Final Report Summary - ACCI (Atmospheric Chemistry-Climate Interactions)
ACCI is a modelling study which has explored the two-way interactions between changes in atmospheric composition and climate. The composition focus is on chemically active gases in the troposphere and stratosphere and, in particular, ozone and methane.
The principle research tool is a chemistry-climate model, UM-UKCA, based on the UK Met Office Unified Model into which detailed chemistry schemes have been incorporated.
Our main findings are:
1. Model projections of climate change depend critically on how changes in ozone are included in the model. For a 4xCO2 experiment, we found an approximately 20% (1C out of a total change of 6C) difference in surface warming between model runs where ozone changed interactively in the model compared with when ozone was unchanged between the base case and the increased CO2. The surface warming was reduced when ozone was calculated interactively.
Further studies showed that the treatment of ozone also impacted the model representation of ENSO under climate change. ENSO modulation was larger when ozone was simply specified.
2. The tropical upper troposphere and lower stratosphere were identified as key regions where the representation of ozone most influenced climate change. One poorly understood process in that region, which we identified, is the production of the oxides of nitrogen by lightning. The tropics are also a region where chemistry-climate interactions will control the evolution of the ozone column during the coming century.
3. Our model studies show that stratospheric ozone is expected to recover during this century but an important new result was that episodes with low ozone in the Arctic could continue to
occur late into the century. These results were based on an unprecedentedly large ensemble of chemistry-climate integrations.
Our stratospheric ozone studies highlighted the role of short-lived bromine and chlorine gases in controlling ozone concentrations in the low stratosphere.
4. Ozone recovery will be little influenced by changes in tropospheric chemistry process, but recovery of stratospheric ozone will have a significant impact on the troposphere. Important processes include changes in the transport of ozone to the troposphere, ozone-induced changes in penetration of UV radiation to the troposphere and the influence of lower stratospheric temperature and wind structure on tropospheric climate.
5. We explored hypothetical geoengineering options involving ‘radiation management’, in which the stratospheric might be engineered (eg by addition of reflecting aerosol particles) to reduce surface temperature to compensate for growing greenhouse gas climate forcing.
Our calculations suggest that, while radiation management may offset global warming, it would not cancel changes in stratospheric ozone due to greenhouse gas increases. This could have severe implication, for example, for tropospheric air quality.
6. Methane is a relatively short-lived greenhouse gas, so that regulation of its emissions could be a particularly effective climate control mechanism during the next few decades. We have improved understanding of the natural emissions of methane. Calculations in support of field measurements have improved our understanding of the seasonality of Arctic wetland emissions and indicate that emissions from the seabed, through the ocean into the atmosphere are currently very low.
The principle research tool is a chemistry-climate model, UM-UKCA, based on the UK Met Office Unified Model into which detailed chemistry schemes have been incorporated.
Our main findings are:
1. Model projections of climate change depend critically on how changes in ozone are included in the model. For a 4xCO2 experiment, we found an approximately 20% (1C out of a total change of 6C) difference in surface warming between model runs where ozone changed interactively in the model compared with when ozone was unchanged between the base case and the increased CO2. The surface warming was reduced when ozone was calculated interactively.
Further studies showed that the treatment of ozone also impacted the model representation of ENSO under climate change. ENSO modulation was larger when ozone was simply specified.
2. The tropical upper troposphere and lower stratosphere were identified as key regions where the representation of ozone most influenced climate change. One poorly understood process in that region, which we identified, is the production of the oxides of nitrogen by lightning. The tropics are also a region where chemistry-climate interactions will control the evolution of the ozone column during the coming century.
3. Our model studies show that stratospheric ozone is expected to recover during this century but an important new result was that episodes with low ozone in the Arctic could continue to
occur late into the century. These results were based on an unprecedentedly large ensemble of chemistry-climate integrations.
Our stratospheric ozone studies highlighted the role of short-lived bromine and chlorine gases in controlling ozone concentrations in the low stratosphere.
4. Ozone recovery will be little influenced by changes in tropospheric chemistry process, but recovery of stratospheric ozone will have a significant impact on the troposphere. Important processes include changes in the transport of ozone to the troposphere, ozone-induced changes in penetration of UV radiation to the troposphere and the influence of lower stratospheric temperature and wind structure on tropospheric climate.
5. We explored hypothetical geoengineering options involving ‘radiation management’, in which the stratospheric might be engineered (eg by addition of reflecting aerosol particles) to reduce surface temperature to compensate for growing greenhouse gas climate forcing.
Our calculations suggest that, while radiation management may offset global warming, it would not cancel changes in stratospheric ozone due to greenhouse gas increases. This could have severe implication, for example, for tropospheric air quality.
6. Methane is a relatively short-lived greenhouse gas, so that regulation of its emissions could be a particularly effective climate control mechanism during the next few decades. We have improved understanding of the natural emissions of methane. Calculations in support of field measurements have improved our understanding of the seasonality of Arctic wetland emissions and indicate that emissions from the seabed, through the ocean into the atmosphere are currently very low.