The Earth’s climate is a complex system consisting of many individual chemical and physical processes that are coupled to each other often in a nonlinear way through feedback mechanisms. State-of-the-art climate models try to include more and more of these processes. Nevertheless, computational power is limited and expensive so that not every variable can be included in the simulation. Scientists therefore have to make choices in order to reproduce Earth’s climate realistically. Especially atmospheric chemical changes and feedbacks are often neglected in climate simulations.

A new modelling study led by scientists from KIT and Cambridge now highlights that especially the representation of stratospheric ozone has a first-order impact on estimates of future global warming. Using a comprehensive atmosphere–ocean chemistry–climate model, the scientists find an increase in global mean surface warming of around 1 °C after 75 years when ozone is prescribed at pre-industrial levels (Figure 1, C) compared with when it is allowed to evolve self-consistently in response to an abrupt quadrupling of CO2 in the atmosphere (4×CO2 forcing; Figure 1, A, B). The difference is primarily attributed to changes in long-wave radiative feedbacks associated with circulation-driven decreases in tropical lower stratospheric ozone and related stratospheric water vapour and cirrus cloud changes.

Apart from protecting the Earth from the Sun’s ultraviolet radiation ozone can be a greenhouse gas as well. It is mainly formed in the tropical stratosphere and subsequently transported to the poles. Under increased CO2 concentrations this so called Brewer Dobson circulation accelerates a ubiquitous feature in climate model. The new study however highlights the fact that in an accelerated circulation less ozone is been formed resulting in a local decrease of ozone and a significant cooling of the lower and middle tropical stratosphere of up to 3.5 °C. An important feedback resulting from this temperature decrease is a relative drying of the stratosphere. Water that is contained in the rising air freezes in the tropopause region at around 18 km height, creating cirrus clouds. These again amplify the tropospheric cooling due to increased albedo.

The study highlights important implications for global model intercomparison studies in which participating models often use simplified treatments of atmospheric composition changes that are not consistent with the specified greenhouse gas forcing scenario or the associated atmospheric circulation changes. The study emphasises the necessity of building accurate climate models in order to be able to assess the effect that carbon emissions have on the Earth’s climate and to predict possible pathways for the future of our climate.

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