Integrated ozone depletion as a metric for ozone recovery.

The Montreal Protocol is successfully protecting the ozone layer. The main halogen gases responsible for stratospheric ozone depletion have been regulated under the Protocol, their combined atmospheric abundances are declining and ozone is increasing in some parts of the atmosphere1. Ozone depletion potentials2-4, relative measures of compounds' abilities to deplete stratospheric ozone, have been a key regulatory component of the Protocol in successfully guiding the phasing out in the manufacture of the most highly depleting substances. However, this latest, recovery phase in monitoring the success of the Protocol calls for further metrics. The 'delay in ozone return' has been widely used to indicate the effect of different emissions or phase-down strategies, but we argue here that it can sometimes be ambiguous or even of no use. Instead, we propose the use of an integrated ozone depletion (IOD) metric to indicate the impact of any new emission. The IOD measures the time-integrated column ozone depletion and depends only on the emission strength and the whole atmosphere and stratospheric lifetimes of the species considered. It provides a useful complementary metric of the impact of specific emissions of an ozone depleting substance for both the scientific and policy communities.

On the role of ozone feedback in the ENSO amplitude response under global warming.

The El Niño-Southern Oscillation (ENSO) in the tropical Pacific Ocean is of key importance to global climate and weather. However, state-of-the-art climate models still disagree on the ENSO's response under climate change. The potential role of atmospheric ozone changes in this context has not been explored before. Here we show that differences between typical model representations of ozone can have a first-order impact on ENSO amplitude projections in climate sensitivity simulations. The vertical temperature gradient of the tropical middle-to-upper troposphere adjusts to ozone changes in the upper troposphere and lower stratosphere, modifying the Walker circulation and consequently tropical Pacific surface temperature gradients. We show that neglecting ozone changes thus results in a significant increase in the number of extreme ENSO events in our model. Climate modeling studies of the ENSO often neglect changes in ozone. We therefore highlight the need to understand better the coupling between ozone, the tropospheric circulation, and climate variability.

Responses of Arctic sea ice to stratospheric ozone depletion.

The Arctic has experienced several extreme springtime stratospheric ozone depletion events over the past four decades, particularly in 1997, 2011 and 2020. However, the impact of this stratospheric ozone depletion on the climate system remains poorly understood. Here we show that the stratospheric ozone depletion causes significant reductions in the sea ice concentration (SIC) and the sea ice thickness (SIT) over the Kara Sea, Laptev Sea and East Siberian Sea from spring to summer. This is partially caused by enhanced ice transport from Barents-Kara Sea and East Siberian Sea to the Fram Strait, which is induced by a strengthened and longer lived polar vortex associated with stratospheric ozone depletion. Additionally, cloud longwave radiation and surface albedo feedbacks enhance the melting of Arctic sea ice, particularly along the coast of the Eurasian continent. This study highlights the need for realistic representation of stratosphere-troposphere interactions in order to accurately predict Arctic sea ice loss.

Prescribing Zonally Asymmetric Ozone Climatologies in Climate Models: Performance Compared to a Chemistry-Climate Model.

Three different methods of specifying ozone in an atmosphere-only version of the HadGEM3-A global circulation model are compared to the coupled chemistry configuration of this model. These methods include a specified zonal-mean ozone climatology, a specified 3-D ozone climatology, and a calculated-asymmetry scheme in which a specified zonal-mean ozone field is adapted online to be consistent with dynamically produced zonal asymmetries. These simulations all use identical boundary conditions and, by construction, have the same climatological zonal-mean ozone, that of the coupled chemistry configuration of the model. Prescribing ozone, regardless of scheme, results in a simulation which is 3-4 times faster than the coupled chemistry-climate model (CCM). Prescribing climatological zonal asymmetries leads to a vortex which is the correct intensity but which is systematically displaced over regions with lower prescribed ozone. When zonal asymmetries in ozone are free to evolve interactively with model dynamics, the modeled wintertime stratospheric vortex shape and mean sea level pressure patterns closely resemble that produced by the full CCM in both hemispheres, in terms of statistically significant differences. Further, we separate out the two distinct pathways by which zonal ozone asymmetries influence modeled dynamics. We present this interactive-ozone zonal-asymmetry scheme as an inexpensive tool for accurately modeling the impacts of dynamically consistent ozone fields as seen in a CCM which ultimately influence mean sea level pressure and tropospheric circulation (particularly during wintertime in the Northern Hemisphere, when ozone asymmetries are generally largest), without the computational burden of simulating interactive chemistry.

