How large is the mitigation potential of natural climate solutions in China?

China is facing a huge challenge in achieving its carbon neutrality goal by 2060 given that it is currently one of the world’s largest greenhouse gas (GHG) emitters and has set a very short timeline for going from peak emissions to carbon neutrality. Natural climate solutions (NCS) that protect, manage, and restore ecosystems have shown substantial potential for increasing carbon sinks or reducing GHG emissions to offset fossil fuel CO2 emissions. This study quantified the mitigation potential of 18 NCS pathways in China at 0.67–1.65 Gigatonne of CO2 equivalent (Gt CO2e) yr−1 averaged from 2020 to 2060, which is even larger than the size of the current national terrestrial carbon sink. Compared with a previous global estimate, our results show a lower mitigation potential in forest ecosystems but a much greater potential for cropland ecosystems in China. From 2020 to 2060, all 18 pathways combined can provide cost-effective mitigation compared to the global social cost of CO2 emission and carbon prices, and 98.6% and 83.3% mitigation potential are cost-effective, respectively. This study further showed that immediate action provides the greatest mitigation. Our estimates highlight the important role of NCS in achieving the national carbon neutrality goal because of their large mitigation potential and cost-effectiveness.


Introduction
As one of the world's largest greenhouse gas (GHG) emitters, China plays an important role in the regulation of atmospheric GHG concentration and climate change. In 2019, the Chinese government set the goal of peaking its carbon emissions by 2030 and achieving carbon neutrality by 2060. This highly ambitious goal is challenged by the substantial amount of China's GHG emissions and the tight timeline for going from peak emissions to neutrality. This timeline of 30 years is much shorter than that of several other top emitters (Friedlingstein et al 2020), such as 75 years for the European Union (EU27), and 45 years for the United States (figure S1(c)). In addition, China's peak emissions was estimated to reach 10.40 Gt CO 2 e yr −1 (Zhang and Chen 2022) which are 1.70-4.12 times those of the other four top emitters (figure S1(b)). China will need to reduce its fossil fuel CO 2 emissions at an average rate of 0.32 Gt CO 2 e yr −1 , almost 2.96-6.53 times that of the other four top emitters (figure S1(d)).
Terrestrial ecosystems play a critical role in meeting this challenge of achieving carbon neutrality. Recently proposed natural climate solutions (NCS) (Griscom et al 2017) to improve ecosystem management have consistently been regarded as one of the most effective mitigation options to enhance terrestrial carbon sinks and reduce GHG emissions (Qin et al 2021a, Huang et al 2022. According to estimates by Griscom et al (2017), the maximum potential of NCS, which is constrained by food security, fiber security, and biodiversity conservation, is 23.8 Gt CO 2 e y −1 globally, more than twice the current global land carbon sink (Friedlingstein et al 2020). The NCS seems to be practical in China, considering that large-scale ecological projects have already been implemented since the early 1980s (Liu et al 2014, Lu et al 2018. In addition, as the world's leading food producer, China uses about 35% of global nitrogen fertilizers (Ju et al 2009, Song et al 2018, which have a large contribution to N 2 O emissions. Previous studies highlighted that China can reduce nitrogen fertilization use by about 30% without negatively impacting crop yields (Yin et al 2021), and thus contribute to substantial N 2 O emission reduction.
However, recent studies produced widely different estimates of the contributions of several major NCS pathways to increase the terrestrial carbon sink in China, indicating that there are large uncertainties. For example, the increase in carbon sink that could come from afforestation in China has been estimated at 0.21 Gt CO 2 e yr −1 by Xu et al (2010) and at 1.26 Gt CO 2 e yr −1 by Griscom et al (2017), an almost sixfold difference. In addition, although NCS pathways have a large mitigation potential, their cost of implementation is also very high. Previous evaluations did not adequately link the potentials of NCS with the national goal of carbon neutrality, and especially did not compare the costs of NCS with those of fossil fuel-related CO 2 emission reduction, providing very limited insights to policy-makers working on balancing both carbon mitigation strategies. Therefore, it is urgently needed to evaluate the cost-effectiveness of NCS pathways by comparing them with the cost of fossil fuel CO 2 emission reductions.
