The Earth’s climate is not controlled solely by the atmosphere but instead to a large degree by the heat store represented by the ocean, which has a 3,300 times greater heat capacity than the atmosphere. Furthermore, with a global circulation time of roughly 1,000 years, compared with one year for the atmosphere, changes in ocean heat release or uptake operate over the longer multidecadal, centennial, and millennial time scales associated with climate (as opposed to weather) change.
Despite its critical importance for climatic studies, we have a poor record of ocean heat observations, and it is only since the inception in 2004 of the ARGO global network of more than 3,000 drifting and diving ocean probes that we have an adequate estimate of ocean temperatures and heat budget. Though ARGO data are in their infancy and still subject to adjustment for errors, early indications are that the oceans are currently cooling (Loehle, 2009).
In an important paper, Shaviv (2008) has explored some of the key issues relating to change in ocean heat as a driver of climate change, particularly in response to solar variations. As background, Shaviv writes, “climatic variations synchronized with solar variations do exist, whether over the solar cycle or over longer time-scales,” citing numerous references in support of this fact. However, many scientists decline to accept the logical derivative of this fact: that solar variations are driving climate changes. Their prime objection is that measured or reconstructed variations in total solar irradiance seem too small to be capable of producing observed climate change.
One way of resolving this dilemma would be to discover some amplification mechanism, but most attempts to identify one have been fraught with difficulty and met with much criticism. In his 2008 paper, however, Shaviv makes a good case for the existence of such an amplifier, as well as providing a potential mechanism that might fill that role.
Specifically, Shaviv’s study aimed to “use the oceans as a calorimeter to measure the radiative forcing variations associated with the solar cycle” via “the study of three independent records: the net heat flux into the oceans over 5 decades, the sea-level change rate based on tide gauge records over the 20th century, and the sea-surface temperature variations,” each of which can be used “to consistently derive the same oceanic heat flux.”
In pursuing this logic, Shaviv demonstrated “there are large variations in the oceanic heat content together with the 11-year solar cycle.” In addition, he reports the three independent datasets “consistently show that the oceans absorb and emit an order of magnitude more heat than could be expected from just the variations in the total solar irradiance,” thus “implying,” as he describes it, “the necessary existence of an amplification mechanism, although without pointing to which one.”
Finding it difficult to resist pointing, however, Shaviv acknowledges his affinity for the solar-wind modulated cosmic ray flux (CRF) hypothesis, which was suggested by Ney (1959), discussed by Dickinson (1975), and championed by Svensmark (1998). Based on “correlations between CRF variations and cloud cover, correlations between non-solar CRF variations and temperature over geological timescales, as well as experimental results showing that the formation of small condensation nuclei could be bottlenecked by the number density of atmospheric ions,” this concept, according to Shaviv, “predicts the correct radiation imbalance observed in the cloud cover variations” that are needed to produce the magnitude of the net heat flux into the oceans associated with the 11-year solar cycle. Shaviv thus concludes the solar-wind modulated CRF hypothesis is “a favorable candidate” as the primary instigator of many climatic phenomena.
The global thermohaline system of circulation of ocean currents, also sometimes called the meridional overturning circulation, provides links for the transfer of heat across, between, and vertically through ocean basins, with complete mixing taking up to 1,000 years and more. Physical forcing of the system is provided by the westerly wind belts of the southern circum-Antarctic Ocean and by the sinking of dense, saline water in the North Atlantic Ocean. Past changes in the flow of this ocean circulation system can be shown to be linked to major climate change; for example, flow speeds of the cold-water Pacific Deep Western Boundary Current increased during past glacial periods (Hall et al., 2001). The IPCC, noting such facts, therefore argues global warming will change the speed of ocean circulation phenomena such as the Gulf Stream in ways that will make the world’s climate less hospitable.
In setting out to assess this argument, Baehr et al. (2007) investigated how quickly changes in the North Atlantic meridional overturning circulation (MOC) could be detected by projecting simulated observations onto a time-independent spatial pattern of natural variability, which was derived by regressing the zonal density gradient along 26°N against the strength of the MOC at 26°N within a model-based control climate simulation, which pattern was compared against observed anomalies found between the 1957 and 2004 hydrographic occupations of this latitudinal section.
Looking to the future, this exercise revealed that Atlantic MOC changes could likely be detected with 95 percent reliability after about 30 years, using continuous observations of zonal density gradients that can be obtained from a recently deployed monitoring array. Looking to the past, they report, “for the five hydrographic occupations of the 26°N transect, none of the analyzed depth ranges shows a significant trend between 1957 and 2004, implying that there was no MOC trend over the past 50 years.” The finding is significant because to this point in time, over which the IPCC claims the Earth has warmed at a rate and to a level of warmth that is unprecedented over the past two millennia, there has been no observable change in the rate of the North Atlantic MOC, suggesting either the IPCC is significantly in error in its characterization of Earth’s current level of warmth or the North Atlantic MOC is not nearly as sensitive to global warming as many climate models employed by the IPCC have suggested it is.
