The Science Needed for Robust, Scalable, and Credible Nature-based Climate Solutions for the United States
The objective of this report, which is co-authored by experts in both NbCS science and implementation, is to describe the technologies, tools and approaches necessary to support robust, scalable, and credible NbCS strategies for the US. The report is organized around the identification of key knowledge gaps and pathways to close them, providing a road map for actionable, cross-sectoral information to foster NbCS strategies that work while avoiding energy wasted on NbCS strategies that have limited environmental benefits or the potential to backfire and exacerbate climate change.
The impacts of climate change are accelerating non-linearly with devastating consequences, and mitigating the problem is fundamental for the national interest and societal well-being. More frequent and intense wildfires, droughts, floods, and heatwaves are already posing grave and interconnected threats to agriculture, human health, biodiversity, and physical infrastructure. The scientific consensus on how to reverse the course of climate change is clear – we need to dramatically reduce, and eventually eliminate, anthropogenic emissions of greenhouse gases from fossil fuel burning, other industrial processes, and land management practices. However, given the relatively slow pace of mitigation to date, emissions reductions alone will likely be insufficient to prevent dangerously high levels of warming, and they will need to be complemented by approaches for removing CO2 directly from the atmosphere.
Land-based carbon removal strategies which harness naturally occurring ecosystem processes have a particularly broad base of support. These Nature-based Climate Solutions (NbCS) are not a panacea for reversing climate change and can only be effective when pursued concurrently with economy-wide decarbonization. Nonetheless, NbCS are part of nearly all net-zero pathways, reflecting the crucial role of terrestrial ecosystems in driving the global carbon cycle. The terrestrial biosphere absorbs roughly 15% of the carbon in the atmosphere each year through photosynthesis, but then returns a nearly equal amount through respiration. These large photosynthesis and respiration fluxes approach a long-term balance under steady atmospheric and climatic conditions.
However, since the Industrial Revolution, the biosphere has been out of equilibrium. Rising atmospheric CO2 and increased nitrogen deposition are increasing photosynthesis more than respiration, such that the rate of net carbon uptake on land has increased over the past century, and even doubled since the 1960s. As a result, terrestrial ecosystems currently absorb 25% to 33% of the CO2 emitted annually by human activities. Important questions remain concerning the cause of this imbalance and the fate of the land carbon sink in a warmer world that will face increasing and competing land use pressures. Nonetheless, right now, terrestrial ecosystems undeniably sequester and store a large fraction of anthropogenic emissions of CO2, substantially slowing the pace of climate change.
Collectively, NbCS represent management approaches and technologies designed to increase net carbon uptake and/or reduce “natural” emissions of methane (CH4), ozone (O3), and nitrous oxide (N2O), which are powerful non-CO2 greenhouse gasses (hereafter GHGs). In general, land based NbCS can be classified into management approaches applicable to forested ecosystems, croplands and grasslands, and terrestrial wetland ecosystems.
While there is ample justification for implementing NbCS based on their co-benefits alone, for NbCS to succeed specifically as climate mitigation tools, they must meet four essential criteria:
- Criteria 1: Lead to enhancements to carbon uptake and/or reductions of non-CO2 GHGs that are additional to what would have occurred in a baseline or counterfactual scenario, and that integrate over all ecosystem sources and sinks.
- Criteria 2: Lead to net cooling such that the biophysical effects on water and energy cycling do not overwhelm the gains in carbon uptake or emissions reductions.
- Criteria 3: Achieve durable carbon storage by accounting for social and environmental risks to the permanence of ecosystem carbon storage and avoided GHG emissions.
- Criteria 4: Account for leakage so that gains in one area are not canceled out by shifting activities to another area.