(Presented to the Board of Supervisors December 6, 2016)
“Events as severe as the 1998 event, the worst on record, are likely to become commonplace within 20 years.” – Ove Hoegh-Guldberg, 1999 
In Figure 1, I’ve updated the global temperature anomaly using the NASA/GISS data set through September, 2016. It remains a stark reminder that global warming did not stop in 1998 as many pundits insisted. Misinformed by such pronouncements, it is difficult for us to appreciate the impact of a few degrees of global warming. Many of the anticipated consequences which are discussed by scientists such as sea level rise occur too gradually to be observed and appreciated. Though it means more nuisance flooding in coastal communities such as Norfolk and more flooding during extreme storms such as super storm Sandy, there are no daily reminders for most of us. Mass coral mortality however has been dramatic, well reported and consequential. Coral reefs are the rain forests of the oceans and harbor about 26% of all marine species. And coral reefs are dying before our eyes as shown in Figure 2.
A mutualistic symbiotic relationship exists between corals and small marine algae which live inside their tissue  shown in Figure 3. The algae use photosynthesis to capture sun light to create organic molecules from carbon dioxide and water as well as trace minerals provided by the host corals. In turn the algae provide most of the energy corals need to grow. When temperatures get too hot, corals expel the algae. Without the algae, the corals turn white which is why this process is called bleaching. The coral can survive in this state for a few weeks. But, if temperatures do not decrease they will die from starvation and disease.
There were no observed or recorded mass coral bleaching events until the 1980s . The first multi-year event on record occurred during 1982-83. Since then we have experienced repeated regional mass bleaching events and in 1997-1998 we experienced the first global coral bleaching event which prompted coral biologist Ove Hoegh-Guldberg to forecast in 1999 that these events would become commonplace and they have. I’ve therefore added a new feature to the temperature anomaly chart I produce each month. The dotted orange line in Figure 1 indicates the surface temperature threshold corresponding to the start of mass bleaching events, about 1 degree Fahrenheit (0.6 Kelvin) above pre-industrial climate.
Coral reef bleaching is not only dependent on surface temperature but also on other stresses such as ocean acidification, pollution, particularly agricultural runoff, disease, invasive species and over fishing. However only atmospheric carbon and its consequences, increased warming and ocean acidification, are global. Since all of these stresses are correlated human activity, it isn’t surprising that coral mortality is correlated to surface temperature. Human activities emit about 11 billion tons of carbon dioxide per year. If this rate were ten to hundred times slower as would be the case if the sources were natural, then the temperature would rise slowly and the oceans would not become acidic because at that slower rate the ocean could chemically buffer the carbon dioxide. In the past corals and other species have easily adapt to the slowly rising temperature. At the rate of human emissions, the oceans become too acidic and the temperature rises too quickly for species to adapt.
I’ve also shown a dotted red line corresponding to a surface temperature of 1.6 F or 0.9 K, the threshold consistent with more server global mass bleaching such as the one occurring during the El Nino of 1997-98 . The latter event killed off 16% of all corals . During the first decade of the twenty-first century the Earth was in a continuous state of mass coral bleaching. In 2006 a local mass bleaching event occurred in the Keppel Islands on the southern end of the Great Barrier Reef which resulted in 30 to 40% coral mortality. In 2010 the second global mass bleaching event occurred and in 2014, a third global bleaching event began, which is still ongoing . Biogeochemist Andrea Grottoli told me that such events were not expected to happen before 2030.
In a 2015 presentation to the Loudoun County Board of Supervisors I had forecast a La Nina event occurring in 2017 and 2018 during which we might have expected corals to get some relief as the surface temperature cooled down. However, NOAA is now expecting the La Nina to be weak  and therefore temperature may remain closer to the climate trend line which as can be seen in Figure 1 is now above the global beaching threshold given by the dotted red line in the figure.
For anyone looking for a consequence of human-caused climate change, corals may be the canaries in our global coal mine. Coral reefs are the rain forests of the ocean comprising much of the ocean’s marine diversity. If they go extinct we will have initiated a fairly major marine mass extinction event.
 Bob Henson, Weather Underground blog, 3:30 PM GMT on March 28, 2016. Longest Coral Bleaching Event on Record Continues to Hammer Reefs. https://www.wunderground.com/blog/JeffMasters/archive.html?year=2016&month=03
 Bob Henson, 5:06 PM GMT on November 11, 2016, Weak La Niña Expected to Persist into 2017, https://www.wunderground.com/blog/JeffMasters/weak-la-nia-expected-to-persist-into-2017