Climatologists, like Walter Broecker and James Hansen, have predicted a warmer climate due to the accumulation of greenhouse gasses in the atmosphere from the burning of fossil fuels. Geologists take a longer view. On a geological timescale, it appears that the Earth has long oscillated between "greenhouse" and "ice house" conditions. During greenhouse periods, the average global temperature has been a balmy 22°C (72°F), quite a bit warmer than the 2012 average of about 14.4°C, one of the hottest years that climatologists have actually recorded. But all of man's time on Earth has so far been spent within a few degrees of ice house conditions.
Greenhouse conditions of the geologic past were warm enough to melt all the world's glaciers and icecaps, causing a sea level rise of about 100 meters. However, global sea level rise may have been more than twice that during the Cambrian and the Ordovician periods. Ed Landing, New York State Paleontologist and Curator of Paleontology, New York State Museum, has a theory to explain the extra water rise: global hyperwarming.
According to Landing, hyperwarming is a feedback effect as shallow seas overlap the continents. Shallow seas absorb sunlight and warm, and will also cause the world ocean to become warmer; there is also an increase in atmospheric water vapor with greater evaporation from the shallow seas and ocean. Water vapor is itself a potent greenhouse gas, causing yet more heating. The extremely high sea levels come about because of thermal expansion of the world ocean --a phenomenon already being detected as a result of modern anthropogenic warming. Some of the evidence that Landing sees for hyperwarming are the layers of black shale and slate laid down over North America, which formed from thick anoxic, carbon rich mud on the bottom of those ancient shallow seas and on lower parts of the ancient continental slopes. Very high temperatures reduce oxygen solubility and allow expansion of low oxygen water into near shore environments.
AAAS Members Central: The first scary notion is the idea that greenhouse conditions are the default state of the planet. So, is it true that the Earth was headed for very much warmer temperatures on a geological time scale, anyway, but we are vastly accelerating the process?
Ed Landing, New York State Paleontologist and Curator of Paleontology, New York State Museum: Greenhouse conditions reflect the continued input of carbon dioxide ultimately from sources in Earth's mantle. The carbon dioxide is then brought to the surface with volcanism at spreading centers. The new carbon dioxide is then taken out of the system by reef production of calcium carbonate (lime), by sequestering by land plants (from peat to coal) and marine phytophankton (which are eaten by planktivores), and the oily fecal debris sinks to the sea floor and, if not oxidized, accumulated as petroleum source rocks.
The typical alternative is "ice house conditions," which appear with global cooling typically brought about as a major continent enters high or polar latitudes and undergoes a period of continental glaciation (as present Antarctica), or the circulation of warm water into a polar sea (the Arctic Ocean) becomes reduced as plate tectonics brings land masses into the polar area to isolate the polar sea and reduce the amount of warm water brought to the pole (north or south) and cause it to cool and likely develop pack ice. Both continental glaciers and pack ice increase albedo and cause cooling by reflecting heat to space.
There doesn't seem to be any evidence that Earth was going to warm "naturally" anytime in the future as the orientation of its axis to the eccliptic and the location of Antarctica and the isolated Arctic Ocean would have been the basis for persistent icehouse conditions
The recent James Hansen/Al Gore/Bill McKibben synthesis is that the steady accumulation of carbon dioxide since about 1850 will "artificially" take Earth out of its 2.6 million year ice house condition and lead to a greenhouse world. The amount of carbon dioxide now in the atmosphere is the greatest it has been for more than 400,000 years (based on ice cores).
Accelerated atmospheric and ocean warming will begin (initially subtly) with the rise of sea-level and the warming of epeiric (on continent) seas. The global hyperwarming feedback mechanism, with time, will take anticipated global temperatures well beyond the ca. 5 degrees C predicted by some of the "worst case" models.
AAASMC: How do we know what the climate was like millions of years ago--oxygen isotopes?
