https://orcid.org/0000-0002-6913-5267
Abstract
Accelerated global urban expansion not only directly occupies surrounding ecosystems, but also induces cascading losses of natural vegetation elsewhere through cropland displacement. Yet, how such effects alter the net primary productivity (NPP) worldwide remains unclear. Here, we quantified the direct and cascading impacts of global urban expansion on terrestrial NPP from 1992 to 2020 and projected the impacts under the shared socioeconomic pathways framework by 2100. We found that global urban expansion caused a cascading loss of 29.2 to 63.9 Tg C/year of terrestrial NPP in the historical period (1992–2020), accounting for 13–29% of the total direct NPP loss. Instead, our projections indicate that during 2020–2100, mainly due to the increased relocation of displaced croplands to low-productive ecosystems, the cascading impacts gradually change from negative to positive, leading to a net NPP increase. Such an increase may offset up to 7% of the total direct NPP loss, better balancing crop compensation with NPP maintenance. Our findings highlight the unexpected large cascading impacts of urban expansion on the carbon cycle and stress the importance of regulating land transitions to curtail land-use emissions.
The accelerated global urban expansion poses widespread challenges to sustainable development. In addition to the direct occupation of the local surrounding vegetation cover, cropland displacement due to urban expansion might cause additional vegetation loss and thus alter the terrestrial net primary productivity (NPP). Our study assesses the cascading impacts of both observed and future urban expansion on NPP through cropland displacement in a continuous and comparable framework. Our findings reveal an important but commonly overlooked dimension of global urban expansion impacts on terrestrial carbon cycling and suggest that urban land development under nonsustainable pathways deserves much greater attention in the future, especially as climate change mitigation efforts seek to curtail land-use change emissions and a growing population strives for food sustainability worldwide.
Introduction
The world has experienced accelerated urbanization in recent decades and will continue in the 21st century (1). Urban expansion causes the destruction and degradation of ecosystems and the decline in biodiversity by directly occupying surrounding natural vegetation (2, 3). Changes in the biophysical and chemical characteristics of the land surface induced by urban expansion can further affect the terrestrial carbon cycle (4) (i.e. direct impacts, Fig. S1A). As an important component of the ecosystem carbon cycle, terrestrial net primary productivity (NPP) is a key indicator of ecosystem response to climate change, anthropogenic disturbances, and other factors (5). The location, frequency, and magnitude of NPP variations directly caused by urban expansion have been extensively discussed in past studies at different scales, suggesting that vegetation loss caused by urbanization can result in an NPP decline (6, 7). In fact, the impact mechanism of urban expansion on vegetation loss can be quite complex (8). It has been evidenced that, due to land competition between agricultural and urban use, croplands have been spatially shifted from areas close to cities to more remote areas in many countries (9, 10). Geographic shifts in cropland driven by urban expansion, namely cropland displacement, also potentially contribute to cascading losses of original natural vegetation elsewhere (i.e. cascading impacts, Fig. S1B).
Cropland displacement can lead to a series of environmental issues such as the loss of natural habitat and the imbalance of yield between lost and new cropland (11). The losses of natural vegetation due to urbanization-induced cropland displacement have been evaluated or predicted in several studies at global (2), national (11), regional (8), and local (12) scales. Some studies further predicted the impact of cascading loss of natural vegetation on ecosystem services in the future (13, 14). However, few studies have assessed the cascading impacts of both observed and future urban expansion on the terrestrial carbon cycle through cropland displacement in a continuous and comparable framework at the global scale. The replacement of vegetation with different carbon functions by displaced lands due to urban growth may have different impacts on the terrestrial carbon cycle (15). Therefore, accurately capturing complex land-use/land-cover (LULC) changes based on reliable historical and future global LULC data with a high spatial resolution is essential to investigate the impact of cropland displacement on the carbon cycle. Moreover, previous studies have suggested that a reasonable development pathway and policy could abate the carbon emission caused by cascading land-use change (16, 17). Therefore, under the social and economic development pathways depicted by different shared socioeconomic pathway (SSP) scenarios (18), the cascading impacts of future urbanization on terrestrial NPP also need to be discussed further.
