Results of an eight-year climate experiment: Danish heath vegetation increases CO2 uptake as atmospheric concentration rises
In a unique experiment, researchers from the University of Copenhagen, DTU and Aarhus University have simulated future climate change on a heath in Jægerspris, Denmark. The heath was exposed to increased CO2, higher temperatures and less rainfall. Despite the change in climate, the plants captured more CO2, which eventually ended up getting stored as carbon in the soil.
Nature helps absorb a large part of the CO2 we emit into the atmosphere. Indeed, terrestrial plants capture roughly one-third of all human emitted CO2. The big question is whether nature will continue to help us in the future as atmospheric CO2 concentrations rise and the global climate changes. Over the course of an eight-year experiment, researchers from the University of Copenhagen, DTU and Aarhus University exposed a tract of heathland in Jægerspris to the type of climate that forecasters say will characterize the Danish climate come 2075: elevated atmospheric CO2 levels, higher temperatures and diminished rainfall.
"We are able to conclude that this ecosystem has shown a marked ability to sequester more CO2 from the atmosphere as concentrations rise and store it in the ground. And, over the experiment’s eight years, we have not observed any evidence that CO2 uptake in plants has been satiated," says Associate Professor Klaus Steenberg Larsen.
More CO2 bound under future climatic conditions than today’s
Researchers used a piping system to continuously blow CO2 over more than 48 test fields, each consisting of nine square meters of heath. At the same time, heat curtains were laid over the test fields to simulate increased temperatures, while rain curtains reduced the amount of growing season precipitation by 10 percent to mimic the climate of the future.
The researchers’ results demonstrate that plants absorb more CO2 even when exposed to climate conditions of the future. Plants used the extra CO2 uptake primarily for subsoil growth and the development of larger root systems, probably to ensure improved access to water or nutrients.
"As with a plant’s branches and leaves, parts of a plant’s root system also die over time. Some of this is transformed by microorganisms in soil, while other parts, that are harder to break down, end up as carbon in soil. In this way, a portion of the absorbed CO2 becomes bound within soil rather than being returned to the atmosphere," explains Klaus Steenberg Larsen.
Danish heathland is not representative of the world
Very few field experiments worldwide have combined drought, increased temperatures and elevated CO2 emissions. According to Klaus Steenberg Larsen, this is a big mistake:
"The heath here is not particularly representative of nature found around the world. Therefore, there is an obvious need to study nature’s responses to climate change in as many ecosystems as possible, such as in forests, wetlands, agricultural areas, etc. We should all be conducting many more experiments on CO2, droughts and rises in temperature around the world, because we simply know too little about the combined effects of elevated CO2 with climatic changes.”
Larsen adds: “This does not change the fact that atmospheric CO2 concentrations are continuing to increase, and that there remains a need for the entire planet to limit CO2 emissions if we are to stave off major climate change."
Klaus Steenberg Larsen
Associate Professor, Department of Geosciences and Natural Resource Management
Mobile:+ 93 56 55 83
Michael Skov Jensen
Press officer, Faculty of Natural Sciences and Life Sciences (SCIENCE)
Mobile:+ 45 93 56 58 97
Facts and background
- Researchers pumped CO2 into the atmosphere above experimental fields to achieve a level of 510 ppm (parts per million), i.e. approximately 100 ppm higher than the current atmospheric CO2
- After eight years, measurements demonstrated that amounts of carbon increased by approximately 1 kg per square meter in the upper 30 cm of soil in fields exposed to increased atmospheric CO2
- Every time we humans emit a ton of CO2 into the atmosphere, nature helps absorb much of it. Roughly, "only" about 400 kg remain in the atmosphere. This part of our emissions is what causes atmospheric CO2 concentrations to rise.
- The world's oceans sequester about 250 kg, primarily because there is a chemical balance between atmospheric and oceanic CO2 Terrestrial ecosystems absorb the remaining approximately 350 kg and substantial uncertainty remains about the processes governing this uptake.
- A natural explanation is that terrestrial plants simply absorb more CO2 through photosynthesis as the concentration of CO2 in the atmosphere becomes more abundant. The researchers also know that it is possible for plants to be more effective in their water relations – how much water they need during growth, because the pores in their leaves used to absorb CO2 also serve to evaporate water. With more CO2 present in the atmosphere, these leaf pores can be kept more closed and absorb the same amount of CO2, while reducing water consumption.
- Finally, we humans have done more than just increase atmospheric CO2. With the advent of artificial fertilizers, we have increased the amount of reactive nitrogen dramatically since the Second World War, which has allowed for increased plant growth globally.