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En kommentar om bioenergi og CO2 [in Norwegian]
I mitt forrige blogginnlegg skrev jeg at vedovnen din påvirker klimaet både mer enn du tror og på flere måter enn du tror. Innlegget var basert på vår ferske studie av klimaeffektene av bioenergi i Norge. Øyvind Kvernvold Myhre kommenterer studien vår i et innlegg i avisa Hadeland (27. april 2018). Jeg ønsker å kommentere et sitat fra Myhre knyttet til studien vår. Myhre skriver:
«Nylig er det publisert en studie fra SINTEF, NTNU og CICERO. (Arvesen, Cherubini, Strømman et al: «Cooling aerosols and changes in albedo counteract warming from CO₂ and black carbon from forest bioenergy in Norway».) Denne studien sier at i et 100-årsperspektiv gir fjernvarme fra ved mindre global oppvarming enn (blant annet) olje. Men på kortere sikt – to til tre årtier – får vi økte utslipp av CO₂, står det. Det skal sies at forskningsmiljøet rundt Cherubini og Strømman ofte framstår som positive til bioenergi fra skog. Det kan komme av at de tar et hundreårsperspektiv. Men oppgaven vår er å redusere utslippene NÅ; ikke vente til etter 2050.»
Myhre tar opp et relevant tema, nemlig tidshorisonten vi bruker når vi beregner påvirkninger på klimaet. Tidshorisonten er viktig fordi ulike typer klimapådriv skaper klimaendringer over forskjellige tidsskalaer. La meg forsøke å forklare med et forenklet eksempel: Å slippe ut CO2 fra fossil energi, er som å skru på en ovn som varmer opp atmosfæren permanent – altså, for alltid. Til sammenligning blir utslipp av CO2 fra bioenergi fra skog som å skru på en ovn som varmer opp atmosfæren en stund, men som gradvis avtar i styrke og etter hvert slukner. Grunnen til at det er slik, er at trær, i motsetning til fossile brensler, vokser opp igjen.
Den relativt kortvarige klimaoppvarmingen fra CO2 fra bioenergi og langvarige (permanente) klimaoppvarmingen fra CO2 fra fossil energi, er illustrert i figuren under.
Sammenligning av temperaturendringene over tid som følge av et utslipp av CO2 fra fossile energi (svart kurve) og CO2 fra bioenergi (for bioenergi fra norsk skog, er det de grønne og røde kurvene som er relevante). Figuren er fra supplementet til Cherubini et al. (https://rdcu.be/O5mn).
Hvis vi velger kort tidshorisont når vi gjør beregningene våre, vil vi ikke fange opp de langvarige klimaeffektene av CO2 fra fossil energi. Motsatt, med en lang tidshorisont, vil vi i større grad fange opp de langvarige effektene av fossil CO2. Siden de relativt kortvarige effektene av CO2 fra bioenergi vil fanges opp uansett, vil en lang tidshorisont stille bioenergi i et fordelaktig lys (sett i forhold til fossil energi) og en kort tidshorisont i et ufordelaktig lys. Men dette er veldig forenklet, for måten tidshorisonten er behandlet på spiller også en rolle. Mer om det i neste avsnitt.
Det er ikke helt enkelt å forstå akkurat hva Myhre mener, fordi han skjødesløst blander referanser til strålingspådriv (som 100-årsperspektivet refererer til), temperaturendring (som «global oppvarming» referer til) og utslipp[1]. Myhre trår feil når han antyder at fordi vi har en tidshorisont på 100 år, får vi resultater som er fordelaktige for bioenergi. Grunnen til at det er et feiltråkk, har med måten tidshorisonten er brukt på å gjøre[2]. Uten å gå inn i en detaljert diskusjon om ulike indikatorer for å beregne klimapåvirkning, er det viktig å påpeke at 100-årsperspektivet som Myhre refererer til, gjelder for integrert strålingspådriv, som er noe annet enn både utslipp og temperaturendring. Forskning har vist at en tidshorisont på 100 år for integrert strålingspådriv, som er det studien vår bruker, gir liknende resultater som en tidshorisont på omtrent 20-40 år for temperaturendring. Er 20-40 år for temperaturendring en lang tidshorisont? Uansett, hovedpoenget er at beregningsmetoden vår for klimapåvirkninger faktisk i betydelig grad vektlegger den relativt kortvarige oppvarmingseffekten av CO2 fra bioenergi, men i svært liten grad fanger opp den ekstremt langvarige oppvarmingseffekten av CO2 fra fossil energi. Med andre ord: beregningsmetoden og tidshorisonten studien vår bruker, gir ikke spesielt fordelaktige resultater for bioenergi.
