The Pause Started In 1993
By Paul Homewood
One of the many papers published to explain the temperature hiatus was one last year from Benjamin Santer and many others, Volcanic contribution to decadal changes in tropospheric temperature, which attempted to blame it on increased volcanic activity.
Despite continued growth in atmospheric levels of greenhouse gases, global mean surface and tropospheric temperatures have shown slower warming since 1998 than previously. Possible explanations for the slow-down include internal climate variability, external cooling influences and observational errors. Several recent modelling studies have examined the contribution of early twenty-first-century volcanic eruptions 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.
What was intriguing was this comment:
Our analysis uses satellite measurements of changes in the temperature of the lower troposphere (TLT) made by Microwave Sounding Units (MSU) on NOAA polar-orbiting satellites. Satellite TLT data have near-global, time-invariant spatial coverage; in contrast, global-mean trends estimated from surface thermometer records can be biased by spatially- and temporally non-random coverage changes.
In other words, they admitted that satellite measurements of tropospheric temperatures was more reliable than surface data. (They referenced the Cowton & Way study).
But of even more interest was this graph:
The top graph shows the familiar trend of RSS and UAH data, with little change since 2001. (Please bear in mind that the study was published in Feb 2014, so temperatures since then have ticked up as the El Nino has developed).
Graph B removes ENSO effects, so, noticeably, the 1998 peak has gone. (In other words, this is theoretically what they believe the temperature record would have said without ENSO changes)
Finally, Graph C removes the effects of El Chichon and Pinatubo as well as ENSO. We find that, once the dip after Pinatubo is removed, there has effectively been no increase in temperatures since around 1993.
It seems that the temperature standstill has lasted longer than we all thought.