Speakers:
Fang Jingyun, Academician of the Chinese Academy of Sciences, President of Yunnan University, and Professor at the School of Urban and Environmental Sciences, Peking University. His main research areas include global change ecology, community ecology and biodiversity, ecological remote sensing, and ecological grassland grazing.
Academician Jingyun explored the carbon sinks of China's terrestrial ecosystems from a carbon neutrality perspective and looked ahead to future research directions.
1. The connotation of carbon emissions and carbon neutrality
Carbon neutrality refers to achieving a balance between carbon emissions caused by fossil fuel use and land use changes with carbon sequestration from land and ocean absorption (i.e., carbon sinks) and other technological methods, resulting in net zero CO2 emissions. From an ecological perspective, there are four core factors for achieving carbon neutrality: first, emission reduction and sink enhancement are the two decisive factors for achieving carbon neutrality. Second, energy conservation, adjusting the energy structure, improving energy utilization efficiency, and developing low-carbon and clean energy are the core of emission reduction. Third, ecological construction, management, and protection are the core of the increased sink. Fourth, carbon capture, utilization, and storage (CCUS) technology only plays a supporting role. Based on this, the general formula for carbon neutrality is as follows: Carbon neutrality = anthropogenic carbon emissions - (marine and territorial carbon sinks + CCUS) = 0. In this formula, anthropogenic carbon emissions include emissions from fossil fuel use and land use. China's carbon neutrality equation is slightly different. Since China already includes land use carbon emissions when estimating land carbon sinks, land use emissions are zero; At the same time, considering that the ocean is a global public resource and countries should have equal access to marine carbon sinks, China's carbon neutrality equilibrium equation is: fossil fuel emissions ≈land carbon sink + global ocean carbon sink ×14/75 + CCUS.
On a global scale, from 2010 to 2019, 86% of anthropogenic carbon emissions came from fossil fuels, about 9.4 Pg C/a, while the remaining 14% came from land use, about 1.6 Pg C/a. Atmospheric storage and land-sea absorption are the main pathways of carbon absorption. The atmosphere absorbs an average of 5.1 PgC per year, accounting for about 46%; land absorbs 3.4 PgC per year, about 31%; and the ocean absorbs 2.5 PgC per year, about 23% (1 PgC = 3.667 billion Pg CO2 = 1 billion tons of carbon = 3.667 billion tons of CO2).
2. Estimation of China's land-based carbon sinks
Over the past 20 years, Chinese scholars have assessed the carbon sink size of China's terrestrial ecosystems based on forest inventory data, ground observation and statistical models, process models, and atmospheric inversion.
Based on forest survey data
Fang Jingyun and others published a paper in Science in 2001, first clarifying that over the past half-century, forest vegetation in China has been an important carbon sink, with an average carbon sink intensity of 0.021 Pg C/a from the 1970s to 1998.
Based on ground observation and statistical models
From 1981 to 2000, China's terrestrial vegetation carbon sink intensity was 0.096-0.106 Pg C/a, offsetting 14-16% of China's fossil fuel carbon emissions during the same period; If soil is considered, the total carbon sink is 0.14-0.18 Pg C/a, which can offset 21-27% of fossil carbon emissions.
Based on ground observations, process models, and atmospheric inversion
Piao et al. (2009), based on ground observations and scale transformation, carbon process model simulation, and atmospheric inversion, estimated the carbon sequestration capacity of China's terrestrial ecosystems from 1981 to 2000 to be 0.173–0.228 Pg C/a, which can offset 28–37% of China's fossil fuel carbon emissions during the same period. This value matches the ratio of global terrestrial ecosystem carbon sinks to human-caused emissions. As shown in Figure 1, compared to Europe and the US, China's terrestrial ecosystem has less carbon sink capacity than the US, but is close to Europe.
Figure 1. Carbon sink capacity of terrestrial ecosystems in China, Europe, and the United States
Source: Piao et al., 2009
Based on large-scale ground surveys
Based on large-scale survey data from the Chinese Academy of Sciences' carbon special project, Fang Jingyun and his team conducted a comprehensive survey and systematic study of carbon storage in China's terrestrial ecosystems. The study found that from 2001 to 2010, China's average annual land-based carbon sink was 0.201 PgC, which is comparable to existing research but reduced the proportion of fossil fuel carbon emissions offset during the same period to 14.1%. As shown in Figure 2, forest ecosystems are the main carbon sinks, contributing about 80% of the total. Farmland and shrubland ecosystems account for 12% and 8% of total carbon sinks respectively, while grassland ecosystems are basically in a carbon-neutral or weak carbon source state.
