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Nearly Half Of 80s Warming Due To Natural Variability, New Study

Reporting Climate Science

Nearly half the warming seen during the 1980s was due to natural variations linked with long term cycles in the Pacific Ocean, according to new research. Natural variability contributes ‘considerably’ to surface warming but the relative importance of natural variation, compared to human-driven warming is declining, researchers claim.


Click to enlarge. Observed and simulated change in global-mean surface temperature. Annual-mean time series relative to 1961–1990 mean derived from observations (black), ASYM-H (red) and ASYM-C (blue) experiments. Shading represents ranges of 95% confidence. Linear trends for 1961–2012 and 2003–2012 are denoted at the top. Time series from the combined CMIP3 and CMIP5 models is also shown by the grey curve, with shading representing one standard deviation. Red and blue vertical dashed lines show the occurrence of El Niño and La Niña events, respectively. Three major volcanic eruptions (Agung, El Chichón and Pinatubo) are indicated by green triangles. Courtesy: authors and Nature Climate Change.

Natural variations have played an important role in both the acceleration of global warming seen at the end of the 20th century and in the so called global warming pause that has continued since the late 1990s, according to researchers.

Pacific Ocean warming and cooling cycles that are linked to deceleration and acceleration in the Pacific trade winds contribute “considerably” to the observed global mean surface air temperature (SAT) record, the scientists report in a paper published in Nature Climate Change.

The team, led by Masahiro Watanabe of the University of Tokyo, used computer models to study the impact of these cycles on the global mean surface temperature for the decades of the 1980s, the 1990s and 2000s.

The results indicate that these natural cycles increased the global average surface temperature by 0.11oC for the 1980s and by 0.13oC for the 1990s while they decreased temperatures by 0.11oC during the 2000s.So natural variations contributed both to the rapid growth in temperatures seen during the 1980s and early 1990s and also to the so called global warming pause seen since the late 1990s.

The researchers suggest that these cycles accounted for 47 per cent of the observed temperature change in the 1980s, 38% in the 1990s and 27 per cent in the 2000s. In other words, natural variations accounted for almost half of the warming seen in the 1980s.

In their paper entitled “Contribution of natural decadal variability to global warming acceleration and hiatus”, the scientists state that “Results indicate that inherent decadal climate variability contributes considerably to the observed global-mean SAT time series, but that its influence on decadal-mean SAT has gradually decreased relative to the rising anthropogenic warming signal”.
The implication of this research is that the pause in global warming is attributable to prolonged La Nina-like (sea surface cooling) conditions or a negative phase of the Interdecadal Pacific Oscillation (IPO) – a long term cycle of Pacific Ocean warming and cooling. The authors also point out that their approach shows the usefulness of coupled atmosphere ocean general circulation model (CGCM) experiments that are partially driven by observed atmosphere ocean states.

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