Water vapor and climate change


Just saw this absolutely ridiculous article posted on Digg, by way of the Guardian: “Water vapour caused one-third of global warming in 1990s, study reveals.”

That’s gotta be one of the more sensationalist titles ever written in the climate change debate, which will help fuel and legitimize claims made by climate change deniers. Anyway, the article does have some interesting nuggets and things that should be discussed.

Scientists have underestimated the role that water vapour plays in determining global temperature changes, according to a new study that could fuel further attacks on the science of climate change.

The research, led by one of the world’s top climate scientists, suggests that almost one-third of the global warming recorded during the 1990s was due to an increase in water vapour in the high atmosphere, not human emissions of greenhouse gases. A subsequent decline in water vapour after 2000 could explain a recent slowdown in global temperature rise, the scientists add.

Basically, scientists need to do a better job modeling how water vapor plays a role in climate change. That said, there are few interesting things to consider, that this article fails to mention:

  • Examples of common greenhouse gases are CH4 (methane), CO2 (carbon dioxide), N2O (nitrous oxide), and H2O (water!).
  • The atmospheric concentration of CO2 and CH4 *is* increasing, mainly due to anthropogenic causes (burning coal, oil, and natural gas).
  • In general, the concentration of H2O in the atmosphere varies (but is dependent on atmospheric temperature), however there is a complex relationship between increased H2O -> increased cloud cover -> increased albedo.

The second thing to consider is the relative impact each of these gases have on trapping heat. In general, all greenhouse gases are compared to CO2 (which has a value of 1.0). This is called the global warming potential.

Methane is 25x stronger at trapping heat than carbon dioxide! Again, the atmospheric concentration of both of these gases is increasing. What is water vapor’s effect on trapping heat?

A GWP is not usually calculated for water vapour. Water vapour has a significant influence with regard to absorbing infrared radiation (which is the green house effect); however its concentration in the atmosphere mainly depends on air temperature. As there is no possibility to directly influence atmospheric water vapour concentration, the GWP-level for water vapour is not calculated.

Anyway, how do we know that concentrations of carbon dioxide and other greenhouse gases are increasing? I wrote an article about that on the Geology News Blog awhile ago. Check it out.

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