Global warming affects photosynthesis in competing ways: higher CO2 levels can boost carbon uptake, but rising temperatures, drought, and warmer nights increasingly undermine that benefit. The net result depends on how hot it gets, what type of plant is involved, and whether water remains available. For many ecosystems, the balance is already tipping toward reduced photosynthetic productivity.
How Heat Disrupts the Core Chemistry
Photosynthesis relies on an enzyme called Rubisco to grab CO2 from the air and convert it into sugars. As temperatures rise, Rubisco works faster, which sounds like good news. But there’s a catch: Rubisco becomes sloppier at higher temperatures. It loses its preference for CO2 and increasingly grabs oxygen instead, triggering a wasteful process called photorespiration that burns through energy without producing anything useful. Under current atmospheric conditions, oxygen interference already suppresses photosynthesis in common crops and wild plants by up to 40%. Heat makes this worse.
Every plant species has a temperature sweet spot where photosynthesis peaks. Warm-temperate rainforest trees, for example, hit their optimum between roughly 26°C and 28°C, while tropical species can function well up to about 39°C. Beyond these optima, photosynthetic rates drop off. The absolute ceiling, the temperature at which the photosynthetic machinery begins to break down irreversibly, averages around 46.7°C in tropical tree leaves. That number matters because canopy leaf temperatures in some tropical forests are already approaching it on the hottest days.
The CO2 Fertilization Effect and Its Limits
More CO2 in the atmosphere does feed photosynthesis, at least initially. Plants absorb CO2 through tiny pores called stomata, and when there’s more CO2 available, the process becomes more efficient. This “fertilization effect” is real and measurable. It has contributed to a greening trend visible from satellites over recent decades.
But the benefit has a ceiling. Research on soybeans grown across a range of CO2 concentrations (from today’s roughly 420 parts per million up to 1,600) found that leaf photosynthesis peaked at about 1,200 ppm. Beyond that point, photosynthesis actually declined. The reasons are structural and genetic: at very high CO2, plants develop fewer stomata, their internal biochemistry becomes less efficient, and the genes responsible for photosynthesis dial down their activity. So the fertilization effect is not a simple “more is better” relationship. It saturates, and at extreme concentrations it reverses.
There’s also a practical constraint. CO2 fertilization works best when plants have plenty of water and nutrients. In a warming world where droughts are becoming more frequent and soils in many regions are nutrient-poor, the extra CO2 often can’t be fully used.
Drought and Stomatal Closure
Warming doesn’t just raise temperatures. It dries the air. As the gap between how much moisture the air holds and how much it could hold (the vapor pressure deficit) widens, plants close their stomata to avoid losing too much water through evaporation. Closed stomata conserve water but simultaneously block CO2 from entering the leaf, starving the photosynthetic machinery.
This creates a painful tradeoff. Keeping stomata open in hot, dry conditions risks dehydration and even lethal overheating, because transpiration (water evaporating from leaves) is a plant’s main cooling mechanism. Closing them preserves water but cuts carbon uptake and removes that cooling effect, pushing leaf temperatures even higher. In regions experiencing more frequent or intense droughts, this stomatal squeeze is becoming a dominant limit on photosynthesis, often more important than temperature alone.
What Warmer Nights Do to Carbon Balance
Global warming raises nighttime temperatures faster than daytime temperatures in many regions. This matters because plants respire around the clock, burning stored sugars for energy, but only photosynthesize during the day. Warmer nights speed up respiration, draining more of the carbon a plant fixed during daylight hours.
One hypothesis suggested that plants might compensate: after a warm night depletes their sugar reserves, they could ramp up photosynthesis the following day to make up the difference. Experiments on subtropical evergreen trees tested this idea over several months and found the compensation was, at best, a brief flash. In one species, nighttime respiration increased by about 20% under warming while daytime photosynthesis dropped by roughly 13%. In another, respiration held steady but photosynthesis still fell by about 10%. The culprit, again, was stomatal closure driven by higher vapor pressure deficit in warmer conditions.
The takeaway is that warmer nights erode a plant’s net carbon gain. Even if daytime photosynthesis stays the same, more carbon is lost at night, meaning the plant accumulates less biomass. And in practice, daytime photosynthesis often declines too.
C3 vs. C4 Plants: Winners and Losers
Not all plants use the same photosynthetic strategy, and global warming affects them differently. Most of the world’s plants, including wheat, rice, soybeans, and nearly all trees, use a pathway called C3 photosynthesis. C3 plants are the ones most vulnerable to the Rubisco-oxygen problem described above: as temperatures climb, photorespiration wastes more and more of their effort.
C4 plants, which include corn, sorghum, sugarcane, and many tropical grasses, evolved a workaround. They concentrate CO2 inside specialized cells before handing it off to Rubisco, essentially shielding the enzyme from oxygen. This makes C4 plants far more efficient in hot, sunny, dry conditions. Rising temperatures hurt C4 crops less than C3 crops.
The flip side is that C3 plants benefit more from rising CO2 concentrations. Because C3 photosynthesis is currently limited by how much CO2 Rubisco can grab in competition with oxygen, flooding the system with extra CO2 gives C3 plants a bigger relative boost. C4 plants, already operating near CO2 saturation thanks to their concentration mechanism, gain less from elevated CO2. The question is whether the CO2 benefit for C3 plants can outpace the heat penalty. In cooler, wetter regions with moderate warming, it might. In hot, drought-prone areas, the heat and water stress are likely to overwhelm any CO2 advantage.
Tropical Forests Face the Tightest Margins
Tropical forests are responsible for a huge share of global photosynthesis and carbon storage, which makes their vulnerability to warming especially significant. Tropical species can photosynthesize at higher temperatures than their temperate counterparts, with optima reaching up to 39°C in some cases. But tropical environments are already warm, so the gap between current conditions and the critical failure point is narrower than you might expect.
A study comparing tropical and warm-temperate rainforest trees found that tropical species could shift their photosynthetic optimum upward as growing temperatures increased, at a rate of about 0.35°C for every 1°C of warming. Subtropical species adjusted even more, at roughly 0.78°C per degree of warming. Warm-temperate species showed almost no ability to adjust. This suggests that some tropical trees have a buffer, but it’s a partial one. The critical temperature for irreversible photosynthetic damage (around 46.7°C on average) is not that far above what canopy leaves already experience during heat waves.
If warming pushes leaf temperatures past that threshold more frequently, the photosynthetic capacity of tropical forests could decline substantially, turning the world’s largest terrestrial carbon sink into a weaker one, or in extreme scenarios, a net carbon source.
The Overall Picture
Global warming pulls photosynthesis in multiple directions at once. Extra CO2 acts as a fertilizer, but only up to a point, and only when water and nutrients aren’t limiting. Rising daytime temperatures push many plants past their photosynthetic sweet spots and amplify wasteful photorespiration in C3 species. Hotter, drier air forces stomata shut, cutting off CO2 supply. Warmer nights accelerate carbon loss through respiration without a matching increase in daytime carbon gain. The plants best equipped to handle the heat, C4 species, are the ones that benefit least from extra CO2.
For temperate and boreal regions with cold winters, moderate warming may extend growing seasons and boost annual photosynthesis in the short term. For tropical and subtropical ecosystems already near their thermal limits, the trajectory is more concerning. The net effect on global photosynthesis will depend on how quickly temperatures rise relative to CO2 concentrations, and on whether rainfall patterns can keep up with plants’ increasing thirst in a warmer world.