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Making Trouble — 5,000 Days? CO2 Targets and How Much Fossil Fuel We Can Burn

Energy & Sustainability Science
Making Trouble — 5,000 Days? CO2 Targets and  How Much Fossil Fuel We Can Burn

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A recent 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 CO2 output. That sounds straightforward, but is that the reality of climate science?

Not really. The problem with expressing the CO2 reduction target as a percentage is that it hides important facts — that we know reasonably well how CO2 ends up in the atmosphere, and that CO2 has a long residence time in the atmosphere — and suggests we’ll somehow be able to continue to emit some amount of CO2 in the future and still be OK.

In the groundbreaking book Energy Policy in the Greenhouse, the authors expressed the CO2 problem in a more honest way. They calculated how much the CO2 concentration in parts per million (ppm) 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 CO2 concentration in the atmosphere:

1 billion tons of carbon = +0.260ppm CO2

1TWyr of coal = +0.198ppm CO2

1TWyr of oil = +0.155ppm CO2

1TWyr of gas = +0.112ppm CO2

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 fuels, which means 10TWyr of energy each year, you can see how it adds up quickly. Remember that CO2 is at about 387ppm today. The above relationship 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 CO2 output 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 CO2 ppm does that correlate to? Warming 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 non-carbon 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 CO2 will be added to the atmosphere to 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 gas to create the new solar cells, electric cars, wind turbines, nuclear power plants, green buildings, and mass transit solutions, until we can make those machines with clean power.

Using conservative assumptions for how much energy it will take to create these things, you see that it will take:

» 0.5ppm of CO2 to completely replace the world’s fleet of around 1 billion cars with small, light, 1,000kg electric vehicles

» 6ppm of CO2 to make 5TW of new solar generating capacity

» 0.5ppm of CO2 to make 3TW of new wind power

» 0.5ppm of CO2 to make 2TW of new geothermal power

» 9ppm of CO2 to make 250 million new “green homes”

So that’s about 16ppm already. Add some nuclear plants and some biofuel refineries, some wave power machines, some new rail, and quickly you’ll see we can easily expend 25ppm of CO2 just building the new infrastructure we need for a new economy. That would take us to about 415ppm from where we are now. Then we’ve only got 250TW years of fossil fuel burning left. That’s 20 years at the rate we’re burning today.

If we can commit to a CO2 ppm target, we can figure out how to get there and how many fossil fuels we’ve got left to use. If we stick with the percentage model, we’re flying blind. This kind of “working backward” analysis is what businesses do when they set a strategic goal, and it’s what we need more of if we’re to solve the climate problem once and for all.

So if you don’t want to fly blind, and you want a strong target so we actually get to where we want to go, you have to answer a few of the questions above. If you say we’ve got 5,000 days left until we have to act decisively, you’re playing a very risky game. After all, 5,000 days is 13.6 years. If we do nothing for 13.6 years, and then we make the decisions above, we only get 7 years of our current energy consumption before we don’t have any more energy to run humanity’s needs. Every joule after that has to be put toward the new infrastructure.

I know there are a lot of crude assumptions here, so feel free to tear those apart; the two main points are that we need to commit to hard numbers of where we wish to go, and we need to consider carefully the cost of building infrastructure, because we only get one shot at this.

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Saul Griffith

DR. SAUL GRIFFITH is founder and principal scientist at Otherlab, an independent R&D lab, where he focuses on engineering solutions for a clean energy, net-zero carbon economy. Occasionally making some pretty cool robots too. Saul got his PhD from MIT, and is a founder or co-founder of,,,,,,, and more. Saul was named a MacArthur Fellow in 2007.

View more articles by Saul Griffith
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