This study is aimed at assessing the direct and cascading impacts of global urban expansion on terrestrial NPP from 1992 to 2100. The direct impacts refer to the direct occupation of vegetation by newly urbanized lands and its effects on terrestrial NPP (Fig. S1A). The cascading impacts refer to the change in terrestrial NPP through cropland displacement to equivalently compensate for the urbanization-induced crop production losses (Fig. S1B). In this process, croplands may be displaced to other locations within the same region, or across regional boundaries to more distant places. Therefore, we performed two separate experiments under different strategies: continental-scale displacement (CSD) strategy and global-scale displacement (GSD) strategy, which assumed that the equivalent crop production could be compensated on each continent or globally, respectively. Here, we quantified the direct and cascading impacts of global urban expansion on terrestrial NPP from 1992 to 2020 using the Europe Space Agency Climate Change Initiative Land Cover (CCI-LC) products and two ecological models, namely the Carnegie–Ames–Stanford Approach and the modified Lund–Potsdam–Jena Dynamic Global Vegetation Model (LPJ-Hydrology). Additionally, using a newly developed 1-km global LULC product (19), we calculated the NPP changes from 2020 to 2100 under the SSP framework and explored the characteristics of direct and cascading impacts of future urban expansion on NPP guided by different development paths.
Results
Cascading loss of natural vegetation exceeds direct loss
Global urban lands increased by 455.7 thousand km2 from 1992 to 2020 (Table S1), with a net expansion rate of 134%. It is noteworthy that 63% of the newly urbanized areas came at the expense of cropland shrinkage. In addition to directly occupying terrestrial vegetation, global urban expansion can also lead to cascading changes in natural vegetation through cropland displacement. Under the CSD strategy, the required newly cultivated croplands to compensate for urbanization-induced crop production losses were 2.3 times (657.0 thousand km2) that of direct cropland loss caused by urbanization in the past 28 years (Fig. 1). Under the GSD strategy, newly cultivated croplands required were 2.6 times (752.1 thousand km2). The above results were calculated using actual crop yield. In the remaining presentation of the results, unless otherwise noted, our calculations were all based on actual yield.
The direct loss of cropland area and crop production induced by urban expansion, and the cascading loss of various natural vegetation areas under the CSD and GSD strategies. The SSP scenario corresponds to the period from 2020 to 2100. The error bars show the total amount of cascading area loss calculated based on potential yield.
For the future decades, along with the declining global urban population (20), the rate of urban expansion will gradually slow down in all scenarios (Fig. S2A). According to our projections, the proportion of cropland occupied by newly urbanized lands will decline over the period 2020–2100 (Fig. S3A), with only 34–39% of new urban lands that may directly come from cropland. As for the cascading changes, the required croplands to compensate are expected to be 1.5–1.7 times that of the lost area under the CSD strategy and 1.8–2.3 times under the GSD strategy (Fig. 1). Considering the uncertainty caused by the level of land-use management, we also calculated the effect of urbanization on cascading vegetation loss utilizing potential crop yield (see Supplementary material, Materials and methods). The results show that enhanced land management can reduce the area of cascading loss of natural vegetation by 10–30% (Fig. 1).
