Anthromes, CO2 and Terrestrial Carbon – Session 7: CO2-fertilization and human land-use contributions to future terrestrial carbon storage
Authors: Roe Stephanie , Cook-Patton Susan , Griffey Vivian , Felzer Benjamin , Koven Charlie 
Published: March 2023

4.00 – 4.20: What is the mitigation potential of improved land-use, and how will it respond to future climate change? , Stephanie Roe

4.20 – 4.40: Agroforestry as a natural climate solution, Susan Cook-Patton

4.40 – 5.00: Global Carbon Dioxide Removal Potential of Trees in Agriculture , Vivian Griffey

5.00 – 5.20: Effect of land use and land cover change and CO2 fertilization on the future carbon sink for the conterminous U.S, Benjamin Felzer

5.20 – 5.40: What will happen to the terrestrial carbon sink once we reach net zero?, Charlie Koven

What is the mitigation potential of improved land-use, and how will it respond to future climate change?

Improved stewardship of land to reduce GHG emissions, enhance carbon removals and protect the residual sink have gained significant attention and importance in delivering on the Paris Agreement goal of limiting warming to 1.5°C and 2°C. To better understand the land sector’s role in mitigation pathways, this presentation will examine mitigation potentials for 20 land-based measures across >200 countries, and compare methods used in the IPCC reports including technical and economic estimates using “bottom-up” sectoral and integrated assessment model approaches. To address one of the main gaps in mitigation potential literature identified by the IPCC, this presentation will also explore the impact of future climate change on land-based sequestration potential.

Agroforestry as a natural climate solution

Restoring tree cover is a prominent climate solution, with the potential to remove gigatonnes of carbon dioxide out of the air. However, achieving this potential will depend on many human decisions about how and where to restore tree cover. In particular, agroforestry is a promising option, given its potential to simultaneously store additional carbon, enhance livelihoods, and support biodiversity. However, substantial uncertainty remains around how much carbon can be captured. One of the central challenges is the sheer diversity of agroforestry practices employed across the globe. Species identity, planting density, and management practices, as well as many other factors, will influence the overall climate mitigation potential of an individual agroforestry system. Although recent reviews have begun to compile carbon sequestration rates and stocks within agroforestry systems, the current evidence base is not fully comprehensive. Individual reviews examine only a subset of the existing literature and typically partition agroforestry systems into coarse categories that do not reflect the diversity of actual on-the-ground practices. As individuals, corporations, and governments decide whether and how to deploy agroforestry as a climate solution during this climate critical decade, there is a strong need for a readily available and comprehensive dataset to better predict climate outcomes across diverse agroforestry systems. We are therefore conducting a systematic review across over 25,000 published studies to find empirical estimates of carbon sequestration rates and stocks in agroforestry systems. To date, we have compiled data from over 1000 papers into a consistent data structure. Our goal is to create a publicly available dataset that can help to accelerate our scientific understanding of the climate mitigation potential of these human-natural ecosystems. Although agroforestry offers high potential as a climate solution, delivering on that promise requires a more precise understanding of how much carbon can be captured, based on the best available data.

Global Carbon Dioxide Removal Potential of Trees in Agriculture

Trees in agricultural systems have been identified as a high-potential Natural Climate Solution with benefits to biodiversity and climate resilience. However, efforts to quantify their CO2e mitigation potential at a global scale have been based on standing biomass, without carefully addressing potential impacts on agricultural production. We estimated the enhanced carbon sequestration potential of increasing tree cover without compromising long-term agricultural yields, starting with a Delphi-style expert elicitation on ideal tree types and densities in major cropping and grazing systems, disaggregated by climatic zones (biomes). Comparing expert-recommended tree density values to current levels, we applied the spatially explicit Cook-Patton (2020) aboveground carbon accumulation rates to create a 30x30m resolution global map of the additional carbon sequestration potential of trees in agriculture over the next 30 years, analyzing the results by biome and country. From the Delphi-style elicitation, we found a suggested pattern of recommended tree cover related to aridity, with the lowest recommended tree cover values in deserts. Carbon sequestration potential per hectare for both crop and grazing lands was generally higher in small tropical countries, such as Laos and Malaysia, but also high in small wet, temperate countries, such as Ireland and Belgium.

Effect of land use and land cover change and CO2 fertilization on the future carbon sink for the conterminous U.S

Historic land use and land cover change (LULCC) has been responsible for the largest release of CO2 from terrestrial ecosystems, while climate change, rising atmospheric CO2 levels, nitrogen deposition, and surface ozone have all contributed to changing carbon dynamics since the 1860s. My current work with the Terrestrial Ecosystems Model (TEM) highlights the importance of land-use legacy on the future carbon sink, even without further LULCC, due to soil nutrients and forest stand-age legacy. The new experiments will start from initial conditions that account for land-use legacy from 1750. This study will involve using the SSP3rcp7 climate from NCAR CESM2 from 2015 – 2099 for the conterminous U.S., downscaled and bias correct to CRU4.04 historical data. The land use transitions are developed from Land Use Harmonization (LUH2) data into land cover cohorts, starting from fractional land cover at 2015. Future U.S. runs with TEM will involve experiments with and without LULCC, and with and without future changes in CO2, to evaluate the effect of LULCC and CO2 fertilization. Current results for the conterminous U.S. without LULCC for the RCP8.5 scenario show increasing vegetation carbon, little change in soil carbon, with increasing cumulative Net Ecosystem Productivity and carbon storage.

What will happen to the terrestrial carbon sink once we reach net zero?

Currently about half of anthropogenic CO2 emissions are taken up by land and ocean sinks, which on land has been driven largely by CO2 fertilization. While a large amount of research has focused on how these sinks may weaken under further unmitigated CO2 emission scenarios, less focus has been on the behavior of these sinks under highly mitigated future scenarios. Here we ask how we expect carbon sinks to behave if we are able to achieve net zero or net negative carbon dioxide emissions, what are some of the key uncertainties governing these coupled carbon-climate responses to net zero and net negative emissions, and how these dynamics may inform both climate policy and the potential for using biospheric sinks as a form of carbon dioxide removal.