The Hottest of Days on Modern Earth

For a long time, I had known superficially about the Messinian salinity crisis, an episode starting around 5-1/2 million years ago–merely a blip in geologic time, and quite modern actually. The Strait of Gibraltar closed because of geologic processes, and the Mediterranean Sea mostly evaporated. My recent trip to Spain for a meteorological conference brought up some side discussion of that astounding event, which re-ignited my geological and meteorological interests in it again.

Think about this: the Mediterranean–a relatively narrow but deep oceanic basin–almost dried up completely, leaving (at best) large, river-fed saline lakes in what is now the deep abyss. What a sight that would have been–if you could travel back in a time machine and survive being there for more than a few minutes! This was a valley of great depth and size with conditions certainly unlike anything known to human experience. What kind of conditions?

The basin’s deepest parts are over 16,000 ft below current sea level in the eastern Mediterranean’s abyss, which

  1. commonly extends below 13,000 ft,
  2. is and was surrounded by hotter and drier land than the western basin and
  3. was cut off from the western basin at peak evaporation by the ridge between Sicily and Tunisia.

How far down did it dry up? That’s unclear, but the sediment-buried Nile Canyon extends about 8,000 feet below Cairo, which is near sea level. This means the sea level had to be at least that much lower than today, not accounting for further downhill drainage northward across the continental slope. [That Nile Canyon dwarfs the Grand Canyon’s modern depth and extent, but is buried under river sediment eroded from East Africa.] The late Miocene Nile’s delta fan is found over 4 km (13,000 ft) below present-day sea level in the eastern Mediterranean basin. That’s the “Messinian sea level” I’ll use here, though it may be a conservatively high water-level estimate. Actual water level in the eastern basin could have been even lower at peak evaporation, given the presence of cross-bedded, eolian (wind-deposited) dust in drill cores from the abyssal plain.

How hot could it get down there in the big hole? The question bugged me on the flights back; but at least it’s straightforward for any atmospheric scientist to estimate (below). The answer: around 170 degrees F, perhaps more–certainly too hot for animals and green plants to tolerate. The air pressure also was more than 1.5 times anything now known in the atmosphere. At some point on your time-machine walk from the dried-up Libyan coast into the basin, all visible life would vanish, the temperature would get dangerously hot, and bare sand, rocks and dried mud would bake under a blistering sun as far as the eye could see on that slope downward to the north. Death Valley? A tiny dimple–child’s play by comparison! You wouldn’t make it alive anywhere near the lakes at the bottom without an air-conditioned decompression suit or enclosed, cooled, sealed vehicle.

Midsummer day temperatures of 35 degrees C (95 degrees F) are common on the southeastern Mediterranean coast at today’s sea level, and often hotter. Still, I made a reasonable assumption that such temperatures would occur, at sea level, on some high summer days during the Messinian salinity crisis, even if an ice age had been underway with glaciers in northern Europe. With most (if not all) of the sea gone, the climate would have been even drier, an idea supported by the presence of evaporites from sedimentary deposits that were located well below present sea levels during that time, and subsequently lifted tectonically. Anhydrite deposits (the precipitation of which requires water temps in the mid-90s F at pressures at or within a few thousand feet below current sea levels) also suggest a very hot and dry condition.

The concept is nothing revolutionary or even-ground-breaking. It’s basic atmospheric thermodynamics. The dry-adiabatic lapse rate (change in temperature of an unsaturated air parcel with height) is 9.8 degrees C per km. That assumes the air isn’t being warmed or cooled by anything but sinking or lifting. We simply take that pretty common summer day’s temperature on the Libyan coast down a dry-adiabatic line 4 km (about 40 degrees C warming). That gives us a temperature of about 75 degrees C at 2.5 miles below current sea level, or about 167 degrees F.

A really hot summer day of 45 degrees C (113 degrees F) sometimes happens there on the modern Libyan coast. That yields an air temperature near 85 degrees C (185 degrees F) along the presumed Messinian-era waterfront! If the Mediterranean dried up completely–river water evaporating before even getting to the deepest parts of the abyss–then temperatures may have crowned 90 degrees C (194 degrees F) on the very worst few days in that basin. That’s still under boiling point for current sea level, as well as for the higher air pressure (1600-1700 mb, roughly, compared to 1013 at current sea level)–but unsurvivable for more than a minute or so without protection.

We only can speculate wildly on what the humidity (or dew point) could have been under such conditions. Nothing very close to that situation exists for us to observe today. The actual values likely would depend on whether any brine lakes lay in the abyss to supply water at the driest stages, though before evaporation would have been complete, some very high dew point values (over 90 degrees F) likely were generated beneath a boundary-layer inversion at great depths. Again, this is just an educated guess. Subsident air from Africa likely would have spread an elevated mixed layer out over the deep basin and set up a cap, comparable to (but deeper and hotter than) than what moves off the Rockies and Mexican plateau, over the central and southern Great Plains moist sector, in North American springtimes.

Wintertime at that latitude (30-40 degrees N), with its short days, low sun angles and efficient nocturnal surface heat loss from the barren desert plain, could have offered a respite. There perhaps was enough of one (with basin-bottom daytime temperatures “only” 30-40 degrees C) to survive with gradual decompression on the way out. Since every wind direction is downslope (warming) in that scenario, however, only a calm winter’s night might ensure “cool” surface temperatures.

One thing is safe to say: the summertime Mediterranean basin during the Messinian salinity crisis offered intolerably hot air with high pressures, the likes of which we don’t see, and which can’t happen anywhere on Earth even under the most extreme climate-warming predictions. That is, until the northward drift of the African plate again pinches off the Strait of Gibraltar and the sea starts drying up again. By that time humans either will be gone or will have found a way to keep the water flowing, either through a blasted-open Strait or the Suez Canal area. Still, it’s amazing to imagine! Another safe bet: civilization would be a lot different today if the Atlantic hadn’t spilled back through a slightly reopened gap between Spain and Morocco.



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