In 2004, Pacala and Socolow laid out a list of ways we can battle global warming using current technologies. They said that to avoid serious trouble, we need to choose seven ‘stabilization wedges’: that is, seven ways to cut carbon emissions by 1 gigatonne per year within 50 years. They listed 15 wedges to choose from, and I’ve told you about them here:
• Part 1 – efficiency and conservation.
• Part 2 – shifting from coal to natural gas, carbon capture and storage.
• Part 3 – nuclear power and renewable energy.
• Part 4 – reforestation, good soil management.
According to Pacala:
The message was a very positive one: “gee, we can solve this problem: there are lots of ways to solve it, and lots of ways for the marketplace to solve it.”
I find that interesting, because to me each wedge seems like a gargantuan enterprise—and taken together, they seem like the Seven Labors of Hercules. They’re technically feasible, but who has the stomach for them? I fear things need to get worse before we come to our senses and take action at the scale that’s required.
Anyway, that’s just me. But three years ago, Pacala publicly reconsidered his ideas for a very different reason. Based on new evidence, he gave a talk at Stanford where he said:
It’s at least possible that we’ve already let this thing go too far, and that the biosphere may start to fall apart on us, even if we do all this. We may have to fall back on some sort of dramatic Plan B. We have to stay vigilant as a species.
You can watch his talk here:
It’s pretty damned interesting: he’s a good speaker.
Here’s a dry summary of a few key points. I won’t try to add caveats: I’m sure he would add some himself in print, but I’d rather keep the message simple. I also won’t try to update his information! Not in this blog entry, anyway. But I’ll ask some questions, and I’ll be delighted if you help me out on those.
First, Pacala’s review of different carbon emissions targets.
The old scientific view, circa 1998: if we could keep the CO2 from doubling from its preindustrial level of 280 parts per million, that would count as a success. Namely, most of the ‘monsters behind the door’ would not come out: continental ice sheets falling into the sea and swamping coastal cities, the collapse of the Atlantic ocean circulation, a drought in the Sahel region of Africa, etcetera.
Many experts say we’d be lucky to get away with CO2 merely doubling. At current burn rates we’ll double it by 2050, and quadruple it by the end of this century. We’ve got enough fossil fuels to send it to seven times its preindustrial levels.
Doubling it would take us to 560 parts per million. A lot of people think that’s too high to be safe. But going for lower levels gets harder:
• In Pacala and Socolow’s original paper, they talked about keeping CO2 below 500 ppm. This would require keeping CO2 emissions constant until 2050. This could be achieved by a radical decarbonization of the economies of rich countries, while allowing carbon emissions in poor countries to grow almost freely until that time.
• For a long time the IPCC and many organizations advocated keeping CO2 below 450 ppm. This would require cutting CO2 emissions by 50% by 2050, which could be achieved by a radical decarbonization in rich countries, and moderate decarbonization in poor countries.
• But by 2008 the IPCC and many groups wanted a cap of 2°C global warming, or keeping CO2 below 430 ppm. This would mean cutting CO2 emissions by 80% by 2050, which would require a radical decarbonization in both rich and poor countries.
The difference here is what poor people have to do. The rich countries need to radically cut carbon emissions in all these scenarios. In the USA, the Lieberman-Warner bill would have forced the complete decarbonization of the economy by 2050.
Then, Pacala spoke about 3 things that make him nervous:
1. Faster emissions growth
A 2007 paper by Canadell et al pointed out that starting in 2000, fossil fuel emissions started growing at 3% per year instead of the earlier figure of 1.5%. This could be due to China’s industrialization. Will this keep up in years to come? If so, the original Pacala-Socolow plan won’t work.
(How much, exactly, did the economic recession change this story?)
2. The ocean sink
Each year fossil fuel burning puts about 8 gigatons of carbon in the atmosphere. The ocean absorbs about 2 gigatons and the land absorbs about 2, leaving about 4 gigatons in the atmosphere.
However, as CO2 emissions rise, the oceanic CO2 sink has been growing less than anticipated. This seems to be due to a change in wind patterns, itself a consequence of global warming.
(What’s the latest story here?)
3. The land sink
As the CO2 levels go up, people expected plants to grow better and suck up more CO2. In the third IPCC report, models predicted that by 2050, plants will be drawing down 6 gigatonnes more carbon per year than they do now! The fourth IPCC report was similar.
This is huge: remember that right now we emit about 8 gigatonnes per year. Indeed, this effect, called CO2 fertilization, could be the difference between the land being a big carbon sink and a big carbon source. Why a carbon source? For one thing, without the plants sucking up CO2, temperatures will rise faster, and the Amazon rainforest may start to die, and permafrost in the Arctic may release more greenhouse gases (especially methane) as it melts.
In a simulation run by Pacala, where he deliberately assumed that plants fail to suck up more carbon dioxide, these effects happened and the biosphere dumped a huge amount of extra CO2 into the atmosphere: the equivalent of 26 stabilization wedges.
So, plans based on the IPCC models are essentially counting on plants to save us from ourselves.
But is there any reason to think plants might not suck up CO2 at the predicted rates?
Maybe. First, people have actually grown forests in doubled CO2 conditions to see how much faster plants grow then. But the classic experiment along these lines used young trees. In 2005, Körner et al did an experiment using mature trees… and they didn’t see them growing any faster!
Second, models in the third IPCC report assumed that as plants grew faster, they’d have no trouble getting all the nitrogen they need. But Hungate et al have argued otherwise. On the other hand, Alexander Barron discovered that some tropical plants were unexpectedly good at ramping up the rate at which they grab ahold of nitrogen from the atmosphere. But on the third hand, that only applies to the tropics. And on the fourth hand—a complicated problem like this requires one of those Indian gods with lots of hands—nitrogen isn’t the only limiting factor to worry about: there’s also phosphorus, for example.
Pacala goes on and discusses even more complicating factors. But his main point is simple. The details of CO2 fertilization matter a lot. It could make the difference between their original plan being roughly good enough… and being nowhere near good enough!
(What’s the latest story here?)