Mitigating Global Warming: The Devil Is in the Pathway

by Bill Chameides | July 16th, 2008
posted by Erica Rowell (Editor)

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Read more comments on this post at Huffington Post

The recent G8 communiqué calling for a 50 percent cut in greenhouse emissions by 2050 has received a lot of attention — some in praise, some not. One major criticism has been that it did not specify the pathway that gets us from present emissions to the target of a 50 percent cut by 2050. A number of people I’ve spoken with were surprised that such interim targets matter. “What difference does it make,” they’ve asked, “how we get to the reduction, as long as we get there?” Well, it makes a big difference. Here’s why.

Let’s start with a simple thought experiment. Suppose you have a bucket and hose, and water from the hose is filling up the bucket. The weight of the bucket at any point in time depends upon the total amount of water in the bucket and not on how fast water is coming out of the hose. Of course the faster the water leaves the hose, the faster the bucket will fill up. And if you don’t want the weight of the bucket to be more than you are able to carry, you have to be sure to stop the flow before the total amount of water in the bucket exceeds that weight limit.

Just like the water in the bucket, the amount of global warming at any point in time depends upon the total amount of greenhouse gases in the atmosphere. It does not depend on the rate at which these gases are emitted.

How Much Carbon Is in the Atmosphere

Today the atmosphere holds roughly 770 billion tons of carbon. This is equivalent to a CO2 concentration of about 385 parts per million (ppm). Let’s back up a moment. Scientists talk about the amount of CO2 in the atmosphere in two ways: in tons of carbon and in the number of CO2 molecules per million molecules of atmosphere. One ppm of CO2 in the atmosphere is equivalent to 2 billion tons of carbon.

We are currently adding 8 billion tons of carbon to the atmosphere each year and of that roughly 4 billion tons stay in the atmosphere for many decades. This corresponds to a 2 ppm rise in the CO2 concentration each year.

What’s the Tipping Point for Atmospheric Carbon?

To prevent dangerous climate change many scientists estimate that:
1) CO2 concentrations can not exceed 450 ppm and
2) total amount of CO2 emitted cannot exceed 280 billion tons of carbon by the end of the century.
These are the two numbers to remember. Scientists estimate we can emit 280 billion tons of carbon (which is more than the difference in tons between 450 ppm and the current concentration of 385 ppm) by allowing for the fact that some of the CO2 gets removed over time. Any way you figure it, this is not very much. For example, at the current rate of emissions — 8 billion tons per year — we would emit 280 billion tons of carbon in only 35 years.

Interim Targets Are Critical

So while cutting emissions by 50 percent by 2050 is important, the real focus needs to be on how much total CO2 we emit between now and 2050. How much we emit depends on the pathway we use to get to the 50 percent reduction.

For example, consider the three pathways illustrated below. All three start in 2010 and achieve the same 50 percent reduction in emissions by 2050 but each shows a different steepness in cuts depending on when reductions actually begin. Pathway A starts with emission cuts in 2020 — that’s about as soon as we can reasonably expect to begin reducing global emissions. Pathways B and C delay to 2030 and 2035, respectively, and then must cut emissions at a faster rate.

Let’s see how each does in terms of avoiding the cap of 280 billion tons. Those of you who took calculus might remember that the total amount of CO2 emitted by each pathway is equal to the area under the curve. So Pathway A leads to a total emission of 260 billion tons of carbon. It stays under the 280 billion ton cap — but just barely. Following this path only leaves 20 billion tons of carbon left to emit over the rest of the century.

By cutting our emissions by 50 percent, we would still be emitting 4 billion tons of carbon in year 2050 and would go through our 20 billion ton leeway in just 5 years! This path is not realistically going to get us to the end of the century with 450 ppm CO2 and that’s why some feel that the G8’s 50 percent emission cut is not enough.

Delaying Makes for a Tougher Path

But Pathways B and C are much worse. Pathway B spews a total of 280 billion tons of carbon into the atmosphere, leaving no room for any additional emissions in the later half of the century. Pathway C emits 290 billion tons, exceeding the cap. So you see that just meeting the 50 percent reduction target will not necessarily be adequate.