Ultraviolet radiation modelling using output from the Chemistry Climate Model Initiative.

We have derived values of the Ultraviolet Index (UVI) at solar noon using the Tropospheric Ultraviolet Model (TUV) driven by ozone, temperature and aerosol fields from climate simulations of the first phase of the Chemistry-Climate Model Initiative (CCMI-1). Since clouds remain one of the largest uncertainties in climate projections, we simulated only the clear-sky UVI. We compared the modelled UVI climatologies against present-day climatological values of UVI derived from both satellite data (the OMI-Aura OMUVBd product) and ground-based measurements (from the NDACC network). Depending on the region, relative differences between the UVI obtained from CCMI/TUV calculations and the ground-based measurements ranged between -5.9% and 10.6%. We then calculated the UVI evolution throughout the 21st century for the four Representative Concentration Pathways (RCPs 2.6, 4.5, 6.0 and 8.5). Compared to 1960s values, we found an average increase in the UVI in 2100 (of 2-4%) in the tropical belt (30°N-30°S). For the mid-latitudes, we observed a 1.8 to 3.4 % increase in the Southern Hemisphere for RCP 2.6, 4.5 and 6.0, and found a 2.3% decrease in RCP 8.5. Higher increases in UVI are projected in the Northern Hemisphere except for RCP 8.5. At high latitudes, ozone recovery is well identified and induces a complete return of mean UVI levels to 1960 values for RCP 8.5 in the Southern Hemisphere. In the Northern Hemisphere, UVI levels in 2100 are higher by 0.5 to 5.5% for RCP 2.6, 4.5 and 6.0 and they are lower by 7.9% for RCP 8.5. We analysed the impacts of greenhouse gases (GHGs) and ozone-depleting substances (ODSs) on UVI from 1960 by comparing CCMI sensitivity simulations (1960-2100) with fixed GHGs or ODSs at their respective 1960 levels. As expected with ODS fixed at their 1960 levels, there is no large decrease in ozone levels and consequently no sudden increase in UVI levels. With fixed GHG, we observed a delayed return of ozone to 1960 values, with a corresponding pattern of change observed on UVI, and looking at the UVI difference between 2090s values and 1960s values, we found an 8 % increase in the tropical belt during the summer of each hemisphere. Finally we show that, while in the Southern Hemisphere the UVI is mainly driven by total ozone column, in the Northern Hemisphere both total ozone column and aerosol optical depth drive UVI levels, with aerosol optical depth having twice as much influence on the UVI as total ozone column does.

The increasing threat to stratospheric ozone from dichloromethane.

It is well established that anthropogenic chlorine-containing chemicals contribute to ozone layer depletion. The successful implementation of the Montreal Protocol has led to reductions in the atmospheric concentration of many ozone-depleting gases, such as chlorofluorocarbons. As a consequence, stratospheric chlorine levels are declining and ozone is projected to return to levels observed pre-1980 later this century. However, recent observations show the atmospheric concentration of dichloromethane-an ozone-depleting gas not controlled by the Montreal Protocol-is increasing rapidly. Using atmospheric model simulations, we show that although currently modest, the impact of dichloromethane on ozone has increased markedly in recent years and if these increases continue into the future, the return of Antarctic ozone to pre-1980 levels could be substantially delayed. Sustained growth in dichloromethane would therefore offset some of the gains achieved by the Montreal Protocol, further delaying recovery of Earth's ozone layer.

A large ozone-circulation feedback and its implications for global warming assessments.