In this study, we evaluated China's climate change mitigation potential from 2020 to 2060 according to 18 NCS pathways, which include pathways for the forest, cropland, wetland, and grassland ecosystems. Specifically, this study compared the mitigation potential of NCS pathways with three action scenarios, and evaluated the impacts of implementation rates on mitigation potential. We further estimated the cost of all 18 NCS pathways and compared them with the social cost of CO 2 emissions (SCC), carbon price, and cost of carbon capture, utilization and storage (CCUS), to provide insights required for evaluating effective NCS pathways for mitigating climate change and achieving the carbon neutrality goal.

Methods
This study estimated the mitigation potential of 18 NCS pathways, including carbon sequestration or GHG emission reductions, on the premise of safeguarding the production of food and fiber. Therefore, we did not reduce existing cropland areas and only included cropland management interventions with no negative impacts on crop yields. The annual mitigation potential (M x ) of a specific pathway to increase the carbon sink or reduce emissions was calculated as the product of annual extent (A x : applicable land area or amount) and intensity (F: avoidable emissions or enhanced carbon sequestration per unit of extent), where M x = A x × F. In this paper, C is used when reporting carbon density, and converted to CO 2 e when calculating the final mitigation potential. Since timely action is likely to be the key to achieving the mitigation goal (Deng et al 2022, Zhu et al 2022, we defined three scenarios to examine the impacts of delayed action on the implementation of NCS. Three scenarios assumed that the potential extent (A p ) of all pathways will be fully implemented by the years 2030 (S30), 2045 (S45), and 2060 (S60), and the implementation rate was assumed to follow a linear variation from 2020 to these three target years. For example, for the afforestation pathway, there are currently a total of 50 Mha of suitable forest areas left for afforestation in China. In the S30 scenario, a total of 50 Mha of suitable forest area (A p ) will be fully afforested by 2030, and the total afforested area has been allocated evenly from 2020 to 2030. Therefore, A x of the NCS pathways will change linearly (figure 1). Eight out of all 18 pathways rely on an increase in carbon sequestration, and thus the extent is set to increase in the scenarios (figure 1(b)). In contrast, the other ten pathways rely on reducing gas emissions, and the extent will be decreased (figure 1(a)). Eventually, A x is calculated accordingly based on A p . Details about A p and F of all pathways have been summarized in the supplementary materials.
The uncertainty of each pathway was from the estimation of the extent and flux density. For most pathways, we estimated A p and F based on the means of historical statistics. Here, we calculated the standard deviation (SD), and convert it to 95% confidence interval (95% CI) as E = 1.96 ± SD/ √ n. As a result, the uncertainty of A x and F can be obtained, then the uncertainty of the total mitigation potential (E Mx ) was calculated as We estimated the cost of each pathway according to the recent literature (supplementary materials). Our estimates were done at the provinciallevel administrative regions. Hong Kong, Macao, and Taiwan were not included in the estimation due to the lack of relevant statistical data. In addition, to evaluate if the NCS pathways are cost-effective, this study compared the cost of all pathways with the SCC, carbon price, and the cost of CCUS. The social cost of CO 2 emissions is defined as all the economic damage that would result from emitting 1 ton of CO 2 e into the atmosphere, indicating the financial benefit of avoiding future projected damage. A recent study showed that the global SCC is about US$ 417 ton CO 2 e −1 , and China's SCC is US$ 24 ton CO 2 e −1 (Ricke et al 2018).
In addition, we also used a predicted carbon price and cost of CCUS to compare with the cost of NCS pathways. The predicted carbon price was derived from He et al (2021) and predicted until 2050 (table S1). The predicted cost of CCUS was derived from Cai et al (2021) (table S1). In this study, all costs and benefits were analyzed on the basis of US$ in 2020.

