A refined method for calculating equivalent effective stratospheric chlorine

Ozone recovery as seen in perspective of the Dobson spectrophotometer measurements at Belsk (52°N, 21°E) in the period 1963–2008

Abstract. Hypothetical reductions in future emissions of ozone-depleting substances (ODSs) and N2O are evaluated in terms of effects on equivalent effective stratospheric chlorine (EESC), globally-averaged total column ozone, and radiative forcing through 2100. Due to the established success of the Montreal Protocol, these actions can have only a fraction of the impact on ozone depletion that regulations already in force have had. If all anthropogenic ODS and N2O emissions were halted beginning in 2011, ozone is calculated to be higher by about 1–2% during the period 2030–2100 compared to a case of no additional restrictions. Direct radiative forcing by 2100 would be about 0.23 W/m2 lower from the elimination of anthropogenic N2O emissions and about 0.005 W/m2 lower from the destruction of the chlorofluorocarbon (CFC) bank. Due to the potential impact of N2O on future ozone levels, we provide an approach to incorporate it into the EESC formulation, which is used extensively in ozone depletion analyses. The ability of EESC to describe total ozone changes arising from additional ODS and N2O controls is also quantified.

Abstract. The rates at which ozone-depleting substances (ODSs) are removed from the atmosphere, which determine the lifetimes of these ODSs, are key factors for determining the rate of ozone layer recovery in the coming decades. We present here a comprehensive uncertainty analysis of future mixing ratios of ODSs, levels of equivalent effective stratospheric chlorine (EESC), ozone depletion potentials, and global warming potentials (GWPs), using, among other information, the 2013 WCRP/SPARC (World Climate Research Programme/Stratospheric Processes and their Role in Climate) assessment of lifetimes of ODSs and their uncertainties. The year EESC returns to pre-1980 levels, a metric commonly used to indicate a level of recovery from ODS-induced ozone depletion, is 2048 for midlatitudes and 2075 for Antarctic conditions based on the lifetimes from the SPARC assessment, which is about 2 and 4 yr, respectively, later than based on the lifetimes from the WMO (World Meteorological Organization) assessment of 2011. However, the uncertainty in this return to 1980 levels is much larger than the shift due to this change in lifetimes. The year EESC returns to pre-1980 levels ranges from 2039 to 2064 (95% confidence interval) for midlatitudes and from 2061 to 2105 for the Antarctic spring. The primary contribution to these ranges comes from the uncertainty in the lifetimes, with smaller contributions from uncertainties in other modeled parameters. The earlier years of the return estimates derived by the uncertainty analysis, i.e., 2039 for midlatitudes and 2061 for Antarctic spring, are comparable to a hypothetical scenario in which emissions of ODSs cease in 2014. The later end of the range, i.e., 2064 for midlatitudes and 2105 for Antarctic spring, can also be obtained by a scenario with an additional emission of about 7 Mt CFC-11 eq. (eq. – equivalent) in 2015, which is the same as about 2 times the projected cumulative anthropogenic emissions of all ODSs from 2014 to 2050, or about 12 times the projected cumulative HCFC emissions from 2014 to 2050.

Trichlorofluoromethane (CFC‐11, CFCl3) is a major anthropogenic ozone‐depleting substance and greenhouse gas, and its production and consumption are controlled under the Montreal Protocol. However, recent studies show that CFC‐11 emissions have been near constant or increasing since 2002. In this study, we use a two‐dimensional chemistry‐climate model to investigate the stratospheric ozone response to a range of future CFC‐11 emissions scenarios. A scenario with future emissions sustained at 10 gigagrams per year (Gg/year) above the baseline WMO (2018) A1 scenario results in minor additional global (90°S–90°N) ozone depletion of 0.13% by 2100, and a 1.5‐year delay in the global ozone recovery to 1980 levels, relative to the baseline. A scenario with 72.5 Gg/year (the 2013–2016 average) sustained to 2100 results in a substantial 15% increase in effective equivalent stratospheric chlorine and nearly 1% additional global ozone depletion by 2100, with a 7.5‐year delay in the recovery to 1980 global ozone levels, relative to the baseline. The ozone response averaged over time has a strong linear dependence on the cumulative amount of future CFC‐11 emissions under a wide range of scenarios. The resulting ozone response sensitivity gives a simple metric relating the time‐averaged ozone change to the cumulative CFC‐11 emissions. This sensitivity has an inverse dependence on future greenhouse gas concentrations (CO2, CH4, and N2O). For the medium Intergovernmental Panel on Climate Change Representative Concentration Pathway‐6.0 scenario, the sensitivity per 1,000 Gg of cumulative CFC‐11 emissions is −0.1% and −1% for global and Antarctic spring ozone, respectively.

