Ozone Variability Following a Near-Future Grand Solar Minimum

The production and loss reactions of stratospheric ozone are photochemical in nature, ebbing and flowing with any variation in the sun’s radiation. The sunspot cycle is characterized by an 11-year periodicity with 6 — 7 % variation at 205 nm throughout. The solar rotational cycle is much shorter and exhibits less variation, with 2 — 3 % changes observed at 205 nm over its 27 day period. Grand solar minima and maxima are unpredictable, but occur frequently (approximately 30 times in the last 10,000 years), often lasting a century or longer, with significant impact on column ozone.

Though total variation of the solar spectrum remains below 0.1% during the 11-year sunspot cycle, the ultraviolet region exhibits much higher volatility. Solar variation is higher at shorter wavelengths and this has an out-sized effect on Chapman chemistry (figure 1); the photolysis of molecular oxygen by light of wavelength less than 240 nm is responsible for the overwhelming majority of the stratospheric ozone production rate. While 6 — 7% variation is observed at 205 nm during the sunspot cycle, this variation increases to approximately 68% at the Lyman-alpha wavelength (121.6 nm). At higher wavelengths, some variation is observed at line emission intensities (e.g., 6% variation at Mg II lines between 279 — 282 nm), but in general, variability drops below 1% at wavelengths greater than 240 nm.

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Figure 1: The Chapman Cycle. Dioxygen is photolyzed by ~ 200 nm light to form O atoms. These O atoms recombine with O2 to produce O3. O3 will absorb ~ 250 nm radiation to produce O2 and atomic oxygen. Because hypsochromic light features more variation in response to solar cycling than bathochromic light, Chapman chemistry is expected to be especially perturbed relative to other photochemical cycles which depend on ‘redder’ light during a grand solar minimum. (Klobas thesis 2018)

During periods of high solar activity, molecular oxygen photolysis rates (and it follows, ozone production rates) are enhanced. Likewise, various photocatalytic ozone loss mechanisms are impacted by solar cycle variability; however, the relevant wavelengths tend to be red-shifted relative to the wavelengths responsible for molecular oxygen photolysis and ozone loss rates are thus less volatile.

The ozone response to the sunspot cycle is latitude dependent. Model simulations (e.g. Brasseur 1993, Fleming et al., 1995, Hood 1997, and Shindell et al., 1999) find ~1% changes in ozone abundance between solar minima and solar maxima conditions in the tropics, but about 2% changes near the poles. These changes are accompanied by changes to the thermal structure of the stratosphere due to the close-coupling of ozone photolysis and stratospheric heating rates, and this feedback effect has been found to be nearly as strong as the photochemical effect in model studies. Particularly, this alteration in stratospheric temperature fields modifies middle atmospheric circulation by impacting the propagation of planetary waves — the so-called top-down mechanism. Variability resultant from solar rotation is found to be less than 0.5%, due mainly to these same dynamical forces.

While interesting for modelers, climatologists, and other workers, these periodic solar variations are liable to have very limited impact on human health at the surface. On the other hand, a strong decrease in solar activity, as is expected following the onset of a grand solar minimum is predicted to reduce global ozone columns by up to 2% worldwide, with the strongest impact of up to 8% over midlatitudes per Anet et al. (2011). The same group followed up in 2013, finding that such an event, if it occurs before the decay of anthropogenic halocarbons to preindustrial levels, may interfere with stratospheric ozone recovery, delaying it by up to a decade or longer.

This is significant in light of the projected super-recovery of stratospheric ozone as a result of climate change (described in, for example, Li et al. 2009).  So I was reading my Google scholar alerts (surprise, surprise) and I was intrigued when I found a paper regarding a potential near-future grand solar minimum.

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Tlatov and Pevtsov use a statistical treatment to predict that the next grand solar minimum will occur prior to the next century, stating that the sun spends about 15% of its lifetime in grand solar minimum phases (and 10% in grand solar maximum phases). Examining carbon-14 proxies of prehistoric solar phases, they establish a 205-year periodic trend upon which they derive indicators of an impending grand solar minimum. They find that nearly all prior grand solar minima occured in troughs of this 205-year periodic cycle, and that the next trough will occur around 2090. Their findings are not out-of-the-blue; several other groups have made similar predictions (e.g., Abreu et al., (2010), Lockwood et al., (2011), and Roth and Joos (2013), among others)

Lubin and coworkers recently (2017) characterized the expected variance in luminosity during a grand solar minimum using astronomical observations of sun-like stars, determining that the solar flux at wavelengths significant to Chapman chemistry will likely decrease by as much as 7% relative to the quiet sun (so-called normal behavior) during such an event — and this leads me to another interesting paper, this time on ozone.

