On November 6, the 27th World Climate Change Conference kicked off in Sharm El-Sheikh, Egypt - at a time when man-made global warming continues to advance. Efforts are still underway to meet the agreed 1.5-degree target. Releasing less carbon dioxide (CO2) into the atmosphere is the main countermeasure. But there is another set screw: short-lived climate-altering substances such as methane, soot and ozone. They, too, are part of the big picture. If these substances were reduced, warming could be reduced by about 0.8 degrees Celsius. Jülich atmospheric scientist Prof. Astrid Kiendler-Scharr was lead author of the chapter on short-lived climate pollutants in the latest report of the Intergovernmental Panel on Climate Change.
In August 2021, the Intergovernmental Panel on Climate Change (IPCC) published the first part of its latest assessment report. The verdict is clear: climate change is progressing rapidly, and every region of the world is already affected. Without immediate and comprehensive countermeasures, our atmosphere is likely to warm to 1.5 degrees Celsius above the pre-industrial temperature over the next 20 years. The consequences of the currently plus 1.1 degrees are already showing up in weather extremes such as heat waves, droughts or heavy rain.
The top priority, therefore, is rapid CO2 neutrality. This is because temperatures have been rising continuously with the amount of CO2 emitted into our atmosphere since pre-industrial times. CO2 is a very long-lived substance. Every gigatonne that we release into the air today will stay there for hundreds of years, heating up the climate for an equally long time.
However, there is another way to flatten the Earth’s temperature curve even further – and, above all, more quickly: the reduction of short-lived climate forcers. These remain in the atmosphere for a few hours only up to a maximum of around ten years. They include methane, ozone, hydrocarbons, aerosols, halogenated compounds and soot. In total, these substances contribute to global warming about as much as CO2. The IPCC concludes in its assessment report that substantially reducing emissions could rapidly allow further warming to be avoiced, that is to say, diminished by 0.8 degrees by the end of the century.
Reducing short-lived climate forcers
Measures to reduce short-lived climate forcers are already available today: particulate filters, systematically switching to modern heating technology and renewable energies, insulating buildings, repairing gas leaks, saving electricity, reducing meat and milk consumption and using public and alternative transport. In order to meet the target of 1.5 degrees Celsius, the nationwide implementation of the measures would have to reduce methane emissions by 40 per cent and soot emissions by 70 per cent by 2030 compared to 2010, and halogenated hydrocarbons would have to be reduced by 90 per cent by 2050.
Many short-lived climate forcers have a massive impact on air quality and thus on the health of people and nature. Among other things, they cause cardiovascular diseases, asthma and lung cancer and are associated with strokes and dementia. The World Health Organization estimates that air pollution causes 7 million premature deaths per year. In addition, ozone, for example, damages plants in many ways and causes annual crop losses amounting to 50 million tonnes.
Jülich researchers have contributed important information to the report: data they collected in extensive measurement campaigns; previously unknown reaction pathways they discovered through elaborate calculations; also, new and improved global climate models – incorporating their insights into the complex world of atmospheric chemistry. However, it is precisely this complexity that makes it difficult to make reliable statements: through various reactions and processes in the atmosphere, many substances depend on and influence each other. Three examples show everything that needs to be considered.
TREND OR FLUCTUATION?
“A good example of how strongly individual substances and processes influence each other is ozone, the most important anthropogenic greenhouse gas after CO2 and methane,” says Dr. Andreas Petzold from the Jülich Institute of Energy and Climate Research (IEK-8). It starts with the formation: the trace gas is not emitted directly, but is formed in the lower regions of the atmosphere through the photochemical degradation of hydrocarbons originating from road traffic, but also from industry and plants.
Petzold and his team have been collecting data on many different substances in the atmosphere for decades. He coordinates the European project IAGOS, short for “In-service Aircraft for a Global Observing System”. “The participating institutions’ equipment has been travelling around the world in commercial airliners for almost 30 years. While in flight, they measure short-lived greenhouse gases such as ozone, water vapour and methane, trace gases such as carbon monoxide and nitrogen oxides, as well as fine dust, ice and cloud particles, not to mention the long-lived carbon dioxide.