The dependence of Antarctic ozone depletion on midlatitude planetary wave activity and South Pole temperatures was examined from 1979–2003 using NCEP/NCAR reanalyses and column ozone data. The annual severity of Antarctic ozone depletion was quantified using the seasonal mean of daily ozone mass deficit (OMD). The dependence of annual mean OMD on effective equivalent stratospheric chlorine (EESC) was removed to produce an anomaly time series (OMD′). Similar anomaly time series for 100 hPa South Pole temperatures (T′) and 20 hPa, 60°S midlatitude planetary wave activity (PWA′) were calculated. Regression of OMD′ against T′ and PWA′ shows that most of the interannual variability in Antarctic ozone depletion can be explained by variability in midlatitude planetary wave activity and South Pole temperatures. To estimate how future changes in South Pole temperatures, midlatitude wave activity and EESC will affect Antarctic ozone depletion, the regression model was applied to T′ and PWA′ values from a chemistry‐climate model run (1975–2019).

Abstract. The combined record of total and profile ozone measurements from the solar backscatter ultraviolet (SBUV) and SBUV/2 series of instruments, known as the SBUV Merged Ozone Data (MOD) product, constitutes the longest satellite-based ozone time series from a single instrument type and as such plays a key role in ozone trend analyses.Following the approach documented in Frith et al. (2014) to analyze the merging uncertainties in the MOD total ozone record, we use Monte Carlo simulations to estimate the potential for uncertainties in the calibration and drift of individual instruments in the profile ozone merged data set. We focus our discussion on the trends and associated merging uncertainty since 2001 in an effort to verify the start of ozone recovery as predicted by chemistry climate models. We find that merging uncertainty dominates the overall estimated uncertainty when considering only the 15 years of data since 2001. We derive trends versus pressure level for the MOD data set that are positive in the upper stratosphere as expected for ozone recovery. These trends appear to be significant when only statistical uncertainties are included but become not significant at the 2σ level when instrument uncertainties are accounted for. However, when we use the entire data set from 1979 through 2015 and fit to the EESC (equivalent effective stratospheric chlorine) we find statistically significant fits throughout the upper stratosphere at all latitudes. This implies that the ozone profile data remain consistent with our expectation that chlorine is the dominant ozone forcing term.

Abstract. We report on updated trends using different merged zonal mean total ozone datasets from satellite and ground-based observations for the period from 1979 to 2020. This work is an update of the trends reported in Weber et al. (2018) using the same datasets up to 2016. Merged datasets used in this study include NASA MOD v8.7 and NOAA Cohesive Data (COH) v8.6, both based on data from the series of Solar Backscatter Ultraviolet (SBUV), SBUV-2, and Ozone Mapping and Profiler Suite (OMPS) satellite instruments (1978–present), as well as the Global Ozone Monitoring Experiment (GOME)-type Total Ozone – Essential Climate Variable (GTO-ECV) and GOME-SCIAMACHY-GOME-2 (GSG) merged datasets (both 1995–present), mainly comprising satellite data from GOME, SCIAMACHY, OMI, GOME-2A, GOME-2B, and TROPOMI. The fifth dataset consists of the annual mean zonal mean data from ground-based measurements collected at the World Ozone and Ultraviolet Radiation Data Centre (WOUDC). Trends were determined by applying a multiple linear regression (MLR) to annual mean zonal mean data. The addition of 4 more years consolidated the fact that total ozone is indeed slowly recovering in both hemispheres as a result of phasing out ozone-depleting substances (ODSs) as mandated by the Montreal Protocol. The near-global (60∘ S–60∘ N) ODS-related ozone trend of the median of all datasets after 1995 was 0.4 ± 0.2 (2σ) %/decade, which is roughly a third of the decreasing rate of 1.5 ± 0.6 %/decade from 1978 until 1995. The ratio of decline and increase is nearly identical to that of the EESC (equivalent effective stratospheric chlorine or stratospheric halogen) change rates before and after 1995, confirming the success of the Montreal Protocol. The observed total ozone time series are also in very good agreement with the median of 17 chemistry climate models from CCMI-1 (Chemistry-Climate Model Initiative Phase 1) with current ODS and GHG (greenhouse gas) scenarios (REF-C2 scenario). The positive ODS-related trends in the Northern Hemisphere (NH) after 1995 are only obtained with a sufficient number of terms in the MLR accounting properly for dynamical ozone changes (Brewer–Dobson circulation, Arctic Oscillation (AO), and Antarctic Oscillation (AAO)). A standard MLR (limited to solar, Quasi-Biennial Oscillation (QBO), volcanic, and El Niño–Southern Oscillation (ENSO)) leads to zero trends, showing that the small positive ODS-related trends have been balanced by negative trend contributions from atmospheric dynamics, resulting in nearly constant total ozone levels since 2000.

