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Ocean Temperatures (The Past Few Decades) -- Summary
What do ocean temperatures tell us about the theory of CO2-induced global warming? In this brief summary, we consider what can and cannot be inferred from studies of the past few decades.

Going back to the turn of the last century, in a Science news story that highlighted the work of Levitus et al. (2000), Richard Kerr's title all but announced the finding of climatology's Holy Grail: "Globe's 'Missing Warming' Found in the Ocean." But was that really the case?

Before considering this question, it is instructive to note that Kerr clearly acknowledged that much of the warming that had long been predicted to occur as a consequence of the historical rise in the atmosphere's CO2 concentration had indeed been missing. That is to say, his purposeful choice of words was admissive of the fact that earth's atmosphere had not warmed by the amount that had long been predicted. But finally, we were told, the "missing warming" had been found; and once again, all was well with climate modeling: disaster was back on track.

So just how much warming was supposedly found? In a detailed analysis of a vast array of oceanic temperatures spanning the globe and extending from the surface down to a depth of 3000 meters, Levitus et al. detected a whopping 0.06C temperature increase between the mid-1950s and mid-1990s. Because the world's oceans have a combined mass some 2500 times greater than that of the atmosphere, however, this number -- as small as it seems -- was truly significant. But was it correct?

Although their data extended back in time several years beyond the point at which they specified the warming to begin, Levitus et al. computed the linear trend in temperature between the lowest valley of their oscillating time series and its highest peak, ensuring that they would obtain the largest warming possible. Consequently, over a moderately longer time period, global ocean warming would have been computed to be much less than what Levitus et al. reported; and the extended length of record would make the rate of warming smaller still. Nevertheless, NASA's James Hansen was quoted by Kerr as saying that the new ocean-warming data "imply that climate sensitivity is not at the low end of the spectrum" that had typically been considered plausible.

But the warped hype did not end with the magnitude of the warming; it continued with its cause. Climate modeler Jerry Mahlman, for example, stated -- according to Kerr -- that the study of Levitus et al. "adds credibility to the belief that most of the warming in the 20th century is anthropogenic." Yet Levitus et al. had clearly stated in their Science paper that "we cannot partition the observed warming to an anthropogenic component or a component associated with natural variability," which brings us to the subject of climate sensitivity. To calculate such a parameter one must have values for both a climate forcing and a climate response. And if one can't identify the source of the forcing, much less its magnitude, it is clearly impossible to calculate a sensitivity.

One year later, Levitus et al. (2001) and Barnett et al. (2001) added to the documentation of the modest warming of the planet's deep oceans. With respect to this accomplishment, Lindzen (2002) wrote that "the fact that models forced by increasing CO2 and tuned by nominal inclusion of aerosol effects to simulate the global mean temperature record for the past century roughly matched the observed deep ocean record was taken as evidence of the correctness of the models and of the anthropogenic origin of the deep ocean warming." However, he took strong exception to this conclusion.

Assuming the deep-ocean temperature measurements and their analysis were correct, Lindzen used a coupled climate model (an energy balance model with a mixed layer diffusive ocean) "to examine whether deep ocean temperature behavior from 1950 to 2000 actually distinguishes between models of radically different sensitivity to doubled CO2." This exercise revealed that the warming of the deep oceans, in Lindzen's words, "is largely independent of model sensitivity," which led him to conclude that "the behavior of deep ocean temperatures is not a test of model sensitivity, but rather a consequence of having the correct global mean surface temperature time history." In this regard, he also noted that "we are dealing with observed surface warming that has been going on for over a century" and that "the oceanic temperature change over the period 1950-2000 reflects earlier temperature changes at the surface."

Further to this point, we note that according to the data of Esper et al. (2002), the earth began to warm in the early 1800s, so that the warming of the 20th century, according to Briffa and Osborn (2002), was "a continuation of a trend that began at the start of the 19th century." Hence, it can be appreciated that the earth had completed the bulk of its post-Little Ice Age temperature rebound well before the bulk of the Industrial Revolution's CO2 emissions had ever entered the atmosphere, i.e., by about 1930. As a result, the modest rise in deep-water temperatures over the past half-century or so tells us nothing about the sensitivity of earth's climate to atmospheric CO2 enrichment, nor does it link the warming to anthropogenic CO2 emissions.

