How is Climate Change Affecting Photosynthesis?

• The short answer is, it depends. Whether climate change affects photosynthesis and what the effects are, depends on many factors.
• The effects of climate change on photosynthesis will depend on soil water, nutrient availability, species, and climactic conditions.
• Plant physiological processes like stomatal conductance, transpiration, and photorespiration moderate climate change effects on photosynthesis.
• More CO2 and higher temperatures directly increase photosynthesis, but the rise is not global.

Society has to be ready to feed the anticipated 70% increase in human population over the next 30 years. At the same time, climate change-driven abiotic stresses like increased heat and extent, as well as the intensity of dry periods due to climate change, could reduce yields by 50%. Photosynthesis is the process that directly or indirectly produces the food we need and is the primary flux that removes carbon dioxide from the atmosphere. Natural ecosystems, which function as carbon sinks could also be affected. So finding out how climate change will affect photosynthesis is crucial in shaping adaptation strategies.

Climate Change and Photosynthesis

Photosynthesis is widely expected to rise due to increased atmospheric carbon dioxide (CO₂). However, the effect of higher temperatures and less soil moisture on photosynthesis and other plant physiological processes is not always positive. So the net balance could be positive, neutral, or negative, depending on several factors.

Photosynthesis is affected by carbon dioxide, light, temperature, and water availability. These factors also interact. This means ecosystems in different climatic regions, having varying soil nutrients and water availability will show diverse responses to climate change.

Carbon Fertilization is Increasing Photosynthesis

In the presence of light, plants combine carbon dioxide and water to produce oxygen and basic carbohydrates that are used to synthesize other bio compounds.

The rise in carbon dioxide increases photosynthesis due to carbon fertilization, though it has been challenging to calculate the exact extent.

Since there are few global-scale photosynthesis records, there is no basis for calculating climate change effects on this scale. So we have many models and simulations predicting climate change’s impact on global photosynthesis. Measurements of actual changes in photosynthesis are possible only at a smaller site level.

Experiments under controlled conditions show that C3 plants can increase their photosynthetic rate by 25 to 75% when doubling CO2 levels in the atmosphere.

Chen et al. 2022, measured photosynthesis at 632 sites worldwide. They used a framework that combines three theories to find the effect of atmospheric CO₂, leaf area index, air temperature, soil water content, specific humidity, surface pressure, and shortwave radiation on carbon fertilization effects.

They found that photosynthesis on a small insitu scale increased by 9.1 g C m−2 year−2 between 2001 to 2014. The carbon fertilization effect was responsible for 44%, and an increase in temperature was responsible for a 28% increase in gross primary production (GPP) at the site level. Increased atmospheric CO₂ leads to higher light use efficiency and carbon assimilation. The temperature rise also has a net positive effect and helps to increase photosynthesis.

Soil water and specific humidity regulate stomatal conductance and influence leaf carbon content. These two factors were responsible for interannual variations in GPP.

The research team incorporated satellite observations and meteorological data in their framework for the global scale. On a worldwide scale, there can be a 4.4 gC m−2 year−2 increase in GPP to produce 4.7% rise in GPP every decade.

A model-based study predicts a 7.1% rise in GPP due to carbon fertilization and temperature but a reduction of the gains by 29–38% due to vegetation climate feedback to give a similar estimate.

Terrestrial ecosystems have fixed around 30% of the anthropogenic carbon emissions in the past decades through an increase in photosynthesis. CO₂ fertilization is more in warm and arid regions and less in cold environments. The highest rates of fertilization benefits will be in the temperate forests, followed by tropical forests and C3 plants. The tundra and boreal forests show minor benefits, and C4 plants have none.

Factors Constraining Photosynthesis

Figure 1: “Schematic diagram of CO₂‘s three pathway influence on terrestrial GPP. The rising atmospheric CO₂ concentration will facilitate plant uptake of CO2 through photosynthesis (fertilization effect). CO₂ also influences plant photosynthesis indirectly through its climate-forcing effect. Its impact on climate through trapping longwave radiation (radiative climate change) can increase Earth’s mean surface temperature and thus influence plant photosynthesis. The response of plants to rising CO₂ can cause an increase in foliage cover and decreases leaf transpiration by reducing stomatal conductance per unit leaf area, which also impacts the climate system (vegetation-climate feedback) and thus influence plant photosynthesis indirectly, Zhu et al. 2017. (Image credits: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1002/2016GL071733)

An increase in photosynthesis leading to a boost in CO₂ capture and plant productivity is interesting as it could slow down the effects of climate change. Whether carbon fertilization will increase productivity and GPP and whether this continues or stops depends on nitrogen and water availability and temperature, see Figure 1.