Temporal trends in global cascading NPP changes
From 1992 to 2020, the overall observed cascading impact was found to be negative. Compared with the direct impact caused by urban expansion during the same period, the net change in terrestrial NPP due to cascading impact was relatively small but not negligible. Under the CSD and GSD strategies, the net cascading NPP loss due to global urban expansion accounted for 13% (29.2 Tg C/year) and 29% (63.9 Tg C/year) of the total direct NPP loss, respectively (Tables S1 and S2). The net cascading NPP loss can be associated with the fact that the relocation of displaced croplands occurred mainly in forests during this period (Fig. S3B). Deforestation for agriculture caused severe and irreversible losses of terrestrial NPP (Table S2). In addition, we noted that the cascading impact on NPP showed different trends over time under the two different displacement strategies (Fig. 2). The results based on the CSD strategy could better reflect the regional heterogeneity. Particularly from 1999 to 2006, Asia and Europe showed quite a difference under the two displacement strategies. On the one hand, during this period, urban expansion in Asia and Europe accounted for 73% of the world’s total. Correspondingly, the amount of cropland to be compensated for in these two regions exceeded 88% of the global total, resulting in quite drastic changes in NPP. On the other hand, more than half of global cropland growth occurred in Africa and South America, mostly largely at the expense of forest resources, and only 16% of global croplands were relocated to bare lands with access to irrigation. All regions therefore exhibited a net cascading loss of terrestrial NPP under the GSD strategy. But under the CSD strategy, 36% of croplands were relocated to bare lands with access to irrigation in each of Asia and Europe, with both regions contributing to the cascading net increase in global NPP over this period (Fig. S6).
Temporal changes in the cascading impacts of global urban expansion on terrestrial NPP from 1992 to 2100. Cascading impacts under A) the CSD strategy and B) the GSD strategy. The error bars show the 95% CI.
However, global terrestrial NPP is expected to be partially compensated in the future. Overall, from 2020 to 2100, global urban expansion may lead to a cascading net increase in terrestrial NPP. Under the CSD strategy, terrestrial NPP is forecasted to increase indirectly by up to 19.0 Tg C/year, while under the GSD strategy, it is expected to increase indirectly by up to 17.8 Tg C/year (Table S2). Such increases are to offset up to 7% of the total direct NPP loss due to global urbanization. Although the crop production loss is equivalently compensated through cropland displacement, the direct loss of terrestrial NPP due to urban expansion is difficult to fully reverse. In general, the cascading NPP growth can be attributed to the increased relocation of displaced croplands to low-productive ecosystems, especially shrublands and grasslands (Figs. 3 and S3B). We further analyzed the temporal response of terrestrial NPP to the cascading impacts of global urban expansion under different SSP scenarios (Fig. 2). The SSP1, SSP2, and SSP4 scenarios all exhibit similar trends under the CSD and GSD strategies, that is, the cascading NPP increase is expected to remain particularly prominent until the 2050s. Then, in the second half of this century, along with the slowing global urban expansion due to declining urban populations (Fig. S2A), the magnitude of cascading NPP changes also gradually softened. Before the 2050s, SSP1 has a higher net growth rate than all the other four scenarios, followed by SSP4. In SSP3, as a result of continued population growth but low population urbanization (21), urban land is growing steadily in relatively small quantities, with only a slight decline in the growth rate (Fig. S2A). Therefore, unlike the three aforementioned scenarios where the NPP changes will tend to zero over time, continuous urbanization-induced cropland loss is keeping the cascading changes in all kinds of natural vegetation at a stable level (Fig. S4), and so does the overall changes (Fig. 2). In the SSP5 scenario, terrestrial NPP is forecasted to maintain a stable cascading net increase under the CSD strategy due to the high level of population urbanization (Fig. 2A). But under the GSD strategy, the cascading impact of urban expansion on NPP in SSP5 is even projected to be severely negative in the 2040s (Fig. 2B), mainly because the relocation of displaced croplands may be more (∼47%, Fig. S3B) to forests. In the second half of this century, the cascading growth rate of NPP under the GSD strategy for the SSP5 scenario is expected to be significantly faster than all the other four scenarios, and its growth rate is increasing over time.
Spatial patterns of relocation of displaced cropland and resulting changes in terrestrial NPP. A) From 1992 to 2020. B–F) from 2020 to 2100 in the SSP1–SSP5 scenario, respectively. For better visualization, we implemented a block average analysis, with a rectangular neighborhood of 10 cells.
