The quiet culprit behind smaller harvests – and, eventually, pricier loaves – isn’t scorching afternoons. It’s what happens after dark. New global research on night temperature and wheat yield confirms it: agronomists didn’t expect the damage to be happening while the sun was down.
A Puzzle in the Field
Picture a wheat field under a clear sky, warm sunshine, and seemingly ideal growing conditions. By every daytime measure, it looks like a season primed for a bumper crop – the kind of harvest that keeps bread on shelves and prices steady. Yet when the combines roll through at harvest, the grain is smaller than expected and the tonnage falls short. For years, this mismatch puzzled agronomists who were trained to watch the thermometer during the hottest part of the day.
For as long as scientists have worried about climate change and food supply, the villain in the story has almost always been the same: blazing daytime heat. Heatwaves, scorching afternoons, record-high daytime temperatures – these are the images that come to mind when people picture a warming world damaging crops.
A new analysis of wheat harvests collected over 42 years, from 255 locations across the globe, tells a different story. The real damage isn’t happening under the midday sun. It’s happening after dark.
Night Temperature and Wheat Yield: The Night Shift Beats the Day Shift
Researchers tracking wheat yields alongside detailed weather data found that nighttime minimum temperature – how warm it stays after the sun goes down – is a far better predictor of how well a wheat crop will turn out than daytime maximum temperature is. Specifically, nighttime lows during the plant’s most sensitive growth phase accounted for about 40% of the differences in yield seen from field to field and year to year. Add in how sunny the days were, and that figure climbs to 52%. Daytime highs, by contrast, explained only about 20%.
That’s a genuinely surprising result. It means the temperature reading most people would assume matters most – the daytime peak – is actually the less important one. What happens while the plant sits in darkness matters roughly twice as much.
Why Warm Nights Hurt: The Plant’s Day-and-Night Economy
To understand why, it helps to think of a wheat plant as running on a kind of day-night shift schedule, with two very different jobs to do.
During daylight hours, the plant is in “earning” mode. Through photosynthesis, it uses sunlight to manufacture sugars – essentially building up a store of chemical energy and raw material. At night, with no sunlight to work with, the plant switches to “spending” mode. It burns through some of that stored energy via a process called dark respiration, using it to repair worn-out proteins, maintain its cell membranes, and handle the general upkeep of staying alive. Even in a good year, under perfectly normal conditions, a plant typically spends 15 to 30% of what it earned during the day just on this nighttime maintenance.
The exact reason warm nights raise that overhead bill isn’t fully settled – measuring photosynthesis and respiration directly wasn’t possible at the global scale of this study, so the mechanism is inferred from decades of separate plant-physiology research rather than measured directly here. The leading explanation: heat causes proteins inside plant cells to break down faster, so the plant has to spend more energy replacing and repairing them – a process known as nocturnal protein turnover. At the same time, heat makes cell membranes more fluid, which appears to make the plant’s energy-generating machinery less efficient, forcing it to burn even more fuel to produce the same result. Multiply this nightly overspending across the weeks it takes a wheat plant to fill out its grain, and the losses add up fast.
Notably, when a related study tested wheat under sustained warm nights directly, the plants showed little ability to acclimate – the extra spending simply continued, night after night, without an apparent workaround.
The Make-or-Break Window: Grain-Filling
Not every stage of a wheat plant’s growth is equally vulnerable to this problem. There’s a specific, roughly three-week stretch – called the grain-filling period – where nighttime warmth does almost all of its damage. This stage kicks off once the wheat head has fully emerged and the plant stops growing new leaves and stems. From that point on, the plant is no longer building its own body; it’s simply funneling its stored sugars into the developing kernels, fattening them up for harvest.
Warm nights sabotage this process in two ways at once. First, they compress the timeline: for every 1°C rise in average nighttime temperature, the grain-filling window shrinks by about 1.6 days, giving the plant less time to pack the grain full. Second, and more significantly, they drive up the metabolic “spending” described above, siphoning away carbon that would otherwise have gone straight into the grain. Researchers weighed the two effects against each other using a statistical technique called a Variable Importance Index, and found that the direct energy cost matters more – it accounts for 56% of the total damage, with the shortened growing window responsible for the remaining 44%. That’s a meaningful clue for where to focus breeding efforts: wheat that spends energy more efficiently at night may matter more than wheat that simply matures faster.
The “Golden Rule”: One Degree, Half a Ton
All of this physiology boils down to a strikingly clean number. Within the temperature range most wheat is actually grown in – nighttime lows between roughly 8°C and 22°C – yield declines in a straight line as nights warm up. The rate: about half a ton of grain per hectare lost for every 1°C increase in average nighttime temperature.
Across the four decades covered by the study, the typical test site warmed by about 1.2°C at night. On its own, that has already cost the world more than 10% of potential wheat yield – and that’s just the average. Some places have fared dramatically worse, and a few have barely been touched at all.
Mapping the Damage
The warming hasn’t hit everywhere equally, which has created a clear divide between regions under serious strain and regions holding steady.
