Mark Lynas's "Six Degrees": A Summary Review

What Is Six Degrees About?

Mark Lynas's Six Degrees* is, first, a graceful yet massive synthesis of a very large selection of scientific research papers; second, an eloquent and honest plea for action on the 'slow-motion crisis' that is climate change; and third, a coherent account of how global warming would affect humans and their world if allowed to proceed.

That makes it something of a modern classic, but not in the sense of being 'evergreen.' Given the rapid pace of climate research, any summary of the 'state of the art' is apt rapidly to become dated. Nor have sociopolitical developments been lacking since Six Degrees's publication in 2008.

Accordingly, I'll try not only to evaluate and summarize the book but also, to a limited degree at least, to update it, comparing its information with recent sources, such as the IPCC Fifth Assessment Report.

Structure of the Book

The central structuring metaphor of Six Degrees is that global warming is hell. Lynas doesn't quite put it so baldly, though a few of his adjective choices clearly imply it. But quotations from Dante's "Inferno" make the point quite clearly by serving as epigraphs for Chapter One, One Degree, and for the final chapter, Choosing Our Future.

Just as Dante's Hell was organized in increasingly dreadful circles, Lynas's account proceeds systematically from the "one-degree world" in which we live now--for global mean temperature is roughly .8 degrees Celsius above pre-Industrial levels--to the "nightmare" world of six degrees. For each level, Lynas sets forth the possible impacts and implications of that level of warming, as known at the time of writing. We’ll step through one chapter at a time. Each chapter also has a table summarizing the impacts. These tables are in separate articles, linked via sidebar capsules.

* Six Degrees: Our Future On A Hotter Planet, by Mark Lynas, National Geographic Society, 2008.

One Degree

In Dante’s vision of Hell, the outer circle was inhabited by 'virtuous Pagans' like Plato, whose only fault was not being Christian. Basically good, even great people, they were punished by nothing more severe than deprivation of contact with God. According to Lynas, the one-degree world, similarly, is 'not so bad.'

There is a laundry list of possible or observed impacts, from the return of the megadroughts western North America experienced during the Medieval Climate Anomaly, to the continuation of the already observed 'death spiral' of the Arctic sea ice, with its implications for Northern hemisphere weather and increased warming of the whole planet. Some, like the megadroughts, could be very serious indeed.

But at this level of warming there are climate 'winners,' too--for instance, the Sahel, the semi-arid transitional zone on the south flank of the Sahara, may become a little moister. For a table listing these impacts, see the article One Degree.

(Update: The boreal forest of Northern Canada may become moister as well, reducing wildfire risk there, even as that risk increases in places like Australia and the Eastern Mediterranean basin. Details in The One Degree World.)

It's just as well that it's not all bad, because the one-degree world is the one we all live in right now. As the current IPCC Assessment Report 5 makes clear, many long-projected impacts of warming are unfolding as expected. Indeed, some, such as Arctic sea ice loss or ice mass losses in Greenland's glaciers, have been proceeding faster than expected.

Two Degrees

The two-degree world is less familiar, but not yet completely strange. Some aspects of the two-degree world--for instance, European heatwaves similar to the lethal 2003 event--are already emerging. Others, like ocean acidification, will become familiar news items to the children and grandchildren of present readers of this article.

While the use of computer climate models is the most familiar method of predicting future climate states, Lynas explains that ancient climates also give important insights into possible future change. For the two-degree world, the analog is the Eemian interglacial, which reached its warmest temperatures--roughly 2 degrees Celsius above 'pre-industrial' levels--around 125,000 years ago. If past patterns turn out to be true precedents for our future, northern China could get very thirsty, adding to the environmental woes already costing China so dearly.

(Update: Northern China is already suffering from severe water shortages. See Two Degrees for details.)

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Water shortages could also be serious problems in Peru (as Andean glaciers disappear) and California (as snowpacks shrink.) Droughts due to declines in precipitation are expected in the Mediterranean basin, as already mentioned, and in parts of India, where increasing temperatures are also expected to challenge the heat tolerances of rice and wheat crops. Unsurprisingly, global food supplies are expected to be stressed as global populations peak this century.

