The first results from the CLOUD experiments have been getting a lot of media attention. The focus of the attention is the Nature paper that was published this week: [news][paper].
The goal of this project is to determine whether cosmic rays have a significant impact on clouds.
Let's boil this down a little. This project is a laboratory experiment at CERN. It is a cloud chamber, basically an isolated volume of air that is precisely controlled for temperature and pressure. They put very pure air into the chamber, add a little background water, and some gases like ozone, sulfuric acid, and ammonia. The chamber is heavily instrumented to look for nucleation, which just means that they try to keep track of particle formation that occurs as the vapors interact and possibly start condensing. They can do this in neutral conditions (like a classical cloud chamber), or they can shine a pion beam into the chamber. That beam is adjusted to mimic cosmic ray bombardment. The goal is to see if cosmic rays produce ions that enhance the formation of particles, which could then go on to become the seeds for cloud droplets.
The answer seems to be that shining that beam into the chamber does produce more particles. This actually isn't a surprise, as far as I can tell. One important point is that nucleation rates, that is the rate of particle formation, are smaller than observed rates unless the temperature is quite low. This means that it is unlikely that cosmic rays ionizing gases near the Earth's surface is a major source of particle formation. Certainly there is particle formation, but it is likely to be a small source of the total number of particles. This result may change when they start adding in organic molecules, but that is future work.
There is better coverage on RealClimate: link.
There is hubbub about this result because there is a crack-pot theory that galactic cosmic rays are a major control of climate because of their impact on cloud formation. There are major flaws with this theory. My own take is that cosmic rays probably do produce some of the particles in the atmosphere that go on to become cloud condensation nuclei, but there are many paths to becoming cloud condensation nuclei, and there are lots and lots of these particles around. In fact, I seriously doubt that cloud formation is frequently affected by the limitation of these aerosol particles. I've been thinking about this in terms of observed cloud properties. The number of cloud droplets is connected to the number of aerosol particles available: over land where there are lots more aerosol particles, there tends to be more, smaller droplets in clouds, while over the remote ocean the clouds are made of fewer, larger droplets. In very polluted conditions, we can observe changes in the cloud properties that follow that same trend. I think the downfall of the cosmic ray theory of cloud formation comes from the fact that out in the middle of the ocean there are still tons of aerosol particles. While many of those particles may come from cosmic ray influenced nucleated vapors, there is no evidence that there is a shortage of other sources of aerosol, so if the intensity of cosmic ray bombardment were to change, it seems unlikely that other sources of aerosol wouldn't fill whatever tiny void that change would make.
Besides this basic criticism (which amounts to the originators of the theory simply having a bit of a myopic view of cloud formation), there is also a clear lack of evidence for cosmic ray intensity modulating cloud/climate. The RealClimate piece covers that. Finally, there is the link to climate change, for which there is absolutely no evidence.
So my summary would be something like: This research presents experimental results that suggest that ionization by cosmic ray-like effects can impact nucleation rates in conditions similar to the Earth's atmosphere. The role of such nucleation enhancement in the Earth's atmosphere remains unclear, especially given that the impact seems most pronounced in conditions that are outside the atmospheric boundary layer. This is a nice contribution to basic aerosol research, which should help to constrain models of aerosol formation. The impacts on cloud formation and the Earth's climate can not be assessed with the data collected so far.
The authors are only slightly overselling their results, which is typical for authors of Nature papers. The lead author's comments can be heard in the embedded YouTube clip. The media coverage, and especially the climate change denier blogosphere, is lighting up like this experiment proves something controversial. It does not.
Showing posts with label chemistry. Show all posts
Showing posts with label chemistry. Show all posts
2011-08-31
2009-09-24
the chemistry of the greenhouse effect
Just a quick mention of an interesting study that I saw today. Three researchers from NASA and Purdue have a paper in the Journal of Physical Chemistry called Identifying the Molecular Origin of Global Warming [LINK]. It was brought to my attention by a news story on NewScientist.com [LINK]. Now based on that title, what would you expect from this paper? The news article doesn't completely make sense to me, perhaps because the writer tries to get so many barely-related things into such a tiny piece. It had me intrigued though, so I went and read the paper.
Before we get to the conclusions of the paper, let's consider what we know about global warming, specifically the greenhouse effect. We know that the most important greenhouse gas in terms of the human impact on climate is carbon dioxide. The way that the greenhouse effect works is that the molecules of carbon dioxide absorb infrared radiation emitted from the Earth's surface. Those molecules then emit infrared radiation at a slightly colder temperature, and they radiate in all directions. This has the effect of storing energy in the system, first by keeping it in molecules (before they radiate it away), and second by radiating back to the atmosphere and surface. So what does this paper have to offer?
