Abstract
Global surface temperature has been increasing since the beginning of the 20th century but with a highly variable warming rate, and the alternation of rapid warming periods with ‘hiatus’ decades is a constant throughout the series. The superimposition of a secular warming trend with natural multidecadal variability is the most accepted explanation for such a pattern. Since the start of the 21st century, the surface global mean temperature has not risen at the same rate as the top-of-atmosphere radiative energy input or greenhouse gas emissions, provoking scientific and social interest in determining the causes of this apparent discrepancy. Multidecadal natural variability is the most commonly proposed cause for the present hiatus period. Here, we analyze the HadCRUT4 surface temperature database with spectral techniques to separate a multidecadal oscillation (MDV) from a secular trend (ST). Both signals combined account for nearly 88% of the total variability of the temperature series showing the main acceleration/deceleration periods already described elsewhere. Three stalling periods with very little warming could be found within the series, from 1878 to 1907, from 1945 to 1969 and from 2001 to the end of the series, all of them coincided with a cooling phase of the MDV. Henceforth, MDV seems to be the main cause of the different hiatus periods shown by the global surface temperature records. However, and contrary to the two previous events, during the current hiatus period, the ST shows a strong fluctuation on the warming rate, with a large acceleration (0.0085°C year−1 to 0.017°C year−1) during 1992–2001 and a sharp deceleration (0.017°C year−1 to 0.003°C year−1) from 2002 onwards. This is the first time in the observational record that the ST shows such variability, so determining the causes and consequences of this change of behavior needs to be addressed by the scientific community.
Highlights
The record of global mean temperature anomalies (GMTA), available at monthly resolution since 1850, is a noisy time series that is complicated to analyze due to the large number of forcing factors
An increased storage of heat in the deep ocean is one of the most recurrent explanations derived from modeling exercises [8,13] and from measurement records [14,15]; negative tropical sea surface temperature anomalies have been recently invoked as a major forcing for the recent warming slow-down [16]
By applying Singular spectrum analysis (SSA) to the latest surface temperature database (HadCRUT4) at annual resolution, it is found that nearly 88% of the total variability of GMTA is due to the combination of secular trend (ST) (78.8%) and multidecadal variability (MDV) (8.6%) [4,6] (Fig. 1)
Summary
The record of global mean temperature anomalies (GMTA), available at monthly resolution since 1850, is a noisy time series that is complicated to analyze due to the large number of forcing factors. Most of the recent research tends to agree with natural variability being the most probable cause of the last 15-year hiatus in surface temperature [4,5] as the MDV is currently on a negative phase [7].
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