Realism vs alarmism: Since the 1980s, there has been an ozone 'hole' over the tropics much larger than the Antarctic one; obviously, no harm has been reported related to this thinning despite affecting almost 50pct of the world's population

Observation of large and all-season ozone losses over the tropics featured. Qing-Bin Lua. AIP Advances 12, 075006 (Jul 5 2022); https://doi.org/10.1063/5.0094629

Abstract: This paper reveals a large and all-season ozone hole in the lower stratosphere over the tropics (30°N–30°S) existing since the 1980s, where an O3 hole is defined as an area of O3 loss larger than 25% compared with the undisturbed atmosphere. The depth of this tropical O3 hole is comparable to that of the well-known springtime Antarctic O3 hole, whereas its area is about seven times that of the latter. Similar to the Antarctic O3 hole, approximately 80% of the normal O3 value is depleted at the center of the tropical O3 hole. The results strongly indicate that both Antarctic and tropical O3 holes must arise from an identical physical mechanism, for which the cosmic-ray-driven electron reaction model shows good agreement with observations. The whole-year large tropical O3 hole could cause a great global concern as it can lead to increases in ground-level ultraviolet radiation and affect 50% of the Earth’s surface area, which is home to approximately 50% of the world’s population. Moreover, the presence of the tropical and polar O3 holes is equivalent to the formation of three “temperature holes” observed in the stratosphere. These findings will have significances in understanding planetary physics, ozone depletion, climate change, and human health.

IV. DISCUSSION

The present finding of a tropical ozone hole is closest to the observation by Polvani et al.²⁶ of large O3 losses at the altitude of 18.5 km or 67/68 hPa over the tropics (30°S–30°N) from 1979 to 1997 with data from three independent datasets (TOST, BDBP, and GOZCARDS)^28–30 although they did not reveal the tropical O3 hole. Polvani et al.²⁶ also showed that global and tropical LST cooling had disappeared since 1997, while the tropical O3 concentration reached the minimum around 2005. When explaining their observed results, however, Polvani et al. argued for the low abundance of active chlorine and hence no local chemical O3 destruction in the tropical lower stratosphere. Instead, they argued that the observed tropical O3 and cooling trends were primarily driven by tropical upwelling caused by ODSs rather than greenhouse gases (GHGs) (mainly non-halogenated GHGs, as widely believed). As we have noted in the Introduction and will further discuss later, however, even a low level of active halogen can cause significant ozone depletion in the tropical lower stratosphere.

Interestingly, Polvani et al.²⁶ also performed simulations from a chemistry–climate model (CCM) with incrementally added single forcings (sea surface temperatures—SSTs, GHGs, ODSs, volcanic eruptions, and solar variations) to detail the contribution of each forcing to tropical ozone and LSTs. Although their simulated results showed that ODSs were the dominant forcing of tropical ozone loss over GHGs, it must be pointed out that their simulated values of sum ozone loss (−28 ± 13 ppbv per decade; see their Table 1) were about five times smaller than their observed results (∼−150 ppbv per decade for the 1980s and 1990s), even ignoring that not all individual ensemble members showed statistically significant trends (see their Table 2). Moreover, in contrast to their claim that tropical lower stratospheric ozone would be closely tied to tropical upwelling w*, their simulated value of the w* increase by ODSs is not dominant but very close to that by GHGs, each force contributing to an increment by ∼0.1 km yr⁻¹ decade⁻¹ at 85 hPa, namely, 0.04 ± 0.09 for SSTs, 0.10 ± 0.11 for SSTs + GHGs, and 0.21 ± 0.11 for SSTs + GHGs + ODSs (see their Table 1). The results of the latter were also consistent with their simulated results of tropical LST trends (see their Fig. 3).

The simulated results of CCMs by Randel and Thompson³⁷ and others³⁸ had some differences from but were overall similar to the above-mentioned CCM results by Polvani et al.²⁶ In most simulated results of CCMs, the strength of tropical upwelling was projected to increase from 1960 to 2100 by ∼2% per decade with the largest trends occurring in JJA, corresponding to tropical O3 reductions at 50 hPa of 0.15–0.35 ppmv (11–25 ppbv per decade).³⁸ This ozone loss trend resulting from CCM simulations is about 10 times less than the observations by Polvani et al.²⁶ and the present observations shown in Figs. 1–5. More crucially, the observed data have robustly shown that significant tropical ozone loss and LST cooling occurred in the 1980s and 1990s only, which is in drastic discrepancy from the simulated results of CCMs. Polvani et al.²⁶ were then led to the open question: How could ODSs affect the stratospheric circulation? They conceded that the underlying mechanism for ODSs being a key forcing for tropical lower stratospheric O3 and temperature trends remained largely unexplored. Knowing the wide belief in CCMs that the key drivers of tropical upwelling and thus tropical O3 or LST trends since 1960 are non-halogenated GHGs (mainly CO2), Polvani et al.²⁶ were forced to suggest that polar O3 depletion caused by ODSs would cause tropical upwelling and hence large tropical O3 losses. This explanation cannot be correct either, as the polar O3 hole is seasonal and appears only in the springtime, whereas the tropical O3 hole is all-season and has no changes in its central location over the seasons and over the decades since its appearance in the 1980s [Figs. 1–3 and 4(e)].