En annen svakhet ved Myhres argumentasjon, er at den sidestiller CO2 fra bioenergi fra skog med CO2 fra fossil energi. På grunn av dette, synes jeg ikke utsagnet «oppgaven vår er å redusere utslippene NÅ» gir særlig mening. Ja, det finnes grunner til å være bekymret for oppvarming som skjer på kort sikt, for eksempel at kortvarige oppvarmingseffekter kan akselerere smelting av isbreer og arktisk sjøis som allerede er i gang. Samtidig er global oppvarming i aller høyeste grad et langsiktig problem: Det er våre barns barn og barnebarn som virkelig vil få føle klimaendringene på kroppen. Og mens oppvarmingen fra fossil CO2 er en irreversibel skade på klimaet, er oppvarmingen fra CO2 fra bioenergi fra skog reversibel – den går over. Dette er en vesensforskjell som også må tas hensyn til.
Kvantifiseringer av karbonbudsjetter – altså hvor mye CO2 vi mennesker kan slippe ut og samtidig nå gitte mål for å begrense global oppvarming – var et sentralt resultat fra den siste hovedrapporten fra FNs klimapanel. Karbonbudsjett er basert på en sammenheng mellom oppsamlede (kumulative) CO2-utslipp og temperaturendring. Som forskning fra NTNU har vist, gjelder imidlertid ikke denne sammenhengen for CO2 fra bioenergi. Årsaken til det er nettopp de ulike tidsskalaene som fossil CO2 og bioenergi-CO2 virker over. Dette illustrerer, igjen, at CO2 fra bioenergi fra skog og CO2 fra fossil energi kan ikke uten videre sidestilles.
Helt til sist: Klimapåvirkningene av bioenergi fra skog er mangefasetterte og kompliserte, avhenger av lokale forhold og involverer en rekke oppvarmende påvirkninger på klimaet (for eksempel utslipp av sot) så vel som nedkjølende (for eksempel albedoendringer). Studien vår (og andre studier, som denne og denne) gir innsikter i noen av fasettene og kompleksitetene. Det er behov for mer forskning for å bedre forståelsen vår av helheten av klimapåvirkningene, og for å finne tiltak som reduserer de uønskede, oppvarmende påvirkningene.
[1] Myhre er også skjødesløs når han sier studien er utført av SINTEF, NTNU og CICERO, mens den faktisk er utført av SINTEF, NTNU, NIBIO og NMBU.
[2] Studien vår bruker indikatoren GWP («Global Warming Potential») for en tidsperiode på 100 år fram i tid. Grunnen til at studien bruker akkurat GWP-indikatoren over 100 år (det finnes andre indikatorer og andre tidshorisonter som også kan brukes), er ikke fordi vi mener det er den riktigste indikatoren, men fordi det er den indikatoren som er blitt lagt til grunn i klimapolitikk så langt.
Den omtalte studien:
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.
Your wood-burning stove causes more climate warming than you might think
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.
The wood stove in my dwelling. Norwegians love their firewood and wood stoves. Out of 2.5 million dwellings in Norway, 1.2 million use firewood for heating. (Norwegian “kosefyring” translates roughly into “firing with wood for the purpose of relaxation and pleasure”.)
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:
A forest and a forest trail. Photo from https://flic.kr/p/UXTVRp.
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.
Annual climate impacts of wood stove bioenergy in Norway. Based on results from Arvesen et al. (2018).
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.
Acknowledgment:
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).
An industrial ecology perspective on climate change mitigation models
Figure source: IPCC AR5 WGIII. Available from: http://www.ipcc.ch/report/ar5/wg3/.
The above IPCC figure depicts possible future trends in greenhouse gas emissions. Underpinning this display is a set of well over a thousand scenarios. The range reflects different technological and economic trajectories, in addition to uncertainties. If we want to limit warming to no more than 2 ℃, we should look at the ‘RCP2.6’ path. How did the climate analysts produce this large collection of future outlooks? Not with crystal balls, but with an army of computer models. Or, somewhat more precisely, integrated assessment models. In a recent article, lead-authored by Stefan Pauliuk, we review these models from the perspective of our field, industrial ecology.
