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Contrary to what is said in the popular media, water (H2O) is the most important greenhouse gas in Earth's atmosphere, not the small amount of demonized CO2. But aside from acting as a greenhouse gas, water vapor plays an active role in shaping global atmospheric circulation and thus Earth's climate. Water does this by undergoing state changes—from liquid to vapor and back again—allowing water vapor to carry significant amounts of latent heat from the warm equatorial regions toward the poles. The importance of this heat transfer mechanism in climate regulation is poorly understood but new data have begun to show the impact is major. One thing is certain, most widely used climate models do not correctly account for the complex dynamics of water vapor.

In a detailed study of the mechanisms and effects of water vapor, to be published in Reviews of Geophysics, Tapio Schneider and Xavier Levine of the California Institute of Technology, and Paul A. O’Gorman of the Massachusetts Institute of Technology, have expanded our knowledge of water's role in climate regulation while showing just how poorly understood the Earth climate system really is. Our level of collective ignorance is explained at the start of the paper: Although the mechanisms are not well understood, it is widely appreciated that heating and cooling of air through phase changes of water are integral to moist convection and dynamics in the equatorial region. But that water vapor plays an active and important role in dynamics globally is less widely appreciated, and how it does so is only beginning to be investigated.

As we reported in The Resilient Earth, the role of water as a greenhouse gas was first methodically investigated by Irish scientist John Tyndall in the mid 19th century. An accomplished mountaineer, Tyndall was fascinated by Louis Agassiz's daring proposal of ice ages, in which glaciers once covered enormous parts of the world. Looking for mechanisms to explain climate change, he established the absorptive power of clear aqueous vapour—water vapor. To investigate this phenomenon he constructed the first spectrophotometer, shown below.


Tyndall's spectrophotometer apparatus.

Tyndall's experiments showed that, in addition to water vapor, a number of other atmospheric gases can absorb heat energy. Correctly identifying water vapor as the strongest absorber of radiant energy, Tyndall marveled at the ability of transparent, colorless gas to trap heat. He suggested this phenomenon was linked to changes in climate—changes that caused glaciers to advance and retreat. In his own words, he stressed the importance of water vapor in the atmosphere.

Aqueous vapour is a blanket more necessary to the vegetable life of England than clothing is to man. Remove for a single summer-night the aqueous vapour from the air which overspreads this country, and you would assuredly destroy every plant capable of being destroyed by a freezing temperature. The warmth of our fields and gardens would pour itself unrequited into space, and the sun would rise upon an island held fast in the iron grip of frost.

When it comes to climate change carbon dioxide is pretty much a one trick pony. It acts as a greenhouse gas, delaying the re-radiation of energy from the Sun back into space and thus raising the average temperature of the atmosphere. It can have secondary effects on plant cover—CO2 is basically plant food—but it does not contributes directly to climate regulation in any other significant way. H2O on the other hand, is not just our atmosphere's major greenhouse gas, it is a multi-talented climate regulator.

When scientists talk about heat in the atmosphere they refer to two major types: sensible heat and latent heat. Sensible heat is thermal energy that causes dry bulb temperature changes in the air. Dry bulb here means that the change in temperature occurs without a change in water vapor content—no state change is involved. In contrast, latent heat requires a state change in a substance. Ice turning into water, or water turning into water vapor are state changes that require the input of energy. The energy becomes latent heat energy during the state change and can be released by reversing the state change. In other words by condensing water vapor back into liquid or freezing liquid water into ice. If you have ever boiled away a pot of water to make steam it should be obvious that water changing state can absorb a lot of thermal energy.


Latent heat potential for water state changes.

The input of energy required by a change of state from liquid to vapor at constant temperature is called the latent heat of vaporization. At normal atmospheric pressure this is 2257 kilo-Joules/kg for water (970.4 Btu/lb for the metric challenged). Energy from the Sun evaporates a lot of water from Earth's oceans, particularly from the tropical zones around the equator. Water vapor, being lighter than air, tends to rise from the surface and is then carried along by currents in the atmosphere. As the water vapor is transported toward the poles it carries with it the latent heat of its state change from liquid to gas. This latent heat is released when atmospheric water vapor condenses and more captured by the cooling of air through evaporation or sublimation of condensate. Both affect atmospheric circulation.

As water vapor is transported by atmospheric circulation it also affects circulation patterns. This in turn impacts atmospheric stability and storm formation. As the study's authors explain: “We discuss how latent heat release is implicated in such circulation changes, particularly through its effect on the atmospheric static stability, and we illustrate the circulation changes through simulations with an idealized general circulation model. This allows us to explore a continuum of climates, constrain macroscopic laws governing this climatic continuum, and place past and possible future climate changes in a broader context.”