“These long-term measurement series enable us to distinguish long-term trends from short-term fluctuations and to understand interrelationships in the complex climate process,” says Petzold. For example, the data show that the seasonal peaks in ozone in the lower atmosphere are shifting. “They occur earlier and earlier in the course of the year,” explains the Jülich researcher. The researchers attribute this trend to climate change: “It gets warmer earlier in the year and there are more hours of sunshine,” he explains. “Both encourage the formation of ozone: the higher temperature gives all ozone formation processes a boost, and the UV light provides the energy for this.”
MORE OZONE THROUGH SHUTDOWN
Another important link emerged during the lockdown in the corona pandemic. In the months of March, April and May 2020, which were particularly affected by the economic shutdown, ozone levels near the ground rose by up to 41 per cent at night and up to 19 per cent during the day. “The lockdown meant that there were fewer traffic exhaust gases as a source for ozone, but at the same time the ozone probably lacked another substance from the exhaust gases as a reaction partner: nitrogen monoxide. As a result, less ozone was broken down and, on balance, the ozone concentration in the conurbations rose slightly,” analyzes Petzold. According to the researcher, this is an important indication that short-lived climate forcers react very quickly to changes, but at the same time can only be considered as parts of an overall system. It also means that as a result of a mobility transition, ozone levels in the cities could initially rise. “However, if the other strong climate drivers such as methane and hydrocarbons are reduced concurrently, the trend will be reversed after about 20 years. By 2100, a total of 0.8 degrees of global warming could be avoided. This is what the IPCC calculations show,” says Petzold.
It gets even more complex with aerosols, a widely ramified family of short-lived climate forcers. They are a mixture of tiny solid or liquid airborne particles. Aerosols come from emissions from biomass burning, exhaust fumes, desert dust or sea spray. They can also form through chemical reactions from substances that plants release into the atmosphere. “In general, we assume that aerosols predominantly cool the climate, but in climate research, aerosols continue to be the great unknowns. Due to the complex interrelations, many questions are still open, such as whether the particles reflect or absorb solar radiation depending on their composition, whether they have a warming or cooling effect, and to what extent they are involved in cloud formation,” says Dr. Alexandra Tsimpidi from IEK-8. She and her team have set themselves the task of making organic aerosols more predictable for climate research.
A challenging goal, as can be seen when looking at the properties that determine how the aerosols work: “This ranges from their physical state, liquid or solid, to their chemical composition and the property of being water-loving or waterrepellent,” lists Alexandra Tsimpidi. Scientists must also bear in mind that aerosols go through life cycles: “They are oxidized, accumulate, break apart, absorb other substances on their journey, react with them and lose others. Previous climate models do not take all this into account to a sufficient degree,” says the researcher.
However, the Jülich researchers have succeeded in uncovering the fundamental processes that determine how aerosols form and grow. To this end, they are conducting experiments in the Jülich atmospheric chamber SAPHIR. In the huge volume of the chamber, a wide variety of air mixtures can be recreated almost naturally, from clean forest air to heavily polluted city air. In these scenarios, researchers then analyze how aerosols form and from which precursors they do so, how they age, with which other substances they react and whether they are suitable as cloud nuclei. “We have applied these and other findings to our global models and can now calculate how air pollution and natural airborne particles affect air quality and the climate. In subsequent simulation calculations, we can then derive forecasts for various future scenarios,” says Tsimpidi.
NATURE’S WASH CYCLE
Not only the climate pollutants need to be considered, however, but also substances that ensure their chemical degradation. An important substance in atmospheric chemistry is the OH radical, which is also known as the “detergent” of the atmosphere. It has the ability to react with almost all atmospheric trace and pollutant gases. Depending on the nitrogen oxide load, it creates or destroys ozone molecules and breaks down traffic emissions as well as methane, the front-runner among the climate-impacting substances. “In order to make reliable predictions about short-lived climate forcers, there is no way around the OH radical,” says Dr. Hendrik Fuchs from IEK-8.
There are always new insights into complex relationships here, too, that then have to be taken into account in the models. For example, Jülich researchers have succeeded in clarifying discrepancies between measurement results and model calculations and, based on their findings, in improving the predictions of global models. The new approach now allows the OH values – and thus the purification capacity of the atmosphere – to be calculated more concretely.
