Climate change is making the tropics hotter

A new study finds that tropical areas often thought to be safe from the effects of climate change are actually experiencing significant changes in rainfall and extreme temperature events.

Researchers studied climate events at the La Selva research station in Costa Rica and the El Verde field station in Puerto Rico. They found that, while average temperatures were not changing much over time, the two locations did experience significantly more days of extreme heat in 2024 compared to long-term data stretching back to 1992. 

“In La Selva prior to 2024, you had an average of eight hot days – days where the temperature exceeded the 95th percentile of temperatures at that site – per year,” said Alonso Ramirez, lead author of the study and a professor in the Department of Applied Ecology at North Carolina State University. “Now that number is up to 30. These spikes in temperature are becoming more severe and more common.”

Notably, the two locations also experienced contrasting precipitation patterns, despite being relatively close to each other. La Selva experienced more heavy-rain events and fewer zero-rain days, while El Verde experienced more dry days. At La Selva, instances where rainfall exceeded the 99th percentile rose from around one event per year between 1983 and 1985 to six per year in 2024. El Verde showed no significant increase in heavy-precipitation events, but saw an increase in dry extremes from 152 to 170 days per year.

“That was surprising, that one is getting wetter and one is getting drier,” said Marcelo Ardon, co-author of the paper and a professor in the Department of Forestry and Environmental Resources and the Center for Geospatial Analytics at NC State. “Given how close they are to each other, we expected to find similar trends across the two sites. This shows how climate change is happening right now, and we can demonstrate that with data that doesn’t stretch that far back.”

These findings demonstrate how climate change is affecting even areas typically thought of as having stable climates.

“A lot of people tend to think of the tropics as being stable and unchanging, especially in protected areas like the ones we’re studying,” said Ramirez. “Conventional wisdom would say that these places should not show much of a change from anthropogenic impacts. Instead, these two sites that are hotspots for research are showing clear changes in climate and in the environment, which then affects the functioning of the ecosystem.”

The paper, “Shifting climate in Caribbean tropical forests: Long-term trends at La Selva (Costa Rica) and El Verde (Puerto Rico)” is published in PLOS Climate. Co-authors include Pavel Garcia of Universidad de San Carlos de Guatemala, Nicholas S. Marzolf of The Jones Center at Ichauway and David Genereux of NC State University.

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Note to Editors: The study abstract follows.

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

Authors: Alonso Ramírez ,Marcelo Ardón, David Genereux, North Carolina State University; Pavel García, Universidad de San Carlos de Guatemala; Nicholas S. Marzolf, The Jones Center at Ichauway.

Published: Oct. 12 in PLOS Climate

DOI: 10.1371/journal.pclm.0001044

Abstract: Climate change is altering ecosystems worldwide, but long-term, site-level hydroclimatic responses remain comparatively under-documented in many tropical regions. We analyzed long-term air and water temperature, precipitation, and stream water level data from two active Caribbean research sites: La Selva Research Station, Costa Rica, and El Verde Field Station, Puerto Rico. Our goal was to assess long-term hydroclimatic trends and changes in extreme events using continuous monitoring from automatic and manual sensors. Long-term records show a mean daily temperature of 25.1°C (1992–2024, range: 19.5-29.6°C) at La Selva and 23.6°C (2000–2024, range: 17.7-28.2°C) at El Verde. Mean annual precipitation is 4166 mm at La Selva (1983–2024) and 3635.8 mm at El Verde (1975–2024). We found a decadal warming trend of 0.29°C at both locations. The El Niño Southern Oscillation (ENSO) influenced air temperature at El Verde, where La Niña (+0.02°C day−1, 95% CI: 0.002–0.042) phase was associated with higher daily temperatures, but had no detectable effect on air temperature at La Selva. A significant increase in the frequency of extreme temperature events (defined as exceeding the 95th, 97th, and 99th percentiles) was observed at both sites. Using the 95th percentile and a fitted generalized linear model, both locations had ~8 extreme temperature events per year in the beginning and increased to ~35 events per year in 2024. Changes in precipitation extremes were site-specific and differed between wet and dry metrics. La Selva showed an increase in wet extremes (heavy precipitation), with 99th percentile rainfall exceedances rising from ~1 event per year (1983–1985) to 6 per year in 2024. In contrast, El Verde showed no significant increase in heavy-precipitation events, but exhibited an increase in dry extremes (dry days <1.4 mm, increasing from 152 to 170 days per year). Stream water temperature had a moderate but significant increase. At La Selva, water temperature increased by 0.2°C, and at El Verde, by 0.9°C. An increasing tendency in high-water-level conditions was observed at both locations, though with marginal statistical support (p = 0.08 for the 99th percentile at La Selva and 95th percentile at El Verde), with estimated durations above these extreme thresholds roughly doubling over the study period. Overall, our study shows an increasing frequency of extreme events, both wet and dry, at these two Caribbean protected locations. These findings suggest important and distinct regional changes in climate over the region, with important consequences for ecosystem conservation and management.