Cover crops can cool the climate - but if it snows, they have a different effect.

Cover crops may very well have a cooling effect on the climate. Thus, they can be an important tool to counteract global warming and make agriculture more climate-friendly.

In a LinkedIn post, Tine Engedal, a postdoc at the Department of Plant and Environmental Sciences, Plant and Soil Sciences at the University of Copenhagen, writes about a new study she co-authored. It shows how significant a cooling effect cover crops can have by reflecting more sunlight than if the farmer leaves the soil bare after harvest.

The albedo effect of cover crops is difficult to account for in climate accounts because of missing data, and the study is a step towards better understanding in this area. Here, the researchers show that the effect can correspond to as much as 84 kg CO₂e per hectare per year, thus exceeding the negative climate impacts from diesel use when establishing cover crops and their nitrous oxide emissions.

The researchers behind the study emphasize, however, that the results are based on a single experiment and should therefore be considered a preliminary estimate.

What is albedo?

The albedo effect can best be described as how good a surface is at reflecting sunlight, and it is measured on a scale from 0 to 1. The closer the albedo is to 0, the more of the sun's energy the surface absorbs and converts into heat.

Dark, bare soil without cover crops will generally have a low albedo and thus absorb a large portion of the energy from the sun. This energy is converted into heat, warming both the earth and the atmosphere. If the Earth's albedo can be increased, more sunlight can be reflected, thereby reducing climate warming.

In the study, bare soil without plant growth had an average albedo of just 0.14, while the albedo increased to between 0.19 and 0.23 when the field was covered with cover crops.

The Snow Paradox

That cover crops have a positive impact on the albedo effect is not entirely straightforward because of something as simple as snow.

Snow on bare ground has, according to the study, one of the highest natural albedo effects, measured at 0.59 in the study. However, the researchers also show that the effect decreases if cover crops protrude through the snow. This darkens the surface, reducing the albedo to between 0.20 and 0.32.

The effect varies depending on when during winter the snow is present. Nine days with cover crops and snow in mid-winter reduced the calculated climate effect by 16% — and in February by 27%, because there is more sunlight later in winter.

In Denmark, however, the snow paradox is less significant. This is mainly due to our relatively mild winters and the fact that snow cover periods are often limited.

Although the albedo effect is highest with snow cover without cover crops, there are on average not enough snow days in Denmark to change the overall picture of the positive effect of cover crops.

Thus, the study indicates that, overall, cover crops can provide a measurable net cooling effect on the climate over the course of a year.

The number of snow cover days has also been decreasing since 1960. During 1961-1990, Denmark averaged 26.4 snow cover days per year, while in 1991-2020, the number fell to 18.6. In the period 2014-2023, it has dropped further to just 8.3. If this trend continues, the snow paradox will likely become less relevant for the albedo effect on Danish fields.

Although the study shows promising results, the researchers themselves point out that the results are "scenario-dependent".

Results are based on a single season, a single location, and one snow measurement. According to the researchers, this means the results cannot be broadly generalized to Danish agriculture but should rather serve as a starting point for further research in the area.

The study was published in the journal Agriculture, Ecosystems & Environment.