Regular annual fluctuations in atmospheric carbon dioxide concentrations occur because

When we think of carbon dioxide in Earth’s atmosphere, we tend to presume that it is constantly on the rise. Whilst this is true in general, if we look closer, we see that the amount actually fluctuates over the course of days, seasons and years. For instance, on local scales more carbon dioxide is present at night than during the day and on continental scales, there is more in winter than in summer. These fluctuations are monitored by the Copernicus Atmosphere Monitoring Service (CAMS) to help scientists improve their understanding of the carbon cycle and to support businesses and policy makers in adapting to climate change.

During the day or in spring and summer, plants take up more carbon dioxide through photosynthesis than they release through respiration [1], and so concentrations of carbon dioxide in the air decrease. Then at night or during autumn and winter, plants reduce or even stop photosynthesising, releasing carbon dioxide back into the air. This is often called the natural carbon cycle.

But this cycle is affected by the carbon dioxide that humans add to the atmosphere when they burn fossil fuels. As Earth’s atmosphere warms up due to global warming, plants bloom earlier and are active for longer, especially at mid- and high-latitudes. In north-western Europe for example, the so-called ‘growing season’ of plants is now about a month longer than it was on average between 1961 and 1990.

Every day, CAMS, which is implemented by the European Centre for Medium-Range Weather Forecasts (ECMWF) on behalf of the EU, estimates the amount of carbon dioxide in the atmosphere to monitor how it changes.

“The classic ‘sawtooth’ graph shows how carbon dioxide levels change over the course of each year,” explains Richard Engelen, Deputy Head of CAMS. “We see that as the growing season of plants lasts longer, plants absorb and then release more carbon dioxide, and the graph’s ‘teeth’ can get bigger.”

The above graph shows the change in carbon dioxide levels measured for the northern hemisphere (red), southern hemisphere (green) and as a global average (black). The ‘sawtooth’ lines show carbon dioxide absorbed and released by plants, but the steady increase is mostly attributed to human emissions. Because the northern hemisphere contains much more land than the southern hemisphere – which is mostly covered by ocean – the amount of carbon dioxide in the atmosphere increases overall during the northern winter [2].

The annual variation depends on location. In tropical areas, plants are active all year round with little variation between summer and winter. However, in these regions other factors determine the balance between photosynthesis and respiration, such as the availability of nutrients and water, which is vital for photosynthesis. Moving towards the poles, larger temperature variations mean that plant activity, and therefore carbon dioxide levels, change more than they do at the equator. And with increasing temperatures due to global warming, more shrubs are also able to grow in the Arctic, which extends the sawtooth even more.

“At ECMWF, we estimate the net absorption and emission of carbon dioxide at Earth’s surface by combining data from satellites and Earth-based detectors with a model that represents the atmosphere and biosphere [3],” explains CAMS scientist Anna Agustí-Panareda. “This allows us to produce a daily estimate of the distribution of carbon dioxide in the atmosphere.”

Changes in carbon dioxide levels in the atmosphere during 2018, showing that as carbon dioxide increases in the northern hemisphere, it decreases in the southern hemisphere and vice versa.

Agustí-Panareda continues: “As well as making short-term forecasts, we also think on longer timescales. We look at data from satellites and ground-based stations that show how carbon dioxide in the atmosphere has varied over the last few decades, and fit the data with so-called atmospheric inversion models. This allows us to estimate the amount of carbon dioxide emitted and absorbed during those longer time periods, helping scientists to improve their understanding of the natural carbon cycle and the way it changes over time.”

A team of scientists recently used the CAMS atmospheric inversion system (run by the French Alternative Energies and Atomic Energy Commission) together with other inverse models to discover that tropical plants now absorb more carbon dioxide than they did in the past. A better grasp of how plants react to increasing carbon dioxide levels will help scientists to more accurately predict future levels in the atmosphere and improve our understanding of the carbon cycle.

“Once we have a very good understanding of how the carbon cycle works, we will be able to figure out exactly how human activities contribute to the amount of carbon dioxide in the atmosphere,” explains Engelen. “CAMS’ work will also feed into a new planned Copernicus service that will focus on these so-called anthropogenic carbon dioxide emissions.”

Links

CAMS forecasting global atmospheric carbon dioxide, Anna Agustí-Panareda: https://www.atmos-chem-phys.net/14/11959/2014/

Notes

[1] When sunlight is available, plants have the energy to photosynthesise, absorbing carbon dioxide and water and turning it into glucose for the plant and oxygen that is released into the atmosphere. Plants use the glucose to build larger molecules, so they can for instance grow, but also to fuel the cells, just as we humans do. They also respire, using oxygen to burn glucose and releasing carbon dioxide. This absorption and release of carbon dioxide happens all the time.

[2] Smaller seasonal variations also occur because the ocean absorbs different amounts of carbon dioxide depending on circulation and biological activity in surface waters, and humans burn a certain amount of fossil fuel to meet their energy demand. Here in Europe we use much more energy in the winter to heat our homes, whereas in other areas of the world, more energy is used in summer to cool down homes.

[3] This computer model is also used to produce daily five-day forecasts of carbon dioxide levels on a global scale. Not only are these forecasts a useful input for satellite algorithms, but they also provide valuable information to scientific field campaigns, for example for detailed aerial observations of carbon dioxide in certain parts of the atmosphere.