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If the cooling fails

When it comes to climate protection, most people think of wind and solar energy, electromobility or heating with heat pumps – i.e. measures aimed at reducing carbon dioxide emissions.

In fact, this greenhouse gas is also the one by far the most important drivers of climate change caused by us. Climate protection is therefore not feasible without significant reductions in carbon dioxide emissions.

However, one should not lose sight of the other mechanisms by which mankind influences the climate, warn Nadine Mengis from the Geomar Helmholtz Center for Ocean research in Kiel and Damon Matthews from Concordia University in Montreal, Canada in the journal “Climate and Atmospheric Science”.

What also arises when coal and oil are burned

In addition to carbon dioxide, soot, nitrogen and sulfur compounds are also produced when coal and petroleum are burned, as well as the gasoline, heating oil or marine diesel produced from them. These compounds form tiny particles and droplets that float in the air and influence the climate there. These aerosols come in a wide range of sizes, shapes and colors and also have different effects depending on the air layer: Black soot particles literally swallow the sunlight and thus heat up the atmosphere.

Also trickle they fall to the ground after just a few days and quickly melt snow cover in higher latitudes, which in turn accelerates further warming. The nitrogen and sulfur compounds give rise to acids in the air that can cool the climate in very high and deeper layers of the air, but in other areas and also depending on their size and shape, absorb heat and thereby heat them. Overall, these aerosols cool the climate. However, this effect is much smaller than the warming of the climate due to the much larger amounts of carbon dioxide emitted at the same time.

Other greenhouse gases that warm the climate

” There are also a number of other greenhouse gases that warm the climate, ”says Jochem Marotzke, who does research at the Max Planck Institute for Meteorology in Hamburg and has been the lead author of the IPCC reports since 2009 co-authored. The most important of these is methane. On the one hand, it comes from natural sources. A good tenth of man-made methane emissions are released from flooded rice fields, where microorganisms decompose plant residues and produce this greenhouse gas.

A great deal of methane also enters the atmosphere from the digestive organs of ruminants such as cattle, sheep and goats. However, these emissions can hardly be reduced because people still have to produce food. On the other hand, it would be easier to reduce the amount of methane that leaks into the air during the production, transport and processing of natural gas. The EU plans to present a strategy for this soon.

The role of laughing gas

Another greenhouse gas is nitrous oxide – laughing gas. Large amounts of this are produced in agriculture from manure and liquid manure, but also when growing legumes such as beans, peas, lentils, soybeans, peanuts and lupins. If a farmer changes from conventional agriculture to organic farming, the nitrous oxide production on each hectare of his fields drops by 40 percent, but not to zero.

On top of that, the yields of organic farmers are usually lower and they therefore have to cultivate larger areas in order to produce the same amount of food as their conventional colleagues. All in all, a complete switch to organic farming would probably halve the nitrous oxide emissions.

At the same time, additional forests would have to be cleared and savannahs plowed in order to secure the food of the world population. Areas were lost that currently still take a lot of carbon dioxide out of the air. One could, however, feed a growing world population in a climate-friendly manner, as the “EAT study” by the Lancet Commission shows. The key to this would be the switch from animal to vegetable protein.

Less emissions from agriculture could be compensated for

Overall, rural activities heat up with an irradiance of 0, 34 watts per square meter, Nadine Mengis and Damon Matthews determine in their study. At the same time, the aerosols that are produced when fossil fuels are burned cool with a power of around 0.4 watts per square meter and thus more than make up for the emissions from agriculture.

If the emissions of carbon dioxide are reduced in the coming years by shutting down coal-fired power plants, replacing oil and gas heating and by other measures, fewer aerosols will be released. Nadine Mengis and Damon Matthews warn that less emissions of methane and nitrous oxide from agriculture will be compensated for and the cooling of the global climate will increasingly fail.

“With this, the two researchers confirm an effect with another climate model, which we already expected 2013 in the IPCC report, ”says Jochem Marotzke. If the emissions of carbon dioxide are reduced, the greenhouse gases produced in agriculture are not only less and less offset by cooling aerosols, but they are also becoming increasingly important in the face of climate change. Nadine Mengis and Damon Matthews are thus pointing to an unsolved problem that will become increasingly important with the increasing success of climate protection: How should climate warming from agriculture be compensated in the future?

“We have to get carbon dioxide from the atmosphere “

” In order to stabilize the climate, we have to get carbon dioxide from the atmosphere, “says Marotzke. In addition to planting trees, there are technical processes that wind and solar power plants could provide the energy for. But there is still a problem: where to put the huge amounts of carbon dioxide fished from the air? “It would be best to inject this carbon dioxide into the underground at suitable points,” says Marotzke.

Citizens' initiatives and environmental groups in Germany have so far resisted this. But now a rethink has started. The WWF, for example, calls for the development of a regulatory framework for injecting carbon dioxide from industry. A long-standing project by the German Geo Research Center in Ketzin near Potsdam has shown that the method is safe.

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