Rethinking AGW, Part 3: The Human Contribution
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For those who haven't been following along, I've been attempting a dispassionate, reasonable evaluation of global warming, from the point of view of an admitted layman. In my last installment, I determined that there has been a measurable increase in average global temperatures, but this week, I'm going to put that aside and focus on the anthropogenic side of the equation. More specifically, I'm looking at manmade greenhouse gas emissions, and I'm going to attempt to put a single number on it, a percentage of how much humanity contributes to the global total. This isn't going to address the potential for runaway effects--how a slight change might snowball out of control into a large one in the future or how tiny effects can upset delicate climatological cycles--nor is it going to attempt to corellate anything to the observed increase I discussed last time.
I'll start with carbon dioxide, since that's the greenhouse gas everybody seems concerned with. Don't worry; I'll get into the other gases--including water vapor--a bit further down, but for now, I want to look at just carbon dioxide. Yes, it is a greenhouse gas, meaning that a dramatic increase in CO2 would, with absolute certainly, translate to a greenhouse effect, to warming. Now, the IPCC and U.S. Department of Energy use estimates of pre-1750 CO2 levels to set a baseline, and using ice core samples (which aren't without controversy, but I'll give them the benefit of the doubt), they have determined that the baseline is 280 ppm (parts per million). According to the National Oceanic and Atmospheric Administration Earth System Research Laboratory, the average 2010 level is 388.5 ppm, an increase of 38.75%.
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We could just cite 38.75% and call it a day, but it would be hasty to conclude that that number represents nothing but the human contribution. All it says is that modern CO2 levels are 38.75% higher than they were before 1750, which is hardly enough data to draw any meaningful conclusions. There are several potential natural causes for the increase in CO2, including ocean degassing (which can be exacerbated by warming, anthropogenic or otherwise) and biogeochemical reactions. According to the IPCC's own figures, mankind only adds about 29 gigatons of CO2 to a system that cycles through 750 gigatons each year through natural processes, a contribution of only 3.9%. Granted, that contribution could be cumulative over time, depending on the ability of the Earth to compensate for it, a topic outside the scope of this particular exercise (I won't forget about it, though, I promise).
Let's take the worst case scenario, that mankind is directly responsible for the entire 38.75% increase in CO2 over the last two and a half centuries. How much of a greenhouse effect are we talking about? In order to answer that question, you have to look at GWP, or Global Warming Potential (which for the sake of this blog I will restrict to a 100 year time scale). Figuring GWP is a complicated bit of math, but the upshot is that you can compare other gases to CO2 in terms of how strong they are as greenhouse gases. CO2 has a GWP of 1, and when you look at the GWP of other greenhouse gases, you start to realize how insignificant CO2 is in the grand scheme of things.
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For example, using IPCC figures, methane (CH4) has a GWP of 25, meaning it is 25 times as powerful as carbon dioxide as a greenhouse gas, and nitrous oxide (N2O) has a GWP of 298. But even those pale in comparison to many halogenated gases (CFCs), which are 100% manmade. One of the most ubiquitous, CFC-12, has a GWP of a whopping 10,900, but by far the worst is sulfur hexaflouride (SF6) which not only has a GWP of a truly staggering 22,800 but also has an average lifespan of 3,200 years. Given these figures, one wonders why people are so worried about a 38.75% increase in atmospheric CO2. Clearly, there are much scarier greenhouse gases out there.
However, consider that CO2 levels are much, much higher than CH4 or N2O levels, which are measured in parts per billion instead of parts per million, and thankfully, CFC concentrations, which are measured in parts per trillion, are infinitessimal compared to CO2. Without getting into too much more math, the bottom line is that CO2 still accounts for more potential warming than all other greenhouse gases (with the exception of water vapor) combined, by a factor of about three to one.
But that doesn't answer the original question: how much warming would a 38.75% increase in CO2 cause? For that, you have to look at something called radiative forcing, which I have to admit is where I get off; it's just too complicated to get my untrained head around. If you want to figure it out yourself, a good place to start would be with the NOAA Annual Greenhouse Gas Index. The best definition I've been able to get for radiative forcing is "the change in net irradiance between different layers of the atmosphere" (Wikipedia), but good luck putting that into concrete, easy-to-understand figures. For that, I may have to talk to a living, breathing climate scientist.
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So let's drop that question and look at the granddaddy of all greenhouse gases: water vapor. Due to how quickly water vapor concentrations can change, it's GWP is impossible to meaningfully calculate, but it is an extremely potent greenhouse gas that mankind has very little control over. Don't take my word for it, either; according to the IPCC, "Water vapour is the most abundant and important greenhouse gas in the atmosphere. However, human activities have only a small direct influence on the amount of atmospheric water vapour." The climate change skeptic could stop here and conclude that, since water vapor is so much more "abundant and important" than CO2, the whole AGW debate is absurd.
That doesn't tell the whole story, though. Water vapor is remarkably easy to understand and predict: if you increase the temperature in a system with liquid water (like the Earth), water will evaporate, creating more water vapor, and if you decrease the temperature, you will decrease the amount of water vapor. It doesn't take much imagination to see how this could be a problem in a climate system undergoing change. If the Earth is getting hotter, then the water cycle will ensure that the change is more pronounced than it otherwise would be. Therefore, if a small change in CO2 is causing a greenhouse effect, than the prevelance of water vapor will only make things worse.
So where does that leave us, in regards to the human contribution? It is certain that mankind is adding greenhouse gases to the system, and CO2 is the most important greenhouse gas in the equation. Depending on how you figure it, we are responsible for anywhere between 3.9% and 38.75% of the CO2 currently in our atmosphere, and this is undoubtably significant in regards to climate change. Still, there's an open question of the severity of that significance, and whether or not it is a major factor in the observed average increase in global temperatures over the last few decades, not to mention all the politics. I will work on these questions in the future.
-e. magill 1/24/2012
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