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 Arsenovic et al (2018) report on the ozone and climate effects of an end-of-century grand solar minimum. They explore the scenario with the SOCOL3-MPIOM model within the RCP4.5 greenhouse gas emissions trajectories. The authors impose a 6.5 w/m^2 reduction in the solar constant (and 9 — 15% in the 180 — 250 nm band), finding that the reduced insolation depresses ozone production rates by up to 8%. This effect counters the expected so-called super-recovery of ozone (an effect of reduced rates of ozone destruction as a result of stratospheric cooling following anthropogenic climate change). As the solar forcing decreases, so too does the heliopause contract. Increased precipitation of galactic rays and decreased flux of solar wind [see, for example, this blog post on Energetic Particle Precipitation (EPP)] will impact the NOx production rates in the thermosphere and mesosphere, likely changing the vertical profile of ozone via chemical processes in addition to the photochemical perturbation of Chapman cycling. The end result finds that total ozone recovery from anthropogenic halogen-induced ozone depletion is postponed — not for a decade or two — but until the grand solar minimum ends, likely many decades later.

If one views the projected anthropogenic halocarbon decay trajectory, figure 2, one sees that stratospheric halogens are expected to remain elevated for many more decades. The implications of a potential grand solar minimum then may be that it prevents the onset of ozone super-recovery for several decades, and perhaps prevent it entirely if humankind can get its act together and reverse the thermal changes to the stratosphere are a result of greenhouse gas emission. At the same time, the overall distribution of ozone is perturbed in the grand solar minimum scenario: the ozone layer is thicker over the high latitudes (though not as thick as in the quiet sun reference scenario) and thinner over equatorial regions. This result occurs because acceleration in the Brewer Dobson circulation continues regardless of the scenario.

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Figure 2: Effective Equivalent Stratospheric Chlorine (EESC), a metric of available stratospheric halogens, as a function of year and greenhouse gas emissions trajectory. (a) computed for a 3-year old parcel of air (ascending). (b) computed for a 5.5-year old parcel of air (subsiding). The green dashed line reflects 1980 values. (Klobas thesis 2018)

The change in ozone distribution and delay of super-recovery is important: the ozone super-recovery will produce differential impact on vitamin D biosynthesis on the basis of (a) an individual’s expression of melanin, (b) the latitude at which an individual lives, and (c) the individual’s ability to consume high-fat foods in which vitamin D is soluble (such as milk). The end result is, if ozone super-recovery occurs, dark-skinned individuals living in high latitudes will suffer from higher rates of vitamin D deficiency — and these individuals tend to have high levels of lactose intolerance, which products are the preferred vehicle for vitamin D supplementation. Because vitamin D deficiency is linked to about a 2% increase in all-cause mortality, the human-health impact may be especially severe among these groups. Ironically, the ozone-depleting effects of a grand solar minimum might actually have a positive outcome, reducing future differential mortality rates on the basis of skin-tone.

Climate Feedbacks from Stratospheric Ozone Loss Following Energetic Particle Precipitation

Incoming energetic particles (EPs), primarily protons and electrons, mediate some chemical and physical processes in the Earth’s atmosphere. Variations in the incoming flux of EPs can thus result in perturbations to the chemical partitioning of the stratosphere.

Though EPs arise from multiple origins, each source process produces a characteristic energy spectrum. Galactic cosmic rays (GCRs) are a highly energetic source of protons originating from outside the solar system, with characteristic energies between 10^{-12} J — 10^2 J per nucleon. Solar cosmic rays (SCRs) originating from coronal mass ejections have energy spectra which typically peak around 10^{-7} J per nucleon. Fluxes of GCRs and SCRs are inversely correlated with 11-year periodicity. When the 11-year sunspot cycle is at a maximum, so too are SCR fluxes, while GCR fluxes are at a minimum due to pressure from solar winds.

Regardless of their origin, sufficiently energetic EPs (> 10^{-11} J) will collide with atmospheric gases and produce charged secondary products, classified by their soft (electrons, positrons, and photons) and hard (muons, pions, etc.) components. These secondary products will then interact with atmospheric species in a variety of ways. One such scheme resulting in the production of NO_\text{x} is presented below.

The reaction pathway is initiated following interaction with energetic secondary electrons, denoted \text{e}^*.

\text{e}^* + \text{N}_2 \rightarrow \text{N}^+ + \text{N} + 2\text{e}^-

\text{e}^* + \text{N}_2 \rightarrow \text{N} + \text{N} + \text{e}^-

\text{e}^* + \text{N}_2 \rightarrow \text{N}_2^+ + 2\text{e}^-

\text{e}^* + \text{O}_2 \rightarrow \text{O}_2^+ + 2\text{e}^-

\text{e}^* + \text{O}_2 \rightarrow \text{O} + \text{O}^+ + 2\text{e}^-
Subsequent recombination/exchange chemistry further enhances atomic nitrogen.

\text{N}_2^+ + \text{O} \rightarrow \text{NO}^+ + \text{N}

\text{N}_2 + \text{N}^+ \rightarrow \text{N}_2^+ + \text{N}
\text{N}_2^+ + \text{e}^- \rightarrow \text{N} + \text{N}
\text{NO}^+ + \text{e}^- \rightarrow \text{N} + \text{O}

\text{N}^+ + \text{O} \rightarrow \text{N} + \text{O}^+

Finally, reaction of atomic nitrogen with molecular oxygen produces nitric oxide.