Moving on to some specific oceanic sites, He et al. (2002) used stable oxygen isotope data acquired from a core of Porites lutea coral on the east of Hainan Island in the South China Sea to develop a 56-year (1943-1998) history of sea surface temperature in that region. This work revealed, in their words, that the sea surface temperature in the 1940s "was warmer than that in the 1980s-1990s," and by as much as 1.5C, which is just another example of the fact that great portions of the world are no warmer now than they were some 50 to 70 years ago.

Motivated by reports of "extraordinary change in the Arctic Ocean observed in recent decades," Polyakov et al. (2003) began their study of this phenomenon by referencing the work of Carmack et al. (1995) and Woodgate et al. (2001), who had reported evidence of positive Atlantic Layer Core Temperature (ALCT) anomalies of up to 1C in the 150- to 800-meter depth interval. Polyakov et al. rightly noted, however, that an evaluation of the significance of these anomalies "requires an understanding of the underlying long-term variability" of the pertinent measurements, which they thus proceeded to provide.

The data employed by Polyakov et al. included temperature and salinity measurements from Russian winter surveys of the central Arctic Ocean carried out over the period 1973-79, which were derived from 1034 oceanographic stations and that constituted "the most complete set of arctic observations." In addition, they utilized forty years of summer and winter observations from the Laptev Sea. Based on these comprehensive measurements, they determined new statistical estimates of long-term variability in both ALCT and sea-surface salinity (SSS). This work demonstrated that the standard dataset that had been used to suggest the existence of the apparent 1C temperature anomalies of the 1990s "considerably underestimates variability," as the observed ALCT anomalies in the late 1970s were fully as great as those of the 1990s.

In discussing their findings, Polyakov et al. wrote that their new statistical analyses placed "strong constraints on our ability to define long-term means," as well as the magnitudes of ALCT and SSS anomalies computed using synoptic measurements from the 1990s referenced to means from earlier climatologies. Consequently, what some had described as "the extraordinary change in the Arctic Ocean observed in recent decades" turned out to be not extraordinary at all; it was merely a reappearance of conditions that had prevailed but a few years earlier.

A little further south, Freeland et al. (2003) analyzed water temperature and salinity measurements that were made at a number of depths over a period of several years along two lines emanating from central Oregon and Vancouver Island westward into the Pacific Ocean. The data obtained from this effort indicated that subsurface waters in an approximate 100-meter-thick layer located between 30 and 150 meters depth off central Oregon were, in the words of the researchers, "unexpectedly cool in July 2002." Specifically, mid-depth temperatures over the outer continental shelf and upper slope were more than 0.5C colder than the historical summer average calculated by Smith et al. (2001) for the period 1961-2000, which Freeland et al. said "might be cooler than a longer-term mean because the 1961-71 decade coincided with a cool phase of the Pacific Decadal Oscillation (Mantua et al., 1997)." At the most offshore station, in fact, they reported that "the upper halocline [was] >1C colder than normal and about 0.5C colder than any prior observation [our italics]." And in addition to being substantially cooler, the anomalous water was also much fresher, and the combined effects of these two phenomena made the water less spicy, as Freeland et al. described it, so much so, in fact, that they referred to the intensity of the spiciness anomaly as "remarkable."

Along the line that ran from the mouth of Juan de Fuca Strait to Station Papa at 50N, 145W in the Gulf of Alaska -- which was sampled regularly between 1959 and 1981, but irregularly thereafter -- similarly low spiciness was observed; and the researchers opined it was the same feature as that detected off the coast of central Oregon. In this case, they reported that "conditions in June 2002 [were] well outside the bounds of all previous experience [our italics]," and that "in summer 2001 the spiciness of this layer was already at the lower bound of previous experience."

Freeland et al. concluded that their data implied that "the waters off Vancouver Island and Oregon in July 2002 were displaced about 500 km south of their normal summer position." Was this observation an indication that the Pacific Ocean was beginning to experience a shift from what Chavez et al. (2003) called a "warm, sardine regime" to a "cool, anchovy regime"? It is tempting to suggest that it was. However, Freeland et al. cautioned against jumping to such a conclusion too quickly, saying there were no obvious signals of such a regime shift in several standard climate indices and that without evidence of a large-scale climate perturbation, the spiciness anomaly might have simply been, well, anomalous. Consequently, although the pattern of Pacific Ocean regime shifts documented by Chavez et al. suggested that a change from warmer to cooler conditions might have been imminent, there was not at that time sufficient climatic evidence to claim that it was indeed in process of occurring.

