I’ve been reading a lot lately about the proposed use of hydrogen fuel cells to provide a clean renewable source of energy, especially for use in cars.
But I vaguely remember something I heard a long time ago about water vapour being a greenhouse gas. Also the only waste product of fuel cells is water vapour.
If this is true, then when we switch our cars from running on petrol to fuel cells won’t we be still be spewing greenhouse gases into the atmosphere?
I hope I am wrong.
Technically, every gas (or vapor) can produce/contribute to the greenhouse effect to some degree. But, some are worse offenders than others… - Jinx
Yes, water vapor is a greenhouse gas. Unlike, say, carbon dioxide, however, water vapor condenses and returns to the surface as liquid water. The equilibrium level of water vapor is determined by temperature and pressure, and isn’t going to change just because we start burning hydrogen.
Not something to worry about.
Water vapor does have effects on climate. I believe it has a slight greenhouse heat-trapping effect, but also when it condenses into clouds it reflects sunlight, causing relative cooling. And changes in humidity can effect climate directly and indirectly in complex ways.
But we don’t really need to worry about water vapor from fuel cells, because the amount created by even a whole country of fuel cells is pretty small, compared to the amount of water vapor created by a lake and a hot day. And anyway water vapor has a mechanism where it leaves the atmosphere as rain (or snow!), so it’s more or less in equilibrium.
Plus, of course, you need to get the hydrogen somewhere. Either from electricity created by wind, solar, nuke, etc., in which case hydrogen is coming from water (that’s split apart using electricity), in which case you’re using as much water as creating, or from fossil fuels, in which case you need to worry more about the carbon dixoide greenhouse effects than the water vapor.
Any molecule that can exist as a gas under atmospheric conditions (e.g. in the lower troposphere, roughly 290K and 1 atm pressure) is a potential greenhouse gas if it is capable of absorbing infrared radiation. A molecule can absorb in the IR if a vibration of its bonds changes its dipole moment. (Explaining which molecules satisfy this criterion is rather complicated; try here and here for some rather technical information on dipole moments and their applications to IR spectroscopy.) One prerequisite is that there must be at least two different elements in the molecule, so diatomic gases such as nitrogen and hydrogen do not absorb in the IR and cannot be greenhouse gases. Water, however, does absorb in the IR (see its spectrum here).
Absorption in the IR and being present as a gas in the atmosphere is not the only requirement for a significant greenhouse gas. The molecule must also have a reasonably long lifetime in the atmosphere; some molecules might be chemically or physically removed in a very short time. Water vapor has a sufficiently long lifetime to accumulate, though of course it has a mechanism for removal (rainfall). Note that the amount of water vapor in the atmosphere is in equilibrium; adding more water vapor to the atmosphere will cause more rain, so the amount of vapor in the atmosphere is kept under control to an extent.
Another factor to consider is where in the IR the molecule absorbs. Compare the IR spectrum of water to this IR spectrum of the atmosphere. This spectrum indicates the ranges where water vapor, ozone and carbon dioxide (the main natural gases that absorb in the IR in the atmosphere) absorb. This spectrum shows that water vapor is responsible for a substantial amount of the natural greenhouse effect, the beneficial process that moderates the temperature of the atmosphere. (With no greenhouse effect, the average surface temperature of the earth would be about 0°C.) However, if you look at this spectrum, you can see that there are some ranges where the atmosphere absorbs relatively little IR. This is called the atmospheric window, and it is of great concern in studying climate change. If humans add gases to the atmosphere that absorb in this ‘atmospheric window’ range, we can add substantially to the greenhouse effect, by making the atmosphere absorb radiation in wavelengths that passed through the natural atmosphere. CFCs happen to absorb in some of these ranges, and so they are very significant as potential greenhouse gases.
One last thing to consider is the effect of human activity on the amount of the gas in the atmosphere. The amount of water vapor in the atmosphere from natural sources is large – so large, in fact, that water absorbs almost all the IR radiation that it possibly can. Adding more water would not produce a large change in the overall absorption of the atmosphere, particularly because water is in equilibrium and adding water causes more rain. The same is true, to a lesser extent, of CO[sub]2[/sub], but there is still cause for concern over that one. CFCs, again, are significant here because they have a very long lifetime (tens of years, at least).
To conclude, yes, water is a greenhouse gas, and it is an important contributor to the natural greenhouse effect that increases the temperature of our planet to a range where life as we know it is possible. Water added to the atmosphere by human activity is not likely to contribute significantly to climate change.
Jinx: No, some gases do not contribute to the greenhouse effect (e.g. nitrogen and hydrogen). The greater the number of atoms in the molecule, the more likely it is to have high greenhouse gas potential (because there are more ways for it to absorb in the IR, particularly in the ‘atmospheric window’ that I mentioned). CFCs are very bad for this. This page has a table of greenhouse gas potentials of several gases, with 1000 kg of CO[sub]2[/sub] = 1 (thus 1 ton of SF[sub]6[/sub] has the greenhouse potential of almost 24,000 tons of carbon dioxide).
Quercus: Good point about water vapor increasing the reflectivity of the atmosphere. There is some evidence that the increased amount of particulate matter in the atmosphere is making clouds more reflective by spreading out the water vapor over more dust particles (making the water droplets smaller and more reflective). Water also moderates the earth’s temperature in some other ways. Liquid water removes CO[sub]2[/sub] from the atmosphere and is the major reason the Earth’s atmosphere has relatively little CO[sub]2[/sub]. And snow and the polar ice reflect incoming radiation back into space.
With hydrogen, what we really need to be concerned about is CO[sub]2[/sub] produced in generating the hydrogen. Production of hydrogen from fossil fuels necessarily creates CO[sub]2[/sub], and producing it electrolytically from water can add CO[sub]2[/sub] to the atmosphere if the electricity is generated from fossil fuels. To really make hydrogen as green as it seems if you only consider the end use (cars that only emit water vapor), we need to find efficient ways of generating hydrogen without generating CO[sub]2[/sub] in the process.