Yesterday’s letter didn’t go as far as I had hoped but it did contain a highlight that is truly extraordinary, something every climate scientist should be acutely aware of and prepared to explain—the peculiar relationship between the hottest surface water temperature in the world today and the hottest land temperature. These two locations exist in the closest possible proximity: Southern Iraq sits directly on the upper end of the Persian Gulf. I can well remember how the two of them had this exact same temperature relationship a year ago. I also know that a little corner of Iran, right next to southern Iraq, has made unverified claims in the past of setting a world record for hottest day ever, which is not necessarily false. Iraq’s beastly summer heat has been well-known for decades, but moisture arising from the Persian Gulf is seldom mentioned as the main reason.
The Persian Gulf, by some standards, is a “large” body of open water, perhaps twice as large as all of the Great Lakes combined. Otherwise, on global water maps, it is just a speck. It is most distinguished by having a location on the very edge of the tropics combined with having a major portion that is almost fully landlocked, with a minimum of throughput currents. This is enough to explain why its surface gets so hot, about three to four degrees higher than any other large body of water. What is implied is a tremendously high rate and volume of evaporation, maybe small in area of coverage but exceptionally high in concentration. When vapors leave the surface we might assume that warm air currents will quickly carry them to higher levels of the atmosphere and spread them out, bearing concentrations that stay well above average. Some will make it up to jetstream levels and be carried to far-off places. Most will stay lower, probably condensing into light clouds over much of the area of spreading. At least that is how it looks to me in this image, where the cloud cover that appears around the Gulf is indeed very light in most places but still detectable:
Now we can get into the real meat of this whole story, because water vapor, with or without cloud condensation, and regardless of altitude, will always enter into the signal of total precipitable water (PW) density recorded over all of Earth’s locations. Here is a current map for the region:
What you can look for is an image of two strong bands of PW, one on each side of the Gulf, with strong concentrations on the north end of each band. That strong concentration at the top or the west-side band sits precisely over the nation of Iraq. I believe the greenhouse energy impact generated by this vapor and its associated light clouds fully account for the high temperatures down below, which are the same today as they were on the map in yesterday’s letter. (On the east side of the gulf there is a comparable stream of PW in the air, but the situation on the ground is entirely different due to the presence of a range of high mountains.)
The most important takeaway from these two letters is that all of the seas to the north, as listed yesterday, have something in common with the Persian Gulf. Their surfaces are rapidly growing warmer, as yesterday’s map shows, and they are already warm enough, seasonally, to be generating extraordinary volumes of evaporation and water vapor. The trend that is now in place has almost surely reached a level of energy production—of the greenhouse type—having the kind of strength needed to create the extreme weather impacts we are reading about. There is a frightening story just today about the wildfires in Turkey: https://www.npr.org/2021/08/02/1024010396/wildfires-turkish-vacation-towns-evacuations-resorts. Intense heatwaves and flooding are also common to many parts of Europe. Their intensity is difficult to explain without taking into account the high volumes of PW emerging from the many local seas, which are only now reaching peak surface temperatures for the season. What happens in the next few weeks will be a thing of interest.
I am more and more convinced that evaporation rates accelerate when water temperatures have risen to these current levels, and I have doubts about claims that set limitations on how much concentrated PW the upper atmosphere can hold in situations like this.
Carl