In their recent editorial, Schulze and colleagues (Schulze et al., 2012) discuss the implications of a large-scale increase in the harvest of forest biomass to provide 20% of the current global primary energy supply. They present several well-founded concerns regarding the economic and ecological sustainability of such a scenario, concerns we largely share. However, the authors express particular apprehension regarding greenhouse gas (GHG) consequences of expanded forest bioenergy production that we do not entirely share. GHGs – while important – should not be the sole evaluation criterion when the objective is climate protection. The peer reviewed literature provides important insights beyond that presented which warrants additional commentary in order to give a more holistic and balanced perspective on the subject of forest bioenergy and climate.
In particular, the authors overlook two attributes fundamental to forest based bioenergy that distinguish it from fossil-fuel energy: (1) forests can affect climate in many ways extending beyond the biosphere-atmosphere cycling of carbon, and sometimes in ways that are in direct opposition to the carbon cycle, (2) CO2 emissions from bioenergy sourced from a sustainably managed forest generates less warming than the same mass of CO2 from fossil fuels.
These attributes can have positive implications for the inclusion of forest based bioenergy as part of an integrated climate mitigation strategy.
The importance of biophysical factors
Changes in biophysical factors such as reflectivity, evaporation, and surface roughness can alter both local and global-average climate more than carbon sequestration does, and often in opposing ways (Bonan, 2008; Jackson et al., 2008; Anderson et al., 2010). When considering the climate forcing caused by changes in surface albedo, the climate benefits of carbon sequestration can be counteracted in boreal and other snow-covered regions, where darker trees trap more heat than snow does (Bonan & Pollard, 1992; Betts, 2000; Claussen et al., 2001; Randerson et al., 2006; Bala et al., 2007). Expansion of forest cover in mid-latitude regions could potentially drive unintended changes in circulation and precipitation in the tropics (Swann et al., 2011). Recent empirical observation affirms earlier hypotheses that differences in biophysical processes between forests and adjacent open areas in northern temperate and boreal regions contribute to local temperature anomalies – even at small (‘hectare’) scales (Lee et al., 2011).
Many uncertainties remain surrounding the magnitude and scale of the climate response to changes in biophysical processes resulting from small-scale forest disturbances (Bonan, 2008) – including uncertainties surrounding climatic effects of secondary organic aerosol particles of biogenic origin (Virtanen et al., 2010). But if the goal is to mitigate climate change and not just atmospheric GHGs, then ignoring biophysical aspects could lead to decisions that fail to deliver the intended climate benefits when forest bioenergy is eschewed in favor of forest protection (Jackson et al., 2008).
Global warming due to CO2 emissions from bioenergy sourced from sustainably managed forests
The main arguments of Schulze et al. (2012) are largely founded on outcomes of scenario-based studies rooted in GHG flux accounting conventions that respect the additionality principle and avoid ‘baseline error’. While these types of ‘what-if’ analyses are useful for gauging the GHG implications of forest bioenergy scenarios, and are not without merit in certain contexts, analyses that embrace fundamental climate physics cannot be disregarded and should be comprehended prior to the additional insights provided by scenario analyses. For example, CO2 emissions from deforestation or combustion of fossil fuels induce a response that warms global average surface temperature for millennia (shown schematically in Fig. 1). On the other hand, CO2 emissions from forest bioenergy warm climate only temporarily if the terrestrial sink is fully regenerated (Cherubini et al., 2012) and may be offset or enhanced by changes to biophysical factors, as described above.
Irrespective of whether ‘additional’ forests are brought into production, management that maintains or enhances sink strength ensures that the global temperature response to CO2 emissions from bioenergy systems is different than the response to equal amounts of fossil CO2 emissions (Fig. 1). Should increases in the demand for forest bioenergy lead to reduced carbon stocks rather than full recovery of the pre-harvest forest, the resulting climate response should still be understood as being principally different (from that of fossil CO2 or from deforestation), having a profile that lies somewhere in between the two cases presented in Fig.1.
Although the provisioning of bioenergy may be more emission-intensive than fossil energy, it is imperative that one-first acknowledges the fundamental distinction in climatic effects of CO2 emissions from the two sources, which, for forest bioenergy, are ultimately determined by forest management activities. Acknowledging this distinction together with biophysical tradeoffs in specific forest management contexts can provide a solid foundation for augmented analyses that seek to compare different sources of energy. Such analyses should be inclusive of various emission intensities, life cycle emissions, conversion efficiencies, market-mediated effects, and other case-specific system parameters. Comprehensive physical analyses should be an integral component of scenario analyses – both of which are crucial to arrive at robust conclusions about the merits of bioenergy versus other energy options.
Discerning a legitimate role for bioenergy
Like Schulze et al. (2012), we recognize the inherent risk of increasing CO2 emissions accompanying large-scale increases in forest harvesting for bioenergy. However, we stress that any assessment of climate risk is incomplete without acknowledging the principle differences between the climate response to CO2 emissions from bioenergy sourced from sustainably managed forests and that of fossil energy. Further, we reiterate that biogeochemical considerations are only part of the picture and that quantifying impacts from changes to biophysical factors is essential to any full assessment of climate risk. Although in many situations biogeochemical impacts will not necessarily be offset or reduced by biophysical factors, we remind the authors that adding geological carbon from fossil fuels into the contemporary carbon cycle and then relying on biospheric sequestration to offset these emissions is also not without risk, since such sequestration is reversible as a result of forest disturbance or a changing climate (Pan et al., 2011).
We agree with Schulze et al. (2012) that we ought not to charge ahead blindly expanding forest bioenergy; but on the other hand, we need to consider all options in addressing the enormous challenges of supplying renewable energy and managing climate change. Diverse strategies that recognize the breadth and complexity of the many climate services forests provide will be essential and will need to be tailored to specific geographic conditions and management contexts. There will be times and places where it is advantageous to contemplate forest bioenergy as part of a climate protection strategy, even if it may have negative short-term elements. Sustainable forest bioenergy strategies could be an investment worth pursuing in order to transform to a renewable-energy-based society. What we need first and foremost are thoughtful, comprehensive climate analyses that address multiple physical mechanisms and timescales and that respect context specificities so that we can distinguish between the good, the bad, and the ugly.