Earth is on the edge of runaway warming

The old picture, with Earth well within
our solar system's habitable zone

How well is Earth's orbit around the sun positioned within the boundaries of the habitable zone? The illustration by the Wikipedia image on the right would give that impression that Earth was comfortably positioned in the middle of this zone.

What is the habitable zone? To be habitable, a planet the size of Earth should be within certain distances from its Sun, in order for liquid water to exist on its surface, for which temperatures must be between freezing point (0° C) and boiling point (100° C) of water.

In the Wikipedia image, the dark green zone indicates that a planet the size of Earth could possess liquid water, which is essential since carbon compounds dissolved in water form the basis of all earthly life, so watery planets are good candidates to support similar carbon-based biochemistries.

If a planet is too far away from the star that heats it, water will freeze. The habitable zone can be extended (light green color) for larger terrestrial planets that could hold on to thicker atmospheres which could theoretically provide sufficient warming and pressure to maintain water at a greater distance from the parent star.

A planet closer to its star than the inner edge of the habitable zone will be too hot. Any water present will boil away or be lost into space entirely. Rising temperatures caused by greenhouse gases could lead to a moist greenhouse with similar results.

The distance between Earth and the Sun is one astronomical unit (1 AU). Mars is often said to have an average distance from the Sun of 1.52 AU. A recent study led by Ravi Kopparapu at Penn State mentions that early Mars was warm enough for liquid water to flow on its surface. However, the present-day solar flux at Mars distance is 0.43 times that of Earth. Therefore, the solar flux received by Mars at 3.8 Gyr was 0.75 × 0.43 = 0.32 times that of Earth. The corresponding outer habitable zone limit today, then, would be about 1.77 AU, i.e. just a bit too far away from the Sun to sustain water in liquid form. Venus, on the other hand, is too close to the Sun (see box below).

Kopparapu calculates that the Solar System’s habitable zone lies between 0.99 AU (92 million mi, 148 million km) and 1.70 AU (158 million mi, 254 million km) from the Sun. In other words, Earth is on the edge of runaway warming.

Image by Kopparapu et al. New calculations show that Earth is positioned on the edge of the habitable zone
(green-shaded region), boundaries of which are determined by the moist-greenhouse
(inner edge, higher flux values) and maximum greenhouse (outer edge, lower flux values)

Kopparapu says that if current IPCC temperature projections of a 4 degrees K (or Celsius) increase by the end of this century are correct, our descendants could start seeing the signatures of a moist greenhouse by 2100.

Kopparapu argues that once the atmosphere makes the transition to a moist greenhouse, the only option would be global geoengineering to reverse the process. In such a moist-greenhouse scenario, not only are the ozone layer and ice caps destroyed, but the oceans would begin evaporating into the atmosphere's upper stratosphere.

Venus' runaway greenhouse effect a warning for Earth by Sam Carana - first posted November 28, 2007, at: http://global-warming.gather.com/viewArticle.action?articleId=281474977189423 Venus was transformed from a haven for water to a fiery hell by an runaway greenhouse effect, concludes the European Space Agency (ESA), after studying data from the Venus Express, which has been orbiting Venus since April 2006. Venus today is a hellish place with surface temperatures of over 400°C (752°Fahrenheit), winds blowing at speeds of over 100 m/s (224 mph) and pressure a hundred times that on Earth, a pressure equivalent, on Earth, to being one km (0.62 miles) under the sea. Hakan Svedhem, ESA scientist and lead author of one of eight studies published on Wednesday in the British journal Nature, says that Earth and Venus have nearly the same mass, size and density, and have about the same amount of carbon dioxide. In the past, Venus was much more Earth-like and was partially covered with water, like oceans, the ESA scientists believe. How could a world so similar to Earth have turned into such a noxious and inhospitable place? The answer is planetary warming. At some point, atmospheric carbon triggered a runaway warming on Venus that boiled away the oceans. As water vapour is a greenhouse gas, this further trapped solar heat, causing the planet to heat up even more. So, more surface water evaporated, and eventually dissipated into space. It was a “positive feedback” -- a vicious circle of self-reinforcing warming which slowly dessicated the planet. “Eventually the oceans began to boil”, said David Grinspoon, a Venus Express interdisciplinary scientist from the Denver Museum of Nature and Science, Colorado, USA. “You wound up with what we call a runaway greenhouse effect”, Hakan Svedhem says. Venus Express found hydrogen and oxygen ions escaping in a two to one ratio, meaning that water vapor in the atmosphere the little that is left of what they believe were once oceans is still disappearing. While most of Earth's carbon store remained locked up in the soil, rocks and oceans, on Venus it went into the atmosphere, resulting in Venus' atmosphere now consisting of about 95% carbon dioxide. “Earth is moving along the curve that connects it to Venus”, warns Dmitry Titov, science coordinator of the Venus Express mission. References - Venus Express - European Space Agency (ESA) http://www.esa.int/SPECIALS/Venus_Express/SEMGK373R8F_0.html - Venus inferno due to 'runaway greenhouse effect', say scientists http://www.physorg.com/news115477239.html - Probe likens young Venus to Earth http://physicsworld.com/cws/article/news/32018 - European mission reports from Venus http://www.nature.com/nature/journal/v437/n7062/full/4371071a.html

References

- Habitable zones around main-sequence stars: new estimates

Ravi Kumar Kopparapu et al. 2013

http://iopscience.iop.org/0004-637X/765/2/131

- Habitable Zone - Wikipedia

http://en.wikipedia.org/wiki/Habitable_zone

- Earth is closer to the edge of Sun's habitable zone

http://physicsworld.com/cws/article/news/2013/mar/25/earth-is-closer-to-the-edge-of-suns-habitable-zone

- Updated model for identifying habitable zones around stars puts Earth on the edge

http://www.gizmag.com/habitable-zone/26051/