Institute for Climate Impact Research, PO Box 601203, 14412 Potsdam, Germany
Received 27 September 2011Accepted for publication 16 November 2011Published 6 December 2011Online at stacks.iop.org/ERL/6/044022
We analyze ﬁve prominent time series of global temperature (over land and ocean) for their common time interval since 1979: three surface temperature records (from NASA/GISS,NOAA/NCDC and HadCRU) and two lower-troposphere (LT) temperature records based on satellite microwave sensors (from RSS and UAH). All ﬁve series show consistent global warming trends ranging from 0.014 to 0.018 K yr
. When the data are adjusted to remove the estimated impact of known factors on short-term temperature variations (El Ni˜no/southern oscillation, volcanic aerosols and solar variability), the global warming signal becomes even more evident as noise is reduced. Lower-troposphere temperature responds more strongly toEl Ni˜no/southern oscillation and to volcanic forcing than surface temperature data. The adjusted data show warming at very similar rates to the unadjusted data, with smaller probable errors, and the warming rate is steady over the whole time interval. In all adjusted series, the two hottest years are 2009 and 2010.
The prime indicator of global warming is,by deﬁnition,global mean temperature. Time series of global temperature show awell-known rise since the early 20th century and most notably since the late 1970s. This widespread temperature increase is corroborated by a range of warming-related impacts:shrinking mountain glaciers, accelerating ice loss from icesheets in Greenland and Antarctica, shrinking Arctic sea ice extent, sea level rise, and a number of well-documented biospheric changes like earlier bud burst and blossoming times in spring (IPCC2007). Despite the unequivocal signs of global warming, somepublic (and to a much lesser extent, scientiﬁc) debate has arisen over discrepancies between the different global temperature records, and over the exact magnitude of, and possible recent changes in, warming rates (Peterson andBaringer2009). To clarify these issues, we analyze the ﬁveleading quasi-global temperature data sets up to and including the year 2010. We focus on the period since 1979, since satellite microwave data are available and the warming trend since that time is at least approximately linear.Much of the variability during that time span can be related to three known causes of short-term temperature variations: El Ni˜no/southern oscillation (ENSO, an internal quasi-oscillatory mode of the ocean–atmosphere system)(Newell and Weare1976, Angell1981,Trenberth
2002), volcanic eruptions (IPCC2007), and solar variations including the solar cycle (IPCC2007,Lean and Rind 2008,2009). This complicates both comparison and trend analysis of the temperature records. Since independent measures of these variations are available, their inﬂuence canto a large extent be removed, leading to adjusted, less noisy global temperature data sets. Therefore we will remove the inﬂuence of these factors on the temperature data sets, not only to isolate the longer-term changes, but also to identify whether different data sets show meaningful differences in their response to these factors. The inﬂuence of exogenous factors will be approximated by multiple regression of temperature against ENSO, volcanic inﬂuence, total solar irradiance (TSI) and a linear time trend to approximate theglobal warming that has occurred during the 32 years subject to analysis.Lean and Rind (2008)performed a multivariate correlation analysis for the period 1889–2006 using the CRU temperature data (Brohan
2006), and found that they could explain 76% of the temperature variance over this period from anthropogenic forcing, El Ni˜no, volcanic aerosols and solar variability. The long-term warming trendalmost exclusively stems from anthropogenic forcing.[….]