Abstract. The concept of Global Warming Potentials (GWPs) has been extensively used in policy consideration as a relative index for comparing the climate impact of an emitted greenhouse gas (GHG), relative to carbon dioxide with equal mass emissions. Ozone depletion due to emission of chlorinated or brominated halocarbons leads to cooling of the climate system in the opposite direction to the direct warming contribution by halocarbons as GHGs. This cooling is a key indirect effect of the halocarbons on climatic radiative forcing, which is accounted for by indirect GWPs. With respect to climate, it is critical to understand net influences considering direct warming and indirect cooling effects especially for Halons due to the greater ozone-depleting efficiency of bromine over chlorine. Until now, the indirect GWPs have been calculated using a parameterized approach based on the concept of Equivalent Effective Stratospheric Chlorine (EESC) and the observed ozone depletion over the last few decades. As a step towards obtaining indirect GWPs through a more robust approach, we use atmospheric models to explicitly calculate the indirect GWPs of Halon-1211 and Halon-1301 for a 100-year time horizon. State-of-the-art global chemistry-transport models (CTMs) were used as the computational tools to derive more realistic ozone depletion changes caused by an added pulse emission of the two major Halons at the surface. The radiative forcings on climate from the ozone changes have been calculated for indirect GWPs using an atmospheric radiative transfer model (RTM). The simulated temporal variations of global average total column Halons after a pulse perturbation follow an exponential decay with an e-folding time which is consistent with the expected chemical lifetimes of the Halons. Our calculated indirect GWPs for the two Halons are much smaller than those from past studies but are within a single standard deviation of WMO (2007) values and the direct GWP values derived agree with the published values. Our model-based assessment of the Halon indirect GWPs thus confirms the significant importance of indirect effects on climate.

The complex morphology of the pulmonary capillary network causes capillary transit times to be dispersed about a mean. It is known that flow-induced decreases in mean capillary transit time are partially offset by capillary recruitment and distension, but the effect of these factors on the rest of the distribution of transit times is unknown. We have studied the relationship between blood flow, capillary recruitment, and the distribution of transit times in isolated canine lungs with videomicroscopy. Doubling baseline lobar blood flow recruited capillaries. All transit times in the distribution decreased, as did relative dispersion. Doubling flow again caused a further decrease in transit times, but neither capillary recruitment nor relative dispersion changed significantly. We conclude that capillary transit times become more homogeneous as lobar flow increases from low to intermediate levels. Further increases in flow across a fully recruited network are associated with decreases in transit times but not with more homogeneous capillary perfusion.

Abstract. The vertical profile of global-mean stratospheric temperature changes has traditionally represented an important diagnostic for the attribution of the cooling effects of stratospheric ozone depletion and CO2 increases. However, CO2-induced cooling alters ozone abundance by perturbing ozone chemistry, thereby coupling the stratospheric ozone and temperature responses to changes in CO2 and ozone-depleting substances (ODSs). Here we untangle the ozone-temperature coupling and show that the attribution of global-mean stratospheric temperature changes to CO2 and ODS changes (which are the true anthropogenic forcing agents) can be quite different from the traditional attribution to CO2 and ozone changes. The significance of these effects is quantified empirically using simulations from a three-dimensional chemistry-climate model. The results confirm the essential validity of the traditional approach in attributing changes during the past period of rapid ODS increases, although we find that about 10% of the upper stratospheric ozone decrease from ODS increases over the period 1975–1995 was offset by the increase in CO2, and the CO2-induced cooling in the upper stratosphere has been somewhat overestimated. When considering ozone recovery, however, the ozone-temperature coupling is a first-order effect; fully 2/5 of the upper stratospheric ozone increase projected to occur from 2010–2040 is attributable to CO2 increases. Thus, it has now become necessary to base attribution of global-mean stratospheric temperature changes on CO2 and ODS changes rather than on CO2 and ozone changes.

Abstract. Decades of hydrograph separation studies have estimated the proportions of recent precipitation in streamflow using end-member mixing of chemical or isotopic tracers. Here I propose an ensemble approach to hydrograph separation that uses regressions between tracer fluctuations in precipitation and discharge to estimate the average fraction of new water (e.g., same-day or same-week precipitation) in streamflow across an ensemble of time steps. The points comprising this ensemble can be selected to isolate conditions of particular interest, making it possible to study how the new water fraction varies as a function of catchment and storm characteristics. Even when new water fractions are highly variable over time, one can show mathematically (and confirm with benchmark tests) that ensemble hydrograph separation will accurately estimate their average. Because ensemble hydrograph separation is based on correlations between tracer fluctuations rather than on tracer mass balances, it does not require that the end-member signatures are constant over time, or that all the end-members are sampled or even known, and it is relatively unaffected by evaporative isotopic fractionation. Ensemble hydrograph separation can also be extended to a multiple regression that estimates the average (or “marginal”) transit time distribution (TTD) directly from observational data. This approach can estimate both “backward” transit time distributions (the fraction of streamflow that originated as rainfall at different lag times) and “forward” transit time distributions (the fraction of rainfall that will become future streamflow at different lag times), with and without volume-weighting, up to a user-determined maximum time lag. The approach makes no assumption about the shapes of the transit time distributions, nor does it assume that they are time-invariant, and it does not require continuous time series of tracer measurements. Benchmark tests with a nonlinear, nonstationary catchment model confirm that ensemble hydrograph separation reliably quantifies both new water fractions and transit time distributions across widely varying catchment behaviors, using either daily or weekly tracer concentrations as input. Numerical experiments with the benchmark model also illustrate how ensemble hydrograph separation can be used to quantify the effects of rainfall intensity, flow regime, and antecedent wetness on new water fractions and transit time distributions.