The study uses calculations of molecular properties to investigate the greenhouse warming potential of different gases. The GWP of a gas is essentially a measure of how strongly a gas absorbs in the infrared, and how much warming it could cause over a given time in the atmosphere (typically 100 years). The way I've seen it presented, carbon dioxide is given a value of 1, and other gases are then shown compared to carbon dioxide. This study shows that there are a couple of families of molecules that have very large GWP, and presents an argument for how it comes about. The gist seems to be that molecules that have carbon-fluorine or carbon-chlorine bonds are particularly good greenhouse gases. These happen to include chlorofluorocarbons (CFCs), perfluorocarbons (PFC) and hydrofluorocarbons (HFCs). The warming potential increases strongly with the number of bonds between the F or Cl and the carbon atom. This seems to be because the vibrational modes of the molecules, which are quite pronounced for these bonds, plus the stretch length of these bonds mean that the infrared interaction is within the atmospheric window. If this is all sounding vague and Greek-ish, it's because I don't completely understand all the terminology in the paper. The point is just that F, and to a lesser degree Cl and H, have strong bonds with C in these molecules, and their vibrational modes are in the atmospheric window, meaning they can absorb strongly in the correct band to make a difference to climate.
The major problem with the paper is that these gases are quite rare in the atmosphere, and it is hard to make the case that they are making a significant difference to the climate. The counter to such an argument is that this paper shows the physical mechanism at the molecular level that is responsible for some gases being very good greenhouse gases. It means that you can basically know from the outset whether some gas, perhaps an industrial product of some sort, will be a strong greenhouse gas. It also can serve as an early warning against over-using these gases, since large increases in their production could have consequences for the climate system.
The secondary problem with this paper is the way it seems to be interpreted in that news story (and thus likely others). It's presented almost like it's the first time we've understood what is going on with the greenhouse effect. Of course, that is rubbish, as we've had a good handle on the basics for over a century, and detailed studies of CO2 for decades. The authors don't try play down their results either, which you can tell just from the title. This is not a world-shattering study; it is a nice piece of chemistry that has some application to climate science.
Before we get to the conclusions of the paper, let's consider what we know about global warming, specifically the greenhouse effect. We know that the most important greenhouse gas in terms of the human impact on climate is carbon dioxide. The way that the greenhouse effect works is that the molecules of carbon dioxide absorb infrared radiation emitted from the Earth's surface. Those molecules then emit infrared radiation at a slightly colder temperature, and they radiate in all directions. This has the effect of storing energy in the system, first by keeping it in molecules (before they radiate it away), and second by radiating back to the atmosphere and surface. So what does this paper have to offer?
The study uses calculations of molecular properties to investigate the greenhouse warming potential of different gases. The GWP of a gas is essentially a measure of how strongly a gas absorbs in the infrared, and how much warming it could cause over a given time in the atmosphere (typically 100 years). The way I've seen it presented, carbon dioxide is given a value of 1, and other gases are then shown compared to carbon dioxide. This study shows that there are a couple of families of molecules that have very large GWP, and presents an argument for how it comes about. The gist seems to be that molecules that have carbon-fluorine or carbon-chlorine bonds are particularly good greenhouse gases. These happen to include chlorofluorocarbons (CFCs), perfluorocarbons (PFC) and hydrofluorocarbons (HFCs). The warming potential increases strongly with the number of bonds between the F or Cl and the carbon atom. This seems to be because the vibrational modes of the molecules, which are quite pronounced for these bonds, plus the stretch length of these bonds mean that the infrared interaction is within the atmospheric window. If this is all sounding vague and Greek-ish, it's because I don't completely understand all the terminology in the paper. The point is just that F, and to a lesser degree Cl and H, have strong bonds with C in these molecules, and their vibrational modes are in the atmospheric window, meaning they can absorb strongly in the correct band to make a difference to climate.
The major problem with the paper is that these gases are quite rare in the atmosphere, and it is hard to make the case that they are making a significant difference to the climate. The counter to such an argument is that this paper shows the physical mechanism at the molecular level that is responsible for some gases being very good greenhouse gases. It means that you can basically know from the outset whether some gas, perhaps an industrial product of some sort, will be a strong greenhouse gas. It also can serve as an early warning against over-using these gases, since large increases in their production could have consequences for the climate system.
The secondary problem with this paper is the way it seems to be interpreted in that news story (and thus likely others). It's presented almost like it's the first time we've understood what is going on with the greenhouse effect. Of course, that is rubbish, as we've had a good handle on the basics for over a century, and detailed studies of CO2 for decades. The authors don't try play down their results either, which you can tell just from the title. This is not a world-shattering study; it is a nice piece of chemistry that has some application to climate science.
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