#NOKXL Over 1 Million Comments! Earth Is Near the Planetary Habitable Zone Limit

The earth is perilously close to becoming an uninhabitable wet greenhouse planet where water escapes to space from a wet stratosphere. A ten fold increase in CO2 from pre-industrial levels would bring the earth to the wet greenhouse limit according to results of an improved model of planetary habitable zones developed by Penn State University planetary scientists.

"Our new climate model predicts that we are closer to the moist-greenhouse scenario than we had thought," says Kasting. In this scenario, the stratosphere becomes wet and fully saturated as the Earth's surface warms. This results in the dissociation of water molecules and the release of hydrogen into space. Depending on the levels of atmospheric saturation, the oceans would be completely lost over timescales as long as several billion years.

The model does not include cloud physics, which is very complicated and not well understood, so there is a margin of error. Despite being so close to the habitable zone limit, life on earth has survived large natural spikes in greenhouse gases. Keystone XL would be game over for a stable climate because it would commit us to a path of exploitation of the dirtiest carbon sources, but life on earth would likely go on.

James Hansen: If released all at once, the known tar sands resource is equivalent to 150 parts per million. As is the case with other fossil fuel sources, the amount in the air declines to about 20 percent after 1,000 years. Of course, only a small fraction of the resource is economically recoverable at the moment. But if you decide you are going to continue your addiction and build a big pipeline to Texas, the economically extractable oil will steadily grow over time. Moreover the known resources would grow because there is plenty more to be discovered.

Every seller will tell you his pile of pollution is small compared to the total pile on Earth, and that is correct.  What makes tar sands particularly odious is that the energy you get out in the end, per unit carbon dioxide, is poor. It's equivalent to burning coal in your automobile. We simply cannot be that stupid if we want to preserve a planet for our children and grandchildren.

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The geologic record shows that the earth has one last line of defense against large releases of greenhouse gases, but that natural defense would be catastrophic to modern human civilization.

Sea level was over 125 meters (about 400 feet) higher, 54 million years ago after massive natural releases of greenhouse gases heated the earth's climate.

An enormous magma plume rose up from the deep mantle under the north Atlantic ocean basin fifty six million years ago. Note that sea level began to rise before the temperature spike at 55 million years ago. This is an indication of a swelling mantle below the north Atlantic before the massive eruption. Apparently, vast flows of basalt destabilized the methane ices and organic carbon on the sea floor. Massive plumes of methane erupted from the ocean into the atmosphere. Huge quantities of carbon dioxide were then released from peat oxidation when (glacier free) Antarctica warmed rapidly. These huge spikes of greenhouse gases raised global temperatures 5°C to 8°C.

The oceans rapidly acidified and warmed. Marine deposits of limestone (calcium carbonate) in shells, organic and inorganic calcium carbonate dissolved in a few thousand years to neutralize the spike in acidity.  However, the neutralization process released CO2 back to the ocean, so atmospheric CO2 stayed high for tens of thousands of years because the oceans quickly stopped taking up CO2 from the atmosphere. Sea levels rose rapidly to heights 100 meters to 125 meters, about 400 feet, above present levels. Large areas of coastline were inundated.

The earth apparently didn't get warm enough to become a wet greenhouse. Rock weathering rates accelerated under the hot, high CO2 atmosphere. Marine deposits from the Paleocene - Eocene boundary show spikes of extreme sedimentation coincident with the spikes in CO2 and temperature. Extreme seasonal precipitation, perhaps in superstorms and supermoonsoons, caused extreme weathering and erosion. Detailed examination of the sedimentary records at multiple sites show strong, probably seasonal, variability between very hot and dry periods and very wet periods. Soils were rapidly eroded from the land and deposited as marine sediments. The extreme rock weathering and marine sedimentation of organic matter removed the atmospheric CO2 over tens of thousands of years and returned the normal balance of salts and alkalinity in the oceans.

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Farmers are now coping with both more droughts and more extreme precipitation events. This year the Mississippi River at St. Louis has gone from record low water to flood in just a few months. The geologic record from the Paleocene-Eocene boundary shows that the extreme weather can be expected to grow even more extreme as greenhouse gas levels rise. If we continue to add greenhouse gases to the atmosphere extreme weather will devastate our farmland and our ability to feed ourselves. Sea level  rise is inevitable in a warming world, but it lags changes in weather and climate. The last time the weather was this warm sea levels were about 6 meters (about 20 feet) higher. If we continue to emit greenhouse gases, rising seas will inundate coastal cities across the planet, displacing billions of people and inflicting trillions of dollars in property losses.

Vast areas, including cities now inhabited by billions of people, were underwater 54 million years ago. This image shows today's continental margins with sea level 125 meters higher.