Decadal modulation of global surface temperature by internal climate variability

Abstract

Despite a steady increase in atmospheric greenhouse gases (GHGs), global-mean surface temperature (T) has shown no discernible warming since about 2000, in sharp contrast to model simulations, which on average project strong warming 1-3 . The recent slowdown in observed surface warming has been attributed to decadal cooling in the tropical Pacific 1,4,5 , intensifying trade winds 5 , changes in El Niño activity 6,7 , increasing volcanic activity 8-10 and decreasing solar irradiance 7 . Earlier periods of arrested warming have been observed but received much less attention than the recent period, and their causes are poorly understood. Here we analyse observed and model-simulated global T fields to quantify the contributions of internal climate variability (ICV) to decadal changes in global-mean T since 1920. We show that the Interdecadal Pacific Oscillation (IPO) has been associated with large T anomalies over both ocean and land. Combined with another leading mode of ICV, the IPO explains most of the di erence between observed and model-simulated rates of decadal change in global-mean T since 1920, and particularly over the so-called 'hiatus' period since about 2000. We conclude that ICV, mainly through the IPO, was largely responsible for the recent slowdown, as well as for earlier slowdowns and accelerations in global-mean T since 1920, with preferred spatial patterns di erent from those associated with GHG-induced warming or aerosol-induced cooling. Recent history suggests that the IPO could reverse course and lead to accelerated global warming in the coming decades.The Pacific Decadal Oscillation (PDO; refs 11,12), or more generally the IPO (refs 13,14), switched from a warm phase to a cold phase around 1999 15 . This switch has been associated with a cooling trend since the early 1990s over the Equatorial Central and Eastern Pacific (ECEP; 15 • S-15 • N, 180 • -80 • W) that has contributed to the recent hiatus in global-mean T (refs 4,5). Modelling studies 1,16,17 have also shown that the IPO can modulate the rate of global warming through changes in ocean heat uptake. Given the welldocumented extra-tropical response to tropical forcings 18,19 , it is not surprising that IPO-associated sea surface temperature (SST) variations in the ECEP have had a large impact on global-mean T (ref. 1). The recent cooling of the ECEP has been accompanied by strengthening trade winds 5 and increasing ocean heat uptake 4,16,17,20 , typical of a La Niña event 21 but over decadal timescales. Although these studies all point to a major contribution of the ECEP to the recent global warming slowdown, it is unclear how much of the observed SST change in the ECEP is associated with ICV, particularly the IPO, and how much is due to external forcing change, such as stratospheric aerosols [7][8][9][10] . Previous analyses 22,23 a b Global-mean temperature anomaly difference (°C) r = 0.80, 0.94 OBS − model OBS EOF1 .2 0.1 0.0 −0.1 −0.2 −0.3 0.8 OBS (GISTEMP) Model (CMIP5) Model + 2 OBS EOFs Global-mean temperature anomaly (°C) 0.5 0..1...

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