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2010 Literature Review Archives - Detection and Attribution

Christidis, N., P.A. Stott, F.W. Zwiers, et al. 2010. Probabilistic estimates of recent changes in temperature: a multi-scale attribution analysis. Climate Dynamics 34:1130-1156.

New advancements in climate change fingerprinting methodologies show promise in attributing regional scale trends in climate to human influences
Human influences are estimated to have more than doubled the likelihood of mean warming in most sub-continental regions except central North America.  The observed cooling in this region is shown to be consistent with what would be expected under conditions with or without anthropogenic forcing.

Over the past decade, various statistical studies have compared trends in observed climate data with outputs from climate model simulations to demonstrate that some of the observed global and continental scale trends over the past half century can be attributed to human influences. However, attributions have not, to-date, been possible at the sub-continental and smaller scales. An international team of scientists, from the U.K., Canada and Japan, have now developed an advanced version of optimal ‘finger-printing’ methodologies that uses global scale analyses to infer information about the likely causes of observed trends at smaller scales.  Using the global detection and attribution analysis to constrain the results, they estimate the expected regional distributions of trends in annual mean surface temperature (Ts) and in the warmest night of the year (WN) in a world with and without anthropogenic forcings.  This information is then used to estimate how human influences have increased the likelihood of regional warming using an index called the Fraction of Attributable Risk (FAR). The results show that anthropogenic forcings more than doubled the likelihood of warming (for both Ts and WN) in most regions of the world. Since high night-time temperatures are characteristic of heat-waves, this result indicates a human-induced increase in heat-wave intensity. Of particular interest is the result for central North America which is at odds with the other sub-continental regions. A cooling has been observed in this region over the study period (1950-1997) which has been referred to as a ‘warming hole’. The results of the analysis show that the observed cooling is not inconsistent with what would be expected under global warming.  While these results suggest positive attribution of anthropogenic forcing effects is now possible at sub-continental scales, the authors note that such results would be more robust if additional simulations of climate change under conditions of anthropogenic and natural forcing alone had been available to use, since the study was limited to output from three climate models.

Solomon, S., R. Rosenlof, R. Portmann, J. Daniel, S. Davis, T. Sanford and G-K. Plattner. 2010. Science Express Jan 28, 2010. Contributions of stratospheric water vapour to decadal changes in the rate of global warming.

A decrease in stratospheric water vapour is shown to have contributed to the flattening of the global warming trend since about the year 2000.
Global average surface temperature has increased by about 0.75°C over the past century. However, over the most recent decade, the trend has been nearly flat. Although this is not in itself surprising, since natural variability internal to the climate system will modulate the effects of external forcing from greenhouse gases and other factors, it still raises questions as to the cause. This study by Solomon et al shows that changes in stratospheric water vapour are likely to have made a sizable contribution to both the strong increasing global warming trend of the two decades prior to 2000, and to the flat trend post-2000. The investigators analysed observations of stratospheric water vapour from balloon and satellite measurements since 1980. The data show a gradual increase in lower stratospheric water vapour of more than 1 ppmv between 1980 and 2000, followed by a sharp, persistent drop of about 0.4 ppmv water vapour after 2000. The effects of these changes are then explored using an Earth System Model of Intermediate Complexity. A baseline simulation involving changes in well-mixed greenhouse gases and aerosols is compared to one that included the changes in stratospheric water vapour. The results showed that reduced forcing associated with the drop in stratospheric water vapour post-2000 decreased the rate of warming from what it would otherwise have been by about 25%. Conversely, the increase in stratospheric water vapour between 1980 and 2000 could have steepened the rate of warming then by about 30%. The cause(s) of the change in stratospheric water vapour are not certain and therefore it is not yet clear whether the changes represent a feedback to climate warming or a source of internal climate variability. The work does highlight the need to improve the representation of stratospheric processes in climate models in order to better understand the factors contributing to observed changes and improve the projections of future change.

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