Model experiments have revealed that stratospheric polar ozone depletion and anthropogenic increase of greenhouse gases (GHG) have both contributed to the observed increase of summertime tropospheric westerlies in the Southern Hemisphere (SH) with the ozone influence dominating. As the stratospheric halogen loading decreases in the future, ozone is expected to return to higher values, with the disappearance of the Antarctic ozone hole. The impact of this ozone recovery on SH climate is investigated using 21st century simulations with a chemistry climate model (CCM). The model response to the ozone recovery by 2100 shows that tropospheric circulation changes during austral summer caused by ozone depletion between 1970 and 2000 almost reverse, despite increasing GHG concentrations. Comparison of the CCM results with multi‐model scenario experiments from the Fourth Assessment Report (AR4) by the Intergovernmental Panel on Climate Change (IPCC) emphasize the importance of stratospheric ozone recovery for Antarctic climate.

This study investigates the response of the Southern Ocean sea surface temperature (SST) and Antarctic sea ice to stratospheric ozone recovery, focusing on the time scale and seasonality of the response. The response is quantified by contrasting two twenty-first-century ensemble simulations conducted with the Goddard Earth Observing System Chemistry–Climate Model: one with decreasing ozone-depleting substances (ODSs) and the other with fixed 2005 levels of ODSs. In our simulations, the response to ozone recovery has large seasonal variations, but it does not show a two-time-scale behavior. Ozone recovery causes Southern Ocean SST warming in austral summer and cooling in other seasons. Ozone recovery mitigates Antarctic sea ice decrease in the twenty-first century in austral spring, fall, and winter. However, the summer Antarctic sea ice extent is not affected by ozone recovery despite strong surface warming, because the warming occurs north of the sea ice edge. The absence of summer sea ice response likely results from the model bias of underestimating summer sea ice climatology. The summer surface warming response is associated with cooling directly below the mixed layer. Temperature tendency budget analysis shows that reduced vertical mixing plays a critical role in driving this vertical dipole temperature response. We also find that the impact of the vertical temperature advection on SST depends not only on changes in upwelling but also on changes in vertical temperature gradient. Significance Statement We study the climate impact of the projected stratospheric ozone recovery in the twenty-first century on Southern Ocean sea surface temperature and Antarctic sea ice using coupled atmosphere–ocean–chemistry model simulations. The model results show that ozone recovery causes weakening of surface winds over the Southern Ocean, which leads to warming of the Southern Ocean surface in summer and cooling in other seasons. Ozone recovery also reduces Antarctic sea ice loss in the twenty-first century. We quantify the relative importance of meridional and vertical advection and vertical mixing in determining the upper Southern Ocean temperature response to stratospheric ozone recovery. These results improve our understanding of how changes in surface winds affect the Southern Ocean temperature, circulation, and Antarctic sea ice.

The length of time that blood remains in the pulmonary capillary is an important variable in gas exchange. We have investigated the distribution of capillary transit times in isolated rabbit lungs perfused with a bicarbonate-free buffer. The time course of gas exchange was monitored by enclosing the lungs in a plethysmograph. A bolus of buffer containing dissolved acetylene was injected into the perfusion system. Exchange of this inert gas occurred as soon as the bolus reached the capillary bed, thereby describing the input function into the bed. A separate bolus injection of bicarbonate solution resulted in production and excretion of CO2 as long as the bolus remained in the capillary bed. The rate of CO2 production was adjusted by partial inhibition of endothelial carbonic anhydrase. The distribution of capillary transit times was computed from a model of CO2 production in the capillary bed and the observed rates and volumes of acetylene and carbon dioxide excretion. The recovered distributions indicate that there is a fairly wide distribution of capillary transit times (relative dispersion, 0.45) around the mean value of 1.71 s (+/- 0.53 [SD]). Only 10% of capillary transit times are less than one half of mean transit time. It is likely that gas exchange reaches equilibrium in the capillary bed except possibly during strenuous exercise or exposure to high altitude or in disease.

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