Another reason global warming has not been too bad yet is because the ocean absorbs most of the earth’s excess heat. But oceans are warming due to greenhouse gas emissions. According to the National Oceanic and Atmospheric Administration, half of the increase in ocean heat content since 1865 has occurred over the past two decades. Warmer water holds less oxygen, but the respiration rate of animals (except for marine mammals) increases with temperature, so they need more oxygen at the same time that less is available. A warmer ocean has less turnover (vertical water movements), which normally brings nutrient-rich water up from deep water to the plankton that photosynthesize near the surface. With fewer nutrients, they photosynthesize less and animals can’t get enough food.
In the Florida Keys, the U.S. Virgin Islands and Maui, coral reef degradation has caused sea floor depths to increase as sand and other sea floor materials have eroded over the past few decades, the USGS study found. In the waters around Maui, the sea floor losses amounted to 81 million cubic meters of sand, rock and other material – about what it would take to fill up the Empire State Building 81 times, the researchers calculated.
We now have a 2017 event, which is not quite as bad as 2016, but certainly worse than the first two events that we studied [in 1998 and 2002]. That is significant because it postpones any hope of recovery. The current bleaching occupies a different geographical footprint from last year, which is bad news because it means between last year and this year a much greater extent of the Great Barrier Reef has now been damaged. In 2017, the hot water was in the middle of the Great Barrier Reef, the central section; last year it was in the north.
“To lose coral reefs is to fundamentally undermine the health of a very large proportion of the human race,” said Ruth Gates, director of the Hawaii Institute of Marine Biology
The damage on the barrier reef is part of a global mass bleaching event that has hit corals hard in many places including Hawai, Fiji and New Caledonia. It is only the third global event in recorded history, with the other two occurring in 1998 and 2010.