Relocation patterns of displaced croplands
We first analyzed the spatial relationship between relocated croplands and urban areas. Relocated croplands were classified into three categories: located in peri-urban areas, located within 100 km of urban centers, and located far from urban areas. The results showed that the spatial relationship between relocated croplands and urban areas varied considerably between regions, but varied little between original vegetation types (Fig. S5). Globally, from 1992 to 2020, 3% of cropland compensation occurred in peri-urban areas, 29% within 100 km of urban centers, and 69% away from cities. Likewise, in the coming decades, 3–6% of cropland will be relocated to peri-urban areas, 28–36% within 100 km of urban centers, and 61–66% away from cities. In Africa and North America, the share of the three types of relocated cropland is similar to that of the globe. In Asia and Europe, the largest amount of cropland is relocated to within 100 km of urban centers. However, in South America and Oceania, the distance between food production and consumption is increasing, as more than 72% of cropland is compensated in areas far from the cities in South America, and even exceeding 92% in Oceania.
We then analyzed the changes in NPP from cropland displacement. In the process of cropland displacement, displaced croplands are relocated to different original natural vegetation elsewhere, leading to different terrestrial NPP responses. It is worth noting that deforestation for agriculture exerts serious negative impacts on terrestrial ecosystems (Table S2). In the past period, Asia was a hotspot for cascading NPP loss due to the relocation of displaced croplands to forests (F2C, Fig. 3). Asia suffered a cascading NPP loss of 56.0 and 69.7 Tg C/year from 1992 to 2020 due to F2C under the CSD and GSD strategies, respectively, which was more than all other regions of the world combined (Fig. S6). In the future, Asia may still be a hotspot for cascading NPP loss due to F2C under the GSD strategy. However, under the CSD strategy, Africa in the SSP4 scenario and North America in the SSP5 scenario may suffer more severe cascading NPP losses than Asia due to the faster urban expansion.
In the past period, the relocation of displaced croplands to shrublands (S2C) resulted in a cascading NPP loss of 2.9 Tg C/year under the CSD strategy but gained a cascading NPP increase of 5.7 Tg C/year under the GSD strategy, which contributed very little to the global overall cascading NPP change. After 2020, the contribution of S2C to the overall cascading NPP changes is forecasted to greatly enhance worldwide under the GSD strategy (Fig. S6B). But under the CSD strategy, S2C is the most dominant contributor to the overall cascading NPP growth in Oceania and South America (Fig. S6A), probably due to the local prevailing vegetation (2).
The relocation of displaced croplands to grasslands (G2C) led to a cascading NPP increase of 14.0 Tg C/year under the CSD strategy and 10.6 Tg C/year under the GSD strategy from 1992 to 2020. Similar to S2C, G2C may play a more important role in the cascading impacts on NPP in the coming decades. In terms of quantity, under both the CSD and GSD strategies, Asia shows the largest cascading NPP growth due to G2C in the other four SSP scenarios except SSP5, followed by North America. In SSP5, half of the global cascading NPP growth due to G2C may come from North America. Additionally, we noticed that from 1992 to 2100, G2C may lead to a cascading loss of terrestrial NPP in Oceania under the CSD strategy in SSP1, SSP2, and SSP3 (Fig. S6A).
The relocation of displaced croplands to bare lands with access to irrigation (B2C) can also induce an increase in terrestrial NPP. Unlike the three displacement activities mentioned above, the contribution of B2C to the global overall cascading NPP change varies little between the historical and future periods (Fig. S3B) but varies significantly between regions. We particularly noted that from 1992 to 2020, B2C led to a cascading NPP increase of more than 9 Tg C/year in Europe under the CSD strategy, making Europe the only region to obtain a net cascading NPP growth in the historical period (Fig. S6A). We noted that in some places in Africa and the Middle East, there are cases of urban expansion pushing cropland toward the desert (Fig. S10). However, bare land irrigation is not the most dominant form of cropland expansion due to the limitations of natural and geographic conditions in these regions and the high cost of constructing and upgrading irrigation infrastructure (Fig. S5). Hence, the contribution of B2C to the cascading NPP increase is not significant.