Marine food sources will be severely stressed, too. Oceans will warm, bleaching coral and degrading reefs, diminishing their touristic value and, worse, their biological productivity. Increased stratification as the ocean surface warms will decrease the upwellings of nutrient-rich cold water, making oceans less productive.

At the same time, acidification will hurt species with calcium carbonate shells, including the plankton which form the entire basis for marine food webs. Already ocean acidity has increased by 30% due to carbon dioxide emissions. As Lynas puts it, "At least half the carbon dioxide released every time you or I jump on a plane or turn up the air conditioner ends up in the oceans... [It] dissolves in water to form carbonic acid, the same weak acid that gives you a fizzy kick every time you swallow a mouthful of carbonated water."

But that's just an overture; Lynas quotes Professor Ken Caldeira: "The current rate of carbon dioxide input is nearly 50 times higher than normal. In less than 100 years, the pH of the ocean could drop by as much as half a unit from its natural 8.2 to about 7.7." That would be a 500% increase.

The precedent of the Eemian suggests that other changes to the ocean, too. The Arctic would likely be committed to a future without sea ice, with intensification of the consequences mentioned above. Ice loss would accelerate for Greenland's glaciers, too. That would mean increases in sea level rise. Currently seal level is rising at just over 3 millimeters a year--around a foot per century. That relatively modest rise has already contributed to the increased flood risks for events such as Superstorm Sandy.

But one modeling study put the threshold level for the eventual near-complete loss of Greenland's ice sheet at a local warming of just 2.7 C--which, due to Arctic amplification, means a global warming of only 1.2 C. Total melting of Greenland--luckily, something that would likely take centuries--would raise sea levels by 7 meters, submerging Miami and most of Manhattan, as well as large chunks of London, Shanghai, Bangkok and Mumbai. Nearly half of humanity could be affected.

So would numerous other species. Polar bears would be under serious threat due to loss of sea ice, as would other Arctic species; and the one-two punch of temperature rises and acidification would pose serious challenges to many marine species. But extinction threats in the two-degree world are not limited to the oceans. The principal investigator of a 2004 study, Chris Thomas, revealed that "Well over a million species could be threatened with extinction as a result of climate change."

Three Degrees

In this chapter, climate regimes we might term 'sort of safe' are left behind. Partly that is because a political consensus of some standing has been that damage below this level might be in some sense acceptable, or at least reasonably survivable. But in part this fact is a reflection of non-linear nature of climate impacts, for above 2 C the risk of encountering what have become known as 'tipping points' rises--and rises unpredictably.

In Six Degrees the primary concern is for 'carbon cycle feedbacks.' In 2000 a paper called "Acceleration of Global Warming Due to Carbon Cycle Feedbacks in a Coupled Climate Model" was published--bibliographically known as Cox et al., (2000.)

Prior to Cox et al, most climate models had simulated the response of atmosphere and ocean to increasing greenhouse gases. But Cox et al was an early product of a new generation of "coupled" climate models. Coupled models added a new level of realism by considering the carbon cycle, in addition to atmosphere and ocean.

For carbon is an important ingredient for all life, and is ubiquitous in sea and sky. It is forever dancing from sky, to living tissues, to the sea--and the specifics depend, in part, upon temperature. For example, as temperatures warm, seawater absorbs less carbon dioxide, and as precipitation patterns change and plants grow (or die), they take up more (or less) carbon. Thus, carbon affects temperature, which affects life, which in turn affects carbon.

What Cox et al. found was startling, for those who spotted the implications. With 3 degrees of warming, "Instead of absorbing CO2, vegetation and soils start releasing it in massive quantities, as soil bacteria work work faster to break down organic matter in a hotter environment, and plant growth goes into reverse." The result, in the model, was the release of an additional 250 ppm of carbon dioxide by 2100, and an additional 1.5 degrees of warming. In other words, the 3 C world was not stable--hitting the 3 degree threshold meant hitting a 'tipping point' which led directly (though not immediately) to the 4 C world.

This effect was primarily due to a huge dieback of the Amazon rain forest. With warming and drying the rainforest collapsed almost completely. Later studies found globally similar effects, albeit in differing amounts. And recent study suggests that the likelihood of an Amazonian collapse may be lower than first thought--welcome news, to be sure.