The rise in global mean temperature of about 0.9 degrees C over the 20th century is one of the most well-known trends in the science of global change. Several modeling and empirical studies suggest that some (~0.3 degrees C) of this warming is due to natural causes like increased solar intensity and decreased vulcanism (which reduces cloud-forming aerosols). Most warming attributed to these factors occurred up to about 1950. The rest of the warming—about 0.6 degrees C, most of which occurs after 1960— can only be explained by the rise in greenhouse gases.
If warming is happening, what about that strange cooling dip from 1940-1970? This has often been attributed to the rise in aerosols from pollution (e.g., power plant smokestacks), which—like volcanoes— form clouds, block solar radiation, and cool temperatures. Once clean air legislation kicked in during the 1970s these pollution aerosols declined, allowing more solar radiation to reach the earth’s surface. Conventional wisdom therefore suggests that cleaning up the atmosphere may have contributed somewhat to climate warming.
A new study in this week’s issue of Nature by David Thompson and colleagues, An abrupt drop in Northern Hemisphere sea surface temperature around 1970, challenges this idea, suggesting that oceans, rather than aerosols, may be the driver of this multi-decadal cooling blip:
The twentieth-century trend in global-mean surface temperature was not monotonic: temperatures rose from the start of the century to the 1940s, fell slightly during the middle part of the century, and rose rapidly from the mid-1970s onwards. The warming–cooling–warming pattern of twentieth-century temperatures is typically interpreted as the superposition of long-term warming due to increasing greenhouse gases and either cooling due to a mid-twentieth century increase of sulphate aerosols in the troposphere, or changes in the climate of the world’s oceans that evolve over decades (oscillatory multidecadal variability). Loadings of sulphate aerosol in the troposphere are thought to have had a particularly important role in the differences in temperature trends between the Northern and Southern hemispheres during the decades following the Second World War. Here we show that the hemispheric differences in temperature trends in the middle of the twentieth century stem largely from a rapid drop in Northern Hemisphere sea surface temperatures of about 0.3 C between about 1968 and 1972. The timescale of the drop is shorter than that associated with either tropospheric aerosol loadings or previous characterizations of oscillatory multidecadal variability. The drop is evident in all available historical sea surface temperature data sets, is not traceable to changes in the attendant metadata, and is not linked to any known biases in surface temperature measurements. The drop is not concentrated in any discrete region of the Northern Hemisphere oceans, but its amplitude is largest over the northern North Atlantic.
This is an interesting development in understanding ocean-atmosphere dynamics. The authors did not go very far in speculating why they thought this observed pattern of North Atlantic cooling occurred, so it’s not yet clear what this means. They offer the following:
The suddenness of the drop in Northern Hemisphere SSTs is reminiscent of ‘abrupt climate change’, such as has been inferred from the palaeoclimate record.
The timescale of the drop is important, because it is considerably shorter than that typically associated with either tropospheric aerosol forcing or oscillatory multidecadal SST variability.
The timing of the drop corresponds closely to a rapid freshening of the northern North Atlantic in the late 1960s/early 1970s (the ‘great salinity anomaly’).
So that potentially rules out things like the North Atlantic Oscillation, Atlantic Multidecadal Oscillation, or Arctic Oscillation but suggests that freshwater inputs from glacial thaw may induce North Atlantic cooling—but likely on a much smaller scale than this.
Thompson, D., Wallace, J., Kennedy, J., & Jones, P. (2010). An abrupt drop in Northern Hemisphere sea surface temperature around 1970 Nature, 467 (7314), 444-447 DOI: 10.1038/nature09394
Image credit: http://commons.wikimedia.org/wiki/File:Instrumental_Temperature_Record_%28NASA%29.svg
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