Displacement cost index
Our results showed that urban expansion has always come at the cost of fertile land resources, such as croplands and forests (Fig. S3A). In particular, the loss of croplands exerts profound impacts on crop production, food supply, and so on. Here, we found that global annual crop production lost ∼140.8 million tons (Mton) from 1992 to 2020 (Fig. 1), consistent with the results reported by van Vliet (2) for the same period. Over the period 2020 to 2100, urban expansion-induced cereal crop production losses will be 41.5–108.5 Mton per year. According to the standard from the Food and Agriculture Organization of the United Nations, the annual food security of 456–623 million people could be guaranteed by the 182.3–249.3 Mton of crop production lost from 1992 to 2100.
The process of cropland displacement to compensate for urbanization-induced crop production loss is accompanied by many issues of concern. The first is the imbalance of yield between lost and new cropland (11). We found that the average yield of newly added croplands is almost always lower than that of lost croplands worldwide, except for the barley yield in SSP3 (Fig. S7). This indicates that the quality of newly added croplands is often poorer than that of the lost croplands, which is in line with a previous study (22). It also means that compensating for the equivalent urbanization-induced crop production loss requires displacing more croplands.
Another issue is how to balance the need to compensate for crop production with the need to maintain terrestrial NPP (23). To explore the tradeoff between crop production compensation and cascading NPP changes, we calculated the displacement cost index (DCI), i.e. the ratio of cascading NPP changes per unit of displaced crop production (Fig. 4). In the past period (1992–2020), the global average DCI under the GSD strategy was −0.46, representing that the displacement of 1 kg of the crop was accompanied by a cascading NPP loss of 0.46 kg C, while under the CSD strategy, each kg of crop displacement resulted in a cascading NPP loss of 0.21 kg C. Under the CSD strategy, Africa paid the highest ecological cost for compensating unit urbanization-induced crop production, with a DCI of −0.93, followed by South America. While Europe was the only region with a positive DCI, the displacement of each kg of crop resulted in a cascading NPP increase of 0.16 kg C.
Continental summaries for the DCI under the CSD and GSD strategies. The SSP scenario corresponds to the period from 2020 to 2100. The error bars show the 95% CI.
In the coming decades, the global average DCI is expected to be positive. Under the GSD strategy, the SSP1 scenario may best balance crop compensation with NPP maintenance, with a global average DCI (0.30) much higher than other SSP scenarios. It is alarming that crop compensation in Africa may have more ecological costs, with DCI always lower than the global average in all five SSP scenarios. Under the CSD strategy, the SSP1 scenario also shows the highest DCI of all SSP scenarios. The global average DCI for the SSP2 scenario may not be very high, but it shows minor inter-regional variability (according to the statistics of the coefficient of variation). The global average DCI for the SSP3 and SSP4 scenarios is expected to rank only behind the SSP1 scenario, but these two scenarios are characterized by significant inter-regional differences. Especially in Africa, compensating for each kg of crop production may still come at the expense of cascading NPP losses of about 0.6 kg C.
Discussion
A long-term high-resolution global LULC dataset plays a critical role in enhancing the reliability of land-related environmental and sustainability studies (24). Based on CCI-LC data, we found that global newly urbanized lands in the historical period were mainly concentrated in the northern temperate zone (4) (Figs. S8 and S9), and occupied a considerable amount of high-quality cropland in North America and Asia (25). However, according to the results from the newly developed global future LULC dataset at a 1-km resolution (19), the future urban expansion will encroach less on croplands. Driven by the growth of urban population, urban expansion in Asia and North America will gradually slow down (Fig. S2), while Africa will become the new hotspot for urban land growth (26) (especially in the tropical northern hemisphere). In Africa, due to the complication in the land tenure systems (27, 28), the impact of urbanization on cropland is relatively small compared with other regions. Instead, nonprivate and high-productive natural vegetation, such as forests in pan-tropical Africa, have attracted the attention of many researchers for their high urbanization rates in the future (29). Using the Lower Guinean forests as an example (Fig. S11), our future LULC products capture well the spatial details of forest loss in Africa due to urban expansion.