On the other hand, in reference to the 1976-77 regime shift in the Pacific, Chavez et al. noted that "it took well over a decade to determine that a regime shift had occurred in the mid-1970s" and, hence, that "a regime or climate shift may even be best determined by monitoring marine organisms rather than climate," as suggested by Hare and Mantua (2000). Enlarging on this concept, they cited several studies that appeared to provide such evidence, including "a dramatic increase in ocean chlorophyll off California," which would seem a logical response to what Freeland et al. described as "an invasion of nutrient-rich Subarctic waters." Other pertinent evidence cited by Chavez et al. included "dramatic increases in baitfish (including northern anchovy) and salmon abundance off Oregon and Washington," as well as "increases in zooplankton abundance and changes in community structure from California to Oregon and British Columbia, with dramatic increases in northern or cooler species [our italics]."

In another regional study, McPhaden and Zhang (2004) reported that between the mid-1970s and mid-1990s, sea surface temperatures in the eastern and central equatorial Pacific Ocean rose by about 0.7C in response to a slowdown of the shallow meridional overturning circulation, and that some scientists had suggested these phenomena were the result of greenhouse gas forcing. However, they also noted the existence of evidence for a late 1990s "regime shift" in the North Pacific (Chavez et al., 2003; Peterson and Schwing, 2003) that could temper, or even refute, the other interpretation of the data.

Since year-to-year fluctuations associated with El Nio and La Nia conditions can greatly influence the state of earth's climate system, the two researchers compared mean conditions in the eastern and central equatorial Pacific Ocean for the six-year period July 1992-June 1998 with the more recent five-year period July 1998-June 2003 (both of which intervals spanned at least one complete ENSO warm and cold phase cycle) in order to gain some insight into the relative merits of these two differing views of the issue, i.e., greenhouse gas-induced warming vs. decadal-scale warming associated with a regime that switched to cooling in the late 1990s. In addition to sea surface temperatures, their investigation utilized hydrographic and wind data spanning the period 1992-2003 in order to calculate geostrophic meridional volume transports in the upper pycnocline of the tropical Pacific.

These data and analyses indicated that "the shallow meridianal overturning circulation in the tropical Pacific Ocean has rebounded since 1998, following 25 years of significantly weaker flow." In fact, McPhaden and Zhang determined it had "recently rebounded to levels almost as high as in the 1970s." Likewise, the area-averaged sea surface temperature in the eastern and central equatorial Pacific Ocean concurrently dropped approximately 0.6C to almost equal the low of the mid-1970s and to actually match the low of the previous regime in the mid-1950s.

With respect to tropical Pacific sea surface temperatures, McPhaden and Zhang concluded that the "precise magnitude of anthropogenic influences will be difficult to extract with confidence from the instrumental record given the rapidity with which observed warming trends can be reversed by natural variations," which was truly an understatement, in view of the fact that it was no warmer in the eastern and central equatorial Pacific Ocean at the time of their analyses than it was a full half-century earlier.

Returning to the global ocean, Lyman et al. (2006) introduced their analysis of the subject by stating that "with over 1000 times the heat capacity of the atmosphere, the World Ocean is the largest repository for changes in global heat content," and that "monitoring ocean heat content is therefore fundamental to detecting and understanding changes in the earth's heat balance." Consequently, as they describe it, "using a broad array of in situ temperature data from expendable bathythermographs, ship board conductivity-temperature-depth sensors, moored buoy thermistor records, and autonomous profiling conductivity-temperature-depth floats," they estimated the global integral of ocean heat content anomaly of the upper 750 meters from the start of 1993 through the end of 2005.