Since Baehr et al. (2007) have used real-world hydrographic transect data to demonstrate “there was no MOC trend over the past 50 years,” we will probably have more time to prepare for any undesirable consequences of a drastic decline in the Atlantic MOC than did the unfortunate folks in the non-award-winning film The Day After Tomorrow.
In a second paper addressing North Atlantic deep water formation and circulation, Vage et al. (2008) write, “in response to global warming, most climate models predict a decline in the Meridional Overturning Circulation, often due to a reduction of Labrador Sea Water” (which is produced in the Labrador and Irminger Seas of the North Atlantic Ocean), noting further, “since the mid-1990s, convection in the Labrador Sea has been shallow—and at times nearly absent.”
This confluence of observations might be interpreted as strengthening claims of an impending climatic disaster. However, Vage et al. document “the return of deep convection to the subpolar gyre in both the Labrador and Irminger seas in the winter of 2007–2008,” using “profiling float data from the Argo program to document deep mixing” as well as “a variety of in situ, satellite and reanalysis data” to provide context for the phenomenon.
The Canadian, Danish, French, and U.S. scientists observed winter mixing to depths of 1,800 m in the Labrador Sea, 1,000 m in the Irminger Sea, and 1,600 m south of Greenland, whereas base-period (the winters of 2001–2006) mixing depths are less than 1,000 m. They also determined, via analyses of heat flux components, “the main cause of the enhanced heat flux was unusually cold air temperatures during [the 2007–2008] winter.”
More specifically, the scientists tell us, “the air temperature recorded at the Prins Christian Sund meteorological station near Cape Farewell was 2.8°C colder in the winter of 2007–2008 than the corresponding mean of the base period.” Furthermore, they say the cooling was “not a local phenomenon,” noting “the global temperature dropped 0.45°C between the winters of 2006–2007 and 2007–2008” and that across northern North America “the mean winter temperature was more than 3°C colder.” In addition, they report “storm tracks, the flux of freshwater to the Labrador Sea and the distribution of pack ice all contributed to an enhanced flux of heat from the sea to the air, making the surface water sufficiently cold and dense to initiate deep convection.” This phenomenon was aided by “very strong westerly winds off the Labrador ice edge” that “boosted the advection of cold air towards the region of deep convection,” thereby providing a sort of perfect storm situation in which everything came together to create an oceanic overturning the likes of which had not been seen since the late 1980s to early 1990s.
In the words of the nine scientists of the research team, “the return of deep convection to the Labrador and Irminger seas in the winter of 2007–2008 was a surprise.” One reason for this reaction, as they describe it, was that “contrary to expectations the transition to a convective state took place abruptly, without going through a phase of preconditioning.”
Baehr, J., Keller, K. and Marotzke, J. 2008. Detecting potential changes in the meridional overturning circulation at 26°N in the Atlantic. Climatic Change 91: 11–27.
Baehr, J., Haak, H., Alderson, S., Cunningham, S.A., Jungclaus, J.H., and Marotzke, J. 2007. Timely detection of changes in the meridional overturning circulation at 26°N in the Atlantic. Journal of Climate 20: 5827–5841.
Dickinson, R.E. 1975. Solar variability and the lower atmosphere. Bulletin of the American Meteorological Society 56: 1240–1248.
Hall, I.R., McCave, I.N., Shackleton, N.J., Weedon, G.P., and Harris, S.E. 2001. Glacial intensification of deep Pacific inflow and ventilation. Nature, 412: 809–811.
Loehle, C. 2009. Cooling of the global ocean since 2003. Energy & Environment 20: 101–104.
Ney, E.P. 1959. Cosmic radiation and weather. Nature 183: 451.
Shaviv, N.J. 2008. Using the oceans as a calorimeter to quantify the solar radiative forcing. Journal of Geophysical Research 113: 10.1029/2007JA012989.
Svensmark, H. 1998. Influence of cosmic rays on earth’s climate. Physical Review Letters 81: 5027–5030.
Vage, K., Pickart, R.S., Thierry, V., Reverdin, G., Lee, C.M., Petrie, B., Agnew, T.A., Wong, A., and Ribergaard, M.H. 2008. Surprising return of deep convection to the subpolar North Atlantic Ocean in winter 2007-2008. Nature Geoscience 2: 67–72.