Australia: The Land Where Time Began

A biography of the Australian continent 

Global Ocean Climate Change

The imprint of anthropogenic climate change is mostly sought in terms of long-term trends in a particular field that is observed, which makes the observing of it by the use of in situ observations difficult. Long time series for the global oceans are not available in many regions, especially away from coasts, islands and shipping lanes that are heavily trafficked, the best available being from tide stations at sites that are scattered around the globe. Satellites provide the best series with good spatial coverage, though this coverage didn't begin until the 1980s. It is possible to discern trends over many decades that are unambiguous as the reconstruction of global ocean heat content, surface temperature, sea level and near-surface salinity with a quality that is high enough, with estimates of error that are quantifiable. The ocean observations provide support for concluding that there is discernable evidence of climate change that is mainly a response to greenhouse gas forcing, when interpreted together with similar time series of atmospheric observations (IPCC, 2007).

The upper 700 m heat content has been increasing since the 1950s, though with a possible decrease through the 1960s. From 1961 to 2003 the heat content increased by about 10 x 1622J, associated with an average increase of temperature of about 0.1oC in the upper 700 m of the ocean (Levitus et al., 2005), with the sea surface temperature (SST) changing by about 0.4oC over that time. The heat content is equivalent to to a sea surface heat flux change of about 0.4 W/m2 (Domingues, et al., 2008), a far smaller change in the heat flux than the error in observations of air-sea flux in the order of 10 W/m2. It is a far more robust indicator of climate change than could possibly be derived from direct observations of air-sea heat flux in the foreseeable future, even though is a formidable task to map and analyse the of the ocean. Since the 1950s the heat content of the entire global system has increased, about 90 % of this heat being absorbed by the oceans because of the much higher heat storage capacity in water than in the atmosphere, sea ice, land ice or the continents. Compared with gas the much greater specific heat of water means that a change of 0.1oC of the upper ocean would be equivalent to a change of almost 100oC change in the temperature of the atmosphere (Levitus et al., 2005).

Over the past 50 years the spatial distribution of the heat content of the ocean and sea surface temperature trend is not uniform. Non-uniform changes are predicted by climate models when projected over the next century, the greatest warming in the Arctic and not much change in the subpolar regions of the North Atlantic and ACC (IPCC, 2007). The warming being observed in the Arctic is more widespread and exaggerated  where there has been a significant retreat of the sea ice cover. In the subpolar North Pacific and the North Atlantic, and in the tropical warm pool of the Pacific heat content appears to have decreased. The situation is somewhat different between the SST trends and the heat content of the water column, with the warm pool warming, and a band of cooling along the ACC in the region where the ocean is warmed by heat fluxes in the annual mean. Wind-driven upwelling, which can decrease SST, is associated with this band. Between the 1950s and 2,000 an increased Antarctic Oscillation AAO (SAM) lowered the sea level pressure and strengthened the westerly winds and upwelling, and it has been suggested that this could have decreased the SST.

According to the authors1 warming has been confined mostly to the upper ocean (Levitus et al., 2005). An ENSO signal in the tropical Pacific Ocean causes the tropical cooling in the world average. In the Northern Hemisphere the subpolar cooling if from the Atlantic Ocean. Around the world temperatures of the Abyssal and bottom waters have been increasing, based on observations that are highly accurate, obtained from research ships (Kawano et al., 2006; Kawano et al., 2010 & Purkey & Johnson, 2010). Though the changes are small they are within the uncertainty of the observations, the largest changes being found near the sources of deep water that are obvious. Far downstream from these sources, in places such as the deep North Pacific, where the bottom water formed hundreds of years ago, can be the result of deep water circulation adjustments such that there is a northward shift of the whole complex of waters without the need of advecting the warmer all the way from the distant source (Nakano & Suginohara, 2002).

