MAKING TROUBLE

BY SAUL GRIFFITH

5,000 Days? CO Targets and

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How Much Fossil Fuel We Can Burn

Arecent Gallup poll shows that 41% of Americans think that reports of climate change are exaggerated, a number that’s grown in the past few years. For someone who has spent a lot of time thinking about climate change and the best way to approach it, this is pretty depressing news.

In order to combat misinformation, it would be really useful to know the urgency of action on climate change: is it needed today, tomorrow, or in 2020?

We often hear the necessary action for climate change expressed as a percentage reduction by a certain year. The current consensus goes something like this: “We need 80% reductions by 2050.” This obscures a whole lot of very important details. Let’s start with the simple one. What’s implied by these statements is that we need to reduce our energy use by 80% of 1990 levels of global CO

2 output. That sounds straightforward, but is that the reality of climate science? Not really. The problem with expressing the CO 2 reduction target as a percentage is that it hides important facts — that we know reasonably well how CO ends up in the atmosphere, and that CO

22 has a long residence time in the atmosphere — and suggests we’ll somehow be able to continue to emit some amount of CO in the future and still be OK.

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In the groundbreaking book Energy Policy in the Greenhouse, the authors expressed the CO problem

2 in a more honest way. They calculated how much the CO concentration in parts per million (ppm)

2 increases for a given amount of energy consumed from different fossil fuel sources. Their numbers are still quite accurate in describing humanity’s influence on CO concentration in the atmosphere:

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1 billion tons of carbon = +0.260ppm CO

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1TWyr of coal = +0.198ppm CO

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1TWyr of oil = +0.155ppm CO

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1TWyr of gas = +0.112ppm CO

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A TWyr is a terawatt-year, or 1,000,000,000,000

( 1 trillion) watts for 1 year, or 3.1556926×10¹⁹ joules.

Given that the world uses more than 10TW of fossil

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fuels, which means 10TWyr of energy each year, you can see how it adds up quickly. Remember that CO is at about 387ppm today. The above relation-

2 ship shows that the way we do things today, we add roughly 1ppm–2ppm every year.

We can turn the analysis around here in a manner that’s really interesting. Averaging the CO output

2 for each of the different fossil fuels, we can now state that for each joule of energy we get from them, we produce a 5× 10-21 ppm increase. This is a tiny, tiny number, but we use a lot of joules. Producing a single can of Coke uses around 5× 106 of them. In this way, we can estimate the effect of billions of small actions. But to return to the question at hand: how soon must we act? We need two more pieces of information. What temperature do we want to stabilize at? And what CO ppm does that correlate to? Warming

2 of 2°C ( 3. 6°F) above preindustrial levels is considered something of a “point of no return” by climate scientists, beyond which we will see very negative consequences. Two degrees implies 450ppm as an upper limit. But that might still be too high, given the even more ambitious 350ppm prescribed by Jim Hansen of NASA and other leading climate thinkers.

So let’s say we accept the risks of 450ppm (and I mean risks — this only gives us a small chance of staying below 2°C of average surface warming). What we get from the above equation is that we only have about 400TWyr left, or 40 years burning at 10TW. Or only 20 years burning at 20TW.

And that’s only the first bit. We also need to decide how much power we’d like from new sources of noncarbon energy that don’t exist yet. If you said you’d like the world to use the same amount of power as it does today in the future (this ignores population growth and growing demand, hoping that efficiency measures offset that), then we’d need 16TW of power, around 12TW of which come from fossil fuels.

Why do we need to know this? It’s to figure out how many ppm of CO will be added to the atmosphere to 2 create our new energy-generating and energy-using infrastructure. What I mean specifically is that at least for a while, we’ll be using coal, oil, and natural