Content - Climate-chemistry interactions
SPARC Theme: Climate-Chemistry Interactions
EZH Zurich, Switzerland
A. R. Ravishankara
NOAA, R/CSD, Boulder CO
- How will stratospheric ozone and other constituents evolve?
- How will changes in stratospheric composition affect climate?
- What are the links between changes in stratospheric ozone, UV radiation and tropospheric chemistry?
Atmospheric chemistry plays a critical role in the perturbation of climate by controlling the magnitudes and distributions of a large number of important climate forcing agents. For example, the abundances of methane and tropospheric ozone also depend on atmospheric chemical processes. Similarly, abundances and distributions of water vapour and ozone in the stratosphere depend critically on the atmospheric chemistry.
With focus on stratospheric ozone, the impact of climate change on atmospheric chemistry and, conversely, the impact of chemical changes on climate have been highlighted in the recent WMO/UNEP “Scientific Assessment of Ozone Depletion: 2010” (WMO, 2011). Major contributions to this assessment derive from SPARC’s activity on chemistry-climate model validation (CCMVal activity). CCMVal investigates the interactions between climate and chemistry, highlighting the sensitivity of stratospheric ozone to changes in chemical composition, e.g. CFCs (chlorofluorocarbns), and climate. Figure 1 depicts past and future changes in ozone and in ozone-depleting substances (ODSs), based on chemistry-climate model (CCM) simulations, which allow a consistent, fully coupled treatment of chemistry and climate. Regional and global projections of ozone and ODSs are shown for the period 1960–2100, referenced to 1960 values. Total ozone decreased after 1960 as stratospheric chlorine and bromine concentrations, expressed as equivalent stratospheric chlorine (ESC), steadily increased. ESC values have peaked and are now in a slow decline, reflecting the successful implementation of the Montreal Protocol (1987) and its subsequent amendments. Correspondingly, all the projections show maximum total ozone depletion around 2000, shortly after which the highest abundances of ESC had been encountered. Thereafter, total ozone increases as ESC slowly declines, except in the tropics.
However, the ozone recovery caused by the decrease in ESC is found to be not globally uniform and cannot be expected to simply lead back to pre-1960 ozone values. Rather, the continued increase of greenhouse gases (CO2, CH4, N2O, and others) is expected to cool the stratosphere and also to accelerate its turnover, i.e. lead to an intensified mass transport from the troposphere to the stratosphere and back, and a lower residence time of the air in the stratosphere. Both, thermal and dynamical changes (cooling and faster exchange) reduce the chemical ozone loss in air masses travelling from the equatorial source regions to the middle and high latitudes.
In all the projections except the Antarctic and the tropics, total ozone returns to 1960 values by midcentury, which is earlier than expected from the decrease in ESC alone. The earlier recovery is attributable to the mentioned thermal and dynamical climate change effects. These effects are expected to even lead to higher total ozone in the mid-latitudes by the end of this century than they were before the large-scale industrial production of ozone-depleting substances (ODSs). This enhancement has also been termed “super-recovery”. In contrast, in the Antarctic the effect of climate change is smaller than in other regions. As a result, Antarctic total ozone in springtime remains being governed by chemical processes, mirroring the changes in ESC, with both closely approaching 1960 values at century’s end. In the tropics, climate change causes total ozone to remain below 1960 values throughout the century. The tropics are the ozone source region, but the intensified circulation transports the air more efficiently away from the tropics towards the mid-latitudes, before it can reach today’s ozone mixing ratios.
While the future evolution of the ozone layer is a good example for climate change influencing atmospheric chemistry, conversely also composition changes reveal strong impacts on climate. According to the Intergovernmental Panel on Climate Change (IPCC, 2007), methane, ozone and halocarbons are the greenhouse gases that directly follow carbon dioxide in terms of strongest increase in radiative forcing due to anthropogenic activities since the industrial revolution. Changes in tropospheric composition alter the stratospheric composition via changes in the input to the stratosphere. Vice versa, changes in the stratosphere affect the troposphere via changes in the input of ozone from the stratosphere and also changes in UV radiation (Figure 2). For example, the transport of more ozone-rich stratospheric air into the troposphere during the past decade has been made partly responsible for tropospheric ozone background values in the northern hemisphere staying at undesirably high levels despite compliance with concerted clean air measures curbing the anthropogenic emissions of tropospheric ozone precursor gases such as NOx and volatile organic compounds (VOCs). In turn, the tropospheric ozone is an important greenhouse gas (Figure 2).
Aerosol is another climate forcing agent. Effects of anthropogenic aerosols on the climate may phase out part of the increased radiative forcing of greenhouse gases since industrialisation. Aerosols can perturb atmospheric radiation through a direct effect of scattering and absorption of radiation (Figure 2). The effects of aerosols depend critically on their chemical composition and mixing state. Aerosols can also have an indirect effect via interaction with clouds (water, ice and cirrus clouds) by acting as Cloud Condensation Nuclei (CCN). Further, clouds can modify aerosols, their optical properties, their size distributions and their ability to act as CCN. The indirect effect, which is a strong function of chemical and physical properties of aerosols, can perturb clouds and the hydrological cycle, two pivotal components of the climate system. Stratospheric aerosols greatly alter the chemistry in that region and lead to such spectacular changes as the Antarctic ozone hole, with major consequences to global climate.
The “Montreal Protocol on Substances that Deplete the Ozone Layer” (a protocol to the Vienna Convention for the Protection of the Ozone Layer) deserves special mentioning, as it did not only protect the ozone layer by phasing out the production of numerous chlorine and bromine containing substances that are responsible for ozone depletion, but also by conserving climate and supporting the Kyoto Protocol, as many ODSs are at the same time strong greenhouse gases. Owing to its widespread adoption and implementation, the Montreal Protocol has been hailed as “perhaps the single most successful international environmental agreement to date” (Kofi Annan, 2003).
Figure 3 provides striking evidence that in a ‘‘world avoided’’ lacking the Montreal Protocol, not only would the depletion of the ozone layer have become much greater than observed in our world today, but also would anthropogenic climate forcing be by more than 50% higher (blue hatched region in comparison with green shaded region). Further climate benefits that would be significant compared with the Kyoto Protocol reduction target could be achieved under the Montreal Protocol by managing the emissions of substitute fluorocarbon gases (HFCs, red shading in Figure 3), as currently discussed by the working group of the parties to the Montreal Protocol.
Finally, it should be noted that many interactions and feedback processes are complex and still poorly understood. Therefore, a clear understanding of the processes acting in the climate system is essential. While long-lived species are subject of the Montreal and Kyoto negotiations, for short-lived species, because of their variability in space and time, even the current contributions to the climate forcings cannot be easily evaluated via their atmospheric observations alone but require modelling of the relevant processes. Currently, there is a great deal of emphasis on the short-lived species because of the possibility of a quick “return” upon some policy action. Furthermore, these short-lived species are the “pollutants” that need to be addressed for human health and other concerns. Therefore, clear understanding of the processes that connect emissions (sources, precursors) to abundances and the processes that connect the abundances to the climate forcings is essential for an accurate prediction of the future climate and an assessment of the impact of climate change and variations on the Earth system. Such questions are at the heart of AC&C (Atmospheric Chemistry and Climate), a joint SPARC-IGAC (International Global Atmospheric Chemistry) project, one of the core projects of the International Geosphere-Biosphere Programme. The AC&C project intends to bridge the gap between tropospheric and stratospheric modelling, forming part of the plan for SPARC’s theme on Climate-Chemistry Interactions.