Regional difference in atmospheric CO2 trends
The CTM modeling estimated that the global mean CO2 concentration rose by 2.05 ppm year− 1 during 2000–2016, which is consistent with that computed from observations (Fig. 1a). Meanwhile, the simulated rate of atmospheric CO2 increase is greater than 2 ppm year− 1 in all regions and is more positively skewed, reaching a maximum value of 3 ppm year− 1 (Fig. 1b). The increasing rates of atmospheric CO2 in South Korea (mean: 2.32 ppm year− 1) and North Korea (2.23 ppm year− 1) are greater than the global 75 percentile value. These countries also share similar monthly variations in atmospheric CO2 (r = 0.98; p < 0.01; two tailed Student’s t-test), especially during the winter when the effect of vegetation activity is negligible (Fig. 1a). However, the variability of the monthly CO2 concentration in South Korea was lower than that in North Korea owing to the greater CO2 drawdown during the summer in North Korea than in South Korea. Moreover, the maximum increase in atmospheric CO2 in South Korea (2.83 ppm year− 1) was greater than that in North Korea (2.40 ppm year− 1) (Fig. 1b).
Changes in national surface CO2 fluxes
The national inventory reported contrasting changes in fossil fuel CO2 (FFCO2) emissions between the two countries from 2000 to 2016 (Fig. 2a). Specifically, in South Korea, FFCO2 emissions increased from 128 MtC year− 1 in 2000–2008 to 156 MtC year− 1 in 2008–2016. In North Korea, however, FFCO2 emissions decreased from 19 MtC year− 1 in the initial nine years of the study period to 11 MtC year− 1 in the final nine years. These contrasting trends are associated with different histories of changes in energy consumption and structure, as more than 90 % of FFCO2 emissions result from energy production in these countries [26]. Economic growth increased energy consumption in South Korea from 220 million tons of oil equivalent (Mtoe) in the initial 9 years to 272 Mtoe in the final 9 years. Further, the ratio of fossil (coal and petroleum) energy consumption decreased by 3 % as a result of increases in natural gas and renewable energy supply, but its magnitude increased by 30 Mtoe between the two periods (Fig. 2b). Conversely, the total energy consumption in North Korea decreased from 16 Mtoe to 13 Mtoe from the initial nine to the final nine years, respectively, as the supply of domestic coal, a major energy source, sharply declined during the study period.
Unlike the FFCO2 emissions, both the process-based models (TRENDY) and inverse modeling (CarbonTracker; CT) results estimated that the amount of terrestrial carbon uptake was similar in South and North Korea. These results also estimated increases in terrestrial carbon uptake in both countries during 2000–2016, although the magnitudes differed among the models (Fig. 2a). In particular, the TRENDY models simulated that terrestrial carbon uptake in South Korea increased from 3.4 ± 2.1 MtC year− 1 in 2000–2008 to 3.9 ± 2.7 MtC year− 1 in 2008–2016, which accounts for 2.5 ± 1.7 % of the mean FFCO2 emissions for the period. Similarly, the CT also estimated that the carbon uptake rose from 4.3 MtC year− 1 to 5.9 MtC year− 1 from the initial nine years to the final nine years, respectively, within an inter-model standard deviation range of the TRENDY models, suggesting that the terrestrial carbon flux in South Korea is well constrained. Similarly, the TRENDY models simulated that the terrestrial carbon uptake in North Korea increased from 3.3 ± 1.8 MtC year− 1 in the initial nine years to 4.2 ± 1.6 MtC year− 1 in the final nine years of the study period, accounting for 38 % ± 15 % of the mean FFCO2 emissions during this period. Further, the CT estimated that the carbon uptake enhanced from 9.4 MtC year− 1 to 13.3 MtC year− 1 between these periods, which was beyond the standard deviation range of the TRENDY models. This notable discrepancy may have resulted from the absence of atmospheric measurements for constraining the terrestrial carbon flux in North Korea. Overall, these results indicate that South Korea continues to deviate further from achieving carbon neutrality, while North Korea has almost achieved carbon neutrality, even though there exist large uncertainties in the terrestrial carbon flux in North Korea.
