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Illustrating global warming—What does "a 2°C increase" refer to?

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UPDATE 1: For fans of the bottom line, click here. For fans of the political bottom line, click here.

UPDATE 2: A complete list of climate series pieces is available here:
The Climate series: a reference post.

The last few posts about global warming (the fast road to climate catastrophe) have referred to a 2°C (3.6°F) or 3°C (5.4°F) increase in global temperature as really really bad — not for the planet (she will do just fine), but for us. Those posts include:

  ■ Quarter to half of species on earth may die from global warming
  ■ What is "climate catastrophe"?
  ■ McKibben 1: Three Numbers—Measuring the march toward climate catastrophe

The first discusses James Hansen's recent paper, in which a 3°C rise in global temperature is seen as catastrophic.

The second spells out what that catastrophe looks like (not pretty).

The third talks about Bill McKibben's Rolling Stone piece and the 2009 Copenhagen conference, which confirmed the previous ceiling of 2°C as the upper limit of acceptable temperature increase.

But this leaves a lot of questions — for example, the temperature increase "ceiling" relative to what? What's our rate of increase? And so on.

So this post will serve to anchor those 2°C and 3°C temperature increase number, and to provide a nice picture to help you sleep at night while you decide how you can help out. (Thanks to Dr. Michael Mann for pointers to all of the following. You can get to know Dr. Mann's thinking, as I did, via this conversation at Virtually Speaking.)

Where does "global temperature increase" start from?

We know from James Hansen that a "global temperature increase" of 3°C (5.4°F) would be an outright disaster — that 20–50% of species would go extinct.

The Copenhagen climate conference of 2009 urged no more than 2°C (3.6°F).

But two degrees from what? There are measurements from any number of starting points (I'll show a few). But the most commonly accepted starting point is the start of the Industrial Revolution (you'll see why in the chart below).

This means, unless otherwise noted, the "zero" point for measuring temperature increase is 1800.

What do graphs of "global temperature increase" show?

Here's an illustration to show what "global temperature increase" looks like. There are many others, but this will get you started.

This is from an excellent (and must-read) report — "The Copenhagen Diagnosis: Climate Science Report" — a scientific paper prepared in advance of the Copenhagen conference in December 2009. Twenty-six scientists contributed to this report. (Full pdf here; figures-only pdf here; other download formats here.)

This recent chart shows observed temperature increase from 1800–2008, with projections backward and forward.

Figure 21. Reconstructed global-average temperature relative to 1800-1900 (blue) and projected global-average temperature out to 2100 (the latter from IPCC AR4). The envelopes B1, A2, A1FI refer to the IPCC AR4 projections using those scenarios. The reconstruction record is taken from Mann et al. (2008).

Four things to notice:
  • The "zero" reference mark is 1800.
  • The fuzzy blue line extending back from 1800 is a reconstruction relative to the 1800 reference index.
  • Starting in 1900, the temperature increase kicks off in earnest. Since this chart is taken from 2008 data, the observations stop at that point.
  • By 2008 according to this chart, we've indeed used up about 0.8°C of the 2°C ceiling. (2008, by the way, was four years ago. We haven't stopped.)
Now look at the projections, the three colored bands.

Those represent scenarios presented in the "IPPC AR4" report ("Intergovernmental Panel on Climate Change, Fourth Assessment Report"), one of the most important climate reports available.

Note the range of temperature increase in these projections — from 2°C to 7°C global temperature rise. For reference, James Hansen's 20–50% mass extinction scenario is a 3°C increase.

About those projections, the Copenhagen Diagnosis says (page 51; my emphasis and paragraphing):
The latest estimates of global mean air temperature projected out to 2100 are shown in Figure 21. The wide range in the projection envelope is primarily due to uncertainty in future emissions.

At the high end of emissions, with business as usual for several decades to come, global mean warming is estimated to reach 4-7°C by 2100, locking in climate change at a scale that would profoundly and adversely affect all of human civilization and all of the world’s major ecosystems.

At the lower end of emissions, something that would require urgent, deep and long-lasting cuts in fossil fuel use, and active preservation of the world’s forests, global mean warming is projected to reach 2-3°C by century’s end. ...

[A] 2°C global temperature rise could lead to sufficient warming over Greenland to eventually melt much of its ice sheet (Oppenheimer and Alley 2005), raising sea level by over six meters and displacing hundreds of millions of people worldwide.
One last thing to note — the long-term average increase will be relentless unless we make changes, but the increases won't be steady year by year. There will be lurches (page 51 again):
Despite the certainty of a long-term warming trend in response to rising greenhouse gases, there is no expectation that the warming will be monotonic and follow the emissions pathway on a year-to-year basis. This is because natural variability and the 11-year solar cycle, as well as sporadic volcanic eruptions, generate short-term variations superimposed on the long term trend (Lean and Rind 2009).
Bottom line

I'm showing you all of this (and there will be more) for a reason. First, your take-aways:

  1. The do-nothing scenario ("business as usual") gives us a global temperature increase of 4°-7°C by 2100. That upper number is an astounding 12½°F.

  2. James Hansen's mass-extinction occurs at 3°C (5½°F).

  3. Staying below 2°C (3½°F) will take "urgent, deep and long-lasting cuts in fossil fuel use, and active preservation of the world’s forests".

  4. This is at heart a political problem, not a scientific one.

If you memorize nothing else, those four points will keep you completely oriented.

About point four — Unless you're a climate professional, I would waste zero time worrying about the science. You can show this information to the curious, but at the first sign of resistance, walk away.

This is no longer about science, but power.

The Koch Bros don't want to be reasonable — they want to force their will on the world. Your crazy right-wing cousin doesn't want to learn anything — he wants to see his social enemies ground under Daddy's big boot. The crazed pre-Raptured "Christian" across the street is just one of the rubes, following the other rubes as they listen to the Preacher praise the rich and help him pick their own pockets.

For none of these groups is the mind involved. This can be solved, but not by talking science. That's how they win.

We win by doing what always works for progressives — finding pressure points and using leverage. Something we rarely do, but something we can learn. In this game, I think there are three pressure points, three groups of actors, each of whom wants something. It's a place to start.

McKibben has thoughts along these lines. I'll add mine as well. The game is not yet over.


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