Advances  in    Our  Understanding  of  the  Coupling  Between  Climate  Forcing  and  UV  Dosage    

Levels  Over  the  US  in  Summer  

Jim Anderson, Harvard University

Summary  of  the  Problem:  We now know, through the union of high altitude in situ aircraft, satellite and NEXRAD weather radar observations, that the stratosphere over the US in summer is vulnerable to ozone loss as a result of a series of factors unique, worldwide, to the central US in summer. The key elements that couple climate forcing to ozone loss in the lower stratosphere over the US in summer are summarized in Figure 1 and reviewed here: 1. Convection, as a result of severe storms over the

Great Plains, delivers both markedly enhanced water vapor, as well as catalytic radical precursors from the lower troposphere, deep into the stratosphere over the US in summer.

2. The depth of convective injection is sufficient to reach altitudes of rapidly increasing available inorganic chlorine that is then catalytically converted on simple, ubiquitous, binary water-sulfate aerosols, to free radical form, ClO.

3. It is the chlorine radical, ClO, that couples to the available BrO radical to form a catalytic cycle rate limited by ClO + BrO → Cl + Br + O2 in combination with ClO + ClO ! ClOOCl that constitutes the catalytic mechanisms capable of removing ozone; and it is the same mechanism that contributes to very large ozone loss over the polar regions in winter.

4. Anti-cyclonic flow in the lower stratosphere over the US in summer, as a result of the North American monsoon, contains that convective injection in a gyre that provides time (days to weeks), for photochemistry to catalytically remove ozone.

5. The remarkable sensitivity of increases in the skin cancer incidence in the US to fractional decreases in ozone column concentration places strict demands on the quantitative understanding required to accurately forecast ozone losses and the requisite increase in skin cancer incidence in the US in a changing climate.

6. The frequency and intensity of severe storms over the Great Plains of the US is increasingly tied to increased forcing of the climate by increasing levels of CO2, CH4 and N2O in the Earth’s atmosphere. As a result, climate forcing is mechanistically linked to forecasts of ozone reduction in the stratosphere over the US in summer.

7. These issues of the close coupling between heterogeneous and homogeneous catalytic ozone loss with the associated aerosols in the lower stratosphere must be linked to emerging strategic plans for climate engineering via solar radiation management (SRM) that proposes the addition of aerosols to the lower stratosphere to reflect solar radiation, thereby reducing shortwave forcing of the climate.

Figure 1. Graphical representation of the link between climate forcing by increased CO2 and CH4 release, convective injection of water deep into the summer stratosphere over the US, entrapment by the anticyclonic North American monsoon flow and the catalytic chemistry linking halogen radicals to ozone loss in the lower stratosphere.

In this Executive Summary we briefly review advances in our understanding through the fall of 2015—advances that are treated in full detail in upcoming publications in the peer-reviewed literature. These developments engage the advancing union of observational techniques, the human health implications of ozone loss, the issue of proposed climate engineering via SRM, and key evidence of the climate structure from the paleorecord. An important consideration associated with this problem of deep stratospheric convective injection in summer includes (a) the sharp gradient in the physical structure of convective spatial overlap with available inorganic chlorine in the stratosphere as well as (b) the sharp gradient in photochemical variables associated with ozone loss that together define the level of concern associated with this problem.

A remarkable attribute of this coupling between increasing climate forcing and human health concerns through increasing skin cancer incidence is the remarkable number of interlocking elements that are involved. This coupling structure is summarized in Figure 2 and graphically captures the key points that constitute the elements of this report.

Figure 2.

Key   Points   Linking   Climate   Forcing   With  Stratospheric   Ozone   Loss   Over   the   US   in  Summer:  Point 1: Review of the unique meteorology over the Great Plains that spawns severe storms over the US in summer Point 2: Flights into the stratosphere over the US in summer repeatedly discovered highly elevated water vapor linking convective storms with the potential for ozone loss. Point 3: In addition to the remarkably deep stratospheric convection, the unique containment of that deep convective injection resulting from the North American monsoon provides extended reaction times for the catalytic removal of ozone. Point 4: NEXRAD weather radar observations over the US in summer provide high accuracy three dimensional geometry of the convective injection of condensed phase water into the stratosphere. Point 5: It has now been established that simple, ubiquitous sulfate-water aerosols catalytically convert inorganic chlorine to free radical (ClO) form that couples (a) convective injection of water into the stratosphere with (b) catalytic ozone loss in the stratosphere. Point 6: Key information from the paleorecord linking elevated CO2 with climate structure demonstrates that at CO2 mixing ratios of ~600ppmv, the Earth may well have resided in a markedly different climate state than is true today. Point 7: The sharp gradient in available inorganic chlorine with respect to both altitude and latitude demonstrates that convective injection of water vapor over the US in summer reaches well into the region of rapidly increasing inorganic chlorine that is available for catalytic conversion to free radical form. Point 8: Linking the unique meteorology and catalytic ozone photochemistry over the US in summer is mechanistically coupled to climate engineering through proposed solar radiation management (SRM) via the addition of sulfate in the lower atmosphere. Point 9: Human health is directly involved with the mechanistic coupling of climate forcing with stratospheric ozone loss through the quantitative coupling of UV dosage with skin cancer incidence in the US.

