An update on the European heatwave (CBS News). This post has good coverage, including comments about causation from several prominent climate scientists.
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–Extra comment: One important detail about heatwaves that scientists tend to overlook is the actual source of the heat energy itself that is manifestly present in a substantially increased way. The sun’s energy delivery has almost no change at all from day to day or week to week in any one place, and the same can be said for the heat-trapping effect of carbon dioxide, or methane, or any other “regular” greenhouse gas. So where does all that extra heat energy come from? There is only one possible answer, and that is the extraordinary greenhouse effect provided by the one truly irregular gas, which also happens to be the most powerful of them all, water vapor.
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Water vapor is unique in several ways, starting with the very short life of any one molecule before dropping out of the sky as precipitation. Prodigious amounts of it are produced daily, of which 80% or more is lifted by evaporation from the tropical oceans. From there it heads toward the poles, carried by an assortment of wind currents at all levels of the atmosphere below the stratosphere (called the troposphere, a term not often used). As it moves, mostly along restricted pathways, it keeps raining out. Very little makes it all the way to the South Pole because adverse winds keep blocking its path. The North Pole is easier to reach, especially in the summer, because there are fewer adverse winds and more that are favorable. At any time of year there is more water vapor likely to be found in northern regions than southern as one approaches the respective poles, but in either case there are patches of more or less water vapor to be found, and those patches tend to keep moving around. But not always. Sometimes they get stuck in one place, and while a patch is stuck it is possible for not much vapor to rain out and at the same time for more vapor to move in, via still open pathways from the main region of source along with the drying out of things below.
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How powerful is water vapor as a greenhouse gas, in terms of its ability to make air warmer as it increases? There is no satisfactory textbook answer that I know of, so I will just give you my own, taken from studying the weather maps in the Climate Reanalyzer website from the University of Maine (
https://climatereanalyzer.org/wx/DailySummary/#t2anom). In the absence of any significant interference from cloud cover, if you successfully double the water vapor content of the atmosphere of any region on Earth, from any starting point, no matter how small, you should see an increase in surface air temperature of about 8C in that region. That does happen, very often indeed, and it may happen in just one day. In some regions which are relatively dry to begin with, you can get two doubles in just a day or two when moist air moves in. Cloud cover is an important caveat, and water vapor often contributes to more cloud cover as well as higher temperature. Very dark clouds can reflect away enough sunlight to offset much of the 8C temperature effect of a water vapor double, or even more in extreme situations.
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Water vapor measurement for this purpose can be taken as the equivalent of the measurement of precipitable water in the atmosphere, expressed as the weight of all the water molecules in a column of air one square meter in area, all the way to the stratosphere. The normal range of these weights over the face of the globe is great, running from highs of 60 or more kilograms per square meter in places close to the Equator to just a few hundredths of one kilogram in the driest part of interior Antarctica. The former is impossible to double because new evaporation easily rains out at that level. The latter can double many times over, and in fact air temperatures over Antarctica are known for their high volatility when moist air is able to penetrate. Everywhere else on the globe things are scaled between these extremes, with volatility always increasing toward the poles from lower and lower starting points. And of course air temperature over land is always more volatile than air over oceans because of water’s greater ability to hold and exchange heat, all day and all night.
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Water vapor is the work horse of weather change and for the same reason climate change as well, constantly engaged. CO2 is the control knob of climate change, operating slowly and methodically over decades and centuries. It creates the kind of environment in the oceans and atmosphere that allows the sources of water vapor to expand in volume of output and from there for the vapor to roam more freely toward places where it has the greatest leverage as a greenhouse gas. All of this information can be observed directly by daily study and interpretation of the above Weather Maps.
Carl