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Blog - stabilization wedges (part 5)

This page is a blog article in progress, written by John Baez. To see the final polished article, go to the Azimuth Blog.

In 2004, Pacala and Socolow laid out a menu of ways we could battle global warming using current technologies:

• Stephen Pacala and Robert Socolow, Stabilization wedges: solving the climate problem for the next 50 years with current technologies, Science 305 (2004), 968-972.

Very briefly, they said we needed seven ‘wedges’—7 ways to cut carbon emissions by 1 gigatonne per year within 50 years. They listed 15 possible wedges, and I 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.”

But three years ago, he publicly reconsidered his ideas in light of new evidence, saying:

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 see his talk here:

• Stephen Pacala, Equitable climate solutions, talk at the Energy Seminar, Woods Institute, Stanford University, 5 November 2008.

It’s well worth watching. Here are some highlights. I will not 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. I’ll be delighted if you help me out on that.

Emissions targets

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: events like melting ice sheets in Antarctica and Greenland, the collapse of the Atlantic ocean circulation, or a drought in the Sahel region of Africa.

Many experts say we’d be lucky to keep CO2 from doubling. At current burn rates we’ll double it by 2050, and quadruple it by the end of this century.

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 a cap of 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 about 430 ppm. This would require cutting CO2 emissions by 80% by 2050, which would 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 done this. (That bill is dead now.)

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 would be drawing down 6 gigatonnes more carbon per year than they do now! This is huge: remember that right now we emit about 8 gigatonnes per year.

Indeed, this effect, called carbon 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.

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, other research 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 carbon fertilization matter a lot. It could make the difference between their original plan being roughly good enough… and being nowhere near good enough!

category: blog