Point   1:   Review   of   the   unique   meteorology  over   the   Great   Plains   that   spawns   severe  storms  over  the  US  in  summer.  

The reason that the Midwest of the US is the site of extraordinarily severe convective storms displayed in Figure 3 stems from a remarkable combination of factors. Those factors are: • The flow of low

level, warm, moist, tropical air from the Gulf of Mexico about 3000 meters in depth that is drawn into the central US by circulation on the western flank of the Bermuda-Azores subtropical cyclone.

• The flow from the west of the sub-tropical jet stream at higher altitudes (10,000 meters) that brings lower temperature air in over the warm, moist maritime air from the Gulf, thereby decreasing the stability of the system by increasing the vertical gradient in temperature.

• The fact that the crossing of the flow from the west of cool, dry air over the lower level warm, moist flow from the south consistently produces vertical wind shear that spatially separates the precipitation in the storm from the updraft.

• The required upward instability is triggered by intense solar heating on the high plains of the Midwest such that the underlying moist tropical air breaks through the shallow temperature inversion between the overlying subtropical jet and the low level northerly flow from the Gulf of Mexico, initiating the explosive updraft, releasing the latent heat of the maritime air flow from the south.

• Convection can also organize over the central US into mesoscale convective systems (MCSs) with areas that may be a thousand times larger than an individual convective cell.

Figure 3.

Point  2:  Flights  into  the  stratosphere  over  the  US  in  summer  repeatedly  discovered  highly  elevated  water  vapor  linking  convective  storms  with  the  potential  for  ozone  loss.    

Aircraft observations in the stratosphere were focused from the late 1980s until the early 2000s on the Antarctic and Arctic regions and the flight trajectories leading from the aircraft bases on the West Coast directly to the Polar regions. Those aircraft observations consistently and repeatedly observed water vapor concentrations limited to the range 4.5 to 5 ppmv in the lower stratosphere, with virtually no major departures except for large dehydration within the Antarctic vortex.

In sharp contrast, in situ observations of water vapor concentrations over the continental US beginning in the early 2000s repeatedly saw regions 100-200 km across with H2O mixing ratios reaching and exceeding 15 ppmv. Simultaneously observed HDO isotopic ratios demonstrated unequivocally that the water vapor was a result of direct convective injection resulting from storm systems over the Great Plains and Eastern US Those first in situ aircraft discoveries are displayed in Figure 4.

Figure 4.

It was also recognized, as a result of NASA’s last stratospheric aircraft mission to the Arctic in 2000, that the catalytic conversion of inorganic chlorine to free radical form was occurring, not on Polar Stratospheric Clouds (PSCs), but rather on simple, ubiquitous sulfate-water aerosols that lead to the catalytic loss of ozone via a combination of chlorine and bromine radicals as shown in Figure 5.

Figure 5.

Point   3:   In   addition   to   the   remarkably   deep  stratospheric   convection   that   occurs  repeatedly  over  the  US   in  summer,   the  unique  containment   of   that   deep   convective   injection  resulting   from   the   North   American   monsoon  provides   extended   reaction   times,   by   creating  a   batch   reactor   for   the   catalytic   removal   of  ozone.  

While the intensity and frequency of severe storms that produce deep stratospheric convective injection over the US in summer is a unique and remarkable attribute of meteorology within the continental US, the anticyclonic flow pattern in the lower stratosphere is also remarkable. This flow pattern has received increasing study in the past few years.

These studies have demonstrated that not only has vigorous convection in the central and eastern US injected a significant amount of tropospheric air into the lower stratosphere, but that the material convectively injected into the lower stratosphere is contained within this anticyclonic gyre. Tracer-tracer observations demonstrate that in midsummer this anti-cyclonic flow is a dominant feature of the stratospheric flow and that air convected deep into the stratosphere reaching the altitudes of rapidly increasing available inorganic chlorine, is captured in this anticyclonic circulation.

Figure 6.

