The Scuba Weather Coral Bleaching Report Card

Corals are the canary in the coal mine of global warming. They are a litmus test of the resolve of world governments, international corporations, and people of the earth to address global climate change.

As a diver you already know that coral bleaching is in the news. Maybe you've recognized it on succesive dives over the years. So how bad is it?

Corals experience thermal stress, the main cause of bleaching, when sea surface temperatures exceed 1°C (1.8°F) above the maximum summertime mean. This stress worsens as the heat anomaly persists. Degree Heating Week (DHW) characterizes how much heat stress has accumulated in an area over the past 12 weeks (3 months) by adding up any temperature exceeding the bleaching threshold during that time period. When the DHW reaches 4°C-weeks (7.2°F-weeks), significant coral bleaching is likely, especially in more sensitive species. When the DHW exceeds 8°C-weeks (14.4°F-weeks), widespread bleaching and mortality from thermal stress may occur.

Here we present a report card on the status of the reefs you dive at. The details can be seen in the table below, but to summarize:

1. Bahamas - In 2023 the coral reefs throughout the Bahamas experienced the highest temperatures in the past forty years. With the single exception of Grand Abaco all the islands experienced temperatures that can cause widespread bleaching and mortality from heat stress. The temperatures in 2024 have lessened.
2. Caribbean - Temperatures throughout the Caribbean in 2024 were the highest recorded in 40 years, some 1.5 - 2 C higher than in 2023. At the time, 2023 was a record year. As reported by the Global Coral Reef Alliance almost all corals in the Caribbean bleached and most died in 2023. The Cayman Islands are the only bright (cold) spot, but still the temperatures were high enough to cause bleaching and mortality from heat stress.
3. Florida Keys - Temperatures in 2024 were less than the 40-year maximums experienced in 2023, the worst coral bleaching event ever recorded and widely reported in the US Media. The coral reefs are slowly recovering from the 2023 event.
4. Indian Ocean - Mauritius and the Maldives continue to hold the award for being the least affected dive sites in the world. Mauritius last experienced high temperatures in 2016 and 2019; since then the temperatures have been mild.

Yearly Peak DHW Temperatures, C

Temperatures that are in bold are the highest experienced in 40 years

Location	2023	2024
Bahamas
Andros	10.22	~3.9
Bimini	16.54	8.85
Cat Island	15.23	~6
Conception Island	15.17	~7.5
Eleuthera	16.92	8.33
Grand Abaco	7.34	<1.0
Grand Bahama	12.09	~5
Great Exuma	14.47	~6
Great Harbor Cay	11.03	~2
Long Island	14.07	~5.5
Nassau	12.67	~4
San Salvador	14.89	~6.5
Shroud Cay	13.79	5.46
Turks & Caicos
	21.29	17.19
Caribbean
Antigua	19.91	22.10
Aruba	19.34	22.17
Barbados	18.60	23.10
Belize	19.69	19.14
Bonaire	19.07	22.32
British Virgin Islands	17.90	21.36
Cayman Islands	17.71	9.79
Curacao	19.67	22.83
Dominica	19.03	20.44
Grenada	17.62	23.01
Guadeloupe	20.95	22.21
Roatan	13.31	14.68
Jamaica	22.17	17.35
Martinique	18.22	21.10
Montserrat	18.99	21.17
St. Lucia	18.45	21.79
Tobago	16.93	21.41
US Virgin Islands	17.18	21.16
Mexico
Cancun	16.44	16.58
Cozumel	16.32	14.70
South America
Galapagos	21.18	17.19
Indian Ocean
Mauritius	0.0	2.0
Maldives	1.0	6.07
North America
Florida Keys	19.55	12.65
Pacific Islands
Fiji	8.79	<1.0
Palau	1.0	9.97
Atlantic
Azores	8.27	18.13



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