Figure 2. Carbon sink capacity of different types of terrestrial ecosystems
Source: Fang et al., 2018
Based on atmospheric CO2 monitoring and atmospheric inversion models
Recently, Wang et al. (2020) assessed China's terrestrial ecosystem carbon sinks based on CO2 monitoring and atmospheric inversion models. As shown in Figure 3, from 2010 to 2016, China's terrestrial ecosystem annual carbon sink was 1.11±3.8 PgC, equivalent to 45% of fossil fuel carbon emissions during the same period. However, this figure is relatively high. 1.1 billion tons of carbon correspond to one-third of the global terrestrial carbon sink, while China's land area accounts for only 6~7% of the global total, with more than half of the land located in arid and semi-arid regions. If 1.1 billion tons of carbon are evenly distributed among terrestrial vegetation, the value is higher than the primary productivity of vegetation, but in reality, carbon sinks are only a very small portion of vegetation productivity, only 5%-10%, so the research conclusions do not match reality. But it is worth noting that the study provides a method for evaluating terrestrial carbon sinks using atmospheric monitoring and atmospheric inversion models.
Figure 3. Annual carbon sinks in China's terrestrial ecosystems from 2010 to 2016
Source: Wang et al., 2020
Recent estimates based on ground observations
Recently, based on ground observation data and empirical models, we estimated carbon sinks for different terrestrial ecosystems in China. We found that in 2020, the estimated value of carbon sinks in China's terrestrial ecosystems was 0.292 Pg C/a, including forests at 0.212 Pg C/a, shrublands at 0.03 Pg C/a, grasslands and deserts at 0.007 Pg C/a, farmland at 0.034 Pg C/a, and wetlands at 0.009 Pg C/a.
Overall, carbon sink estimates vary greatly across different periods and methods, with a more reliable range of 0.12–0.30 Pg C/a (440–1.1 billion tons CO2/a). Therefore, reducing estimation errors is a key task going forward.
Meanwhile, an international research team led and organized by Professor Fang Jingyun also conducted a comprehensive assessment of global forest ecosystem carbon sinks (Figure 4). The study found that the global total forest carbon sink is 4Pg C/a, equivalent to half of global fossil fuel emissions. Tropical forests are near-neutral carbon sinks, changing the traditional view that "tropical forests are carbon sources."
Figure 4. Forest carbon sink capacity in different regions and periods worldwide
Source: Pan et al., 2011
In addition, Professor Fang Jingyun undertook a consulting project for the Chinese Academy of Sciences, conducting a comprehensive and systematic assessment of carbon emissions in China and globally, laying some groundwork for China's subsequent climate change policy formulation. Based on existing research findings, they recently published the book "China and Global Carbon Emissions," a relatively comprehensive report reflecting China's and global carbon emissions and carbon budgets.
III. Key Issues and Research Content
Currently, the estimation and forecasting of terrestrial ecosystem carbon sinks still face the following issues: First, almost all ecosystem types and current and future changes in soil carbon storage remain unclear. Second, biomass carbon sinks and their mechanisms to respond to future climate change are unclear. Third, how do soil carbon emissions change? How does soil respiration respond to future climate change? Furthermore, how can surface organic carbon be redistributed across different ecosystems? Finally, how do various media physically and chemically adsorb carbon on carbon?
From the perspective of ecosystem carbon sinks, future scientific questions will focus on studying the current status, future potential, mechanisms, and enhancement pathways of carbon sinks. Specifically, the research content includes the following four aspects: First, the regulatory mechanisms of functional attributes and diversity on the carbon cycle. Second, the mechanisms by which vegetation structure and environmental changes affect carbon sinks. Third, the driving mechanisms of microorganisms in key processes of the carbon cycle. Fourth, the technological approaches for enhancing sinks based on ecosystem management.
First, China's terrestrial ecosystems are significant carbon sinks but face significant uncertainty, with the largest possible estimates being 0.12-0.30 PgC/a. Second, emission reduction and sink enhancement are the two decisive factors for achieving carbon neutrality, with the core of sink enhancement being ecological construction, management, and protection. Finally, future research should strengthen precise estimation and forecasting of carbon sinks and their potential, as well as research on sink enhancement technologies and realization pathways.
(This article is compiled from the live speech records and PPTs.) ）
Compiled by:
Xu Qingwen, 2020 master's student at the Center for Urban Development and Land Policy Research, Peking University-Lincoln Institute.