Integrated assessment models (IAMs) are widely used to explore strategies for halting climate change. The models mimic human decision and mechanisms in natural systems over long time-scales. They operate by selecting or substituting alternatives – this can be natural resource alternatives or technology alternatives – so that costs are minimized, while respecting constraints, such as limited emission budgets. Among numerous applications, IAMs have been used to explore which fossil fuel reserves need to remain unexploited in a 2 ℃ future, to study the potential role of natural gas as a “bridge fuel” during a shift to a low-carbon society, and to compare energy system transformations in 2 ℃ or 1.5 ℃ futures.
In industrial ecology, on the other hand, we study how energy and matter flow through society, how they are transformed or used in a network of industrial processes to satisfy human needs and desires, and how the natural environment is affected as a result. Key industrial ecology methods include material flow analysis (MFA), environmental input-output analysis (IOA) and environmental life cycle assessment (LCA). Some examples of applications of industrial ecology methods are: to analyse emissions associated with products and services consumed by households; to investigate resource use, emissions and wastes associated with developing and operating material stocks in the future; to study increased material efficiency (that is, to use less materials to deliver a given service) as a strategy to reduce emissions; and to analyse environmental impacts associated with future electricity supply.
Common interests
The IAM and industrial ecology fields are both concerned with understanding industrial systems. Such systems help to create goods and services that humans utilize. They also create emissions and waste.
Looking into the future in order to evaluate possible strategies for sustainable development is, in a way, the heart of what IAMs do. Looking into the future is also at the core of what dynamic MFA is about, and is a growing trend in IOA and LCA. Furthermore, there is a shared ambition to address various types of environmental concerns, including greenhouse gas emissions, air pollution and water and land use.
Distinct approaches
Despite their common interests, each modelling approach has distinct differences. IAMs are strong at representing the dynamics that shape evolutions in human and natural systems. They are cost-led and parsimonious, weighing the costs of alternative means to an end in order to identify lowest-cost solutions. However, with few exceptions, IAMs lack explicit descriptions of physical linkages related to capital stocks and materials. This includes relationships between capital stocks and the materials you need to build the stocks, between the stocks/materials and emissions associated with producing them, and the factors that govern material cycles.
On the other hand, industrial ecology methods are less comprehensive in scope (and you could also say less integrated) than the IAMs. They focus their attention on specific types of linkages in industrial or ecological systems (as indicated in Figure 2 in our article), and often on specific products (LCA) or materials (MFA). Dynamic MFA is the only industrial ecology method that can generate scenarios itself; scenario-based LCAs and IOAs rely on exogenous scenarios as data inputs.
Potentials for interaction
Potential synergies between the two approaches exist. We believe that IAMs can generate more robust and credible emissions mitigation scenarios by adding industrial ecology linkages related to capital stocks and materials. We argue that, one the one hand, this can open the door for widening the set of potential mitigation solutions in the models, because material efficiency solutions – such as recycling, lifetime extensions, or using lightweight materials – can be added. On the other hand, it can introduce constrained availability of scrap for recycling as a new impediment to mitigation. In both cases where new solutions or obstacles come into play, model outcomes may become more realistic and useful for decision-making.
Industrial ecology can make use of IAM scenarios to improve its capability to analyse future change. One way to do this is to apply industrial ecology methods to analyse IAM scenarios; another way is to integrating IAM scenario data into industrial ecology core databases. IAMs provide insights into cost-minimizing strategies – a major concern of policy makers – something that industrial ecology does not offer frequently.
Environmental impact assessment methods developed for LCA have a potential to broaden the range of impact types considered in IAMs, while IAMs can be used to capture cross-sectoral interactions that matter for environmental impacts, such as between food and bioenergy production. Further discussions of potentials for interaction and improvement are available in our article.
In summary, IAM and industrial ecology share several important common interests, but employ entirely different approaches to achieve them. In our article, we call for more interaction between the integrated assessment and industrial ecology communities to the benefit of sustainability science as a whole.
Full article citation: Pauliuk, S., Arvesen, A., Stadler, K., Hertwich, E.G., 2017. Industrial ecology in integrated assessment models. Nature Climate Change 7, 13-20.
Read-only free version: http://rdcu.be/ohpz
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