The major differences were discovered during campaigns conducted by Jülich researchers together with colleagues from Peking University in China. The measured OH levels were up to five times higher than the model predictions. The researchers then re-examined their models and considered possible missing reactions.
The combination of experiments in the Jülich atmospheric chamber SAPHIR and extensive chemical calculations then revealed the secret of the additional OH radicals: “The cause is isoprene,” says Hendrik Fuchs, referring to the most significant hydrocarbon released by plants into the atmosphere. “If there are no other reaction partners available, such as nitrogen oxides from traffic, the OH radical reacts with isoprene in a previously unknown cascade of reactions, at the end of which considerably more OH is produced than was previously thought,” Fuchs explains. His colleague Dr. Domenico Taraborrelli has packed these findings into the new global model for atmospheric chemistry, so that measured values and model calculations now match. The new model was used, for example, to evaluate data from measurement flights with the Zeppelin NT over Europe. This showed that there are considerably more OH radicals in forested regions than previous models had predicted.
The measuring platforms
Forest and heat
One point that researchers must also consider in their models is the consequences of climate change, since these consequences also have an effect on substances and atmospheric chemistry. Dr. Thorsten Hohaus from IEK-8 is investigating this, using forests as an example. “During periods of drought or heat, like we had in Europe in 2018 and 2019, trees come under stress. This means that forest emissions change, and that this, in turn, influences processes involving important substances such as the OH radical and aerosols,” says the Jülich researcher. He and his colleagues at IEK-8 are studying pine trees in an air-conditioned container connected to the SAPHIR atmospheric chamber. Their aim is to better understand the effect on aerosol formation and composition due to typical stress conditions and the possible feedback mechanisms on the interaction of vegetation and climate that could result from this.
THE CLIMATE OF THE FUTURE
“The examples show how important every detail is in atmospheric chemistry. The more precisely we understand how individual substances are interrelated and which reactions are linked, the more reliable forecasts for the future will be,” says Prof. Astrid Kiendler-Scharr, director at IEK-8. She is lead author of the current IPCC report’s chapter on short-lived climate forcers, so the data, findings and models of the Jülich researchers are an important basis for the statements of the IPCC.
„“Thanks to the findings and increased computing power, we can now carry out simulations with which we can calculate in detail the temperature development of the next 80 to 100 years, for example. This is exactly what was done for the IPCC report,” says Astrid Kiendler-Scharr. The result: if mankind continues on as per usual, the unabated emission of short-lived climate forcers will contribute 0.8 degrees to the temperature increase by 2100. “If we only take the reflecting aerosols out of the system, there will be a slight warming of the climate. Removing methane, soot and halogenated hydrocarbons, on the other hand, would lead to a reduction in temperature of around 0.6 degrees. If we reduce all short-lived climate forcers, we could even go down by 0.8 degrees,” summarizes the Jülich researcher.
However, even if all requirements were implemented immediately, it would continue to get warmer until 2040 before a sustainable cooling effect would set in. This has to do with the different life cycles and reaction processes of the substances: “It will take a few years for all the reductions to gradually take effect, but the model calculations show it is possible,” Kiendler-Scharr emphasizes. Now it is up to politicians to draw conclusions from the findings and take action.
Emission control with a toxic side effect
What happens to chemical compounds that are not degraded by the OH radical is shown in studies by Dr. Domenico Taraborrelli (IEK-8): due to their long lifespan, they end up in the upper troposphere and even in the stratosphere, where they may trigger further chemical processes. Take isocyanic acid, for example, which is found in high concentrations in urban air. The cell toxin is suspected of causing inflammatory processes in the body, such as cardiovascular diseases or rheumatism. It is formed from urea compounds, among other things. In the exhaust gas aftertreatment of diesel vehicles, these ensure that up to 90 per cent less nitrogen oxides are emitted. However, a large amount of isocyanic acid is also produced in regions with a high rate of biomass burning, such as through forest fires or slash-and-burn.
TEXT: BRIGITTE STAHL-BUSSE I IMAGES: FORSCHUNGSZENTRUM JÜLICH/RALF-UWE LIMBACH, FORSCHUNGSZENTRUM JÜLICH/SASCHA KREKLAU, FORSCHUNGSZENTRUM JÜLICH/WILHELM PETER SCHNEIDER, DEUTSCHE LUFTHANSA, HONGLOUWAWA