Thus rather than being swept zonally and diluted, the injected mixture of water and boundary layer source species is contained within a “batch reactor” that, while not as isolated as the Antarctic and Arctic winter vortices, never-the-less serves to contain the reactive mixture for periods of up to three weeks in July and August over the US.

Point   4:   NEXRAD   weather   radar   observations  over   the  US   in   summer   provide   high   accuracy  three  dimensional   geometry   of   the   convective  injection   of   condensed   phase   water   into   the  stratosphere.  

The US NEXRAD network of weather radars has opened a new chapter in high accuracy analysis of deep convection east of the Rocky Mountains in the continental US Analysis of multiple radars provides the information needed to specify penetration depth into the stratosphere of individual events, as well as seasonal dependence of frequency and intensity of the storm systems in the stratosphere. An example of a NEXRAD curtain profile of a convective storm is displayed in Figure 7. NEXRAD network measurements, have established that: • the NA sector centered over the US Great

Plains is unique with respect to deep convective injection of water vapor in the summer months and the frequency of deep injection has increased in the last decade.

• convective injection of condensed water reaches altitudes of 18 to 20 km; well into the level of rapidly increasing inorganic chlorine.

• the anti-cyclonic NA monsoon flow in the lower stratosphere integrates material collectively injected into the stratosphere over the US for up to three weeks in July and August.

Figure 7.

This has important implications in that a small number of convective events, captured in the anti-cyclonic gyre, can have a significant impact on the ozone column concentration over the US in mid- summer.

Point   5:   It   has   now   been   established   that  simple,   ubiquitous   sulfate-­‐water   aerosols  catalytically   convert   inorganic   chlorine   to   free  radical   (ClO)   form   that   couples   (a)   convective  injection   of   water   into   the   stratosphere   with  (b)  catalytic  ozone  loss  in  the  stratosphere.  

Large ozone losses occur as a result of heterogeneous reactions involving inorganic chlorine on simple binary sulfate-water aerosols. These reactions serve primarily to transform inorganic chlorine into the primary catalytically active chlorine radical, ClO. The dominant pathway for this “chlorine activation” is on cold sulfate-water aerosols. Thus it is both temperature and water vapor in combination with simple binary sulfate-water aerosols that primarily determine the kinetics for rapid chlorine activation. Convective injection of water vapor into the stratosphere over the US raises the threshold temperature for chlorine activation and thus initiates the same chain of chemical reactions that remove ozone in winter over the Arctic and Antarctic.

The impact on ozone loss rates for convectively enhanced water vapor is displayed by comparing Figures 8A for conditions of 200K and 5 ppmv water with 12 ppmv water (typical convective enhancement) in 8B. The rate limiting ClO radical emerges within a fraction of a diurnal cycle following convective injection, and as a result, the catalytic loss of ozone can increase by two orders of magnitude over that for the unperturbed case.

Figure 8.

Point  6:  Key   information  from  the  paleorecord  linking  elevated  CO2  with  climate  structure.  

The paleorecord demonstrates that at elevated levels of CO2 of 850 to 1000 ppmv during the late Eocene the Earth was in a distinctly different climate state as reflected in conditions surrounding the Arctic Ocean shown in Figure 9. A climate state characterized by a small temperature gradient between the tropics and polar regions, sea level heights 100 m above those of today, virtually no ice structure in either hemisphere, and a stratosphere considerably more moist than that of today.

Ozone survived under these conditions because, with natural levels of chlorine and bromine in the stratosphere, NOx was capable of titrating halogen radicals from the lower stratosphere. Not so today. With chlorine and bromine mixing ratios 6 to 7 times pre-industrial levels, ClO titrates NOx from the system in the presence of elevated H2O as displayed explicitly in Figure 8 panel B. Yet with carbon dioxide mixing ratios passing 400 ppmv this past year, and increases in CH4, N2O, CFCs and HCFCs adding another effective 90 ppmv, we now have 490 ppmv of effective CO2, [(CO2)eff], in the atmosphere. The Earth has not seen these levels of [(CO2)eff], for 10 million years. Convective injection of water vapor north of the subtropical jet may constitute a key pathway to a climate state characterized by a diminished temperature gradient between the tropics and polar regions and associated moist stratosphere. Moreover, recent evidence has shown that for [(CO2)eff] of ~600 ppmv, virtually no ice existed in the Northern Hemisphere.

Figure 9.

Point   7:   The   sharp   gradient   in   available  inorganic  chlorine  with  respect  to  both  altitude  and   latitude   demonstrates   that   convective  injection  of  water  vapor  over  the  US  in  summer  reaches   well   into   the   region   of   rapidly  increasing  inorganic  chlorine.  

