The Sky is the Limit: Comparing China’s ADIZ Intrusions

Intrusions by Chinese military aircraft into Taiwan's air defense identification zone (ADIZ) have increased dramatically in the last four years, contributing to global fears about a potential conflict in the Taiwan Strait. Amidst this historic upturn, Japan in April reported the lowest annual number of air scrambles responding to foreign military activity within its ADIZ in a decade. Could China's increased activity around Taiwan be diverting resources from its intrusions into Japan's ADIZ in the East China Sea?

A comparative study of public data released by Japan and Taiwan suggests that, despite significant increases to its activity around Taiwan, China is not yet operating at capacity, nor is there currently evidence of a sustained downturn in activity targeted at Japan's ADIZ. This presents a challenging road ahead for both Tokyo and Taipei's strained resources to respond to China's increasing military air activity.

A Comparative Look

AMTI analyzed publicly available data on Chinese military intrusions in Japan and Taiwan's ADIZ to better understand the relationship between the two. Taiwan's Ministry of National Defense has released daily reports on Chinese air activities within its ADIZ since September 2020, reporting on the number and type of aircraft involved as well as publishing maps of their flight paths. While AMTI examined these individual reports, this study also relied heavily on the Taiwan ADIZ Violations Database, a public dataset managed by PLATracker which compiles each of the Ministry of National Defense's daily bulletins into a single dataset that categorizes each intrusion by location and aircraft type. Japan's Ministry of Defense has published quarterly and annual reports on foreign military activity within its ADIZ for decades, but reports on the number of corresponding air scrambles conducted by the Japan Air Self-Defense Force rather than specific details on each intruding aircraft. Because a single scramble may be conducted in response to multiple intruding aircraft, Japan's data is not equivalent to Taiwan's in scale, but a comparison can nevertheless provide insights on comparative trends in China's ADIZ intrusions.

Intrusions in Taiwan's ADIZ have increased substantially since Taiwan began reporting data, rising from a monthly average of 81 intrusions in 2021 to 178 through the first half of 2024. The biggest shift, and highest peak of activity, occurred in August 2022 following Speaker of the House Nancy Pelosi's visit to Taipei. Intrusions around Japan have followed a different trajectory, falling from a monthly average of 58 in 2021 to just 32 through the first half of 2024. At first glance this shift would seem to support a theory that increased activity around Taiwan has diverted available aircraft and resources from flights in Japan's ADIZ. But during the biggest spike in Taiwan intrusions in August 2022, intrusions in Japan's ADIZ (63) remained higher than average (46), suggesting that the PLA Airforce and PLA Navy maintain latent capacity to conduct high levels of operations around both Taiwan and Japan.

Comparison of Taiwan and Japan's monthly data also reveals a positive correlation in month-to-month totals: outside of the August 2022 spike, which was not accompanied by a spike in intrusions in Japan's ADIZ, peaks and valleys for monthly intrusions tend to line up between both datasets from 2022 through the present. While it is possible for flights to pass through both ADIZs, and thus contribute to both Taiwan and Japan's figures, an examination of Taiwan's data by location suggests that this is uncommon: only 5.5 percent of violations reported by Taiwan entered the eastern parts of Taiwan's ADIZ, with the vast majority occurring instead in Taiwan's southwest ADIZ and, after August 2022, across the median line. Maps of flight paths included with each daily report also consistently show that aircraft entering from the southwest or across the median line generally left the ADIZ the direction from which they entered, rather than continuing south or east. With dual-violation flights seemingly rare, the general correlation between Taiwan and Japan's monthly intrusions data may have more to do with internal factors in China's planning and operational procedures.

Another sign that increased activity around Taiwan doesn't necessarily translate to reduction in air activity in Japan's ADIZ comes from longer term annual trends in Japan's scramble data. It was widely reported in April that Japan's total annual scrambles during April 2023 - March 2024 (Japan's fiscal year 2023) were the lowest in a decade. But this is not the case when looking only at scrambles responding to Chinese intrusions, which were lower in 2020 before rising to the second highest ever in 2021. While 2023's total of 479 scrambles is on the lower end of recent annual totals, it remains to be seen whether this is indicative of a sustained downturn or is simply another of several fluctuations that have occurred over the past decade of Chinese intrusions.

The ADIZ and International Law

Japan and Taiwan's ADIZs exist within a murky international legal context when it comes to regulating airspace outside of national jurisdiction. Although they have become widely adopted since the United States declared the first ADIZ in 1950, no international agreement or codified body of international law governs their establishment or the specific requirements they impose on aircraft operating within them. This has led to disagreements between states on where they can be established and how they should be enforced.

Beijing drew condemnation from Washington in 2013 when it declared an ADIZ in the East China Sea that required the identification of all foreign aircraft regardless of destination, as the United States maintains that only aircraft intending to enter a country's airspace should be subject to identification requirements. The ADIZ also drew the ire of Seoul and Tokyo, as it overlapped significantly with their own preexisting ADIZs. U.S., Japanese, and South Korean military aircraft all purposely violated China's East China Sea ADIZ following its declaration, and China has never attempted to fully enforce its strict requirements. While in this case the erosion of China's East China Sea ADIZ was a victory for those who opposed its geographic scope and identification requirements, it nevertheless serves as a reminder that, without international consensus or legal underpinnings, there is little to defend an ADIZ besides consistent enforcement by the declaring party.

Conclusions

Both Japan and Taiwan face significant capacity challenges from China's sustained ADIZ intrusions. While the latest data points from Japan do show a slight decrease in Chinese air activity in the East China Sea, data points from the surge in activity in 2022 suggest that China maintains the capacity to increase its operations around both Japan and Taiwan, and that it has not yet reached the point of having to divert resources from one in order to increase operations around the other. Planners in both Tokyo and Taipei will have to continue to account for the costs of daily scrambles in terms of aircraft maintenance and force readiness if they hope to continue to enforce their ADIZs amidst this new normal of Chinese intrusions.

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