Mitigation potential of NCSs
There is a large mitigation potential from NCS in China by increasing the terrestrial carbon sink (eight pathways) or reducing GHG emissions (ten pathways). Under the scenario with immediate action (S30), i.e. fully implementing all NCS pathways before 2030, the accumulated mitigation potential is 55.5 (47-65.8) Gt CO 2 e from 2020 to 2060 ( figure 2(a)). Under the other two scenarios-delayed actions by completing all pathways until 2045 (S45) and 2060 (S60)-the accumulated mitigation potentials are 43.6 (37-51.8) Gt CO 2 e and 31.6 (26.7-37.6) Gt CO 2 e, respectively (figure 2(a)). The accumulated mitigation potential of the S30 scenario is about 1.3 and 1.7 times that of the S45 and S60 scenarios, respectively. In addition, benefiting from fast action, the S30 scenario can reach 90% of the mitigation potential by 2045, thus much earlier than the target year of carbon neutrality (i.e. 2060) ( figure  S2). In contrast, the two delayed scenarios reach the maximum carbon sink at a much later date (figure S2). The annual mean mitigation potentials of the three scenarios are in the range of 0.67-1.65 Gt CO 2 e yr −1 , which is comparable with the current terrestrial carbon sink in China (i.e. 0.69-0.95 Gt CO 2 e yr −1 ) .
The 18 NCS pathways show large differences in mitigation potential. The pathways implemented in forests contribute 48.6%-49.2% of all mitigation potential and cropland ecosystems contribute 44.7%-45.2%, while grasslands and wetlands combined only contribute about 6% (figure 2(b)). As one of the most important pathways, biochar application has a mitigation potential of 0.25-0.47 Gt CO 2 e yr −1 averaged from 2020 to 2060, accounting for 32%-33% of all pathways (figure 2(b)). Afforestation and avoided fuelwood harvest are two of the most important forest management pathways, and account for 14.8%-15.8% and 13.8%-14% of the total six forest pathways, respectively (figure 2(b)).

Costs of NCSs
Besides the large mitigation potential for increasing terrestrial carbon sinks or reducing GHG emissions, NCS pathways are also cost-effective compared to the SCC, carbon price, and the cost of CCUS (see section 2). Here we show the cost differences of various pathways under the S30 scenario, as the differences among the three scenarios are small. The cost of all 18 pathways ranges from US$ −304.2 ton CO 2 e −1 to US$ 1023 ton CO 2 e −1 (figure 3(a)). The negative cost indicates that the pathway is profitable. Compared to global SCC (US$ 417 ton CO 2 e −1 ) (Ricke et al 2018), 92.4% of the mitigation potential contributed by NCS pathways is projected to become costeffective in 2025, and almost all mitigations will be cost-effective after 2030 ( figure 3(b)). Compared with China's SCC (US$ 24 ton CO 2 e −1 ) (Ricke et al 2018), 63.8%, 56.7% and 75.7% of the mitigation potential associated with NCS will become cost-effective in 2030, 2045, and 2060, respectively ( figure 3(b)).  In addition, we compared the NCS cost with the predicted carbon price and cost of CCUS to evaluate if the NCS pathways are cost-effective at achieving the carbon neutrality goal. Predicted carbon prices from 2025 to 2050 show an increasing trend from US$ 21.71 ton CO 2 e −1 in 2025 to US$ 1167.85 ton CO 2 e −1 in 2050 (table S1). On the contrary, the mean cost of all 18 pathways decreases contemporarily ( figure S3). Therefore, all the mitigation potential of NCS can be cost-effective by 2050 compared to the carbon price ( figure 3(b)). CCUS encompasses methods and technologies to remove CO 2 from flue gas and from the atmosphere, followed by recycling CO 2 and determining safe and permanent storage options. CCUS is an important pathway to achieving carbon neutrality, and the predicted cost of CCUS will decrease from US$ 48.67-120.89 ton CO 2 e −1 in 2030 to US$ 21.98-64.37 ton CO 2 e −1 in 2060 as a result of technological improvements (Cai et al 2021). Compared to CCUS, the cost of NCS shows a larger decrease; therefore, by 2060, there will be a larger fraction (75.7%-82%) of cost-effective NCS mitigation ( figure 3(b)).

