Stratospheric Sulfur and its Role in Climate

Science Rationale

Recent ground‐based and satellite based measurements revealed that from the late 1990s to 2009, the stratospheric aerosol layer has increased by 4‐7% per year, depending on location. The causes of those increases remain however unclear as the processes influencing the background state of the stratospheric aerosol layer are not completely understood. Without major volcanic eruptions, the stratospheric aerosol layer is controlled by (a) transport of aerosol precursors (particularly sulfur containing species) and possibly aerosol particles from the troposphere across the TTL into the stratosphere, and (b) modest volcanic eruptions (a volcanic explosivity index (VEI) of 3) that inject sulfur into the tropical troposphere, followed by transport across the TTL, or directly into the lower stratosphere. From there, transport by the Brewer‐Dobson circulation distributes the sulfur throughout the stratosphere (see Figure 1 for a schematic overview). Furthermore, quasi‐isentropic transport from the tropical upper troposphere and convection outside the deep tropics (including the Asian monsoon) have been documented as potential sources of aerosols for the lowermost stratosphere .

Our understanding of the transport processes in the TTL has tremendously improved recently ‐ the whole concept of the existence of the TTL has just been developed within the past decade. On the other hand, most studies of the stratospheric sulfur budget were carried out before an understanding of TTL processes emerged and the implications for the stratospheric sulfur budget are not at all clear and studies looking at these implications are sparse. A key outstanding issue is that climate‐change‐induced changes in the chemical and transport properties of the TTL may result in further climate change by modifying the delivery of natural and human‐derived sulfur to the stratosphere.

Although several microphysical schemes are currently available for global climate models, up to now, there are only a few global aerosol models that explicitly treat microphysical processes interactively including a temporal varying aerosol size distribution. As a result, long term IPCC/CMIP5 scenario runs, for example generally lack this capacity and this leaves a potentially significant climate forcing source ineffectually treated. Even the existing simplified schemes are too computationally expensive to carry out the required ensemble runs of several hundred years. Therefore aerosol climatologies, with their own deficiencies, are commonly used in future scenario runs as a constant stratospheric background. This limits our ability to assess the potential impact of anthropogenic sulfur emissions on climate and our understanding of chemistry/climate feedbacks that result from changes in the delivery of sulfur to the stratosphere and its redistribution there. The proposed project aims to coordinate targeted studies using satellite, in‐situ, ground-based, and airplane based measurements, together with transport modeling tools to investigate the validity of these stratospheric sulfur initialization assumptions and to improve our understanding of atmospheric processes controlling the flux of sulfur into the stratosphere.