Landing: Yes, in part--with water molecules having two types of oxygen isotopes--a light oxygen 16 and a heavier oxygen 18. The heavier water molecular with oxygen 18 evaporates less easily than the water molecule with oxygen isotopic weight 16. The oxygen will be incorporated into calcium carbonate (the "carbonate" "CO3" has oxygen) deposits by organisms (like corals and mollusk shells) and appear in non-biological formations like stalagtites and stalagmites in caves. So, if marine organism shells show more oxygen 18, this means that it was warmer and water with oxygen 16 was evaporated away. At the same time, stalagtites, stalagmites and limey soil will have more oxygen 16 and less oxygen 18 incorporated into their calcium carbonate. The ratio of oxygen 16 to oxygen 18 is used to calculate actual temperatures. In the very Early Ordovician "Schaghticoke dysoxic/anoxic" event in the hyperwarming paper, temperatures of 40 °C are reported.
There a bunch of other qualitative tricks.
AAASMC: The global maximum average during greenhouse conditions seems to be fairly constant at 22 °C (pretty hot!). Are there homeostatic mechanisms that account for this?
Landing: Yes, an essential homeostasis--except when ice house conditions develop as a result of plate tectonics (discussed above) or it slips into global hyperwarming .
The essential "homeostasis" is a balance between solar input, the production of greenhouse gas (particularly carbon dioxide), and life sequestering carbon as calcium carbonate and organic chemicals, as methane and peat. All of the planets have developed a homeostasis of sorts, with Venus, seemingly without carbon-based life, in a condition of extremely high temperatures, while Mars, having lost most of the greenhouse gases carbon dioxide and water vapor to space, is exceptionally cold.
AAASMC: The extra 100 meters of sea level rise during hyperwarming is solely due to thermal expansion of water? How hot does the ocean have to be to get that much expansion?
Landing: The thermal expansion of the deep sea is proposed to have led to sea levels 700-800 feet above that present in the Early Paleozoic--melting of all modern ice means only a 300 foot inundation. The extra several hundred feet can only be accounted for by thermal expansion. I am beginning work with a climate modeler to see just how much global sea level would rise with hyperwarming.
AAASMC So black slate, formed from thick anoxic mud, is a primary indicator of hyperwarming eras, correct?
Landing: The areal extent of black, organic-rich mud drastically increases with hyperwarming as the mid-water hypoxic water mass thickens and intensifies with less storminess (and reduced oxygen solubility with higher temperatures. Thus, the hypoxic water mass expands (thickens) and intensifies to move down the continental slope and into very shallow shelf waters.
Thus, black organic-rich mudrocks that are the key sources of petroleum and natural gas are the markers of the essentially unbearable hyperwarming world. The burial of carbon in these black mudrocks eventually brings hyperwarming to an end.
AAASMC: How do atmospheric carbon dioxide levels fall to create the ice house conditions?
Landing: Atmospheric carbon dioxide levels fall as a result of the sequestering of biological carbon in sediments. Again, I want to defer to future modeling to see if hyperwarming ends in a "mere" greenhouse or might lead to an ice house. Here's speculation—if there is thermal inertia in the global climate system (the world ocean stays hot though atmospheric carbon has fallen with the burial of organic carbon), lingering high global sea level would allow continued burial of carbon compounds so that carbon dioxide would be so low that the atmosphere would go into an ice house condition.
I have some evidence for this—with black mud deposition persisting on the ancient North American continental slope into the earliest Ordovician, and at a time that global sea level was falling.
AAASMC: Are you, so far, a lone voice crying in the wilderness, or has there been some general acceptance of the hyperwarming idea?
Landing: Proposals start with one or a few people. The article came out on-line in November 2011, and was the first proposal of a non-greenhouse gas control of Earth (and planetary?) climate. Yes, I am "evangelizing," but honestly report a good reception for the talk at Lamont-Doherty Earth Observatory last spring and an invitation to give the talk again this spring.
AAASMC: Given the already rapid anthropogenic release of carbon into the atmosphere, how long before the next hyperwarming period kicks in?
Landing: Greenhouse warming and melting of ice on land will be followed by a hitherto unexpected acceleration of sea level rise and global temperature rise. The onset of hyperwarming will be prompted by the sea-level rise due to anthropogenic warming and melting of grounded ice, but I think a real quantitative model will show that it will become the dominant climate and sea-level driver. With one prediction that all Antarctic and Greenland ice could be gone in 300 years, you just have to think that the conditions for global hyperwarming are then well established.