You probably noticed the prolonged period of cold weather across much of the country from the end of December through the middle of January. Many of the college football bowl games held in Florida were played in less-than-ideal conditions with temperatures hovering in the 40s or 50s. Certainly not the weather one expects when they visit Florida to escape the chill in the North. Even the Florida Keys were not immune to the cool temperatures. From January 2-14, Marathon, FL recorded temperatures 8 to 23 degrees below normal each day. On January 10, the temperature never rose above 50F. I’d hate to be the hotel concierge that had to explain this to the vacationing guests.
In addition to the chill in the air, there was a chill in the surrounding water. Water warms and cools much more slowly than air, but when temperatures hover well below normal for two weeks, the shallow water surrounding the Keys can begin to cool as well. According to this article from the AP in the Miami Herald, water temperatures cooled to as low as 52F in parts of the Florida Keys National Marine Sanctuary during the recent cold spell. This change in temperature resulted in bleaching throughout the coral reef. An assessment of how much of the reef was killed is ongoing.
In the past, most bleached coral events have been associated with increasing water temperatures. In fact, it’s simply a strong deviation in temperature (usually 1 degree) from the mean that can lead to a bleaching of any coral reef. According to the article, a cold-weather event has not occurred since the 1970s in Florida.