This ambitious undertaking revealed that from 1993 to 2003 the heat content of the upper 750 meters of the world ocean increased by 8.1 (1.4) x 1022 J, but that "this increase was followed by a decrease of 3.2 (1.1) x 1022 J between 2003 and 2005," which decrease, in their words, "represents a substantial loss of heat over a 2-year period, amounting to about one fifth of the long-term upper-ocean heat gain between 1955 and 2003 reported by Levitus et al. (2005)." They also found that "the maximum cooling occurs at about 400 m," and that "the cooling signal is still strong at 750 m and appears to extend deeper." In fact, they report that preliminary estimates "show that additional cooling occurred between depths of 750 and 1400 m." As for the source of the cooling, they say it "could be the result of a net loss of heat from the earth to space."

Lyman et al. note that the physical causes of the type of variability they discovered "are not yet well understood," and that "this variability is not adequately simulated in the current generation of coupled climate models used to study the impact of anthropogenic influences on climate," which shortcoming, as they describe it, "may complicate detection and attribution of human-induced climate influences." This statement suggests to us that they feel there has not yet been an adequate demonstration of human-induced influences on world ocean temperatures. In addition, it would appear there currently is little hope of finding such a connection in sub-sets of world ocean data any time soon, for they report that "the relatively small magnitude of the globally averaged signal is dwarfed by much larger regional variations in ocean heat content anomaly." In fact, whereas they report that "the recent decrease in heat content amounts to an average cooling rate of -1.0 0.3 W/m2 (of the earth's total surface area) from 2003 to 2005," regional variations "sometimes exceed the equivalent of a local air-sea heat flux anomaly of 50 W/m2 applied continuously over 2 years."

Noting that the global-scale study of the world's oceans conducted by Levitus et al. (2005) suggested a significant increase in the heat content of the upper 3-km layer between 1957 and 1997, Gouretski and Koltermann (2007) began their analysis of the subject by stating that Levitus et al. did not take into account "possible temperature biases associated with differing instrumentation," which -- considering the great importance of the topic -- is what they thus did. And because the large database employed by Levitus et al. was derived from five main types of instruments -- Mechanical and Expandable Bathythermographs (MBTs and XBTs), hydrographic bottles (Nansen and Rosette), Conductivity-Temperature-Depth (CTD) instruments, and profiling floats -- they analyzed temperature offsets among them and applied their findings to temporal trends in the degree of each type of instrument's usage over the period in question.

This work revealed that XBT data comprised the largest proportion of the total database, and that "with XBT temperatures being positively biased by 0.2-0.4C on average," this bias resulted in "a significant World Ocean warming artifact when time periods before and after introduction of XBTs [were] compared." More specifically, they determined that when using the bias-correction techniques they developed, the ocean heat content increase since the 1950s was reduced by a factor of 0.62; and they say that "such corrections if applied would correspondingly reduce the estimate of the ocean warming in Levitus et al. (2005) calculations." Consequently, on the basis of Gouretski and Koltermann's meticulous work, it would appear that the warming of the global ocean over the last half of the 20th century that had been calculated by Levitus et al. (2005) was way over-blown.

Also studying the global ocean were Harrison and Carson (2007), who sorted individual temperature observations in the World Ocean Database 2001 into 1x1 and 2x2 bins, after which (working only with bins having at least five observations per decade for four of the five decades since 1950) they calculated 51-year temperature trends for depths of 100, 300 and 500 meters, as well as sequential 20-year trends, i.e., 1950-1970, 1955-1975, 1960-1980 ... 1980-2000, for the same depths. Then, based on the results that were statistically significant at the 90% confidence level, they determined their implications, which were that the upper ocean "is replete with variability in space and time, and multi-decadal variability is quite energetic almost everywhere." In fact, they found that 95% of the 2x2 regions they studied "had both warming and cooling trends over sequential 20-year periods," and that "the 51-year trends are determined in a number of regions by large trends over 20- to 25-year sub-periods." Hence, they concluded that "trends based on records of one or two decades in length are unlikely to represent accurately longer-term trends," and, therefore, that "it is unwise to attempt to infer long-term trends based on data from only one or two decades." In addition, they note that "the magnitude of the 20-year trend variability is great enough to call into question how well even the statistically significant 51-year trends ... represent longer-term trends." This situation thus suggests to us that we really don't know what may be happening, in the mean, with respect to the temperature trajectory of the global ocean; and this would appear to be Harrison and Carson's conclusion as well.