In addition to directly occupying natural vegetation, urbanization also leads to a cascading loss of other natural vegetation elsewhere. Our results stress that future relocation of displaced croplands may be more to ecosystems with lower productivity than forests (Fig. S5). One possible reason is that increasing temperatures in the future enhance cropland suitability in mid- to high-latitude regions such as the US Northern Great Plains, the Mongolian Plateau, and the southern Cerrado, threatening more subtropical and temperate grasslands and shrublands (30, 31). Additionally, such changes can also be attributed to the effectiveness of forest protection policies. Taking South America as an example, the Brazilian government’s protection of the Amazon forest is much stricter than that of the Cerrado savanna (32). Especially after the Soy Moratorium was promulgated, agricultural production began to shift from the Amazon to other ecological regions (33). In addition to the Cerrado, the Colombian Llanos, the Pampas and Chaco, and the savanna of Uruguay are also facing increasingly severe agricultural encroachment (34–36). These projected future changes do not occur suddenly. Our study reports subtle changes in global trends after 2016 (Fig. S3B), and these changes may appear earlier in some regions such as the United States (37) and North Africa (38). A study based on the IMAGE (Integrated Model to Assess the Global Environment) model also suggests that in the following decades, high-biodiversity wilderness areas, such as North American deserts and savannas of Miombo–Mopane, could be disproportionately affected by agricultural encroachment (39).
During the period of intense urbanization from 1992 to 2020, the contribution of urban expansion to global cropland loss was about 30%. This proportion can then drop to 7–10% due to the slowdown in urban growth in the coming decades. Besides urban expansion, natural hazards such as floods and droughts and the restoration of natural vegetation are also important factors leading to cropland loss and compensation (40–42). Here, we only quantified the cascading natural vegetation losses and NPP changes due to urbanization; however, the cascading impacts of other contributing factors should be proportional. Thus, cascading land-use changes from cropland displacement may have far more profound impacts on the carbon cycle of terrestrial ecosystems than we have reported above. Previous studies have suggested that policy regulation is effective in mitigating the negative impact of cascading land-use changes. For example, Khanna et al. (16) argued that carbon emissions due to cascading land-use changes can be reduced through the formulation of sound development policies. Lambin et al. (43) proposed that forest preservation and food production can be reconciled through policies that manage land-use transitions. According to our results, under the development pathways and policy guidance set by different SSPs, the cascading impacts of global urbanization on terrestrial NPP can vary widely. In the sustainability pathway (SSP1), due to the higher Reducing Emissions from Deforestation and Forest Degradation protection level and the decline in the consumption of animal products (44), the relocation of displaced croplands to forests may decrease, and instead, more grasslands would be occupied by displaced croplands. Therefore, the net cascading NPP growth in the SSP1 scenario may be significant. SSP2 is considered to be a pathway that maintains historical development features (45). That is, even if the current development mode is not changed, the world may still face minor cascading NPP growth in the future. In a de-globalized world (SSP3), many problems will appear around the world, including the highest population growth, slow development of agricultural technology, and resource-intensive consumption (46). These problems will lead to continuous urban expansion, low intensity of cropland, and high demand for pastures. Grasslands in this scenario may therefore be relatively preserved. Correspondingly, the massive displaced croplands may be more relocated to bare lands with access to irrigation, resulting in a rapid increase in terrestrial NPP. As the development of different income regions is uneven, land-use policies will be complex in SSP4, and the regional differences in cascading impacts may be large (47). Particularly, we note that cascading NPP changes due to rapid urban expansion may be more dramatic in low- and medium-income regions with unstable land-use patterns such as Africa and South America. Moreover, in SSP5, land-cover change will be lax regulated (48), so the cascading impacts on terrestrial NPP will be greater than that in other scenarios. Overall, we found that the SSP1 can achieve a better balance between urbanization and ecological protection and can be a more favorable development pathway to mitigate the negative effects of urbanization on the ecosystem carbon cycle (49).
