Each extra day above 30°C cuts rice yields by 1%, study finds — double what climate models predicted

Every additional day that temperatures exceed 30 degrees Celsius during the rice-growing season reduces yields by around 1%, a new study has found.

What matters is not how hot a summer was on average, but how many days crossed the 30-degree threshold. Recalculated on that basis, a 1-degree rise in global average temperature reduces world rice output by 8.1% — more than double the 3.8% figure that international climate models have been projecting.

According to Science Advances (Vol. 12, No. 31), existing crop models have been underestimating heat damage to rice by roughly half.

The findings come from a joint research team that included professors Zhou Feng and Wang Xuhui of Peking University, along with researchers from Columbia University, the Potsdam Institute for Climate Impact Research (PIK) in Germany, and the International Rice Research Institute (IRRI) in the Philippines.

Days above 30°C matter more than average temperature

Previous assessments of climate change's impact on rice harvests conflated two distinct phenomena: a rise in seasonal average temperatures and the frequency of heat waves.

The two harm rice in different ways. Higher average temperatures cause the plant to mature faster and develop less biomass — a gradual, cumulative loss. Short heat waves, by contrast, strike at the critical stages when ears emerge and grains fill.

Lumping the two together produced wildly inconsistent results: earlier studies' estimates ranged from a 2.3% to an 8.3% yield loss per degree of warming.

The research team gathered experimental data from paddy fields where heat was applied directly. They drew 128 observations from 17 sites in existing literature and added 86 more from their own experiments at 12 major rice-growing locations in China. The team said the dataset represents about 80% of the climatic conditions found in rice-growing regions worldwide.

The analysis showed that yields fell 0.9 to 1.6% for each additional day above 30 degrees Celsius. Below that threshold, extra days of heat produced no measurable change in output. In more than 70% of the study sites, exposure to such heat waves accounted for more than half of total yield losses.

As heat waves intensified, rice plants grew — but the grains were empty

The team broke down where the damage actually occurs by separating two components: the total biomass a rice plant produces and the share of that biomass that ends up as grain.

It was the latter that shrank. Plants continued to grow reasonably well, but the portion converted into grain disappeared — like a tree that leafs out abundantly but bears no fruit.

Looking at growth stages, yields fell 1.1 to 1.8% per day when temperatures exceeded 30 degrees Celsius during the ear-emergence and grain-filling period. When heat struck around flowering time, pollen lost its viability and grains failed to set at all, leaving hollow husks. The pathways through which nitrogen moves from the stem into the grain were also blocked.

The pattern differed during the early vegetative stage, when leaves and stems are forming. Each additional day above 35 degrees reduced plant biomass by 1.4% and yields by 1.5%, accompanied by declines in photosynthetic capacity, chlorophyll content and leaf area — though the damage was less severe than during the reproductive stage.

The team benchmarked seven crop models commonly used to forecast harvests around the world against the experimental results.

Most showed almost no response to heat waves. Even as the number of days above 30 degrees increased, their yield projections barely moved. Only three of the seven registered any response at all, and even those estimated a reduction of just 0.1 to 0.4% per day — less than half the observed figure of 0.9%.

The gap widened further at extreme heat above 35 degrees. The models projected a decline of 0.3 to 1.3% per day, while the observed figure was 1.6%.

Rice farming abroad. [Getty Images Bank]

Similar results seen in South Korea

Regional projections shifted sharply under the new framework. Forecast yield losses for Pakistan, India and Bangladesh jumped more than threefold, from 2.1% to 6.6%. Thailand, the Philippines and Myanmar saw their figures rise from 3.7% to 7.7%, reflecting the greater number of days already above 30 degrees in those already-hot regions.

East Asian countries such as China and Japan, by contrast, saw upward revisions of less than 2 percentage points.

The factor driving regional differences turned out to be unexpected. When the team examined climate conditions, soil characteristics and farming practices, soil quality emerged as the strongest predictor of how much damage the same heat caused. Fields with higher organic matter and clay content showed greater resilience. Climate conditions and fertilizer use offered little explanatory power.

The team recommended spreading heat-tolerant varieties, improving water management and improving soil quality as region-specific responses.

Meanwhile, similar results have emerged in South Korea. The Rural Development Administration released simulation results Tuesday that reproduced mid-21st-century climate conditions. Under a scenario with temperatures 2.9 degrees Celsius above normal and carbon dioxide concentrations of 537 ppm, the dry weight of rice plants increased by 9.45% — meaning the plants grew larger. Yet the proportion of properly filled grains fell by around 4%, and the rate of cracked grains rose by 3.5%.

However, the study has limitations. The experiments were conducted in irrigated paddies where only temperature was varied. While that isolation allowed the heat effect to be measured cleanly, the conditions differ from those of actual farms. In irrigated fields, water evaporation keeps temperatures around the plants lower than the surrounding air — an effect not reflected in the calculations. Rain-fed paddies, conversely, may suffer greater damage as heat and water stress compound each other.

Reference

DOI: 10.1126/sciadv.aed9226

Yiwei Jian et al., "Vulnerability to high temperature shapes global warming impacts on rice yield." Science Advances 12, eaed9226 (2026).

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