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The Two Degree World

Updated on November 14, 2016

This Hub is a supplement to my summary review of the book Six Degrees by Mark Lynas. (The complete review and summary are linked at the bottom of this table, above the links and comments.)

The table below summarizes the potential effects of warming the globe two Celsius degrees beyond pre-Industrial temperatures. I hope it will be helpful; many people still think that a particular amount of warming will have the same effect as would a change of that amount during a typical day. But that is not true--a shift of the mean temperature has much bigger effects than a shift of maximum temperature for one particular day.

Many reasons make this true. For one, threshold temperatures exist where conditions change abruptly--an obvious example is the freezing/melting point for water. A 'small' shift near this point can obviously mean a very large change in, say, the number of frost days each year in a particular location. (It's worth remembering here that frosts have their good as well as bad points--they may kill some plants, while being essential to other plants' fruiting process.)

For another thing, temperature distributions tend to follow the famous 'bell curve' shape. That means that small shifts in the mean temperature can effect big shifts in the likelihood of extreme temperatures--say, those exceeding typical human skin temperatures. So a small rise in the average temperature can mean a great many more 'heat-stress' days each year.

Updates will be posted as possible--Six Degrees was published in 2008, and both the science and events have moved on since that time. Look for them following the table below.

How climate shifts can 'amplify' extreme weather.  Image courtesy IPCC.
How climate shifts can 'amplify' extreme weather. Image courtesy IPCC.
China's Thirsty Cities, p. 73
Is the Eemian climate of ca. 125,000 years ago analogous to a 2 C world? If so, northern Chinese cities could suffer severe drought.
Acidic Oceans, p. 75
Carbon doesn't just warm; it decreases the ocean's natural alkalinity, too--this is "ocean acidification." So far, the increase in acidity amounts to about 30%, which could rise to about 500% within a century. This would put the whole oceanic food web at risk.
The Mercury Rises In Europe, p. 79
2003 was 2.3 C above the continental seasoon norm. If that became the new norm--possible by 2040--then half the summers would be warmer than 2003. Update note: See discussion of Mora et al, below.
Mediterranean Sunburn, p. 83
A 2C Med. Drought and wildfire are the worst consequences. Update note: European report would be relevant here.
The Coral and the Ice Cap, p. 85
The Eemian era was the previous interglacial, 125,000 years ago. It was roughly2 C warmer at its warmest peak, so as an 'analogue' it can provide guidance to the implications of a 2C world. Sea level rise is striking; our coastal cities are flooded by 2140. Update: SLR commitment paper would be relevant here.
Last Stand of the Polar Bear, p. 94
Economic exploitation and political conflict in the Arctic. Extinction risks; Arctic amplification means that 2 C of warming implies a 3.2 to 6.6 C Arctic.
Indian Summer, p. 100
Rice and wheat production declines as heat tolerance is exceeded. Deforestation, drought and flood are risks in India and Pakistan particularly.
Peru's Melting Point, p. 102
Vulnerability of coastal Andean cities, especially Lima but also Trujillo to water supply woes as Andean glaciers vanish. Drought and loss of hydropower are serious threats.
Sun and Snow in Calfornia, p. 107
A similar scenario is expected to play out in California--another coastal desert--due to loss of snowpack. Drought will be a challenge throughout the American west.
Feeding the 8 Billion, p. 110
Regional food winners and losers in the 2 C world. On balance it's going to be harder to feed everybody, especially in the subtropics.
Silent Summer, p.113
Extinctions increase. The rate of warming greatly exceeds the ability to shift range in many cases. Millions of species are at risk of extinction, comprising a sizable proportion of all extant life.
From Mora et al, 2013:  "The projected timing of climate departure from recent variability"
From Mora et al, 2013: "The projected timing of climate departure from recent variability"

A 2013 study gives a provocative look at the timing of future warming under a couple of different emissions scenarios. Mora et al analyzed past and future (modeled) climate states, determining for key places and environments when future mean temperatures began to continuously exceed historical precedents. As their abstract puts it:

Here we present a new index of the year when the projected mean climate of a given location moves to a state continuously outside the bounds of historical variability under alternative greenhouse gas emissions scenarios. Using 1860 to 2005 as the historical period, this index has a global mean of 2069 (±18 years s.d.) for near-surface air temperature under an emissions stabilization scenario and 2047 (±14 years s.d.) under a ‘business-as-usual’ scenario.

Put plainly if a bit approximately, if we continue to do what we are doing today, the world of 2047 will bear little resemblance to the one we experience today.

Because the tropics see little variability naturally, they will be the first to pass beyond the bounds of historical climate. Manokwari, Indonesia, is projected to do so as early as 2020, with Jakarta and Lagos, Nigeria, following in 2029. Mexico City is slated for 2031; Mumbai, Cairo, and Baghdad follow in 2036.

Many US cities, from Tampa to New York City, 'cross over' in 2047, following cities such as Perth, Pretoria, Rome, and Santiago, Chile--not to forget Beijing and Tokyo!--by a few years. In the next decade, the moderating influence of Rio de Janeiro's seaside location is finally defeated, with London's falling shortly thereafter. In the '60s comes the turn of Moscow and Reykjavik.

It's sobering to think that most people alive today can reasonably expect to see a good chunk of this process take place:

“The results shocked us. Regardless of the scenario, changes will be coming soon,” said lead author Mora. “Within my generation, whatever climate we were used to will be a thing of the past.”

Small consolation, that the Mora et al timeline for the Alps to exceed historic norms is a few years later than the 2040 date mentioned by Lynas as 'possible' for the Alps to experience a +2.3 C rise. It's not surprising that that is so, of course; by definition the 2003 heatwave is now part of the historical envelope.


The Mora et al. paper discussed above has provoked a lively debate in the pages of Nature, the journal which published it.

A group of researchers critiqued the original paper, writing a comment paper in the same journal. The thrust of the comment was that Mora et al. was too confident about the timing of what yet another researcher termed, colorfully, the 'time of expulsion' (from current climate norms.) While agreeing with Mora that there will be a 'time of expulsion' for each locale on Earth, Hawkins and his colleagues think that it is not quite so easy to calculate just when that will be.

As an example, "the Mora paper argues Lagos will experience an unprecedented climate by 2043, plus or minus two years, if we mitigate greenhouse gas emissions to some extent; Hawkins and colleagues think the answer is more like 2024-2052." That is, in the case of Lagos, the best guess date is correct, but the uncertainty is not plus or minus two years; it should be plus or minus nine.

Though there may be some policy (or public relations?) consequences from this difference, clearly the 'big picture' doesn't much change.

Hawkin's comment paper is linked in the capsule below. An NERC blog post discussing the debate may also be accessible at another link; it is given immediately below. (Sorry; it is not 'embeddable;' interested readers will need to copy and paste it for themselves.)


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