Abstract
We examine whether significant changes in ocean temperatures can be detected in recent decades and if so whether they can be attributed to anthropogenic or natural factors. We compare ocean temperature changes for 1960–2005 in four observational datasets and in historical simulations by atmosphere-ocean general circulation models (AOGCMs) from the Coupled Model Intercomparison Project phase 5 (CMIP5). Observations and CMIP5 models show that the upper 2000 m has warmed with a signal that has a well-defined geographical pattern that gradually propagates to deeper layers over time. Greenhouse gas forcing has contributed most to increasing the temperature of the ocean, a warming which has been offset by other anthropogenic forcing (mainly aerosols), and volcanic eruptions which cause episodic cooling. By characterizing the ocean temperature change response to these forcings we construct multi-model mean fingerprints of time-depth changes in temperature and carry out two detection and attribution analysis. We consider first a two-signal separation into anthropogenic and natural forcings. Then, for the first time, we consider a three signal separation into greenhouse gas, anthropogenic aerosols and natural forcings. We show that all three signals are simultaneously detectable. Using multiple depth levels decreases the uncertainty of the results. Limiting the observations and model fields to locations where there are observations increases the detectability of the signal.
Highlights
Observational estimates show that approximately 93% of the energy entering the climate system since the 1950s has gone into the oceans (e.g. Levitus et al 2000, 2005, 2012; Ishii and Kimoto 2009; Lyman et al 2010; Trenberth 2010)
ocean heat content (OHC) is characterised by an overall warming trend, with superimposed variability and some cooling events attributed to major volcanic eruptions (Agung in 1963, El Chichón in 1982 and Pinatubo in 1991) and El Niño Southern Oscillation (ENSO) variability (e.g. 1998)
Barnett et al (2001) pioneered ocean temperature detection and attribution studies by showing that OHC estimated in the Levitus et al (2000) dataset agrees with historical simulations from a climate models only when the anthropogenic forcing is included
Summary
Observational estimates show that approximately 93% of the energy entering the climate system since the 1950s has gone into the oceans (e.g. Levitus et al 2000, 2005, 2012; Ishii and Kimoto 2009; Lyman et al 2010; Trenberth 2010). Barnett et al (2001) pioneered ocean temperature detection and attribution studies by showing that OHC estimated in the Levitus et al (2000) dataset agrees with historical simulations from a climate models only when the anthropogenic forcing is included. With the ongoing improvements of observational datasets (new data and new instrumental bias corrections) and climate models (CMIP3 and CMIP5), Gleckler et al (2012) and Pierce et al (2012) use higher temporal resolution and a multi-model framework (earlier studies used few models) to reiterate earlier conclusions and show that the anthropogenic signal is detected even more robustly Consistent with these results, but using a different statistical method (optimal fingerprinting, the method used in this paper), Reichert et al (2002) detected the anthropogenic fingerprint in temperature change integrated over the upper 300 m and 3000 m. The three-signal combination has not been attempted before for ocean temperature change
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