouse effect), they completely ignore many important aspects such as clouds, dynamics, and the pole to equator gradients of energy which ultimately describe our climate.”
Wolf and his colleague Brian Toon, also of UC Boulder, used complex, three-dimensional climate models in order to bring more detail into the picture.
“Three-dimensional models, as we refer to them, are general circulation models of climate. They include a fully, spatially-resolved, rotating planet, with clouds, oceans, sea-ice, weather, etc.,” Wolf told Astrobiology Magazine. “The three-dimensional general circulation model I used has also been used for problems of modern climate. General circulation models are considered the most advanced type of climate models.”
The added detail of the 3-D models showed that the Earth could remain habitable for longer than previously expected.
“According to my work, the Earth may remain ‘habitable’ for at least another 1.5 billion years, when the Sun is approximately 15.5 percent brighter than today,” said Wolf. “This is the limit of our current study.”
It’s important to note that a habitable Earth in terms of astrobiology is not necessarily habitable for human beings.
“When we think about exoplanets or the future Earth, scientists refer to a planet as habitable if it has the ability to maintain liquid water at its surface,” says Wolf. “However, a planet may maintain liquid water at the surface while still having a climate which is unfriendly to humans.”The Sun will grow into a Red Giant star in 5 billion years. This image compares the size of the Sun today (yellow dot on the left) to the size of the Sun as a Red Giant. Credit: Department of Physics, NCKU
Coming from the angle of Earth’s climate, the study by Wolf and colleagues also has wider implications.
“Scientists today use climate models of various types (1D and 3D) to examine the runaway glaciation and runaway greenhouse thresholds for Earth, and then we can apply these concepts to our observations of extrasolar planets,” said Wolf.
The Earth orbits around the Sun in a region known as the ‘habitable zone,’ where the energy from the Sun is just right for liquid water to remain stable at the planet’s surface. Life as we know it requires water to survive, so identifying the ‘habitable zone’ around distant stars is the first step in the hunt for Earth-like worlds.
The study, “Delayed onset of runaway and moist greenhouse climates for Earth,” was published in Geophysical Research Letters, Vol. 41, No. 1.
For the full article, visit astrobio.net.
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