Provincial-level mitigation potential
There are large differences in NCS's mitigation potential across various provinces. Compared to the carbon price, in 2050, almost all NCS pathways are cost-effective. Under the S30 scenario, the largest potential is 0.19 Gt CO 2 e yr −1 for Inner Mongolia in 2050, which is 40 times higher than the smallest one (i.e. Beijing) (figure 4(a), table S2). In addition, Heilongjiang, Henan, and Shandong show a large mitigation potential, reaching 0.14, 0.09, and 0.08 Gt CO 2 e yr −1 , respectively (figure 4). In contrast, Beijing, Shanghai, and Tianjin show a low mitigation potential. In general, provincial mitigation potentials are positively related to their arable land areas ( figure S4). Thirty-one investigated provinces and municipalities belong to either cropland or forestdominated provinces ( figure S5). On average, in over 11 provinces and municipalities, cropland NCS pathways contribute more than 50% of mitigation potential. In the other 15 provinces, forest NCS pathways dominate the mitigation potential ( figure S5).
In addition, there are large differences in the mean cost of all 18 pathways. The mean cost of all pathways ranges from US$ −47.6 ton CO 2 e −1 to US$ 55.1 ton CO 2 e −1 across all investigated provinces ( figure 4(b)). Shandong, Henan, and Jiangsu provinces show the highest gain in NCS pathways, with more than US$ 40 ton CO 2 e in profits, and the cost of mitigation in Tibet is the largest, with US$ 55.1 ton CO 2 e −1 ( figure 4(b)). In contrast, the provincial-level abatement costs show a negative relationship with cropland areas (figure S4), pointing to the important role of agricultural ecosystems in determining NCS's potential.

Discussion
This study demonstrates the large mitigation potential of NCS in China. Our estimates show that all 18 pathways can provide a total mitigation potential of 0.67-1.65 Gt CO 2 e yr −1 averaged from 2020 to 2060, which is slightly larger than the current terrestrial carbon sink in China (i.e. 0.69-0.95 Gt CO 2 e yr −1 ) . None of the pathways would threaten food security, fiber security, or biodiversity conservation. Compared to the SCC, carbon price, and cost of carbon capture and storage, the NCS pathways with the most potential are still cost-effective (figure 3). For example, all mitigation potentials are cost-effective in 2050 compared to the carbon price and can double China's current terrestrial carbon sink. Our results highlight that NCS is one of the most important solutions for achieving a carbon neutrality goal because of both the large magnitude effect and cost-effectiveness (Qin et al 2021a).
Recent studies have formulated the pathway for reducing fossil-fuel CO 2 emissions, but the role of NCS pathways has not been fully considered (Chen and Lin 2021, Wang et al 2021. For example, a recent study provided future roadmaps for reducing CO 2 emissions for various sectors to achieve the carbon neutrality goal, but treated the terrestrial carbon sink as a constant (Wang et al 2021), which may largely underestimate the mitigation potential of terrestrial ecosystems. As terrestrial ecosystems play an irreplaceable role in offsetting fossil fuel CO 2 emissions to achieve the carbon neutrality goal, the underestimations of the mitigation potential of terrestrial ecosystems may put a great mitigation burden to other sectors (i.e. energy, industry, transport, and construction). This study highlights that future studies should adequately incorporate the contributions of NCS to terrestrial carbon sink to accurately formulate pathways for achieving carbon neutrality with the lowest economic costs.
This study also shows that earlier implementation of NCS will contribute to a larger mitigation potential. For example, we found that under the S30 scenario, the full implementation of all NCS pathways can reach 90% of the maximum potential by 2045, advancing the target year of carbon neutrality by 15 years from its current goal of 2060. Delayed implementation of NCS pathways may push back the timeline of China's carbon neutrality goal. One of the most important reasons for this difference is that the carbon sink magnitude of an afforestation project is quite small during the initial years and increases gradually with stand age (Liu et al 2014). In addition, although most pathways do not have lagged mitigation effects, the implementation to large extent is still very challenging. The Chinese central government requires the local governments to formulate their own roadmaps for carbon neutrality; however, NCS has been seldom considered by the current roadmaps. Previous studies, as well as ours, suggest that immediate actions of NCS are important for mitigating climate change (Qin et al 2021b).
