Model shows wetlands cooled the 2023 Cambodia heatwave by about 2.3 to 2.9 °C

Why it matters

The results suggest wetlands may help limit extreme heat in tropical deltas, which supports protecting and restoring them as part of adaptation. The authors stress the numbers are upper-bound model estimates, not real-world measurements of cooling from past wetland loss.

What they did

The team used the WRF weather model coupled with the Noah-MP land surface scheme at 1 km resolution over the Cambodia Lower Mekong Delta. They ran three sets of 30 time-lagged simulations of the May 2023 heatwave: baseline, historical (1990) wetland extent, and current (2020) wetland extent. Wetland cells were set to a wetland land-use type with fully saturated soil. They then added warming from 29 CMIP6 models under SSP2-4.5 to the air temperature fields, with and without current wetlands, and computed a simplified heat-stress index (sWBGT).

Key findings

• Wetlands cooled near-surface air by 2.89 ± 0.10 °C (historical extent) and 2.28 ± 0.28 °C (current extent) over wetland areas, with peak-hour values of 1 to 5 °C.
• Cooling spread beyond wetland edges, and the footprint was wider with the larger historical wetland extent.
• Where wetlands were removed between the two scenarios, air temperature rose by 2.71 ± 0.09 °C.
• With the added warming, wetlands still cooled by 1 to 3 °C, and removing them gave more warming than keeping them.
• Wetlands cut sWBGT by about 1 to 2 °C, which was not enough to change heat-stress categories.

Limitations

• Soils were set to full saturation, so the cooling is an idealized upper bound. Converted land such as rice paddies and fish ponds, which hold water, was treated as dry cropland, which may overstate the contrast.
• Only one heatwave was simulated, and model physics uncertainty was not tested. The model was checked only against one airport station and ERA5-Land reanalysis, and wetland moisture and heat fluxes were not validated.
• The future runs changed temperature only, not humidity, winds or sea surface temperature. The sWBGT index ignores radiation and wind and does not estimate health or labor effects.

Glossary

• Pseudo Global Warming (PGW): A method that adds projected warming from climate models to the inputs of a real past event, to see how that event would look in a warmer climate.
• Time-lagged ensemble: A group of runs of the same model started at different times, used to reduce the effect of random model variability.
• sWBGT: A simplified heat-stress index calculated from air temperature and humidity only.
• Noah-MP: A land surface model that calculates how soil, plants and water exchange heat and moisture with the air.