GFDL - Geophysical Fluid Dynamics Laboratory

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Regional rainfall decline in Australia attributed to anthropogenic greenhouse gases and ozone levels

Key Findings

  • An important new tool for prediction of regional-scale climate change has been developed at GFDL: a new high-resolution climate model (CM2.5), able to simulate regional-scale precipitation with fidelity that is considerably better than previous generation models.
  • Model simulations show that the observed long-term decline in winter rainfall over parts of southern Australia, especially southwestern Australia, is a response to anthropogenic increases in greenhouse gases and reductions in stratospheric ozone.
  • Model projections indicate an amplification of this drying trend, in both magnitude and spatial extent, through the 21st century.

Thomas L. Delworth and Fanrong Zeng. Nature Geoscience. DOI: 10.1038/ngeo2201.

Summary

A suite of simulations, done with a new high-resolution climate model (CM2.5) developed at GFDL, were used to study the observed long-term decline of winter rainfall over parts of southern Australia. In response to anthropogenic increases in greenhouse gases and reduction in stratospheric ozone, the model is able to capture many aspects of the observed drying, especially over southwest Australia. The model projects a continued decline in winter rainfall throughout the rest of the 21st century, with significant implications for regional water resources.

In addition to a control simulation, ensembles of simulations were performed that included various combinations of changing radiative forcing, including those from natural sources (volcanic eruptions and solar irradiance changes) and anthropogenic sources (changes in greenhouse gases, aerosols, ozone, and land use changes). The model does not reproduce the observed drying in response to natural forcings, indicating that the observed drying is likely a response to human activity.

Potential future changes in water resources are an immense societal challenge. Improving our ability to simulate observed changes is a vital step towards reliable projections of future changes. This is particularly critical on regional scales, which are most relevant for resource planning and adaptation. This study demonstrates a remarkable advance in NOAA’s capability to simulate observed water resource changes and project future water resource availability on a regional scale.

Changes in Fall-Winter precipitation over Australia from observations (top panel), a model simulation of the past century (middle panel), and a model projection for the middle of the 21st century (bottom panel). The agreement between observed and model simulated precipitation changes supports the idea that human activity has contributed to the observed drying in southwestern Australia, and that this drying trend will amplify and expand in the 21st century.