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The Five Degree World

Updated on September 15, 2015

This Hub is part of a larger series devoted to Mark Lynas's 2008 classic on the potential effects of climate change, Six Degrees. (See the link to the main article.) Here is presented a summary table for Chapter Five, outlining the effects of a five-degree warming, as presented in the book.

Additionally, updates will be posted as possible, commenting upon newer science bearing upon the points Mr. Lynas made. Note: the first update was posted 12/16/13, using information from the Fifth IPCC Assessment Report, Chapter 5, and has been followed by a second update detailing a new result from researchers.

The five degree world is the most extreme amount of warming for which relatively ample climate model studies are available. But it is also a world for which ancient climates offer insight--and those insights are not cheery.

Ellesmere Island fossil sites; the arc across the center of the image represents the Arctic Circle.  "Tiktaalik" is a 370-million year old lobe-fin, adapted to "oxygen-poor shallow-water habitats."  Image courtesy Wikimedia Commons.
Ellesmere Island fossil sites; the arc across the center of the image represents the Arctic Circle. "Tiktaalik" is a 370-million year old lobe-fin, adapted to "oxygen-poor shallow-water habitats." Image courtesy Wikimedia Commons.
A New World, p.215
The 5 C world is "largely unrecognizable." Hadley Cell expansion creates "two globe-girdling belts of perennial drought." Elsewhere, flooding is the perennial risk, and (continental) "Inland areas see temperatures 10 degrees of more higher than now." "Humans are herded into shrinking 'zones of habitability'." Agriculture & residence might be forced into the Russian & Canadian north. The boreal forest would be under severe pressure, possibly implying a further carbon feedback.
A Blast From the Past, p. 220
Eocene climate. In 1975 Mary Dawson & Robert West found Eocene alligator bones on Ellesmere Island. Paleocene-Eocene boundary mass extinctions, and causes: methane hydrate release? volcanic combustion of Greenlandic coal beds? Still mysterious, but during the Paleocene-Eocene Thermal Maximum (PETM), roughly 55 million years ago, "sea temperatures close to the North Pole rose as high as 23 C, warmer than much of the Mediterranean is today." PETM is considered as analog to present (or prophecy of our future.) But while PETM total Carbon releases were greater than today--concentrations hit 1000 ppm--the modern rate of rise is ca. 30x greater. The "clathrate gun" idea--in which submarine methane hydrates destabilize, leading to rapid and large methane release--is contemplated.
Tsunami Warning, p. 228
Description of the ancient submarine landslide, the 'Storegga slide' and associated tsunami. Links between methane hydrates and submarine landslides are explored. Does causality (potentially) run both ways?
The Prospect for Humanity, p. 230
PETM transition took 10,000 years--some North American plants shifted range 1,500 km during that time, raising the question of rate of change in the modern era, and the (im)possibility of adaptation. Contracting 'zones of hability'--will this 'enforce localism?' Economic dislocation on huge scale seems inescapable. Climate 'refuges'--Lesotho, Ethipian highlands. Northern Europe, Patagonia, Tierra del Fuego--maybe the Antarctic peninsula? Tasmania and New Zealand's South Island might also be possibilities.
Survival, p. 234
"Isolationist survivalism" might be an option in such places as Montana, but few have the necessary skills, and ecological disruption would complicate hunting and gathering. Most parts of the world lack the unpeopled terrain. There, "stockpiling" is the obvious strategy--but each stockpile becomes a target, and defending it indefinitely seems unworkable.

Update: Information from the IPCC's Fifth Assessment Report, Chapter 5

Chapter 5 of 'AR5' describes work examining ancient climates on Earth, and the lessons that they hold for us today--and particularly work carried out since the last Assessment Report in 2007. Most of the emphasis is on the Last Glacial Minimum and the Mid-Pliocene Warm Period. However, page 5-46 has this statement on the Pliocene:

Proxy records suggest that the WAIS [West Antarctic Ice Sheet] might have collapsed during past interglacials (Naish et al., 2009b; Vaughan et al., 2011) and was absent during warm periods of the Pliocene when CO2 concentration was 350–450 ppm... and global sea level was higher than present... These reconstructions and one icesheet model simulation (Pollard and DeConto, 2009) suggest that WAIS is very sensitive to the subsurface ocean temperature. This implies, with medium confidence, that a large part of the WAIS will be eventually lost if the atmospheric CO2 concentration stays within, or above, the range of 350–450 ppm for several millennia.

That's clearly consistent with the earlier picture, as painted by Lynas.

For context, here's a graph of the PETM, drawn from the 2007 Assessment Report, AR4:


Note the phrase "approximately 5 C global warming." The bottom panel shows the evidence of ocean acidification, an important clue that the warming was due to carbon dioxide.


A new research paper has shed some light on a still-worse case than the PETM, the end-Permian extinction--the worst known extinction event in Earth's long history.

Dr. Seth Bowring and Sam Burgess, both of MIT, and Shu-Zhong Shen, of Nanjing Institute of Geology and Paleontology, have examined the timing of extinction at the end of the Permian period with greater precision than before, determining it took just 60,000 years, plus or minus 48,000. This is a geological eyeblink. Dr. Bowring asked:

“How do you kill 96 percent of everything that lived in the oceans in tens of thousands of years? It could be that an exceptional extinction requires an exceptional explanation.”

The researchers also determined that the extinction was preceded by a pulse of 'light carbon' 10,000 years before the extinction itself. This pulse:

...likely reflects a massive addition of carbon dioxide to the atmosphere. This dramatic change may have led to widespread ocean acidification and increased sea temperatures by 10 degrees Celsius or more, killing the majority of sea life.

The cause of this carbon dioxide pulse remains unknown--but the leading candidate, just as discussed by Mark Lynas, remains the Siberian Traps mega-eruption. The researchers plan to apply their techniques to pinning down the timing of that eruption more accurately, with a view to assessing whether it was indeed the culprit in the worst mass extinction event in over 500 million years.

Links to the paper and accompanying press release are given in the sidebar, above right.

A specimen of Allognathosuchus, thought to be the species to which the fossil remnants found by Dawson and West belong.  (Specimen is at the Paleontology Museum of Zurich.)  Photo courtesy  "Ghedoghedo" and Wikimedia Commons.
A specimen of Allognathosuchus, thought to be the species to which the fossil remnants found by Dawson and West belong. (Specimen is at the Paleontology Museum of Zurich.) Photo courtesy "Ghedoghedo" and Wikimedia Commons.

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