State-of-the-art climate models now include more climate processes which are simulated at higher spatial resolution than ever1. Nevertheless, some processes, such as atmospheric chemical feedbacks, are still computationally expensive and are often ignored in climate simulations1,2. Here we present evidence that how stratospheric ozone is represented in climate models can have a first order impact on estimates of effective climate sensitivity. Using a comprehensive atmosphere-ocean chemistry-climate model, we find an increase in global mean surface warming of around 1°C (~20%) after 75 years when ozone is prescribed at pre-industrial levels compared with when it is allowed to evolve self-consistently in response to an abrupt 4×CO2 forcing. The difference is primarily attributed to changes in longwave radiative feedbacks associated with circulation-driven decreases in tropical lower stratospheric ozone and related stratospheric water vapour and cirrus cloud changes. This has important implications for global model intercomparison studies1,2 in which participating models often use simplified treatments of atmospheric composition changes that are neither consistent with the specified greenhouse gas forcing scenario nor with the associated atmospheric circulation feedbacks3-5.

Skin cancer risks avoided by the Montreal Protocol--worldwide modeling integrating coupled climate-chemistry models with a risk model for UV.

The assessment model for ultraviolet radiation and risk "AMOUR" is applied to output from two chemistry-climate models (CCMs). Results from the UK Chemistry and Aerosols CCM are used to quantify the worldwide skin cancer risk avoided by the Montreal Protocol and its amendments: by the year 2030, two million cases of skin cancer have been prevented yearly, which is 14% fewer skin cancer cases per year. In the "World Avoided," excess skin cancer incidence will continue to grow dramatically after 2030. Results from the CCM E39C-A are used to estimate skin cancer risk that had already been inevitably committed once ozone depletion was recognized: excess incidence will peak mid 21st century and then recover or even super-recover at the end of the century. When compared with a "No Depletion" scenario, with ozone undepleted and cloud characteristics as in the 1960s throughout, excess incidence (extra yearly cases skin cancer per million people) of the "Full Compliance with Montreal Protocol" scenario is in the ranges: New Zealand: 100-150, Congo: -10-0, Patagonia: 20-50, Western Europe: 30-40, China: 90-120, South-West USA: 80-110, Mediterranean: 90-100 and North-East Australia: 170-200. This is up to 4% of total local incidence in the Full Compliance scenario in the peak year.

Effects of land use on surface-atmosphere exchanges of trace gases and energy in Borneo: comparing fluxes over oil palm plantations and a rainforest.

This paper reports measurements of land-atmosphere fluxes of sensible and latent heat, momentum, CO(2), volatile organic compounds (VOCs), NO, NO(2), N(2)O and O(3) over a 30 m high rainforest canopy and a 12 m high oil palm plantation in the same region of Sabah in Borneo between April and July 2008. The daytime maximum CO(2) flux to the two canopies differs by approximately a factor of 2, 1200 mg C m(-2) h(-1) for the oil palm and 700 mg C m(-2) h(-1) for the rainforest, with the oil palm plantation showing a substantially greater quantum efficiency. Total VOC emissions are also larger over the oil palm than over the rainforest by a factor of 3. Emissions of isoprene from the oil palm canopy represented 80 per cent of the VOC emissions and exceeded those over the rainforest in similar light and temperature conditions by on average a factor of 5. Substantial emissions of estragole (1-allyl-4-methoxybenzene) from the oil palm plantation were detected and no trace of this VOC was detected in or above the rainforest. Deposition velocities for O(3) to the rainforest were a factor of 2 larger than over oil palm. Emissions of nitrous oxide were larger from the soils of the oil palm plantation than from the soils of the rainforest by approximately 25 per cent. It is clear from the measurements that the large change in the species composition generated by replacing rainforest with oil palm leads to profound changes in the net exchange of most of the trace gases measured, and thus on the chemical composition of the boundary layer over these surfaces.

The stability of the stratospheric ozone layer during the end-Permian eruption of the Siberian Traps.

