Challenges to the CO2 Global Warming Hypothesis: (10) Global Warming Comes from Water Vapor, Not CO2

In something of a twist to my series on challenges to the CO₂ global warming hypothesis, this post describes a new paper that attributes modern global warming entirely to water vapor, not CO₂.

Water vapor (H₂O) is in fact the major greenhouse gas in the earth’s atmosphere and accounts for about 70% of the Earth’s natural greenhouse effect. Water droplets in clouds account for another 20%, while CO₂ contributes only a small percentage, between 4 and 8%, of the total. The natural greenhouse effect keeps the planet at a comfortable enough temperature for living organisms to survive, rather than 33 degrees Celsius (59 degrees Fahrenheit) cooler.

According to the CO₂ hypothesis, it’s the additional greenhouse effect of CO₂ and other gases from human activities that is responsible for the current warming (ignoring El Niño) of about 1.0 degrees Celsius (1.8 degrees Fahrenheit) since the preindustrial era. Because elevated CO₂ on its own causes only a tiny increase in temperature, the hypothesis postulates that the increase from CO₂ is amplified by water vapor in the atmosphere and by clouds – a positive feedback effect.

The paper’s authors, Canadian researchers H. Douglas Lightfoot and Gerald Ratzer, don’t dispute that the natural greenhouse effect exists, as do other, heretical challenges described previously in this series. But the authors ignore the postulated water vapor amplification of CO₂ greenhouse warming, and claim that increased water vapor alone accounts for today’s warmer world. It’s well known that extra water vapor is produced by the sun’s evaporation of seawater.

The basis of Lightfoot and Ratzer’s conclusion is something called the psychrometric chart, which is a rather intimidating tool used by architects and engineers in designing heating and cooling systems for buildings. The chart, illustrated below, is a mathematical model of the atmosphere’s thermodynamic properties, including heat content (enthalpy), temperature and relative humidity.

As inputs to their psychrometric model, the researchers used temperature and relative humidity measurements recorded on the 21st of the month over a 12-month period at 20 different locations: four north of the Arctic Circle, six in north mid-latitudes, three on the equator, one in the Sahara Desert, five in south mid-latitudes and one in Antarctica.

As indicated in the figure above, one output of the model from these inputs is the mass of water vapor in grams per kilogram of dry air. The corresponding mass of CO₂ per kilogram of dry air at each location was calculated from Mauna Loa CO₂ data in ppm (parts per million).

Their results revealed that the ratio of water vapor molecules to CO₂ molecules ranges from 0.3 in polar regions to 108 in the tropics. Then, in a somewhat obscure argument, Lightfoot and Ratzer compared these ratios to calculated spectra for outgoing radiation at the top of the atmosphere. Three spectra – for the Sahara Desert, the Mediterranean, and Antarctica – are shown in the next figure.

The significant dip in the Sahara Desert spectrum arises from absorption by CO₂ of outgoing radiation whose emission would otherwise cool the earth. You can see that in Antarctica, the dip is absent and replaced by a bulge. This bulge has been explained by William Happer and William van Wijngaarden as being a result of the radiation to space by greenhouse gases over wintertime Antarctica exceeding radiation by the cold ice surface.

Yet Lightfoot and Ratzer assert that the dip must be unrelated to CO₂ because their psychrometric model shows there are 0.3 to 40 molecules of water vapor per CO₂ molecule in Antarctica, compared with a much higher 84 to 108 in the tropical Sahara where the dip is substantial. Therefore, they say, the warming effect of CO₂ must be negligible.

As I see it, however, there are at least two fallacies in the researchers’ arguments, First, the psychrometric model is an inadequate representation of the earth’s climate. Although the model takes account of both convective heat and latent heat (from evaporation of H₂O) in the atmosphere, it ignores multiple feedback processes, including the all-important water vapor feedback mentioned above. Other feedbacks include the temperature/altitude (lapse rate) feedback, high- and low-cloud feedback, and the carbon cycle feedback.

A more important objection is that the assertion about water vapor causing global warming represents a circular argument.

According to Lightfoot and Ratzer’s paper, any warming above that provided by the natural greenhouse effect comes solely from the sun. On average, they correctly state, about 26% of the sun’s incoming energy goes into evaporation of water (mostly seawater) to water vapor. The psychrometric model links the increase in water vapor to a gain in temperature.

But the Clausius-Clapeyron equation tells us that warmer air holds more moisture, about 7% more for each degree Celsius of temperature rise. So an increase in temperature raises the water vapor level in the atmosphere – not the other way around. Lightfoot and Ratzer’s claim is circular reasoning.

Next: Rapid Climate Change Is Not Unique to the Present