In the tropical Caribbean Sea region, it’s typically warm and humid on land but rarely endlessly hot — relatively stable water temperatures promote conditions that don’t often change drastically day-to-day or even month-to-month.

But that climate norm has been turned on its head over the past two years, with record-breaking heat that ramped up in the spring of 2023 and has continued unabated since; conditions fueled by human-caused climate change. Many places including Puerto Rico, the Virgin Islands and others in the archipelago set heat records in 2023 only to see them overturned last year.

An unusually powerful and resilient marine heat wave contributed to keeping the region toasty. But it wasn’t the only factor.

While the surge in warmth can be partly explained by a typical global response to last winter’s El Niño, and should thus be waning as patterns change, it has persisted in an extreme state for much longer than expected. And it is so far unclear if and when temperatures in the Caribbean will finally edge back downward, although trends over time suggest more warmth like that seen recently, regardless of El Niño conditions.

It seems unlikely that the cold air invading much of North America over the next several weeks will have major impact on the ongoing situation as a warm-weather pattern controls much of the Caribbean and tropical Atlantic. However, cooler-than-typical weather is anticipated in the north and west portion of the region, causing at least a localized change of pace from the recent onslaught.

Atypical heat began in 2023

With the onset of a strong El Niño weather pattern heading into late 2023, ocean and land temperatures climbed across much of the Earth.

While it is typical to see fluctuations in a prevailing temperature pattern year-to-year, and there is often general warming during El Niño, climate change’s role in fueling record heat was hard to ignore. The cumulative number of days with above-average temperatures in the water and on land reached previously unseen levels, as shown in the analysis of sea surface temperatures below.

By May 2023, records were beginning to fall on a daily basis across the basin. Then, it kept going and going.

John Morales, an operational meteorologist who has been forecasting for Puerto Rico for 40 years, said he had “never seen a persistent streak of record-setting heat like what’s been happening over the last two years.”

“The daytime highs are debilitating, given the humidity that makes for very high heat-index values,” he said in an email. “And overnight minimums remain at stifling levels, which can be particularly dangerous.”

But that notable heat in 2023 was just a preview.

Heat roared through 2024

In San Juan, Puerto Rico, annual temperature averages range from 83 to 89 degrees and a low temperature range of 72 to 78. A relatively small change can have astounding impact.

In recent weeks, it felt more like summer than a winter December. This includes a record hot December night with 79 degrees in San Juan.

The southern Caribbean island of Curaçao broke its December record for high temperatures 10 times in the first 20 days of the month, according to climate historian Maximiliano Herrera. It was 93 degrees there shortly before Christmas.

A similar pattern was seen in November, October and September.

Over the summer, numerous record warm lows (think mid-80s) and record highs (mid-90s) were set. The Dominican Republic hit 107 in August, which is the highest reliable temperature on record in the Caribbean, Herrera said. San Juan fell to 83 on multiple occasions, tying for its warmest low on record.

Morales, the operational meteorologist, is based in Florida but frequently travels to Puerto Rico and the Dominican Republic. He said that last year, locals had a memorable way of describing the heat they felt.

“[P]eople are indeed very much aware of the heat and how unusual it is,” he said. In a region used to warmth and humidity, Morales said he noticed residents frequently post on social media, wondering about the persistent barrage, something that he had not previously experienced.

“There is a sense that this may be a permanent shift and a symptom of climate change,” he added.

With 2024 now over, 138 record warm lows and 37 record highs fell in San Juan, according to NOAA data. This came on top of 73 record warm lows and 29 record warm highs in 2023. The sheer number of records over such a short period of time is quite significant.

What is driving the warmth?

Some of the elements driving the pattern of warmth include a persistent marine heat wave, extended runs of higher-than-normal atmospheric pressure across the tropical Atlantic Ocean and bouts of sunshine and light wind that keep water temperatures well above average.

A marine heat wave — an expansive blob of unusual oceanic heat — is typically defined as seas’ surface temperatures being in the top 10 percent of what has been recorded across a wide area for a prolonged period. This has been the story of 2024 in and around the Caribbean Sea.

Because ocean water covers more than 70 percent of Earth, what happens there is critically important to temperatures and humidity on land, with coastal heat waves sometimes fueling terrestrial ones.

Weather systems can sometimes linger, producing persistent sunny and wind-free days, and providing the kinds of conditions that drive marine heat. These systems can sometimes straddle the land and the ocean, leading to a connected heat wave that transports higher-than-usual humidity inland from the waters.

In addition to the weaker-than-normal winds and the unrelenting marine heat wave, there’s, of course, the human-driven increase in global temperatures.

While temperature swings in the tropics are not as intense as high latitudes in a warming world, the available space for a livable climate is smaller. Some studies have found that increased atmospheric high pressure is likely in the tropics and subtropics in a warmer world, a feedback mechanism that could initiate additional warming by stifling clouds and storms in addition to shifting storm tracks.

Will the warmth continue?

While history suggests the heat should wane in 2025, even before the new year, there was less-than-expected waning from peak temperatures. That could throw a wrench in scientists’ expectations of what may happen based on precedent.

Leading climate model guidance suggests warmer-than-average seas will probably continue to be a factor for the region until at least May, or as far as the model looks into the future.

Pointing to Climate Central’s Climate Shift Index, Morales highlighted sea surface temperatures more than 2 degrees Fahrenheit above normal in the region, and noted they are made 1,000 times more likely due to global warming.

“While we’ve seen some pullback around the world from the record hot ocean temperatures of the past two years, the Caribbean hasn’t budged,” he said.

Based on long-term trends, he added, “this won’t end anytime soon and is likely to get worse.”