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Modelling past and future climate change in the stratosphere

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Climate change impacts the whole atmosphere, from the surface to the thermosphere. The changing temperatures and wind fields in the stratosphere will influence the rate of recovery of the ozone layer. Climate models are developed worldwide to improve our understanding of past climate change and project the future state of the atmosphere under varying scenarios, but this requires careful validations against observation-based datasets such as those created by BIRA-IASB. In order to fully address these topics, the Institute has recently installed and started research with the Whole Atmosphere Community Climate Model (WACCM).
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Ozone layer and climate change: a complicated story

Industrial chlorofluorocarbons that cause ozone depletion have been phased out under the Montreal Protocol.

A chemically driven recovery of polar ozone is expected in response to this historic agreement. On the other hand, the ever-increasing abundance of greenhouse gases changes climate not only close to the surface but also in the stratosphere, where it is expected to decrease temperatures and change the circulation (i.e. prevailing wind regimes). Does climate change in the stratosphere slow down the recovery of the ozone layer?

Volcanic emissions of sulfur also have an important impact on the ozone layer - as shown by the large eruption of Mount Pinatubo in 1991.

Past and future contributions of BIRA-IASB

For more than 15 years, BIRA-IASB has been developing and using the BASCOE model and data assimilation system to disentangle the various causes of long-term changes in stratospheric ozone.

To calculate chemistry and transport in the stratosphere, this model depends on temperature and wind fields which it cannot compute autonomously. This information is read from input datasets which are created by meteorological centers using observations by meteorological satellites. Thanks to the precision of these datasets, BASCOE is a very useful tool to study past and current changes in stratospheric composition. Yet it suffers from a fundamental limitation: it cannot project into the future.

Future projections are the realm of chemistry-climate models, and BIRA-IASB needed such a tool to fully address the issue of ozone depletion and its ongoing recovery. Climate models have become fundamental assets in the international assessments of climate change and our means to mitigate it. Since they are validated with observations of the past, this has been a strong motivation for BIRA-IASB to collect observations of atmospheric composition.

For all these reasons, BIRA-IASB selected and started to work with one of the few climate models able to fully account for stratospheric chemistry: the Whole Atmosphere Community Climate Model (WACCM), a component of the Community Earth System Model developed in the USA.

Work has just started with two doctoral students in collaboration with University of Li├Ęge (ULg). They intend to answer an apparently simple question: does climate change also impact large-scale circulation in the stratosphere? This study, which compares WACCM reconstructions of the past stratosphere with BASCOE analyses and independent measurements collected by ULg, has already seen publication of preliminary results in 2018 in the peer-reviewed journal Atmospheric Chemistry and Physics.

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Figure 2 caption (legend)
Vertical profile of the differences between the age of stratospheric air in the mid-latitudes and in the tropics for the period 2002-2007. The modelled updraft of stratospheric air in the tropics is faster than according to observations (black dots with their uncertainties in grey), whether using meteorological reanalyses with the BASCOE transport model (blue lines) or the climate model WACCM (red line). See Chabrillat et al. (ACP, 2018) for more information.
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