The present observed results in Figs. 1–6 and Figs. S1–S4 strongly indicate that, like the Antarctic O3 hole that was once incorrectly explained by the misconceived air transport mechanism (“dynamical theory”), the tropical O3 hole must not result from changes in normal atmospheric circulation patterns over the tropics since the 1960s or 1970s but result from an identical physical/chemical mechanism to that for the polar O3 hole. Obviously, the tropical O3 hole varies closely with the atmospheric level of CFCs [as seen in Fig. 4(b)], so it must originate from a CFC-related mechanism. The postulated stratospheric cooling and tropical upwelling effects of increasing non-halogenated GHGs have disappeared in observed O3 and temperature data for the Antarctic lower stratosphere^6–8 and for the tropical lower stratosphere.²⁶ It is obvious that the simulated results from CCMs^26,37,38 do not agree with the observed results shown in Figs. 4(b), 5(a), 5(b), and 6(f), which show that the negative O3 trends were about 10 times larger (−25 to −30% per decade) in the 1980s and 1990s and there have been no significant O3 or LST trends in the tropical/Antarctic since the mid-1990s. The latter are actually consistent with the observations summarized in the newest IPCC report (Chap. 2).³⁶ Moreover, the proposed enhanced tropical upwelling directly contradicts with the observed CFC depletion in the lower tropical stratosphere [Fig. 4(d)], as increased upward motion would transport CFC-rich air from the troposphere. Indeed, the observed data robustly show no shifts in the positions of both Antarctic and tropical O3 holes that have constantly been centered in the altitude region corresponding to the CR ionization peak since the 1960s/1980s and circularly symmetric O3 depletion cyclones are formed with the largest depletion at the centers [Figs. 1–3 and 4(e)]. These major features cannot be explained by tropical upwelling due to non-halogenated GHGs (mainly CO2) that have kept rising since the industrial revolution starting in 1760. All the observed data strongly indicate that tropical upwelling cannot be the major mechanism for the observed large, deep, and all-season tropical O3 hole. The simultaneous depletions of both CFCs and O3 in the lower tropical stratosphere are most likely due to a physical reaction mechanism that occurs locally. For the latter, the CRE mechanism, supported by the observed data in Figs. 1–6 and the substantial datasets obtained from both laboratory and atmospheric measurements,^{3–9,39–41} has provided the best and predictive model.

It is well known that the presence of PSCs is crucial for the formation of the Antarctic O3 hole.^42–45 It was proposed that on the surfaces of PSCs, chlorine reservoir molecules (HCl and ClONO2) are converted into photoactive forms (Cl2) that can then undergo photolysis to destroy O3. There are two types of PSCs, namely, Type I and Type II PSC. The composition of Type II PSC is water ice, while Type I PSC is composed of mixtures of nitric acid (HNO3), water vapor (H2O), and sulfuric acid (H2SO4). The temperatures required for the formation of Type I and II PSCs are 195 and 188 K, respectively. Thus, it is very likely that TSCs, at least Type I PSC-like TSCs, can also form in the tropical lower stratosphere over the seasons due to the observed low temperatures of 190–200 K [Figs. S4 and Fig. 4(f)]. CRs may also play a certain role in forming PSCs and PSC-like TSCs.^16,19 Note that the tropical lower stratosphere is very different from the polar lower stratosphere in both composition and climate. The former is rich in CFCs and other halogen-containing gases, whereas the latter are composed of inorganic chlorine species and lower-level CFCs. However, the CRE mechanism put forward two decades ago has proposed that O3-depleting reactions of both CFCs and inorganic halogen species can effectively occur on the surfaces of PSCs.^{3–9,39–41} Therefore, there are required and sufficient conditions for O3-depleting reactions occurring on the surfaces of proposed PSC-like TSCs in the tropical lower stratosphere. As noted in the Introduction, the constant co-presence of low-temperature TSCs and intense sunlight should lead to a unique active halogen evolution in the tropical lower stratosphere, in which halogen-catalyzed reactions are much more efficient for O3 destruction than those in the polar lower stratosphere.

The tropics (30°N–30°S) constitutes 50% of the Earth’s surface area, which is home to about 50% of the world’s population. O3 depletion in the tropics could cause a great global concern. In areas where O3 depletion is observed to be smaller in absolute O3 value, UV-B increases are more difficult to detect as the detection can be complicated by changes in cloudiness, local pollution, and other difficulties. However, it is generally agreed that the depletion of the O3 layer leads to an increase in ground-level UV radiation because ozone is an effective absorber of solar UV radiation. Exposure to enhanced UV-B levels could increase the incidence of skin cancer and cataracts in humans, weaken human immune systems, decrease agricultural productivity, and negatively affect sensitive aquatic organisms and ecosystems.⁴⁶ Indeed, there was a report called HIPERION published by the Ecuadorian Space Agency in 2008.⁴⁷ The study using ground measurements in Ecuador and satellite data for several countries over 28 years found that the UV radiation reaching equatorial latitudes was far greater than expected, with the UV index as high as 24 in Quito. This Ecuadorian report concluded that O3 depletion levels over equatorial regions are already endangering large populations in the regions. Further delicate studies of O3 depletion, UV radiation change, increased cancer risks, and other negative effects on health and ecosystems in the tropical regions will be of great interest and significance.

Another important result is that the global lower stratospheric temperature is essentially governed by the O3 layer, which is expected as ozone is the main and dominant molecule that absorbs solar radiation in the stratosphere. As a result, the presence of the tropical and polar O3 holes will play a major role in stratospheric cooling and regulating the global lower stratospheric temperature, as seen previously^6–9,26 and in the results shown in Fig. S4 and Figs. 4(f) and 6(e)–6(f). As seen in Fig. S4 and Fig. 4(f), this is equivalent to the formation of three “temperature holes” in the stratosphere, corresponding to the Antarctic, tropical, and Arctic O3 holes, respectively. This interesting result will be further explored in a subsequent paper.