At the extreme end sits Sudan, where nights warmed by roughly 2.5°C over the study period and yield losses exceeded 40% at most test sites – the worst outcome recorded anywhere. Bolivia has suffered comparably severe losses, around 41%, despite a much smaller temperature rise of under 1°C – a sign that Bolivia’s already low baseline yields make percentage losses look especially steep even without extreme warming. Central Europe (Czechia and Poland) and Mongolia have lost somewhere between 18% and 24% of potential yield. In Czechia and Poland that’s driven by nighttime warming running above 2°C; in Mongolia, as with Bolivia, a comparatively modest temperature rise combined with a low baseline yield inflates the percentage loss. The Mediterranean basin and Mesopotamia have experienced some of the sharpest nighttime warming recorded anywhere in the study – generally above 1.5°C – translating into yield losses in the 13-to-20% range. In South Asia, the Indo-Gangetic Plain spanning India and Pakistan has lost somewhere between 10% and 17% of potential yield, and the warming there has been notably worse in the high-output northern farming belts of Punjab and Haryana than farther south.
On the other end of the spectrum, Chile and Canada have barely warmed at night – increases of roughly 0.1-0.2°C – and their yield losses sit close to 1% or below. Argentina has also fared much better than most of the world, but its picture is a bit more mixed than Chile’s or Canada’s: nighttime temperatures there rose by about 0.8°C, and yield losses came in around 8% – real and measurable, even if far smaller than the losses seen across Europe, Africa, and the Middle East. Chile, notably, also has the highest average reported wheat yield of any country in the dataset, a combination made possible by pairing cool nights with abundant sunshine.
Sunlight as a Shield – But Not a Cure
There is a silver lining, of sorts: strong daytime sunlight can meaningfully cushion the blow of warm nights. More sunlight means more energy income for the plant during the day, giving it a bigger surplus to draw on when covering its nightly “spending.”
The scale of this effect is substantial. At a fairly warm nighttime temperature of 18°C, yields more than doubled – jumping from 2.6 to 5.9 tons per hectare – simply by boosting daily sunlight from 15 to 31 megajoules per square meter. This is a big part of why sun-drenched growing regions like Egypt and Mexico’s Yaqui Valley manage to produce strong harvests even with relatively warm nights.
It’s worth being clear about the limits of this effect, though. Sunlight doesn’t neutralize the harm caused by heat – the underlying damage from warm nights is still there. What it does is hand the plant a larger overall energy budget, so there’s more left over after the heat-related costs are paid. It’s less like removing a fee and more like giving someone a bigger paycheck so a fixed bill hurts less. Wheat’s ability to make good use of strong sunlight likely traces back to its evolutionary roots in the Fertile Crescent, where cool winters gave way to bright, dry stretches during grain development – an ancient adaptation that still pays off today, even though modern nights are warmer than anything the plant evolved to handle.
A Blind Spot in Global Forecasting
Perhaps the most important implication of this research has to do with how the world predicts future food supplies. Most of the crop simulation models that shape international food-security planning are built around daytime processes: how efficiently plants convert sunlight into biomass, and how they cope with daytime heat stress. Nighttime respiration barely factors in.
That blind spot has led some influential modeling efforts to forecast rising or stable wheat yields in places like the Mediterranean and the Middle East out to 2100 – partly based on the assumption that higher atmospheric carbon dioxide will boost plant growth. But the historical data tells a different story: those same regions have already seen yield declines of 10 to 20% since 1980, considerably steeper than what the leading crop models estimate for the coming decades. The researchers argue the models are missing the respiration story, and that separate research suggests rising temperatures may cancel out much of the benefit plants were expected to get from extra CO2.
The upshot is uncomfortable: the models currently used to plan for future food security may be too optimistic, in part because they’re built around the wrong part of the day.
Where This Leaves Us
If nighttime warmth really is driving global wheat losses more than daytime heat, three responses stand out as priorities.
The first is breeding. Researchers argue for a push to find and cultivate wheat varieties that are naturally more efficient at night – plants that respire less, or that repair heat damage without burning through as much energy, along with genetic variation in how well different wheat lines can acclimate their metabolism to sustained warm nights. Modern tools like genomic selection and gene editing could help identify and scale up these traits far faster than traditional breeding. One heat-sensitive target worth watching: starch synthase, the enzyme responsible for building starch in the grain, which is especially vulnerable to heat damage. There’s already a precedent for this kind of selection working: breeders who selected ryegrass lines for lower dark-respiration rates found those lines went on to produce the highest yields, a proof of concept that “low-respiring” breeding can pay off in a real crop.
The second is farming practice. Techniques grouped under “conservation agriculture,” especially leaving crop residue on fields instead of removing it, help retain soil moisture and buffer against temperature swings, giving crops a bit more protection against thermal stress.
The third is modeling itself. The tools used to forecast global harvests need to better account for nighttime respiration and the real cost of rising minimum temperatures, rather than continuing to focus almost exclusively on daytime, sunlight-driven assumptions.
The Takeaway
Wheat evolved in the Fertile Crescent under a climate of cool nights and bright, hot days – a rhythm baked into its biology over thousands of years. The modern climate is quietly breaking that rhythm, not through the dramatic heat waves that make headlines, but through a slow, steady rise in nighttime warmth that has been eating away at harvests for decades, largely unnoticed. Recognizing that is the first step toward doing something about it.
None of this shows up as a dramatic headline the way a heat wave does. It shows up quietly, one degree at a time, in slightly smaller grain, slightly thinner margins for farmers, and – eventually – slightly higher prices for something as basic as a loaf of bread.
Source
Study: Night temperature determines nearly half of wheat yield variation globally
Authors: Urs Schulthess, Matthew Paul Reynolds, Owen Kenneth Atkin, Ernesto Giron, Senthold Asseng, Sieglinde Snapp (2026)
Read the full paper: https://www.biorxiv.org/content/10.64898/2025.12.19.695361v3
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