Water as a Greenhouse Gas

Climate change and the human contribution to the greenhouse effect has been a hot topic recently and people are gradually becoming more aware of the dangers of green house gasses.

But not many of us suspect the old H₂O, to be contributing to the problem... to find out more; Azi Khatiri went to the department of Chemistry at the University of Cambridge to speak to professor Rod Jones.

Rod - Just about everybody knows that CO₂is the most important green house forcing gas, what they don't know is that water vapour in the atmosphere is about twice as effective as CO₂as a greenhouse warming gas. CO₂is put into the atmosphere by both natural and human activities. Water on the other hand evaporates from the surface and its atmospheric concentration is simply determined by the temperature of the atmosphere. So as we put more CO₂and other greenhouse gases into the atmosphere, we force the temperature to rise and therefore the atmosphere can hold more water. The additional water leads to additional warming which is an amplification factor or a feedback effect on the original forcing.

Azi - So the more CO₂and other greenhouse gases we put in the atmosphere the warmer the atmosphere gets and the more water we evaporate, which then goes back into the atmosphere and warms it all up again?

Rod - Warms it further, yes.

Azi - What happens as radiation from the sun comes into the atmosphere and how does it interact with the water molecules?

Rod - Basically, the sun's radiation warms the surface and the Earth's surface then emits infrared radiation which is at a longer wavelength than the visible light that we can see. It's that long wavelength radiation which is trapped by the water molecules in the same way that it's trapped by CO₂and other greenhouse forcing gases. The work that we are doing here is looking at part of the water vapour problem. Water vapour is a very interesting molecule in that it doesn't just absorb as an individual molecule but it can cluster together; ultimately, it can cluster together to form clouds which have a huge impact on the radiative properties of the atmosphere. We are looking at the first stages of that where two water vapour molecules come together to form a dimer.

There are two points about the dimer; one is that the absorption features are very different from the absorption features of the water molecule on its own. The second point is that as water increases in the atmosphere the concentration of the water dimer is expected to increase proportionally with the concentration of water vapour squared.

Azi - How does the water dimer differ to the water monomer, the single water molecule, in its absorption effects?

Rod - The absorption of the dimer has the potential of being stronger per molecule than of the monomer. So the purpose of our study is to measure the absorption properties of the water vapour dimer, so that we can ultimately put those into climate models which are used to predict future changes.

The work we do is in a laser laboratory where one of my PhD students, Alex Shillings has been working for a couple of years... we're just going to have to go through the laser interlock door... hopefully we can find Alex.

Azi - Hello Alex!

Alex - Hello, I'm Alex Shillings, and I'm a PhD student working on the Broadband Cavity Ring Down Spectroscopy experiment, and we are looking for the water dimer absorption.

This grey box you see here, this is a YAG laser producing green laser light. This green light is then injected into the blue box you see here, which is a dye laser. So we put green light in and we get red laser light out.

Azi - So what happens then to the light that comes out of the dye laser?

Alex - It comes around those turning mirrors and is injected into the cavity. Once inside the light bounces around many, many times between the mirrors; we can get an effective path of the light of 30 or 40 km even though the dimensions of the experiment is only about 2m long.

Azi - That's effectively re-producing what happens to the light in the atmosphere!

Alex - Absolutely right. That's one of the beauties of why this experiment is so appropriate for measuring species that could be important for absorbing radiation in the atmosphere, since we get paths that are commensurate with or even exceed paths of sunlight through the atmosphere.

Azi - So you put whatever you want to measure in the tube and in this case it's water vapour isn't it?

Alex - Yes, that's correct. The actual thing that we measure is the time dependence of the light leaking out of this cavity, that's basically how quickly the light decays away. We actually measure the difference between a cavity with water vapour in, or an empty cavity filled with just air, and the difference between the two loss rates gives us a direct measure of how much radiation whatever is in the cavity is absorbing.

Azi - So effectively you compare the cavity with and without the species that you want to measure and the difference between with and without, will tell you how much your species has absorbed?

亚历克斯——这是正确的。水二聚体two water monomer molecules that are stuck together by a hydrogen bond. This alters the bond between the hydrogen and the oxygen in one of the water molecules, and because this bond is altered, this leads to a new absorption feature. This is the dimer absorption feature we are looking for.

Azi - It's very interesting that you can pick up specific vibrations from the water dimer, which as Rod was telling me is very important in the greenhouse effect. Are you working in collaboration with atmospheric modellers? For example to put your findings into ways of understanding the greenhouse effect?

Alex - It has been a bit of embarrassment for climate science that since the greenhouse effect was discovered there has been a disagreement between the amount of radiation we measure the atmosphere absorbing; and the amount of radiation we predict the atmosphere should absorb from our knowledge of the composition of the atmosphere. Hopefully our measurements should be able to bridge that gap to some extent, and once we have a better understanding of the atmospheric system, we will be better equipped to deal with the climate change problems that we are almost bound to be facing in the next century.