Attribution of twentieth century temperature change to natural and anthropogenic causes

  • Stott P
  • Tett S
  • Jones G
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We analyse possible causes of twentieth cen-tury near-surface temperature change. We useàop-timal detection'' methodology to compare seasonal and annual data from the coupled atmosphere-ocean general circulation model HadCM2 with observations averaged over a range of spatial and temporal scales. The results indicate that the increases in temperature observed in the latter half of the century have been caused by warming from anthropogenic increases in greenhouse gases o€set by cooling from tropospheric sulfate aerosols rather than natural variability, either internal or externally forced. We also ®nd that greenhouse gases are likely to have contributed signi®cantly to the warming in the ®rst half of the century. In addition, natural e€ects may have contributed to this warming. Assuming one particular reconstruction of total solar irradiance to be correct implies, when we take the seasonal cycle into account, that solar e€ects have contributed signi®cantly to the warming observed in the early part of the century, regardless of any relative error in the amplitudes of the anthropogenic forcings prescribed in the model. How-ever, this is not the case with an alternative reconstruc-tion of total solar irradiance, based more on the amplitude than the length of the solar cycle. We also ®nd evidence for volcanic in¯uences on twentieth century near-surface temperatures. The signature of the eruption of Mount Pinatubo is detected using annual-mean data. We also ®nd evidence for a volcanic in¯uence on warming in the ®rst half of the century associated with a reduction in mid-century volcanism.

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  • P A Stott

  • S F B Tett

  • G S Jones

  • M R Allen

  • W J Ingram

  • J F B Mitchell

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