Between 1985 and 1998 it has been demonstrated that there have been weak, large-scale trends in the salinity of the upper ocean (Boyer et al., 2005) as well as more recent results from (Durack & Wijffels, 2010), representing a redistribution of fresh water and not a net change in the fresh water content. The average salinity of the ocean should respond to a net melting of land ice by decreasing, which is expected to be a result of global warming. It is shown by quick calculations of the impact of such melting that detection is feasible if the melting is large enough, though at the time the book was written such a change in the salinity of the ocean had not been observed, and salinity has increased weakly in the Indian Ocean and the Atlantic Ocean but decreased in the Pacific Ocean. According to the authors1 this suggests there has been an increase in the evaporation and precipitation cycle. They suggest this could result from atmospheric warming, which would allow it to hold more water thereby cycling it greater amounts from regions of evaporation to regions of precipitation (Bindoff et al., 2007; Talley, 2008).

Since the beginning of reconstructed records 130 years ago global mean sea level has increased. Though the uncertainties of this reconstruction are large the increasingly accurate datasets in more recent times also show the same trend of increase. In 1961a period of improved data coverage began and since that time the average yearly rate of increase has been about 2 mm. Since even better observations began in 1993 the rate of increase has been about 3 mm/year (Bindoff, 2007). About half the sea level rise since 1993 is being attributed to thermal expansion changes resulting from the warming ocean and about half to the melting of glaciers and ice caps. There is a large-scale spatial variation in the change of sea level, as could be expected from the large thermal expansion contribution to the change in sea level.

There have also been changes in the chemical constituents of the ocean, with much attention being focused on carbon parameters as the ocean is a sink for excess anthropogenic carbon dioxide (Sabine et al., 2004) to the extent that the ability to quantify the uptake excess CO2 by the ocean is important for climate change future projections. The acidity of the ocean is increased by an increase of CO2 dissolved in the ocean, which has also been receiving wide attention (Feely et al., 2004; National Research Council, 2010). There have also been changes in the distribution of oxygen in the ocean. The infrequent research ship reoccupations of long sections are the way in which nutrient and oxygen changes have been measured. These observations have shown that at the base of the pycnocline oxygen levels have declined in the mid latitudes over the past several decades. The changes, that have been attributed to circulation of the ocean and winter outcrop densities, have been found to be widespread in the northern and subtropical sections of the North Pacific (Deutsch et al., 2005). Similar oxygen declines in the northwestern North Atlantic have also been attributed to similar factors (Johnson & Gruber, 2007). The outcropping isopycnals in the circulation have also been found to shift as the surface of the ocean warms. The ventilated isopycnals in the subtropical gyres become less dense, which leads to less oxygen on the underlying isopycnals that would have been ventilated vigorously in previous decades. Oxygen declines in the pycnocline in the ACC in the Southern Ocean have been observed (Aoki, Bindoff & Church, 2005). In the tropics the great oxygen minima in the denitrification regions of the upper ocean have been expanding (Stramma et al., 2008).

Therefore over the past several decades the ocean has been warming and its salinity has been observed to have been redistributed in a manner that is consistent with an atmosphere that is warmer and more humid, and the sea level has been rising as a response to ocean warming and the melting of ice on land, and the oxygen content in the upper ocean has been declining.

The authors1 ask if these signals are indicative of climate change. They1 say there are 2 major issues: the data sets are not optimised for global spatial coverage or continuous temporal coverage, and interpretation in terms of climate change have usually been based on the fitting of linear trends to time series. In recent years the data sets have become much better and confirm the long-term trends or provide much more information that allows understanding of the observed trends. Trend fitting is subject to 2 problems, even if the data coverage was perfect: then climate state at the end points of the time series and signal interpretation as a trend instead of being part of a longer term climate variation. To determine between natural and anthropogenic climate variability, "attribution" studies are therefore useful. This has been approached (Barnett et al., 2005) by examining how the natural variability of climate in terms of mode, volcanic activity and solar activity affect regional changes in the temperature change of the upper ocean in an ocean model. At the local level anthropogenic climate change is not necessarily distinguishable from natural variability, though when it is averaged over larger regions, especially entire ocean basins to obtain robust results, at a 90 % confidence level (Barnett et al., 2005), showed that the pattern of temperature change that was observed is not consistent with variability resulting from solely natural sources, the conclusion being that the ocean is showing the imprint of climate change.

Sources & Further reading

  1. Talley, Lynne D., Pickard, George L., Emery, William J., and Swift, James H., 2011, Descriptive Physical Oceanography: An Introduction 6th ed.., Academic Press.
Author: M. H. Monroe
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