Focusing on water vapor dynamics—the study of the dynamic effects of heating and cooling of air through phase changes of water—the study emphasis large scales, from extratropical storms (~1000km) to the planetary scale of the Hadley circulation. Again quoting the researchers, “This allows us to examine critically, and ultimately to reject, some widely held beliefs, such as that the Hadley circulation would generally become weaker as the climate warms, or that extratropical storms would generally be stronger than they are today in a climate like that of the LGM with larger pole-equator surface temperature contrasts.” Here LGM stands for Last Glacial Maximum, which occurred around 20,000 years ago.

The Hadley circulation pattern dominates the tropical atmosphere. Named after George Hadley, who first described it as an explanation for the trade winds, these circulation cells consists of rising motion near the equator, poleward flow 10-15 kilometers above the surface, descending motion in the subtropics, and equatorward flow near the surface. This circulation is intimately related to the trade winds, tropical rainbelts, subtropical deserts and the jet streams. Hadley circulation is so important to Earth's climate system that entire scientific conferences are held to study it.


Atmospheric circulation patterns showing the Hadley cells. NASA.

This long and fascinating paper (22 pages) has much more to say about water vapor, latent heat and changes in the Hadley circulation. Other topics are touched on as well, far to many to cover in a single blog post. One topic that I found particularly interesting was in section 4, regarding extratropical circulations, which the rest of this post will concentrate on.

While water vapor's role in tropical dynamics is fairly well known, its role in extratropical dynamics is less clear. “The unclear role of water vapor in extratropical dynamics in the present climate and its changed importance in colder or warmer climates are principal challenges in understanding extratropical circulations and their response to climate changes,” state the authors. In the present climate, about half of the total atmospheric energy flux in mid-latitudes can be attributed to latent heat release in extratropical circulations. Clearly water vapor must play a significant role in the extratropical atmosphere of the mid-latitudes. Interestingly, not all of the results of this study are as simple as previously assumed.

One unambiguous result was that extratropical storm tracks generally shifted toward the poles in simulations of global warming scenarios. A number of possible mechanisms for this effect are discussed but in the end “there currently is no comprehensive theory for the position of storm tracks.” As an example of how complex and confusing the effects of water vapor have on a changing climate consider the report's findings regarding storminess:

The extratropical transient eddy kinetic energy, a measure of storminess, scales with the dry mean available potential energy. Near the present climate, both energies decrease as the climate warms, because meridional potential temperature gradients decrease and the static stability increases as the poleward and upward transport of latent heat strengthens. In colder climates, however, both energies can also decrease as the climate cools.

Because water vapor has a big impact on atmospheric circulation it also impacts tropical storm formation. As stated by Hye-Mi Kim et al.: “Strengthening or weakening of the vertical wind shear occurs largely through changes in the upper-level westerly flow and is thought to be a major factor inhibiting or enhancing the formation and intensification of cyclones” (see “Impact of Shifting Patterns of Pacific Ocean Warming on North Atlantic Tropical Cyclones”). In particular, this has been tied to vertical wind shear, which can inhibit the formation of tropical cyclones. Now it seems that a warming climate doesn't result in more storms outside of the tropics either. But in an example of how counter intuitive and complex atmospheric circulation can be, a colder climate can also reduce the amount of storminess. As with the erroneous predictions of increased frequency and intensity for tropical storms by global warming proponents, simple blanket statements about how changes in temperature affects our environment are most often wrong.

In the end this paper raises more questions than it answers, something many good scientific investigations do. At the end of the paper the authors pose the question, “what controls the static stability of the subtropical and extratropical atmosphere?” After listing five major unsettled questions regarding atmospheric circulation and the dynamic effects of water vapor, the study's authors summarize their findings: “The lack of a theory for the subtropical and extratropical static stability runs through several of the open questions. Devising a theory that is general enough to be applicable to relatively dry and moist atmospheres remains as one of the central challenges in understanding the global circulation of the atmosphere and climate changes.”


Artistic image of water vapor using data from the Aqua satellite's Atmospheric Infrared Sounder. NASA.

Without such a theory it is impossible to predict changes in atmospheric circulation, it is impossible to predict changes in the hydrological cycle, it is impossible to predict storm frequencies, intensities and tracks. Future climate cannot be predicted without a theory explaining how climate works, yet the IPCC has confidently made predictions regarding changes in storms, precipitation and climate for decades, even centuries into the future.

If something as seemingly simple as water vapor can have such complex and bewildering impacts on Earth's climate why does the IPCC and the climate crisis crowd continue to insist that all fault lies with CO2? It could be that even they realize that blaming global warming on water vapor would give them no political leverage. After all, 70% of our planet's surface is covered with water and not even the most wild-eyed geoengineering proponent would propose we attempt to control the amount of water vapor in the atmosphere.

What does this have to say about all of those general circulation models (GCM) used by the IPCC to divine the future of Earth's climate? It means they can't accurately simulate our planet's climate engine because they don't know how the atmosphere works. If they don't know how climate works today, how can they tell us what the climate will be like 100 years in the future? The predictors of future climate disaster may as well be using tarot cards.

Be safe, enjoy the interglacial and stay skeptical.