Similarly, a recent study estimated the mitigation potential of NCS in China, and reported a mean mitigation potential of 0.6-1.4 Gt CO 2 e yr −1 during 2020-2060 (Lu et al 2022). Although the total potentials estimated by this study are comparable with the estimates by Lu et al (2022), there are still large differences among several major pathways, primarily due to the different assumptions assigned and different data applied to the pathways. For example, the estimated mitigation potential of 'cropland nutrient management' by Lu et al (2022) is over six times higher than our estimate. In estimating the amount of nitrogen fertilizer application that could be reduced, Lu et al used a projection of future nitrogen fertilizer application in China (Gu et al 2015), while we used the historical nitrogen fertilizer application provided by the Chinese Agricultural Statistical Yearbook. We have reason to believe that it is an urgent task to produce accurate basic datasets about ecological management to improve the estimates of mitigation potentials of NCS. More importantly, this study highlights the importance of timely action, as many studies recently reported, delayed action could significantly reduce the mitigation impacts NCS could have on climate (Qin et al 2021b, Deng et al 2022, Zhu et al 2022. Compared with Griscom et al (2017), this study estimated a lower mitigation potential of NCS pathways for forest ecosystems but a higher mitigation potential for cropland ecosystems in China. Griscom et al (2017) estimated a mitigation potential of 1.26 Gt CO 2 e yr −1 only by afforestation and reforestation in China, which is higher than the current net carbon sink of all terrestrial ecosystems of China (i.e. 0.69-0.95 Gt CO 2 e yr −1 ) . With further considerations on the extent of afforestation and carbon sequestration dynamics, our estimate is 0.32 Gt CO 2 e yr −1 at most, 25% of Griscom et al (2017). In addition, NCS pathways for agricultural ecosystems show a comparable mitigation potential with forest ecosystems in China (figure 2), which is different from the contributions on a global scale (Griscom et al 2017). Large crop production in China may be one of the most important causes for the dominant role of NCS in cropland ecosystems. According to statistical data, China is the world's largest producer of food, and the total production of primary crops in China was about 0.66 Gt in 2019, accounting for 24.4% of global production (FAO 2020). According to our estimates, there are 5.55 × 10 8 tons of crop residues that can be used for biochar production. Overuse of nitrogen fertilizers in China is another important reason for the large mitigation potential of agricultural NCS. Nitrogen fertilizer application can directly lead to N 2 O emissions with an average emission factor of about 0.3%-3% (Shcherbak et al 2014).
China consumes approximately 30% of global nitrogen fertilizers, and their overuse results in high N 2 O emission levels (Ju et al 2009, Song et al 2018. It has been estimated that a 21%-33% reduction in current nitrogen fertilizer rates would have no impact on crop yields (Zhang et al 2020, Yin et al 2021, Qin et al 2021c. Although this study updated the mitigation potential of NCS by integrating the results from the latest studies, the potential may still be underestimated due to several knowledge gaps. First, this study estimated the mitigation potential of NCS on the premise of safeguarding food production; therefore, we kept the cropland area constant. However, the continuous increases in agricultural production and food yield due to agricultural technological improvements may allow reducing cropland areas to implement afforestation pathways that increase terrestrial carbon sinks. For example, during the past 30 years, China's crop production has increased at the rate of 2.78% (Yuan et al 2018). Second, several pathways may improve the profitability of forestry and arable agriculture, which can offset their own costs and make the implementation widespread (Liu et al 2014). Numerous studies have reported that biochar applications can effectively enhance agricultural yields and quality while reducing fertilizer and cultivation costs (Collison et al 2009). Although a growing number of studies have been conducted to evaluate the comprehensive economic benefits, they are still difficult because of a scarcity of long-term field data on crop yield enhancements and uncertainty of biochar supply and production costs (Roberts et al 2010). Therefore, field observations and experiments are urgently needed to support a comprehensive evaluation aiming to estimate the implementation potential of NCS pathways accurately.

Data availability statement
Any data that support the findings of this study are included within the article and/or the supplementary materials.