\text{N} + \text{O}_2 \rightarrow \text{NO} + \text{O}

Similarly, EPP events may produce HO_\text{x}. For more details, Mironova et al. (2015) provide a very thorough review on the interaction of EPPs with the atmosphere.

Once formed, HO_\text{x} and NO_\text{x} will engage mesospheric ozone loss processes. A schematic for the HO_\text{x}-mediated destruction of ozone in the middle atmosphere is presented in figure 1 — and NO_\text{x}-mediated processes will be quite similar. Ozone loss processes produced in the immediate environment of EPP interactions are frequently called direct effects. Direct effects from NO_\text{x} and HO_\text{x} produced by EPPs are a significant source of variability in mesospheric ozone (upwards of 25% of mesospheric ozone).

HOxcoloredFigure 1: A graph theory representation of the HOx-mediated catalytic destruction of ozone in the middle stratosphere. Klobas thesis 2018.

Mesospheric HO_\text{x} from EPPs is not long-lived and does not have effects beyond the immediate environment in which it is formed; however, NO_\text{x} has a long lifetime and will transport to the middle stratosphere, where it will further engage ozone loss processing. Ozone loss following transport from the region in which the NO_\text{x} was formed is referred to as an indirect effect of EPPs. Ozone losses from indirect effects are estimated to be as large as 15% in the middle and upper stratosphere.

The action of these events is largely confined to the polar regions because (1) EPs are entrained to geomagnetic fields which direct particle fluxes to the magnetic poles, and (2) mesospheric and stratospheric subsidence of NO_\text{x} occurs at the poles.

One of the most fascinating papers I’ve ever read on the potentially disastrous effects of EPPs concerns a seemingly unlikely situation: a magnetic field reversal of the Earth concurrent with the transit of the Solar System through a region of high interstellar density.  The story goes like this:

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The density of the interstellar medium is heterogeneous. If the solar system were to transit a region with higher density, enhanced rates of EP precipitation would be expected as the heliosphere contracts. The coincidence of both situations seems highly improbable at first glance; however, a review of the expected frequencies and durations of these events provides the evidence to the contrary. The solar system has transited regions of enhanced density (\geq 100 H atoms/cm^3 vs 0.3 H atoms/cm^3 currently) \approx 135 times in the previous 4.5 Gy and the average duration of a transit was \approx 1 My. The Earth’s magnetic field reverses stochastically, however on average once every 300 ky, the event itself lasting several thousand years. A statistical treatment thus indicates that there may have been as many as 7 instances in the prior 250 My of a magnetic field reversal coinciding with the transit of the solar system through a region of enhanced density — and that EP fluxes would remain elevated for up to a period of thousands of years.

Such 1000-year periods of enhanced GCR precipitation are projected to lead to widespread, long-term depletions of the stratospheric ozone layer. Pavlov et al. (2005) use a 2-D dynamical/chemical model to evaluate the new steady-state ozone solution following a 300-fold enhancement of GCR, finding that total column ozone decreases by 40\% globally, and by up to 80% at high latitudes after a 5 — 10 year adjustment period.

It’s a completely insane story with extraordinarily dire consequences. The fact that it’s not only possible, but expected to occur on a basis as frequent as the collision of a 5 km asteroid (one third of a Chicxulub impactor), with 1000-year implications is alarming; while Bruce Willis might be able to save us from an incoming asteroid, there’s nothing we can do to restart the geomagnetic field or redirect the velocity of the Solar System through less dense space.

To put this all in context, the Chicxulub event is estimated to have perturbed the ozone layer, the climate, and pretty much everything on Earth for about a decade or so, but I’ll write a post about that another time..

Two papers recently appeared on my google scholar alerts on the topic of EPPs.

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The first, by Andersson et al. (2018), provides an accounting of total chemical forcing from EPP processes on ozone, while exploring the effect of medium-energy electrons (MEE, 300 — 1000 keV), whose effect has only recently been incorporated into chemical models [WACCM, this paper, SOCOL in Arsenovic et al. (2016)]. Andersson and workers find that the inclusion of MEE results in a strong enhancement of indirect ozone processing rates as low as 30 km — accounting for up to 8% of the ozone depending on the pressure level.

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The second, by Meraner and Schmidt (2018), explores the climate impact of the radiative forcing effects from EPP-induced ozone loss. The authors find that the winter stratosphere increases in temperature as a result of ozone loss and gradient-driven transport is subsequently reduced.This effect could slightly reduce the strength of the polar vortex, though they state that the modeled response is small and that the effect of EPPs on climate may be minimal. This finding was surprising to me as previous works (though with less-sophisticated models) predicted stratospheric cooling  with resultant strengthening of the polar vortex and a much larger surface warming effect.