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Interactions between physical climate and atmospheric chemistry

 
   
 
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page last modified:
April 2005
   
Related to the Support to Protocol monitoring service of TEMIS are the interactions between physical climate and atmospheric chemistry. The following is an introductory text on this subject, adapted from section 2.7 of A global strategy for atmospheric interdisciplinary research in the European research area -- Air pollution report no. 76 (EUR 19436, 2001, European Commission).

 


 
Introduction

Climate-chemistry interactions open a relatively new facet of atmospheric research. These interactions need to be understood in order to optimise any measures for the preservation of cleaner air or to slow down climate changes, as required in the Kyoto protocol and others. The air composition may change due to natural variations (volcanoes, natural biomass burning, lightning, etc.) or due to anthropogenic emissions (e.g., from surface and from air traffic). These changes affect the concentration of radiatively active gases (such as methane, ozone, water vapour), aerosols and clouds, and, hence, impact climate. On the other hand, climate change affects atmospheric composition. For example, increased greenhouse gases cool the stratosphere and may lead to enhanced ozone depletion. Also, climate changes in the troposphere may cause more thunderstorms, more lightning, more nitrogen oxides formation, more upper tropospheric ozone, and hence enhanced radiative forcing. The observed, but unexplained apparent increase of the water vapour concentration in the stratosphere may in the future be as important as the past ozone change, because the increased water vapour may cause a radiative forcing comparable to that by changes in stratospheric ozone. These are just a few of many interaction processes, which are presently far from being understood and which require further research.

 
The current situation

Natural and anthropogenically driven dynamic variability in the atmosphere is one of the major determinants of the variability of the chemical composition of the atmosphere, both directly and indirectly through the influence on aerosol/cloud formation and chemical processes. These phenomena all act on spatial scales ranging from a few metres in the vertical to the entire globe, and influence both the stratosphere and the troposphere. This fundamental variability of the stratosphere and troposphere is not properly understood. Given the possible long-term changes in stratospheric circulation and their relation in the ozone trends over the last decades, more research is needed to characterise the variability of tracers of dynamics in the current atmosphere, using a balance of field measurements and model studies, along with analysis of available satellite observations. In addition, one can not exclude the risk of sudden, possibly irreversible, changes in circulation and composition similar to the one already observed in the springtime polar stratosphere.

 
Future changes

The changes in atmospheric structure caused by climate change could profoundly alter the future composition and variability of the atmosphere. [...] As global warming increases in the [21st] century, the first-order atmospheric changes that impact tropospheric chemistry are the anticipated rise in temperature and water vapour, which will modify the atmospheric loading and distribution of important trace species like ozone.

Thunderstorms, and their associated lightning, are the component of the physical climate system that directly provides a source of a key chemical species, i.e., NOx. The magnitude and distribution of the lightning NOx source is larger in magnitude than the anthropogenic perturbations in the upper troposphere, e.g., that of aviation NOx emissions, but the lightning sources and aviation sources are to a large extent separated in space. The lightning source of NOx is not well established, however, in spite of a number of field studies over Europe and in North America. The link of lightning with deep convection opens up the possibility that this source of NOx would vary with climate change. No proper quantitative evaluation (e.g., the occurrence of severe storms or the production of NOx) can yet be made, however.

Ecosystems respond to near-surface pollution of species like ozone, NO2, acidic gases and aerosols, and to inadvertent fertilization through deposition of reactive nitrogen emitted as NOx or NH3. The response can take the form of die-back, reduced growth, or changed inter-species competition that may alter trace-gas surface exchange through emissions or surface deposition. In a larger feedback loop, human-induced climate change will cause the global system to move to a new physical state defined by changed temperatures, humidity, clouds, precipitation and so on. This new state may in turn induce a variety of changes in global ecosystems, both natural and managed, and hence alter trace-gas surface exchange processes. The coupling of this feedback system -- between build up of greenhouse gases, human-induced climate change, ecosystem responses, trace-gas exchange at the surface, and back to atmospheric composition -- opens a research arena which at present is in its infancy, but which needs to be explored to integrate the Earth system components into the understanding of atmospheric change.