Measurements provide new insights into emissions and the future climate

Atmospheric carbon dioxide (CO₂) concentrations have now risen above 425 parts per million (ppm). This is considerably higher than at any point in the past 800,000 years, which is as far back as researchers can reliably determine historical CO₂ levels by analysing air bubbles trapped in Antarctic ice. The last time CO₂ concentrations were this high was probably around three million years ago, a period in Earth's history when the global temperature was 2-3 degrees higher, polar ice sheets were considerably smaller and sea levels were up to 20 meters higher than today.

Today, monitoring stations around the world allow us to track changes in the atmosphere. Europe's largest network of monitoring stations is operated by ICOS, the Integrated Carbon Observation System, an organisation with 180 stations in 16 European countries. Its Swedish component is coordinated from the Department of Earth and Environmental Sciences at Lund University.

Monitoring in the Paris agreement

Every two years, ICOS organises a scientific conference, and this year more than 500 researchers gathered at AF-borgen in Lund. The conference focuses on greenhouse gases: where they come from, how they can be measured and how they interact with the Earth's ecosystems. 

One major theme discussed at the conference was how measurements can complement the statistics countries use when reporting their greenhouse gas emissions.

“When the Paris Agreement was concluded in 2016, 195 countries agreed to limit global warming to below two degrees. But another central part of the agreement concerned how countries should report their emissions, namely through so-called greenhouse gas inventories, a method that has both advantages and disadvantages,” says Carlos Gomez, researcher at the Department of Earth and Environmental Sciences at Lund University.

Statistics with a two year delay

Greenhouse gas inventories are based on statistics and calculations. For example, statistics can show how much petrol has been sold in a country during a given year. This figure can then be multiplied by an emission factor to calculate the amount of greenhouse gases released when the petrol is burned.

“Inventories are useful because they allow us to compare and track emissions from different sectors and countries over time. But the method also has several limitations. For example, the results are only as reliable as the statistics they are based on. On a country scale uncertainties can be in the range of 5–10 per cent, but on a finer spatial and temporal scale, for instance monthly emissions at city level, the uncertainties can be significantly larger," says Carlos Gomez.

Inventories report emissions on an annual basis, but often with a one to two-year delay. If we want more precise information about where and when emissions occur, other methods are needed. 

“By combining measurements from ICOS stations with mathematical models, my research aims to identify where and when emissions occur on a much finer scale. This allows us to target policies and measures more effectively,” says Carlos Gomez.

Measurements, however, should not be seen as a replacement for inventories. Carlos Gomez explains that the two methods complement each other. Inventories can provide a resource-efficient way of describing emissions from different sectors and activities in society, while measurements can be used to verify and improve our understanding of how large the emissions are, and where and when they occur.

Sensors in Munich and Paris

A clear example of how greenhouse gas measurements can be put to practical use was provided by the ICOS Cities project, which was presented at the conference. Networks of sensors were installed in 15 European cities to measure greenhouse gas emissions.

“Around 70 per cent of the world’s greenhouse gas emissions occur in cities. However, the greenhouse gas inventories that policymakers and city officials usually rely on can have a delay of up to two years, preventing us from seeing the immediate effects of climate policies. We have developed a method that makes it possible to monitor urban emissions in near real time,” says Claudio D’Onofrio, researcher at the ICOS Carbon Portal and Lund University and project leader of ICOS Cities.

In Paris, measurements from the project were able to confirm a reported 25 per cent reduction in CO_{2 -}emissions, showing that the city’s climate efforts are producing tangible results. The high temporal and spatial resolution of the project’s sensor networks also made it possible to identify individual emission sources and leaks. In Munich, the system was able to locate previously unknown emission sources such as a gas leak, an abandoned landfill and a sewage treatment plant. 

Drawing on the experience gained in the participating cities, the ICOS Cities project has also contributed to the development of the World Meteorological Organization’s (WMO) guidelines for monitoring urban emissions - library.wmo.int, a new global standard for monitoring greenhouse gas emissions in cities.

Droughts and smaller carbon sinks

Another way in which ICOS measurements are used is to measure the carbon balance of different ecosystems, such as forests, wetlands and marine environments. These data are crucial both for developing future climate scenarios and for understanding how we should manage our forests as climate change continues.

Much of the research discussed at the conference focuses on long-term changes in different ecosystems, how they are affected by climate change, and how they in turn affect the climate by releasing or absorbing carbon.

“Over the past decade, we have learned a lot about how Europe's forests respond to drought and heat waves. The ICOS measurements reveal how much a drought reduces the forests' capacity to lock away carbon. For example, during the dry year 2018, there were spruce forests in Sweden that did not remove any CO₂ from the air as they usually would. This is important knowledge when deciding how to manage our forests and which tree species to plant,” says Natascha Kljun, Professor at the Department of Earth and Environmental Sciences at Lund University.
 

Many forests recover after periods of drought and continue to grow and store more carbon. But there are indications that forests in northern Europe may be losing their capacity to act as strong carbon sinks. The extent of this change, however, is still being investigated and debated.

“This is one of the big questions in this field of research at the moment: to what extent can we rely on our forests to continue storing carbon in the future as climate change continues?” says Natascha Kljun.

Crucial for climate models

ICOS measurements are also fundamental to ecosystem models used to develop the climate scenarios presented by the IPCC, such as LPJ-GUESS, which originated at Lund University. The measurements provide data against which the models can be developed and tested. By examining how vegetation and the carbon cycle have responded to changes in the climate, researchers can improve the models used to explore how ecosystems and the carbon cycle may respond under different future climate scenarios. 

Learn more about ICOS on their website - icos-cp.eu