Abstract
Given the short time-frame to limit global warming, and the current emissions gap, it is critical to prioritise mitigation actions. To date, scant attention has been paid to the mitigation benefits of primary forest protection. We estimated tropical forest ecosystem carbon stocks and flows. The ecosystem carbon stock of primary tropical forests is estimated at 141–159 Pg C (billion tonnes of carbon) which is some 49–53% of all tropical forest carbon, the living biomass component of which alone is 91–103% of the remaining carbon budget to limit global warming to below 1.5 degrees above pre-industrial levels. Furthermore, tropical forests have ongoing sequestration rates 0.47–1.3 Pg C yr−1, equivalent to 8–13% of annual global anthropogenic CO2 (carbon dioxide) emissions. We examined three main forest-based strategies used in the land sector—halting deforestation, increasing forest restoration and improving the sustainable management of production forests. The mitigation benefits of primary forest protection are contingent upon how degradation is defined and accounted for, while those from restoration also depend on how restoration is understood and applied. Through proforestation, reduced carbon stocks in secondary forests can regrow to their natural carbon carrying capacity or primary forest state. We evaluated published data from studies comparing logged and unlogged forests. On average, primary forests store around 35% more carbon. While comparisons are confounded by a range of factors, reported biomass carbon recovery rates were from 40 to 100+ years. There is a substantive portfolio of forest-based mitigation actions and interventions available to policy and decision-makers, depending on national circumstances, in addition to SFM and plantation focused approaches, that can be grouped into four main strategies: protection; proforestation, reforestation and restoration; reform of guidelines, accounting rules and default values; landscape conservation planning. Given the emissions gap, mitigation strategies that merely reduce the rate of emissions against historic or projected reference levels are insufficient. Mitigation strategies are needed that explicitly avoid emissions where possible as well as enabling ongoing sequestration.
Highlights
Evidence that the planet has already warmed 1 °C above pre-industrial levels (Millar et al 2017), and findings that annual global greenhouse gas (GHG) emissions rose to an all-time high in 2018 (Global Carbon Project 2018) underscore recent studies assessing the deep and rapid cuts in greenhouse gas emissions needed to achieve the goals of the United Nations Framework Convention on Climate Change (UNFCCC) Paris Agreement (Millar et al 2017; Rockström et al 2017; Rogelj et al 2015) of limiting the increase in global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to no more than 1.5 °C by the end of the century
After clarifying our use of forest definitions, this paper is structured in four parts: we examine the mitigation value of tropical primary forests; we evaluate the current focus of forest-based mitigation strategies which are based on halting deforestation, increasing forest restoration and improving the sustainable management of production forests; the section that follows addresses the issue of emissions from degradation and selective logging impacts; and we compare the carbon stocks of primary forests and production forests, along with data on regrowth rate and reduced impact logging
Extant tropical forest ecosystem carbon stocks were calculated to be in the range 306–324 Pg C, with living biomass carbon of 204–221 Pg C
Summary
Evidence that the planet has already warmed 1 °C above pre-industrial levels (Millar et al 2017), and findings that annual global greenhouse gas (GHG) emissions rose to an all-time high in 2018 (Global Carbon Project 2018) underscore recent studies assessing the deep and rapid cuts in greenhouse gas emissions needed to achieve the goals of the United Nations Framework Convention on Climate Change (UNFCCC) Paris Agreement (Millar et al 2017; Rockström et al 2017; Rogelj et al 2015) of limiting the increase in global average temperature to well below 2 °C above pre-industrial levels and pursuing efforts to limit the temperature increase to no more than 1.5 °C by the end of the century. Given the very short mitigation time horizon, and the emissions and sequestration gap, it is critical to identify strategies that can help accelerate the transition to net-zero emissions and avoid the severe climate-related impacts of an exceeding 1.5 °C of global warming This urgency has prompted greater attention to forest-based mitigation actions given, among other things, the current gross carbon sink in forests recovering from harvests and on abandoned agricultural lands of 4.4 Pg C y−1 (i.e. per year), and it has been estimated that stopping deforestation and allowing secondary forests to grow would yield cumulative negative emissions between 2016 and 2100 of about 120 PgC, globally (Houghton and Nassikas 2018). While a range of forest-based mitigation strategies are recognised, the role of primary forest protection to date has not been explicitly considered in international policy negotiations (Mackey et al 2015; Watson et al 2018; Funk et al 2019)
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