The seven billion humans on planet Earth consume 49,400 trillion cubic feet (1.4 quadrillion liters) of oxygen every year, enough to fill seven million Hindenburgs. About half of this oxygen comes from the ocean. Yet the recently concluded, decade-long Census of Marine Life reports that phytoplankton have been decreasing by approximately 1 percent every year since 1900. These microscopic plants produce most of the oxygen that comes from the sea. The exact causes of this decline are unknown, but rising temperatures are suspect. Ocean oxygen production is now likely about one-third of historical levels. In 2050, just as the world’s population is projected to hit a peak of nine billion people, phytoplankton will be down to 22 percent of historical averages.

The Oceans Future project, one of the Census’s 14 field assessments, suggests that 90 percent of the big fish are...

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