Caribbean Rainforest Stations Reveal a Warming Climate of Extremes

Deep in the lowland rainforests of Costa Rica and Puerto Rico, two of the tropics’ most storied research stations have been quietly recording the weather for decades. Now, a team of scientists has mined those archives to reveal a troubling pattern: the climate of these protected Caribbean forests is not simply getting warmer, it is becoming dramatically more extreme. The study, published in PLOS Climate, analyzed long-term air and water temperature, precipitation, and stream water level data from La Selva Research Station in Costa Rica and El Verde Field Station in Puerto Rico, two sites that have hosted continuous ecological monitoring since the mid-twentieth century.

The research team, led by Alonso Ramírez and including Marcelo Ardón, Pavel García, Nicholas S. Marzolf, and David Genereux, set out to fill a persistent gap in climate science. While global temperature trends are well documented, site-level hydroclimatic responses, the combined behavior of temperature, rainfall, and streamflow at individual locations, remain comparatively under-documented in many tropical regions. This matters because tropical forests are among the most biodiverse ecosystems on Earth, and the organisms living in them are often finely tuned to narrow climatic conditions. Understanding how the actual climate experienced by a forest is changing requires exactly the kind of long, continuous, local records that these two stations have accumulated.

The baseline numbers themselves tell a story of two remarkably wet, warm places. At La Selva, mean daily air temperature averaged 25.1 degrees Celsius over the period from 1992 to 2024, with daily values ranging from 19.5 to 29.6 degrees. At El Verde, the mean was 23.6 degrees Celsius from 2000 to 2024, spanning 17.7 to 28.2 degrees. Rainfall is abundant at both sites: mean annual precipitation reaches 4166 millimeters at La Selva based on records from 1983 to 2024, and 3635.8 millimeters at El Verde over the longer 1975 to 2024 record. These are forests where water is rarely scarce on an annual basis, which makes the emerging patterns of drying and flooding all the more significant.

The most consistent signal across both sites is a steady warming trend of 0.29 degrees Celsius per decade. That figure may sound modest, but compounded over the four decades of record it represents a meaningful shift in the thermal environment of these forests. More striking is what has happened to temperature extremes. The researchers defined extreme temperature events as days exceeding the 95th, 97th, and 99th percentiles of the historical distribution, and they found a significant increase in the frequency of such events at both locations. Using the 95th percentile threshold and a fitted generalized linear model, they estimated that both sites experienced roughly 8 extreme temperature events per year at the beginning of their records, climbing to approximately 35 events per year by 2024. In other words, days that once would have counted as unusually hot now occur more than four times as often.

Not every climate driver acted identically at the two sites, however. The El Niño Southern Oscillation, the great Pacific-driven seesaw that shapes weather across the tropics, left its fingerprint on air temperature at El Verde but not at La Selva. During La Niña phases, daily temperatures at the Puerto Rican site were higher, with an estimated effect of 0.02 degrees Celsius per day and a 95 percent confidence interval of 0.002 to 0.042. La Selva, by contrast, showed no detectable ENSO influence on its air temperature. This contrast highlights how even two Caribbean forests separated by the width of a sea can respond differently to the same global climate oscillations, a reminder that regional and local factors mediate how planetary-scale phenomena are expressed on the ground.

Precipitation extremes told an even more site-specific story. At La Selva, the trend ran wet: heavy rainfall events, defined as exceedances of the 99th percentile, rose from roughly 1 event per year in the early 1980s to 6 per year in 2024. The Costa Rican forest is now being hit by intense downpours far more often than it was four decades ago. El Verde showed no significant increase in heavy-precipitation events, but it did exhibit a marked increase in dry extremes. The number of dry days, defined as days receiving less than 1.4 millimeters of rain, increased from about 152 days per year to 170 days per year. A forest that receives well over three and a half meters of rain annually is nonetheless spending nearly three additional weeks each year without meaningful rainfall.

The streams flowing through these forests recorded their own version of the story. Stream water temperature increased moderately but significantly at both sites, rising by 0.2 degrees Celsius at La Selva and by 0.9 degrees Celsius at El Verde. Because stream temperature governs the metabolism of aquatic insects, fish, and the microbial communities that process organic matter, even sub-degree warming can ripple through food webs. Aquatic insects with narrow thermal tolerances, common in pristine tropical headwater streams, may find their habitat shrinking as waters warm, with consequences that propagate to the riparian birds and lizards that feed on their adult stages.

Stream water levels added another layer of concern. Both locations showed an increasing tendency toward high-water conditions, although the statistical support was marginal: p equals 0.08 for the 99th percentile at La Selva and for the 95th percentile at El Verde. Despite that borderline significance, the estimated durations spent above these extreme thresholds roughly doubled over the study period. Doubling the time that streams spend at flood stage, even if the evidence is not yet conclusive, suggests that the hydrology of these forested catchments is shifting toward flashier, more volatile behavior, consistent with the intensification of the hydrological cycle expected under a warming climate.

Taken together, the findings paint a picture of a climate that is changing not just in its averages but in its character. The two protected sites, buffered from direct deforestation and land-use pressure, are experiencing an increasing frequency of extreme events, both wet and dry, alongside a shared decadal warming signal. The divergence between the sites, with La Selva trending toward wetter extremes and El Verde toward drier ones, suggests distinct regional changes in climate over the Caribbean basin rather than a uniform shift. For conservation and management, this is a crucial insight: strategies designed around average conditions may fail to protect ecosystems whose greatest stress now comes from the tails of the distribution, the hottest days, the driest stretches, and the heaviest downpours.

The study also underscores the irreplaceable value of long-term ecological monitoring. Without decades of continuous records from automatic and manual sensors at places like La Selva and El Verde, the slow tightening grip of climate extremes on tropical forests would remain invisible, hidden beneath the noise of year-to-year variability. As climate change accelerates, these stations serve as sentinels, and their data as an early warning system for the world’s tropical forests. The message from the Caribbean is clear: even in the heart of protected rainforest, the climate of the past is no longer a reliable guide to the future, and the extremes of the coming decades are already arriving ahead of schedule.

Subject of Research: Long-term hydroclimatic trends and extreme events in Caribbean tropical forest research stations

Article Title: Shifting climate in Caribbean tropical forests: Long-term trends at La Selva (Costa Rica) and El Verde (Puerto Rico)

Article References: Ramírez, A., Ardón, M., García, P., Marzolf, N. S., & Genereux, D. (2026). Shifting climate in Caribbean tropical forests: Long-term trends at La Selva (Costa Rica) and El Verde (Puerto Rico). PLOS Climate, 5(10), e0001044. https://doi.org/10.1371/journal.pclm.0001044

Image Credits: AI Generated

DOI: 10.1371/journal.pclm.0001044

Keywords: climate change, tropical forests, La Selva, El Verde, Puerto Rico, Costa Rica, ENSO, temperature extremes, precipitation, stream temperature, hydrology, PLOS Climate