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Plants do a lot of work for us, producing the air we breathe, the food we eat, and even some of our medicine. But when it comes to removing carbon dioxide in the atmosphere, we may have been overestimating their ability.

Photosynthesis acts as the lungs of our planet 鈥� plants use light and carbon dioxide (CO鈧�) to make the sugars they need to grow, releasing oxygen in the process. When atmospheric CO鈧� concentrations increase, as they have been thanks to humans burning fossil fuels, one might think that plants are enjoying a smorgasbord of food for unlimited growth. But a new study published in shows this excess of riches is not as effective as previously thought.

Since CO鈧� is the main source of food for plants, increasing levels of it directly stimulate the photosynthetic rate of most plants. This , known as the 鈥淐O鈧� fertilisation effect鈥�, enhances growth in many of earth鈥檚 plant species, with the effects seen most clearly in and young trees, and less so in .

The amount of CO鈧� used by photosynthesis and stored in vegetation and soils has grown over the past , and now absorbs at least a quarter of human emissions in an average year. We鈥檝e been assuming that this benefit will continue to increase as CO鈧� concentrations rise, but data collected over a 33-year period show us that might not be true.

Fertilisation is in decline

Estimating the size of the global CO鈧�-fertilisation effect accurately is no easy task. We have to understand what limits photosynthesis from one region to another, and at every scale from molecules within a leaf through to whole ecosystems.

The big research team behind the new Science study used a combination of data from satellites and on-the-ground observations and models of the carbon cycle. Using this powerful toolkit, they found that the fertilisation effect declined across much of the globe from 1982 to 2015 鈥� a trend that correlates well with observed changes in and available .

In many ways, the combination of these different tools helps to paint a more complete picture of how the world鈥檚 ecosystems are photosynthesising. The researchers a collection of long-term measurements from like the one pictured below which continuously monitor the CO鈧� and water used by plants and are dotted across earth鈥檚 biomes and provide the best means of measuring photosynthesis at the ecosystem scale.

Flux towers are limited in their measurement range (1 km or so) 鈥� but the data these towers collect helps verify the satellite estimates of how much photosynthesis is going on. With satellites and flux towers now providing records since the 1990s (and earlier in some cases), scientists are able to assess long-term trends in global photosynthesis. These can then be compared to 鈥渕odels鈥� 鈥� the computer-based simulations predicting plant鈥揺nvironment interactions 鈥� as the researchers did in this recent study.

What might the models be missing?

Researchers in the latest study found that the decrease in CO鈧� fertilisation was related to the availability of nutrients and water, which the computer simulations might not be accounting for properly. We know that nutrients such as nitrogen and phosphorus are ) in some areas 鈥� which may be unaccounted for. Plants can also acclimate, or change how they grow, when the environment changes.

Just like we can spend less on groceries when food is plentiful, plants in photosynthesis when they are grown at high CO鈧�. When this happens, CO鈧� fertilisation is than before. Because have a stronger response than others, the response can be difficult to account for in computer simulations.

For many years, some people have assumed that carbon fertilisation will mitigate climate change by slowing the rate at which CO鈧� is increasing in the atmosphere. Although the effect is built into the models used to predict future climates, the argument has by those who believe the world is overreacting to climate change.

But if the new study is right, and we have indeed been overestimating the amount of carbon that plants will pull from the atmosphere in the future, even our most cautious climate projections have likely been optimistic.

, Lecturer, and , Research Fellow,

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