Jeff Berardelli is WFLA’s Chief Meteorologist and Climate Specialist

TAMPA, Fla. (WFLA) — How much of a difference does a warming climate make when it comes to coral? This summer is proving it’s the difference between life and death.

That’s because climate change has pushed the summer baseline temperature, from what used to be comfortable for coral, into bleaching territory just about every summer. So now, any unusual weather pattern like we are experiencing this summer can turn a modest bleaching event into a struggle for survival.

Here’s a short and simple explanation.

In the 1980s summer water temperatures in the Florida Keys typically peaked at 84 degrees Fahrenheit — a temperature at which coral could survive and thrive.

Since the 1980s, average water temperatures in the Keys area have warmed by around 3 degrees Fahrenheit (1.7 degrees Celsius) in mid-summer due to climate change. So now, maximum sea surface temperatures in the Keys during summer have warmed to an average of around 87.

That temperature is plenty warm for bleaching if the heat persists long enough. So now, almost every summer Keys coral are under the threat of bleaching. This summer, they are being annihilated by water temperatures in the low 90s.

It’s easy to see the influence of climate heating by looking at the chart below from NOAA’s Coral Reef Watch program. Temperatures have risen from 29.2 C in the mid-1980s to 30.9 C in the past few years – from 84 to 87 degrees Fahrenheit. And that doesn’t even factor in the unprecedented hot water this summer (2023 is the thick black line).

In the illustration below, the horizontal straight blue line is the water temperature in the 1980s. The horizontal straight red line is the summer temperature today. A jump of 3 degrees Fahrenheit in just four decades.

Seasonal Sea Surface Temperatures averaged across the Florida Keys. Image: NOAA Coral Reef Watch with annotation added by Jeff Berardelli

So now each summer we expect a Florida Keys sea surface temperature to average near 87 – just from a few decades of climate change. That temperature alone can cause extensive bleaching.

But in some summers, like this one, there will inevitably be unusual weather patterns. This summer we’ve had a persistent, stagnant weather pattern with light winds over South Florida. That has spiked water temperatures on the reefs another few degrees to near or above 90.

These water temperatures are a death knell for coral.

Sea Surface Temperatures in the Florida Keys August 2nd 2023. Image: WFLA

For weeks, scientists diving into the reefs have been reporting extensive bleaching and mortality. Once vibrant reefs are turning into Ghost reefs. So much so that it’s an all-hands-on-deck situation, with hundreds of coral scientists on rescue missions to pluck coral from the oceans and transport them into cooler land-based tanks.

Image: Bill Precht

Beyond just trying to keep some of the coral alive, they are banking their genes just in case they lose coral species altogether. And given the unprecedented nature of this marine heatwave, that’s a distinct possibility.

Now let’s pretend climate change did not warm the water since the 1980s. Even with the unusual weather pattern this summer, starting at the 1980s base summer water temperature of 84, we’d have Keys ocean temperatures in the upper 80s. Dangerous for sure, but not an existential threat for the entire coral reef system.

The Florida Fish and Wildlife Research Institute put out a statement on their Facebook page today. It reads, “Major bleaching events have been largely cyclical, coinciding with El Niño years. But, with the added impact of ocean warming from climate change, these events may prove to be more and more devastating over time. Between 1983-2011, there were only 5 summers with documented bleaching in Florida. But, since 2011, every year has seen some level of bleaching. Corals experience high stress from other impacts as well, including water quality issues, pollution, and disease outbreaks. With the decline in coral cover that we’ve already seen in the past few decades, our reefs can’t afford to lose more corals.”