Observations of the distribution of inorganic chlorine by three different observing systems (in situ aircraft, MLS satellite and ACE satellite observations) present an emerging picture of the altitude and latitude gradient of Cly over the US in summer shown in Figure 10. This provides the information necessary to overlay the altitude of convective water vapor injection with the distribution of inorganic chlorine. The convective injection of H2O into the stratosphere over the US in summer reaches altitudes sufficient to engage inorganic chlorine in the range of 1000 - 2000 pptv.

Figure 10.

Point   8:   Linking   the   unique   meteorology   and  catalytic  ozone  photochemistry  over   the  US   in  summer   with   proposed   solar   radiation  management  via  the  addition  of  sulfate  to  the  lower  atmosphere.  

Efforts to engineer the climate by solar radiation management (SRM) are under serious consideration because reduction in incoming shortwave radiation is one of the only methods that can reverse the rapid irreversible loss of the Earth’s cryosystems — cryosystems that exert an inordinate control over the global climate structure of the Earth. However, the addition of sulfate to the lower stratosphere as displayed in Figure 11, directly engages the same

heterogeneous and homogeneous catalytic cycles for removal of ozone in the lower stratosphere that are treated throughout this document. This results because an increase in the reactive surface area of sulfate aerosols in the stratosphere increases the critical temperature for conversion of inorganic chlorine to free radical form as displayed in Figure 12. The figure captures the impact of increased sulfate reactive surface area by displaying, in the shaded gray area, the dependence of the conversion of inorganic chlorine to free radical form as a function of temperature, water vapor and sulfate reactive surface area. Thus sulfate addition to the lower stratosphere to reduce shortwave forcing of the climate has the potential to increase ozone loss.

Figure 12.

Point   9:   Human   Health:   the   quantitative  coupling  of  ozone  column  loss  with  skin  cancer  incidence  in  the  US.  

The issue of changes in column ozone leading to changes in UV dosage levels in summer engages directly the issue of human health. Skin cancer is the most rapidly growing form of human cancer with 3.5 million new cases a year in the US alone. The incidence of new skin cancer cases has increased 300% between 1992 and 2013. The medical community has established the relationship between fractional decreases in column ozone and fractional increases in the incidence of new skin cancer cases. Based on direct medical evidence, for every 1% decrease in column ozone over the US in summer, a 3% increase in the incidence of new skin cancer cases results each year. Thus a 1% decrease in column ozone translates to an increase of approximately 100,000 new skin cancer cases a year in the US alone. This relationship combined with the fact that the treatment of skin cancer has increased by 300% between 1992 and 2013 and now registers 3.5 million new cases per year in the US alone, provides the information to plot the number of new skin cancer cases per year versus the percent decrease in column ozone over the US in summer. In addition to the remarkable number of cases involved, this underscores the degree of quantitative understanding required to forecast UV column concentration changes in response to rapid increases in climate forcing by CO2 and CH4 increases. The relationship between the number of additional new cases of skin cancer each year in the US is plotted against the percent decrease in ozone column concentration in Figure 13.

Figure 11.

Figure 13.

Key  Unanswered  Scientific  Questions  in  the  Coupling  of  Climate  Forcing  with  Ozone  Loss  in  the  Stratosphere  Over  the  US  in  Summer  We can summarize the factors that have emerged regarding (a) the dynamical structure of the stratosphere over the US in summer and (b) the heterogeneous and homogeneous catalytic chemistry of the lower stratosphere that, taken together, converge to imply that the column concentration of ozone in July and August is vulnerable to reductions involving the same reactions responsible for ozone loss over the Arctic in winter. The mechanisms that link climate change with ozone loss in the stratosphere are summarized in Figure 14.

The recognition of the unique attributes of the dynamics and catalytic chemistry of the lower stratosphere over the US in summer creates the imperative to answer the following questions.

1. What combination of physical mechanisms is responsible for the observed deep stratospheric convective injection of water vapor into the stratosphere over the US in summer?

2. How will the processes that compel that deep stratospheric injection respond to increased forcing of the climate by the addition of increasing amounts of CO2, CH4, N2O, etc. to the atmosphere?

3. What is the structure of the 3D velocity fields within the storm structure in the stratosphere? What factors control the degree of irreversible injection of water and radical precursors into the stratosphere?

Figure 14. There are unique aspects of the stratosphere over the US in summer that conspire to produce the situation wherein the ozone column is vulnerable to reduction resulting from the combination of convective injection, containment by the anti-cyclonic flow in the lower stratosphere and catalytic photochemistry resulting from the conversion of inorganic chlorine to free radical form in simple binary sulfate-water aerosols.

