But fortunately, your wood stove causes more climate cooling than you might think too! Our new study published in Nature’s Scientific Reports sheds light on the many ways in which residential wood combustion in Norway affects climate.
So, I’ve stated that burning firewood causes both more climate warming and cooling than you might think. Several questions then beg to be answered: How is the warming brought about? How is the cooling brought about? And if we subtract the cooling from the warming, what is the net effect? And might this net effect be more significant than you think?
I’ll address these questions but, before entering into specifics, let me introduce the two fundamental and general ways in which humans affect the climate.
How humans affect the climate
Fundamentally, humans today cause climate impacts in two ways:
A) By changing the amount of incoming solar radiation that is reflected back to space
B) By changing the amount of outgoing thermal radiation emitted from the Earth to space
The most well-known way that humans change climate is via emissions of carbon dioxide (CO2), a so-called greenhouse gas. In the atmosphere, CO2 absorbs thermal radiation, thus reducing the amount of radiation the Earth can emit to space. The same is true for other greenhouse gases, such as water vapour and methane, but the magnitude and duration of the effects vary from one gas to another. The greenhouse effect falls under category B.
Now we need an example for category A. Since the example for category B was related to emissions, let’s use a different example for category A.
Consider this picture of a forest:
As you can see, the trees appear dark (dark green). This is because the trees mostly absorb, rather than reflect, sunlight (green parts of light are reflected though, which is why the trees appear green and not black).
The snow-covered forest trail in the picture, on the other hand, appears bright. This is because snow mostly reflects, rather than absorbs, light.
We can assume that humans have cleared the trees and vegetation to create the forest trail. Consequently, a smaller fraction of the surface is dark (dark green) and a greater fraction is bright; and less energy from sunlight is absorbed and more sunlight is reflected back to space. Reflecting the sunlight back means it doesn’t heat the Earth and the end result is climate cooling – and there we have our example of a category A effect.
Next, let’s turn our attention to the climate effects associated with wood-fired heating stoves, keeping the above categories A and B in mind.
How wood stoves affect the climate
Here are four notable types of climate effects caused by Norwegian residential wood burning:
Number 1: the greenhouse effect connected to CO2 and other heat-trapping gases. Contrary to popular belief, CO2 from burning renewable biomass such as wood can contribute to climate warming. Trees – being renewable – will reabsorb the CO2, but because they need time to do so, there can be a temporary warming effect. Although different to that of fossil CO2 (because fossil fuel stocks do not regrow), there is still an effect.
In addition to CO2, wood stoves emit the greenhouse gases methane (CH4) and nitrous oxide (N2O). Stoves also emit carbon monoxide (CO) and non-methane hydrocarbons (NMVOC), precursors to ozone (O3), another greenhouse gas. We recall that the greenhouse effect (irrespective of whether it is from CO2, CH4, N2O and O3) is a category B effect.
Number 2: harvesting wood from forests typically leads to an increase in how much sunlight is reflected from the Earth surface, and hence climate cooling, in the same way as described in our forest trail example earlier. This is the category A principle.
Number 3: wood stoves emit tiny particles called black carbon. Black carbon gets its name because it absorbs visible light (hence “black”) and is pure carbon. By absorbing sunlight, black carbon in the air exerts a climate warming effect belonging to our category A. But there is more to black carbon: when it’s deposited on snow, the snow becomes darker and loses some of its ability to reflect sunlight – another category A warming effect. Additionally, black carbon interacts with clouds and thereby influences the climate system in complex ways.
Number 4: black carbon has a sibling, called organic carbon. Opposite to sunlight-absorbing black carbon, organic carbon particles in air scatter sunlight back to space. In other words, organic carbon results in a climate cooling effect, one belonging to category A.
Estimating the climate effects of wood stoves
The figure below shows estimated climate effects associated with residential wood stoves in Norway in 2010. I’ll say more about the figure itself, but first a quick word on the comprehensive analysis underpinning it: Key elements of the analysis include an original set of emission factors for different classes of wood stoves, a mapping of wood harvest, supply and wood-stove burning activities, and a unique set of global warming potential (GWP) values. Our study combines these elements in order to analyse climate impacts of firewood burning on a national scale. GWP is just one measure that can be employed in order to study climate effects of different climate-altering pollutants and activities by one common unit, CO2-equivalents (CO2e). In our study, we use GWP evaluated on a time horizon of 100 years (GWP100), as GWP100 has been the metric of choice within climate policy to date.
As you can see from the figure, the climate warming effects of black carbon amount to 1.6 billion tonnes (Mt) CO2e, which makes black carbon the single most important cause of climate warming impacts in our analysis. Another major cause of warming is CO2 from biomass, contributing roughly half the amount of warming as black carbon, while CH4, N2O and fossil CO2 from supply chain activities together contribute roghly half the amount of warming as CO2 from biomass. Considering all climate alterations, combined warming amounts to 3.1 Mt CO2e.
Is 3.1 Mt CO2e a lot? To get a sense of the magnitude of this number, consider that Norway’s current total emissions of fossil CO2 – that is, from industry, transport and all other sources – amount to 45 Mt CO2. Also, bear in mind Norway’s pledge under the Paris agreement to reduce greenhouse gas emissions substantially (40% by 2030). Given that perspective, I would say 3.1 Mt CO2e is quite a lot.
Fortunately, as the figure shows, land surface changes and organic carbon come to the rescue by bringing about considerable climate cooling effects. More precisely, we estimate combined cooling effects of 1.7 Mt CO2e. In other words, combined cooling offsets more than half of the combined warming effects of wood stoves.
Where does that leave us in terms of the net (warming minus cooling) effect? The answer is 3.1 – 1.7 = 1.4 Mt CO2e, corresponding to 3% of Norway’s total emissions of fossil CO2. Is that more or less than you thought?
A complex picture
Finally, our findings display a complex picture, and are subject to large uncertainty. For example, how you operate the stove has an effect, as does where you harvest the firewood from. Additional results that reveal these and further complexities can be found in our paper – which might be a more interesting read than you might think?
Full article citation:
Arvesen, A. Cherubini, F., del Alamo Serrano, G., Astrup, R., Becidan, M., Belbo, H., Goile, F., Grytli, T., Guest, G., Lausselet, C., Rørstad, P.-K., Rydså, L., Seljeskog, M., Skreiberg, Ø., Vezhapparambu, S., Strømman, A.H. 2018. Cooling aerosols and changes in albedo counteract warming from CO2 and black carbon from forest bioenergy in Norway. Scientific Reports. DOI: 10.1038/s41598-018-21559-8.
The research was a collaboration between the Norwegian University of Science and Technology (NTNU), SINTEF Energy Research, Norwegian Institute of Bioeconomy Research (NIBIO), Norwegian University of Life Sciences (NMBU). It was funded by the Research Council of Norway through the Bioenergy Innovation Centre (CenBio).