The discovery of mutated palynomorphs in end-Permian rocks led to the hypothesis that the eruption of the Siberian Traps through older organic-rich sediments synthesized and released massive quantities of organohalogens, which caused widespread O3 depletion and allowed increased terrestrial incidence of harmful ultraviolet-B radiation (UV-B, 280-315nm; Visscher et al. 2004 Proc. Natl Acad. Sci. USA 101, 12952-12956). Here, we use an extended version of the Cambridge two-dimensional chemistry-transport model to evaluate quantitatively this possibility along with two other potential causes of O3 loss at this time: (i) direct effects of HCl release by the Siberian Traps and (ii) the indirect release of organohalogens from dispersed organic matter. According to our simulations, CH3Cl released from the heating of coals alone caused comparatively minor O3 depletion (5-20% maximum) because this mechanism fails to deliver sufficiently large amounts of Cl into the stratosphere. The unusual explosive nature of the Siberian Traps, combined with the direct release of large quantities of HCl, depleted the model O3 layer in the high northern latitudes by 33-55%, given a main eruptive phase of less than or equal to 200kyr. Nevertheless, O3 depletion was most extensive when HCl release from the Siberian Traps was combined with massive CH3Cl release synthesized from a large reservoir of dispersed organic matter in Siberian rocks. This suite of model experiments produced column O3 depletion of 70-85% and 55-80% in the high northern and southern latitudes, respectively, given eruption durations of 100-200kyr. On longer eruption time scales of 400-600kyr, corresponding O3 depletion was 30-40% and 20-30%, respectively. Calculated year-round increases in total near-surface biologically effective (BE) UV-B radiation following these reductions in O3 layer range from 30-60 (kJm(-2)d(-1))BE up to 50-100 (kJm(-2)d(-1))BE. These ranges of daily UV-B doses appear sufficient to exert mutagenic effects on plants, especially if sustained over tens of thousands of years, unlike either rising temperatures or SO2 concentrations.

Climate/chemistry feedbacks and biogenic emissions.

The oxidizing capacity of the atmosphere is affected by anthropogenic emissions and is projected to change in the future. Model calculations indicate that the change in surface ozone at some locations could be large and have significant implications for human health. The calculations depend on the precise scenarios used for the anthropogenic emissions and on the details of the feedback processes included in the model. One important factor is how natural biogenic emissions will change in the future. We carry out a sensitivity calculation to address the possible increase in isoprene emissions consequent on increased surface temperature in a future climate. The changes in ozone are significant but depend crucially on the background chemical regime. In these calculations, we find that increased isoprene will increase ozone in the Northern Hemisphere but decrease ozone in the tropics. We also consider the role of bromine compounds in tropospheric chemistry and consider cases where, in a future climate, the impact of bromine could change.

Critical issues in trace gas biogeochemistry and global change.

The atmospheric composition of trace gases and aerosols is determined by the emission of compounds from the marine and terrestrial biospheres, anthropogenic sources and their chemistry and deposition processes. Biogenic emissions depend upon physiological processes and climate, and the atmospheric chemistry is governed by climate and feedbacks involving greenhouse gases themselves. Understanding and predicting the biogeochemistry of trace gases in past, present and future climates therefore demands an interdisciplinary approach integrating across physiology, atmospheric chemistry, physics and meteorology. Here, we highlight critical issues raised by recent findings in all of these key areas to provide a framework for better understanding the past and possible future evolution of the atmosphere. Incorporating recent experimental and observational findings, especially the influence of CO2 on trace gas emissions from marine algae and terrestrial plants, into earth system models remains a major research priority. As we move towards this goal, archives of the concentration and isotopes of N2O and CH4 from polar ice cores extending back over 650,000 years will provide a valuable benchmark for evaluating such models. In the Pre-Quaternary, synthesis of theoretical modelling with geochemical and palaeontological evidence is also uncovering the roles played by trace gases in episodes of abrupt climatic warming and ozone depletion. Finally, observations and palaeorecords across a range of timescales allow assessment of the Earth's climate sensitivity, a metric influencing our ability to decide what constitutes 'dangerous' climate change.