However, from another point of view, the aforementioned NPP increase in the future may occur at the cost of some other ecosystem services (50). For instance, the highest biodiversity loss was produced due to global cropland expansion on mosaic grassland, natural grassland, and dense forest (51). Pan et al. (52) also pointed out that agricultural expansion in drylands can result in the loss of soil and water conservation, carbon storage, and biodiversity. We predicted a cascading NPP increase in some biodiversity hotspots such as Central America, Eastern Australia, and the Cerrado (Figs. S12–S14), which are also regions reported by several studies to face severe biodiversity loss due to agricultural development in the future (53–55). Additionally, cropland displacement increases the need for grain transportation. Increased grain transportation has further led to higher energy consumption and carbon emissions, which are harmful to the environment (56). Therefore, additional research and policies are necessary to steer the development trajectories of cropland displacement (57) and take ecosystem services as effective tools to guide land-use planning (14).
Our study highlights the significant but long-ignored cascading impacts of urban expansion on terrestrial NPP through cropland displacement. It offers a new dimension for understanding the complex impacts of urbanization on the terrestrial ecosystem carbon cycle. However, there are several limitations to our study. First, to capture a more detailed land change, we used 300-m or 1-km resolution land-use products, which have a resolution mismatch with the 8-km NPP data. Although we approached this problem by calculating the urban expansion intensity and extracting the grids with a single land type, there may still be some uncertainty in the results. Second, we mainly focused on land displacement to analyze the cascading impacts of urban expansion. However, other factors, such as sustainable intensification of croplands or changes in food consumption, can also contribute to the compensation of crop production losses from global urban expansion (54, 58). The causal links between land occupation and these factors need to be established in future studies.
Materials and methods
Our study analyzed the direct and cascading changes in agricultural and natural land covers (including forest, shrubland grassland, and bare land) caused by global urban expansion, and the impact of these changes on terrestrial NPP over the period 1992–2100 under the SSPs. The SSPs describe future socioeconomic development pathways from the perspective of demographics, economy, lifestyle, institution, technology, environment, and resources (18). Our experiment involved two parts: (ⅰ) the analysis of the impact of urbanization and its consequent cropland displacement on different land covers and (ⅱ) the calculation of direct and cascading NPP changes using a global 8-km fishnet. Our experiment design ensured that the observed and future impacts of urban expansion were continuous and comparable. Detailed descriptions of our data sources and analyses can be found in the Supplementary material, Materials and methods.
Supplementary Material
Supplementary material is available at PNAS Nexus online.
Funding
This work was funded by the National Science Fund for Distinguished Young Scholars (Grant No. 42225107) and the National Key Research and Development Program of China (Grant Nos. 2022YFB3903402 and 2019YFA0607203).
Author Contributions
X. Liu and X. Li designed the research. K.L., C.W., and Y.C. performed experiments and computational analysis. K.L. and F.P. drafted the paper. K.L., F.P., X. Liu, Z.Z., P.Z., C.F., X.X., S.W., J.M., X.C., L.Z., Q.S., and X. Li contributed to the interpretation and the preparation of the manuscript. All authors contributed to the final draft of the manuscript.
Data Availability
The datasets that support the findings of this study are available in the Supplementary material. All of them can be obtained from publicly available datasets.
References
Author notes
K.L. and F.P. contributed equally to this work.
Competing Interest: The authors declare no competing interests.