Increasing Temperature

Figure 2: “Photosynthetic responses to temperature in Nerium oleander at normal (a) and elevated CO₂ (b) in an experiment. Redrawn from Berry and Björkman (1980). Original data were obtained by Björkman et al. (1975, 1978),” Kirschbaum 2004. (Image credits: DOI 10.1055/s-2004-820883)

The direct benefits of climate change on photosynthesis are positive. An increase in temperature and CO₂ can increase photosynthesis, see Figure 2. However, the vegetation-climate feedback, where plants also affect climate, can lead to increasing climate change due to higher temperatures.

The increase in temperature can make plants transpire more due to a rise in vapor pressure deficits of air to increase evapotranspiration. However, the heat could also make plants close their stomata to increase water use efficiency by reducing transpiration. Evapotranspiration is an important part of the global water cycle, and as a result of less transpiration, there is a decrease in atmospheric moisture and regional precipitation. Since evapotranspiration also cools neighboring areas, any reduction of the process makes the locality warmer and boosts climate change.

Due to this vegetation-climate feedback, more CO2 fertilization benefits will occur in tropical regions that are warmer and drier than in the northern areas. The vegetation-climate feedback makes northern regions drier and reduces photosynthesis. In drier areas, the increase in the canopy due to photosynthesis increases atmospheric moisture and precipitation so that these regions will see improved photosynthesis.

Also, an increase in temperature boosts photosynthesis rates and speeds up a plant’s growth cycle. The shortened lifespan reduces the time for photosynthesis and biomass accumulation. this can lead to reduced crop yields.

Plants can acclimatize to higher temperatures and continue to conduct photosynthesis. But species and growing conditions will moderate plant-level responses to a rise in temperature, as each species has a different optimum temperature.

The Nitrogen-Carbon Ratio

A fixed nitrogen-to-carbon ratio exists for living organisms. More CO₂ will be sequestered only if there is enough nitrogen to keep up with the increase in CO₂. If and when the nitrogen available to plants becomes scarce, the plants will not be able to benefit from the extra CO₂ in the atmosphere.

However, a temperature rise will also increase the mineralization rate of nutrients or nutrient use efficiency in the soil. So in nutrient-limited regions, an increase in temperature will indirectly increase photosynthesis by making more nutrients available to plants. Water use efficiency (WUE) and nutrient use efficiency (NUE) are inversely correlated. With increasing WUE and decreasing NUE, photosynthesis will grow to a certain point. The changing Spatio-temporal water and nutrient availability patterns will enable plants and communities to shift and lead to changes in net photosynthesis.

Reduced soil water content

The associated reduction in rainfall due to climate hampers CO₂ fertilization the most. This can occur due to various reasons.

One of them is the season: In the cool boreal, where photosynthesis is limited by temperature, warmer temperatures of  3.4^oC increase photosynthesis in one-third of the days. But dry periods during the growing season can reverse the trend and reduce photosynthesis by two-thirds of the days with dry weather.

In drier regions, higher temperatures and CO₂ levels have increased water use efficiency by making leaves close their stomata, but in wetter areas, there is no change in transpiration.

It is also possible that in regions like North America and Eurasia, where the growing seasons increase due to more extended warmer periods, plants will use more soil water and leave less for runoff and percolation, reducing moisture in the streams. Whereas the tropics are expected to get more rain due to climate change, the leaf area index increase will be offset by a reduction in transpiration rates, so there is no change in the runoff.

The boost in photosynthesis seen at a small scale due to carbon fertilization must be balanced with the large-scale negative vegetation-climate feedback and changes to soil moisture. These adverse effects must be considered to make an accurate prediction of future rates of photosynthesis.

Climate Change Also Favors Photorespiration

When leaves close their stomata due to high temperatures and less soil water, the CO₂ levels in leaves fall. The lack of CO₂ causes the RuBisCO enzyme in the Calvin Cycle to react with oxygen instead of CO₂ in a process called photorespiration. The process uses energy, and part of the fixed CO₂ is released. Photorespiration can reduce carbon fixation from photosynthesis by 20-50% in grains like wheat and soybean.

The affinity of RuBisCO for CO₂ also decreases due to temperature rises, increasing photorespiration. But elevated CO₂ levels at higher temperatures lower photorespiration. So high CO₂ levels at high temperatures still increase photosynthesis, while high CO₂ levels at low temperatures reduce the photosynthesis rate.

The complex interactions of CO₂ levels and temperature must be considered while predicting climate change impact, and scientists should not study the effects of the two factors separately. Combining photosynthesis with photorespiration will be important for finding the trends in GPP and breeding for varieties that thrive in climate change.

Measuring Photosynthesis

Measuring leaf, plant, and canopy photosynthesis accurately insitu and for use in global models has been crucial in predicting future trends in photosynthesis in response to climate change. Infrared Gas Analyzers like the CI-340 Handheld Photosynthesis System, which can simultaneously measure all gas exchanges, including stomatal conductance and transpiration, are an asset in the field and laboratory. Tools like C-340 will play a crucial role in monitoring climate change effects.

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