Analysis: ‘No Underlying Global Temperature Increase For 20 Years’

No Underlying Global Temperature Increase For 20 Years

By Paul Homewood I was reminded about a paper from last year, authored by Ben Santer amongst others, Volcanic contribution to decadal changes in tropospheric temperature. The paper looked at recent temperature trends in the lower troposphere, and attempted to isolate the effect of volcanoes and ENSO changes. ABSTRACT Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previously1, 2, 3, 4, 5. Possible explanations for the slow-down include internal climate variability3, 4, 6, 7, external cooling influences1, 2, 4, 8, 9, 10, 11 and observational errors12, 13. Several recent modelling studies have examined the contribution of early twenty-first-century volcanic eruptions1, 2, 4, 8 to the muted surface warming. Here we present a detailed analysis of the impact of recent volcanic forcing on tropospheric temperature, based on observations as well as climate model simulations. We identify statistically significant correlations between observations of stratospheric aerosol optical depth and satellite-based estimates of both tropospheric temperature and short-wave fluxes at the top of the atmosphere. We show that climate model simulations without the effects of early twenty-first-century volcanic eruptions overestimate the tropospheric warming observed since 1998. In two simulations with more realistic volcanic influences following the 1991 Pinatubo eruption, differences between simulated and observed tropospheric temperature trends over the period 1998 to 2012 are up to 15% smaller, with large uncertainties in the magnitude of the effect. To reduce these uncertainties, better observations of eruption-specific properties of volcanic aerosols are needed, as well as improved representation of these eruption-specific properties in climate model simulations. Ignore the misdirection about early twenty-first-century volcanic eruptions. What the paper addresses are the El Chichon and Pinatubo eruptions of 1982 and 1991 respectively, generally reckoned to be the two biggest eruptions, in terms of stratospheric input, of the last century. Santer’s charts below show the raw data, after adjusting for ENSO, and finally adjusting for the volcanic eruptions as well. Note that the black line represents the average of the models. After taking out the effect of ENSO and eruptions, it is apparent that temperatures have been flat since the early 1990’s; indeed they have arguably been falling since. This is significant. We are often told that the 17-year pause, with which we are all familiar, is solely dependent on cherry picking the big El Nino year of 1998 as a starting point. What Santer’s study shows is that there has been no underlying upward trend in global temperatures for more than 20 years.

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Marc Morano

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