Water vapor and global warming


With Copenhagen on the horizon, is it time to check over the physics and chemistry behind the climate change debate? SciScoop has spoken to several scientists recently who would say so, some of whom consider the climate change debate nothing more than a hell-on-earth scenario to give governments a taxation stick with which to beat us. The main point of contention seems to be that water vapor is a much more predominant player in global warming than the trace amounts of carbon dioxide, methane and other greenhouse gases.

But, before we jumped on the denialist bandwagon, we did a reality check with a climate scientist who put the record straight and pointed out that most of the arguments put forward ignored important issues such as the fact that the greenhouse effect is an entirely natural phenomenon. Rather, it’s the human-produced or anthropic “enhanced” effect, caused by our burning fossil fuels, raising beef cattle etc that is the anthropic component we should be very concerned about.

Climate change now?

Carbon dioxide accounts for only one in every 4000 molecules in the air; water vapor accounts for one in every 20. Carbon dioxide absorbs only a quarter as much energy from sunlight as water vapor, molecule for molecule; suggesting that water vapor is responsible for the majority of atmospheric heating.

Indeed, the IPCC (International Panel on Climate Change) essentially agrees:

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. Indirectly, humans have the potential to affect water vapour substantially by changing climate. For example, a warmer atmosphere contains more water vapour. Human activities also influence water vapour through methane emissions, because methane undergoes chemical destruction in the stratosphere, producing a small amount of water vapour.

So, where does carbon come into the equation, if at all? Even late nineteenth century chemist Svante Arrhenius famously outlined a mathematical argument proving that raising atmospheric carbon dioxide levels could not be used to heat Sweden so it could again grow bananas as it had in antiquity.

Scientists have been analyzing these energies since 1857 and British physicist John Tyndall who wrote, “Water vapor is the principle gas heating the atmosphere.” He too made absolutely no reference to carbon dioxide because he considered it insignificant.

Well, superficially it would seem that carbon is trivial. The atmosphere is 78% nitrogen, 18% oxygen, around 1-2% water vapor, 1% trace gases not relevant to atmospheric physics and a mere 0.038% carbon dioxide and 0.00017% methane.

Atmospheric heating is done by infrared, IR, heat energy from sunlight, with wavelengths between 1 and 16 micrometers, which is invisible but we feel as heat on our skin. Nitrogen and oxygen absorb no IR from sunlight; they are transparent to it.

But, climate change is happening, global average temperatures are rising and ice sheets are melting. Burning fossil fuels releases carbon dioxide into the atmosphere and belching beef cattle release methane.

The greenhouse effect is, as we said, an entirely natural phenomenon. If we didn’t have the natural greenhouse effect, the earth’s average temperature would be about -25 Celsius, instead of a relatively mild almost 15 Celsius. There would not be life on earth as we know it, and the earth would most likely be a frozen ball of ice. The greenhouse effect is caused by naturally occurring levels of greenhouse gases in the atmosphere, including water vapor, carbon dioxide, methane, nitrous oxide, and several others in trace amounts.

The actual amounts of these various gases in the atmosphere does not complete the picture in terms of the greenhouse effect and the climate debate. What is important is how powerful the gases are, not their absolute concentrations.

Back in the 1700s and 1800s, scientists did think that water vapor accounted for all of the greenhouse effect. It is such an effective absorber, and occurs in relatively high concentrations near the earth’s surface, that how could it not be? But then they began to realize, on the basis of high-altitude balloon measurements, that water vapor levels drop off very quickly with height; whereas carbon dioxide, methane, and other gases remain well-mixed throughout the lower atmosphere. Not only that, but greenhouse gases are much more effective higher up in the atmosphere where it’s colder, as opposed to near the warm surface of the earth.

Since water vapor is such a comprehensive absorber, many of the [spectroscopic] absorption lines of these gases do overlap with it, so it’s difficult to pin an exact number on how much any individual gas contributes to the natural greenhouse effect. However, with radiative transfer models it is possible to calculate the strength of the greenhouse effect if we just had other greenhouse gases, without water vapor or clouds. When we do so, we get about 35% of the total. We can also calculate the strength if we just had water vapor, but no other gases. When we do this, we get about 85%. So the truth is that water vapor and clouds are responsible for somewhere between 65 and 85% of the natural greenhouse effect. This is not the 99.9% effect claimed by skeptics.

The current problem has arisen because we are artificially “enhancing” this natural balanced phenomenon through our carbon emissions from vehicles, industry and power generation using fossil fuels. Indeed, we’ve been upsetting this balance significantly since the middle of the twentieth century and some would argue since the dawn of the industrial revolution in the nineteenth.

By massively increasing the amount of non-water vapor greenhouse gases in the atmosphere, we are increasing the amount of heat trapped by the atmosphere. Not only that, but warmer temperatures are causing more water to evaporate into the atmosphere. A British scientist proved this in 2007 (see footnote reference). So not only are humans artificially enhancing the non-water part of the greenhouse effect, we are also increasing levels of water vapor as well. And that has to be a lose-lose situation.

Research Blogging IconWillett, K., Gillett, N., Jones, P., & Thorne, P. (2007). Attribution of observed surface humidity changes to human influence Nature, 449 (7163), 710-712 DOI: 10.1038/nature06207