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Mitigating Global Warming: The Devil Is in the Pathway

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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.

Carbon Target Chart

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?

Dr. Bill Chameides is the dean of Duke University's Nicholas School of the Environment. He blogs regularly at www.thegreengrok.com.

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