Forest-based climate mitigation: a systems perspective focused on bio-energy and carbon

Forests are currently estimated to store 861 GtC globally, and have absorbed nearly 200 GtC over the past 150 years, half of all fossil fuel emissions. Their essential role in the terrestrial carbon cycle makes forests central to climate mitigation. For example, through maintaining natural carbon stores, replacing fossil carbon-based building materials with timber-based materials, and bio-energy production without or with Carbon Capture and Storage (BECCS). Debates around forest-based bio-energy are highly contentious and often lack a systems perspective, omitting crucial processes required for holistic analysis of climate-relevant impacts. A systems perspective enables to quantify when and where, and the bounding conditions for carbon neutrality, moving towards transient and uncertainty-aware rather than static calculations of atmospheric CO₂ concentration. By using a systems perspective, a better understanding will better define and constrain the fate of carbon. Three fundamental debates surround forest-based bio-energy:

•  Is wood harvesting carbon-neutral A simple question, but the answer depends on the definition of carbon-neutrality used. We reviewed the literature to seven definitions, herein we focus on: i) IPCC: Harvesting counts as carbon loss, and there is an assumption that burning wood is carbon-neutral; further carbon credits and debts should be linked to the carbon cycle, ii) Carbon payback: Emissions must be reabsorbed by new growth, and assume to take 40 years.
• What are the uncertainties associated with predictions of forest climate mitigation potential? Earth observations and models have shown that the is slowing down, and sinks have reversed to sources. Causes are multiple, including heat waves, droughts, fires and disease. Old growth forests’ role has become clarified, with increasing evidence that they continue to take up carbon, especially under carbon and nitrogen fertilization. Yet, effectivity of bio-energy options should consider both the role of old growth forests and the slow carbon cycle, failing to re-introduce carbon back over decadal to centennial timeframes.
• What are the land area requirements of forest-based energy demands?. Globally, only 3% of our current forests are plantations, an area far from that needed to meet energy needs. Plantation forests have limited potential for climate mitigation due to their assimilation rates, harvesting regimes, and heightened fire risk, among others. Multiple future scenarios use abondoned land for expanding energy crops, yet without an examination of the efficiency of photosynthesis versus that of photovoltaic solar panels (0.1% versus 20%).

With our systems perspective, we compare the carbon balance between forests that are managed for bioenergy and that of forests that remain intact. In this presentation we only focus on carbon with a focus on residual flows. Our results question if the promotion of bioenergy from forests through the Renewable Energy Directive can level off all trade-offs. While forests are crucial for climate adaptation and restoration such as climate-smart forestry, biodiversity-friendly afforestation, nature-based climate solutions, a one-size fits all approach may be detrimental especially in the long run.