4. What is the structure of the wave breaking events associated with the convective intrusion? How do these wave breaking events affect the vertical kinetic energy of the deep convective events?

5. What are the concentrations of species co-injected with water by the convective event? 6. What are the concentrations of (a) the reactive chemical precursors that are normally removed by

photolysis or oxidation in the troposphere and (b) the chemical tracers brought in by the convective storm or drawn in by the perturbation associated with the convection?

7. How does the structure of the convective injection alter the potential temperature surfaces in the vicinity of the injection?

8. What is the subsequent fate of the chemical cocktail convectively injected into the stratosphere in the hours and days following the convective injection?

9. Does the catalytic photochemistry linking sulfate aerosols and water vapor to the conversion of inorganic chlorine to free radical form mirror the same chemistry as a function of temperature and water vapor that was verified in the stratosphere over the Arctic?

10. Does the region of convective injection into the lower stratosphere decrease in temperature as a function of time due to enhanced radiative cooling in agreement with calculated cooling rates?

11. What is the observed covariance of the local ozone concentration with the rate limiting free radicals ClO and BrO as a function of time following convective injection?

12. What combination of observables are best suited for testing the response of stratospheric ozone to increasing convection into the stratosphere? What is required to develop a trusted forecast of ozone column concentrations over the US in summer in the face of increased climate forcing by CO2 and CH4.

Final  Point:  Considerations  for  Setting  a  Strategic  Path  Forward  that  Effectively  Links                    Scientific  Analysis  With  Public  Policy  

• The ozone shield is arguably the most delicate aspect of habitability on the planet’s surface. If the entire stratospheric ozone column were brought to standard temperature and pressure, that ozone column would be ~ 0.5 cm in depth. A 5% reduction in column ozone concentration over the US translates to a 15% increase in new skin cancer cases each year — i. e. slightly exceeding 500,000 additional skin cancer cases per annum in the US alone. This defines the accuracy to which the forecast of ozone column concentration changes must be quantitatively trusted in response to increased forcing of the climate by CO2, CH4, N2O, etc.

• We have experience with, and have witnessed directly, the dramatic loss of ozone within the polar vortices in both hemispheres in late winter and early spring each year. The scientific community has established the detailed mechanistic link between the available inorganic chlorine in the stratosphere and the catalytic conversion of that inorganic chlorine to free radical form on simple, ubiquitous sulfate-water aerosols in the lower stratosphere. With the combination of in situ aircraft, satellite and NEXRAD observations, we now know that the conditions required for catalytic removal of ozone can be met over the US in summer as a result of deep stratospheric convective injection of water vapor and radical precursors in combination with extended containment of that injection by the anticyclonic flow in the lower stratosphere over the continental US resulting from the North American monsoon in July and August.

• The dependence of the frequency and intensity of severe storms over the US is increasingly tied in the scientific literature to the increased forcing of the climate by the release of greenhouse gases from fossil fuel combustion. Given the millennial scale lifetime of carbon dioxide in the atmosphere, the response of the climate system is irreversible on the decadal time scale of importance to society. It follows that so too are reductions in ozone column concentration and requisite increases in UV dosage levels irreversible with increased forcing of the climate.

• Although reasoned consideration compels action to deal with this issue, experience has shown that public policy changes will not occur without the unequivocal in situ observation of the covariance between (1) the rate limiting radicals responsible for ozone catalytic loss and (2) the rate of ozone removal in the stratosphere over the US in summer. We learned this lesson well during the successful development of the Montreal Protocol with the subsequent London and Copenhagen amendments. This constitutes, therefore, a key strategic consideration moving forward.

• The earlier we obtain unequivocal scientific evidence linking climate forcing to chlorine and bromine catalytic loss of ozone over the US in summer, the greater will be our options regarding strategic decisions to address risk associated with the irreversible nature of the problem.

• In order to effectively interrogate this phenomenon of deep stratospheric injection coupled to the catalytic removal of ozone in the lower stratosphere over the US a new observing system is required that is capable of effectively interrogating, in situ, the dynamical structure of convective injection and then following the subsequent catalytic chemistry as it unfolds in the hours, days and weeks following injection. The StratoCruiser system displayed in the adjoining figure provides the required observing capability.

Figure 15. The development of long duration super pressure balloons provides six weeks of observing time in the stratosphere. When combined with solar-electric propulsion and the previously tested “reel down” capability for in situ vertical soundings of the stratosphere it becomes possible to investigate the physics of deep convective injection and the subsequent catalytic photochemistry of ozone loss in July and August over the US.