Atlantropa, Herman Sörgel's proposal to turn vast swathes of the Mediterranean Sea into livable dry land by installing five hydroelectric dams (the main one across the Strait of Gibraltar), is an awful idea for a great MANY reasons. Ignoring all the pesky social and political problems with the concept, what's the worst possible environmental outcome of a successful Atlantropa project?
Many articles mention that the project would raise the sea level around the world with understandably disastrous consequences, but Popular Mechanics implies an even worse scenario:
The reduced weight of water over the volcanic Mediterranean sea floor would probably lead to violent eruptions and earthquakes. (Popular Mechanics March 1977, "Power from the sea: Fact or Fancy" p 193)
I can easily imagine such geologic upheaval causing tsunamis and worldwide atmospheric effects, not unlike the 1883 eruption of Krakatoa. Others have claimed it would cause a European ice age. I expect there's a myriad other factors I haven't run into yet.
What's the worst plausible outcome of a successful attempt to create Atlantropa, with regard to global physical/environmental disasters (not social or political), both short-term and over the next decades? If it matters, assume it's implemented in the late 1940s without any environmental impact oversight whatsoever.