I remember when I first read about the massive dead zone in the Gulf of Mexico. As someone who spent a lot of time on the Gulf Coast, I was particularly concerned about something that could disrupt the ecosystem there. At the time it was unclear whether this issue had existed for a long time and what the exact cause might be.
It turns out that these zones are caused by rapid growth of phytoplankton that nourished by ever-increasing amounts of sewage, pollutants, and fertilizer that enter bodies of water through runoff. This rapid growth exhausts the oxygen in the area which then leads to hypoxia--a dead zone.
What's even more disturbing is that these dead zones are growing and multiplying at a incredible rate. Where there were 150 dead zones in 2004, there are now 200. A UN report details this situation.
Most of the discussion about these dead zones is limited to the impact on humanity that they pose, such as reduced fishing output. But if there were a 200-mile wide hole in the atmosphere were all life was extinguished, wouldn't there be a bit more alarm? This seems to me to be a hugely significant signal that the ecosphere is sending us--but we appear to be mostly deaf.