One year later, Carson and Harrison (2008) derived and analyzed ocean temperature trends over the period 1955-2003 at depths ranging from 50 to 1000 meters in order to "test the sensitivity of trends to various data processing methods." This they did by (1) utilizing the World Ocean Database 2005 (Boyer et al., 2006), (2) employing the analytical approach of Harrison and Carson (2007), and (3) comparing their results (primarily) with those of Levitus et al. (2005).

In terms of Carson and Harrison's analytical procedure, the results indicated, in their words, that "most of the ocean does not have significant 50-year trends at the 90% confidence level (CL)." In fact, they say that "only 30% of the ocean at 50 meters has 90% CL trends, and the percentage decreases significantly with increasing depth." In comparison with prior calculated trends, they also report that the results "can differ substantially, even in the areas with statistically significant trends," noting finally that "trends based on the more interpolated analyses," such as those of Levitus et al. (2005), "show more warming." Thus, the two researchers conclude, and rightly so, that "ocean heat content integrals and integral trends may be substantially more uncertain than has yet been acknowledged."

In concluding this summary of the global ocean's thermal behavior over the past few decades, we discuss two papers that deal more with processes than with history. The first of these papers is that of Kleypas et al. (2008), who looked for evidence of an ocean thermostat by analyzing patterns of sea surface temperature (SST) increases in the tropics over the past five decades, focusing their attention on the western Pacific warm pool (WPWP), because, in their words, "this is a region where maximum SSTs are thought to be limited by negative feedbacks," as described in the writings of Reginald Newell (1979) -- who they cite -- and who in collaboration with Thomas Dopplick employed what he had learned of the subject to demonstrate -- nearly three decades ago -- that the degree of CO2-induced global warming predicted by the climate models of that day was far greater (and is greater still today) than what is allowed by the real world (Newell and Dopplick, 1979), as is further described in the historical narrative of Idso (1982).

Kleypas et al. say their analysis indicates that "the warmest parts of the WPWP have warmed less than elsewhere in the tropical oceans," which fact "supports the existence of thermostat mechanisms that act to depress warming beyond certain temperature thresholds." In addition, they report that "coral reefs within or near the WPWP have had fewer reported bleaching events relative to reefs in other regions," which is also indicative of the existence of an upper-limiting temperature above which SSTs typically do not rise, presumably because of the "kicking-in" of the oceanic thermostat when they approach 30C in the region the three researchers describe as "the center of coral reef biodiversity," which likely merits that description because of the effectiveness of the hypothesized thermostat.

These findings tend to support the thesis put forward years ago by both Newell and Dopplick (1979) and Idso (1980, 1982, 1989), i.e., that rather than the earth possessing some thermal "tipping point" above which global warming dramatically accelerates, the planet's climatic system is organized so as to do just the opposite and greatly attenuate warming above a certain level.

Last of all, we come to the study of Shaviv (2008), who begins a most intriguing paper by noting that "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, many more of which can be found under the general heading of Solar Influence in our Subject Index. Nevertheless, it is difficult for certain people (such as climate alarmists) to accept the logical derivative of this fact, i.e., that solar variations are driving major climate changes, the prime problem being that measured or reconstructed variations in total solar irradiance seem far too small to be able to produce the observed climatic changes.

One potential 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 this particular instance, however, Shaviv makes a good case for at least the existence of such an amplifier, and he points us in the direction of a sensible candidate to fill this role.

Shaviv's course of action was 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, in his words, "to consistently derive the same oceanic heat flux."

In traveling this path, Shaviv demonstrates "there are large variations in the oceanic heat content together with the 11-year solar cycle." In addition, he reports that the three independent data sets "consistently show that the oceans absorb and emit an order of magnitude more heat [our italics] 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 Dickenson (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 that the solar-wind modulated CRF hypothesis is "a favorable candidate" for primary instigator of all of the many climatic phenomena described in the Solar Influence section of our Subject Index. And he well could be right.

In conclusion, even with all the data that have been acquired over the past half-century, it is still amazingly difficult to state with much confidence exactly what the world's oceans are doing in terms of the storage and loss of heat. And to state why they are doing whatever it is they are doing is more difficult still.

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Last updated 12 August 2009