The temperature depends on how much carbon we burn, not how fast [View all]

[GliderGuider]

Last edited Sat Nov 24, 2012, 02:49 AM - Edit history (2)

This is a crucial insight, it seems to me. The dwell time of CO2 in the atmosphere is measured in centuries and millennia. So if we really have 800 Gt of carbon or so in the ground, and we burn it all, we're going to get a temperature rise of at least +3.5C, no matter how fast or slowly we burn it.

In an effort to recover from my earlier data-driven gaffe, I've looked at the rate of increase of each of our three fossil fuels, and found something interesting. The rate of growth for oil and gas is essentially dead linear over the last 30 years. That means it should be relatively easy to at least keep their growth from going super-linear, and maybe even lower the slope a bit.

Coal is another story. At the moment its growth is decidedly super-linear, and that spells Trouble (right here in River City) over the next 50 or 60 years. As a first mitigation step, maybe we could bring the growth of coal back to a linear increase. It just might be possible to get global support for doing that when the funhouse ride I call "World of Hurt" begins around 2030. Unfortunately, even if we succeed it won't actually help in the long run. If we burn it all, we get the warming - it just takes a little longer.

I did the following two graphs (you knew there'd be more graphs, right?) to illustrate my point. I modeled the carbon emissions from oil and gas on their current linear slope from today until the reserves are exhausted in 2060. Then I modeled coal as rising exponentially as it is doing now until 2030, then dropping back to a linear growth rate until the world's coal reserves are exhausted (also, remarkably, in about 2060). We emit a total of about 800 Gt of carbon between now and 2060, and then everything just ... stops.



When I modeled the CO2 concentration I used historical carbon data from CDIAC going back to 1751 to establish the curves, and I used the observed data from Mauna Loa as a validation check. As you can see below, the model matches the observed levels exactly. CO2 levels rise until just after all the reserves are exhausted in 2060. In this simulation the CO2 concentration stabilizes at 643 ppm, a little over twice the pre-industrial level.



I assumed a climate sensitivity of +3.5C per doubling for this test. The temperature increase attributable to just CO2 (the green dashed line) rises from +0.8C today to just over +3.5C by the end of the century. In my model it also keeps rising slowly for about another 100 years, because the planet needs some time to stabilize after the CO2 shock.

The dashed red line simulates the various feedbacks that I expect to kick in once we pass +1.0C. Since they will be temperature-related, I used a feedback term that adds a cumulative 1% per year to the temperature rise. I have no way of knowing if that's even close the right value, but I'm convinced that the effect will be significant in the coming years. That drives the temperature up another couple of degrees by the end of the century. This line will keep rising as more and more methane is melted out of it's various hidey-holes over the coming millennia. Treat it as something to think about rather than any attempt at realistic prediction.

A number of messages fall out of this exercise:

• The decades from 2040 to 2060 could easily be the breaking point for civilization, as we warm the planet from two to three degrees in just two decades.
  
• We face a herculean task just to get coal use off its current acceleration and down onto a linear growth rate.
  
• Unless we can figure out how to leave carbon in the ground, we will warm the planet by 3 to 4 degrees at the very minimum.
  
• We have very little time we have left to do anything substantial. After burning fossil fuels for 250 years, we have just 18 years left (at the very most) in which to make mind-bendingly enormous changes to the way we use energy. The end of the track in now clearly in sight.
  
• Keep your eye on the north. What happens to Arctic methane in the next eight to ten years will spell the difference between a sliver of a chance and none at all.

Here's a final comment about EROI (Energy Return on Investment) and climate change. As we burn through our reserves, fossil fuels become more energy-intensive to extract. More energy is used to extract less accessible resources, leaving less net energy available for end users like you and me. This is part of basic Peak Oil theory, and applies to any fossil energy source.

That effect reduces the amount of useful work a civilization can do as the net energy declines, but it has less effect on CO2 emissions. The only thing that will reduce our emissions is slowing the rate at which we dig or pump the stuff out of the ground. Even if the rate at which we can dig the stuff up drops to a mere trickle at some point, as long as we keep doing it we'll keep warming the planet. Declining net energy may become an issue for us humans in the next 20 years, but unless we figure out how to stop digging it up, the planet will just keep on warming as the physics predicts.