Of course one way to make up for the extra emissions caused by delaying at the front end in Pathways B and C would be to impose a more stringent emissions reduction target at the backend. If Pathway C called for a 70 percent reduction in emissions instead of 50 percent by 2050 it would just squeak under the 280 billion ton cap.The longer we delay, the faster we will need to bring emissions down once we get started.

A further complication is that developed countries are expected generally to take a larger share of the burden of reducing emissions than developing countries. This means that countries like the United States will need to reduce emissions by more than 50 percent if the global target is a 50 percent reduction.

Starting Emissions Cuts Sooner Makes Target Easier

It’s kind of a pay-me-a-little-bit-now or pay-me-much-more later kind of thing — actually more like pay-me-a little-bit-now or your kids get to pay-me-much-more later. The longer we delay the less time we will have to make the needed reductions, and thus the more difficult it will be to meet any target — which means we won’t keep CO2 levels below 450 ppm.

Remember this is the level most scientists agree we must not exceed if we want to avoid the worst of climate change. On the other hand, if we get started soon we will have the luxury of going slow and steady. Wasn’t there an Aesop fable about how a turtle won a race that way?

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  1. Daniel Wedgewood
    Jul 16, 2008

    Do you think it’s possible (practically speaking) to reduce emissions soon enough to prevent us from going over the 450 ppm threshold? And what about methods or new technologies that consume CO2 from the atmosphere? Are those too far in the future to yield measurable results? What happens if the 450 ppm threshold is exceeded, and then 20 years later the atmospheric concentration drops down below 450 again (through improved emission reductions and/or increased CO2 consumption activities)? Will the dranatic climate changes get reduced and reversed in linear time?” title=”Reduction and Consumption

    • Erica Rowell
      Jul 16, 2008

      Dr. Bill Chameides responds – Thanks for keeping the conversation going, Daniel. Let me break up my answer into several parts. First off, yes, it is possible to cut emissions enough to keep us under the 450 ppm threshold. It’s not going to be easy, and we have to get started soon. Next, you ask about technologies under development to keep CO2 out of the atmosphere. It’s a long shot, I think, because the technology is very far from being economically viable, but this could maybe help. But bear in mind, taking CO2 out of the atmosphere isn’t enough; you also have to store it somewhere like deep in the earth’s crust. (Check out this piece on carbon sequestration I co-wrote –,28804,1730759_1731383_1731989,00.html.) Finally, the concern about exceeding 450 ppm is that irreversible changes would occur like the melting of the Greenland ice sheet. There has been some discussion in the scientific community about so-called “overshoot pathways” where 450 ppm is exceeded for a short period of time and then comes back down. I don’t think we know yet how safe these overshoot scenarios would be.” title=”Reduction and Consumption — Some Answers

      • Daniel Wedgewood
        Jul 16, 2008

        Thanks for the reference to the article – I read it and found it informative. I don’t suppose you have a list of articles authored by you and your colleagues? After some reflection on that, what about the possibilities of consuming the CO2 instead of just storing it away? For instance, microbes might be modified that consume CO2 to sustain their life. Or nano-technology might be developed to do the same thing. Both would have to be controllable, of course – we don’t want all of the atmospheric CO2 to be devoured. Another scenario would be the genetic modification of trees to consume more CO2 – to make them better at filtering the atmosphere. They would be easier to control than invisible machines or microbes. All of this is prognostication, but right now so is the belief of curtailing of CO2 emisions.” title=”CO2 Consumption

        • Erica Rowell
          Jul 17, 2008

          Dr. Bill Chameides responds – Daniel, 1. With regard to other articles, take a look at for more info. 2. Consuming or storing CO2 – using technology or biology – is one way of addressing the problem. The technology option (see for example ) is a possibility we should not rule out but is not yet ready for prime time. And even if it became viable, it would require finding a place to store the carbon dioxide — for example deep in the earth’s crust. The biological option is actually something we are doing now (see ) — think no-till farming and forests. This can help and is already doing so, but it is unlikely to be the whole solution. Genetic modification — kind of a hybrid between technology and biology — is an interesting possibility. Let’s wait and see. 3. Agree it is all prognostication. The most disturbing prognosis is what might happen if we don’t curtail CO2 emissions.” title=”More info

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