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The Relative Roles of Ozone and Other Greenhouse Gases in Climate Change in the Stratosphere
GFDL 2002 third quarter milestone
Purpose:
Over the past two decades, scientists studying the Earth's stratosphere have observed a depletion of the ozone layer and a steady increase in the concentrations of long-lived, well-mixed gases ( carbon dioxide, methane, nitrous oxide and halocarbons ). During this same period, satellite and radiosonde observations reveal that the global stratosphere has cooled at a rate of roughly 0.5 degrees Centigrade per decade.
The present study presents results from numerical experiments with the GFDL stratospheric general circulation model ( GCM ) to investigate the stratospheric climate impacts of changes in the trace gases. These simulations are used to evaluate the extent to which the observed temperature cooling can be attributed to the trace gas changes, taking into account uncertainties in both model and observational trend estimates. This detection-attribution study is an integral part of the search for causes of the observed changes in climate.
Figure 1. Averaged temperature trends, shown by latitude and month, in the lower stratosphere (15-21 km) as simulated by the model using observed changes in ozone and well-mixed greenhouse gas concentrations over the period 1979-2000. Shaded regions denote statistically significant results.
Efforts:
A major value of numerical simulations using GFDL's GCMs is the ability to simulate climate impacts due to greenhouse gases and to perform a causal attribution study of the observed climate change. The SKYHI GCM used for the present study was developed specifically to study the global stratospheric climate. In the present undertaking, simulations were performed with the 1979 and present-day ( i.e., 2000 ) observed concentrations of the trace gases. The difference yields a measure of the changes effected to the stratospheric climate over the considered two-decade time period ( Figure 1 ). In order to evaluate the relative roles of ozone and the other trace gases, simulations were performed considering the observed ozone losses alone, and with ozone losses plus the well-mixed greenhouse gases ( Figure 2 ).
Figure 2. Attribution of cooling trend in the stratosphere. Horizontal bars indicate 95% confidence levels about the mean values. Observations are obtained from satellite measurements using the Microwave Sounding Unit ( MSU ) and Stratospheric Sounding Unit (SSU) aboard NOAA satellites. The model simulations illustrate the effects of ( 1 ) ozone changes alone and ( 2 ) changes in ozone and well-mixed greenhouse gases ( WMGGs ).
Customers:
The results have a direct bearing on our understanding of the state of the stratosphere and on the causal attribution of the observed changes over the past two decades. The results obtained here are incorporated in the World Meteorological Organization Stratospheric Ozone Assessment, recently concluded in Les Diablerets, Switzerland ( June 2002 ). They also contributed to the Intergovernmental Panel on Climate Change ( 2001 ) results on trends in stratospheric temperatures.
Significance:
Both ozone and the well-mixed greenhouse gases contribute in an important manner to the cooling of the stratosphere ( Figure B ). Ozone effects are dominant in the lower stratosphere ( around 20 km. ); well-mixed gases are more important in the middle stratosphere ( around 35 km ), while all gases contribute importantly in the upper stratosphere ( around 45 km ). As ozone depletion is linked to the halocarbons, the global stratospheric cooling is attributable in the main to anthropogenic emissions. Another important implication of the trace-gas induced cooling trend in the stratosphere is its likely association with the initiation of ozone depletion in the Arctic and the sustained appearance of 'ozone holes' there during winter / spring.
Success:
The development and application of the model, and the diagnostic analyses comprising comparisons with observations and subsequent inferences, lend robustness to our understanding of the changes in the vertical profile of the atmosphere over the past two decades. While there has been an unambiguous cooling of virtually the entire global stratosphere in the observed record, it has now become possible to attribute this cooling to changes in concentrations of ozone and well-mixed greenhouse gases after taking the observational and model uncertainties into account
Next Steps:
Further developments include a test of the model's ability to simulate the interannual variations of the stratospheric climate and the stratosphere-troposphere interactions that lead to such phenomena as the Arctic Oscillation. These investigations will also focus on identifying the causes of variations in the stratospheric climate on various time scales.