Causes of regional difference in atmospheric CO2 trends
To examine the role of regional CO2 flux changes on atmospheric CO2 variations over the Korean Peninsula, a set of CTM simulations were conducted for 2000–2016 (details in the “Data and methods” section). In the ALLtransient simulation, wherein all variables are transient, the increasing rate of atmospheric CO2 is gradually lowering from west to east over the surrounding areas of the Korean Peninsula during 2000–2016 owing to the heavily industrialized provinces (e.g., Liaoning) in northeastern China (Figs. 3a, 4a, b) [27, 28]. In addition, distinctly different spatial patterns of atmospheric CO2 trends were present between these countries. Specifically, an increase of more than 2.4 ppm year− 1 occurred in the outskirts of Seoul and certain industrial complexes (e.g., Yeosu and Ulsan), located in the northwest and southeast parts of South Korea, respectively. In contrast, the central and northeast regions of North Korea presented a relatively lower increase (2.2 ppm year− 1) than the adjacent surrounding areas.
Sensitivity simulations, which evaluate the effect of surface CO2 fluxes and atmospheric transport on atmospheric CO2 variations, revealed that the increasing FFCO2 emissions in South Korea is the major driver of regional differences in atmospheric CO2 trends between these countries. In particular, the increase in FFCO2 emissions, particularly in major cities, rose the regional and country atmospheric CO2 concentrations by more than 0.3 ppm year− 1 and 0.12 ppm year− 1, respectively, accounting for 5 % of the net increase in national atmospheric CO2 (Figs. 3b, d, 4b). The increasing rate of atmospheric CO2 was relatively lower in Seoul than in the surrounding areas because emissions in this area have decreased owing to the government’s policy to shift industrial facilities from Seoul to its outskirts (Figs. 3b, 4b). In the northeastern part of South Korea, approximately 0.2 ppm year− 1 of the national increase appears to be caused by one strong point source. However, no possible sources of FFCO2, such as coal-burning power plants, are present in this mountainous area. Thus, this area may have been mis-allocated; hence, cautious interpretation of the spatial map is necessary. Conversely, in North Korea, decreases in FFCO2 emissions, particularly in the Pyongyang metropolitan area, which includes several primary source regions, reduced the CO2 concentrations in the region by more than 0.1 ppm year− 1, presenting a small nationwide impact of − 0.03 ppm year− 1 (Figs. 3e, g, 4b). Moreover, the CT estimates, which are greater than those of the TRENDY multi-model mean, indicate that increases in terrestrial CO2 uptake over widely distributed forests in the two countries decreased atmospheric CO2 by up to 0.04 ppm year− 1 (Fig. 3c, f). Although the nationwide effect of terrestrial uptake in South Korea (–0.02 ppm year− 1) is greater than that in North Korea (–0.01 ppm year− 1), its magnitude is too small to offset the increase in CO2 concentrations induced by increasing FFCO2 emissions (Fig. 3d, g).
The changes in transported CO2 similarly rose the atmospheric CO2 concentrations in both South and North Korea by 2.23 ppm year− 1 and 2.27 ppm year− 1, respectively (Fig. 5d, e). The greater increase in atmospheric CO2 in North Korea is the result of its geographic proximity to major carbon sources in northeastern China and South Korea. Although the changes in transported CO2 did not cause distinct regional differences in atmospheric CO2 trends, they comprise more than 95 % of net increases in atmospheric CO2 in these countries. Specifically, the increase in FFCO2 emissions in China, accounting for 65 % of the global FFCO2 increases in the period as derived from the national FFCO2 emission inventory [26], caused South and North Korea to present greater increasing rates of atmospheric CO2 than the global mean (0.56 ppm year− 1), rising at rates of 0.68 ppm year− 1 and 0.70 ppm year− 1, respectively (Fig. 5c, d, e). Nevertheless, the contribution of rising FFCO2 emissions from China to the increases in atmospheric CO2 over the Korean Peninsula is relatively smaller (30–31 %) than the contribution of global FFCO2 increases. This is because the atmospheric CO2 concentration is bound to increase every year, even if global FFCO2 emissions remain at 2000 levels, because FFCO2 emissions have been greater than the natural carbon absorption since industrialization [1].