The Heat Is Online

Researchers Find 40 years of Deep Ocean Heating

From Science -- March 24, 2000

CLIMATE CHANGE:
Globe's 'Missing Warming' Found in the Ocean

Richard A. Kerr, Science Magazine

Greenhouse skeptics often point to the relatively modest atmospheric warming of the past few decades as evidence of the climatic impotence of greenhouse gases.

Climate modelers respond that much of the heat trapped by greenhouse gases should be going into the ocean, delaying but not preventing some of the atmospheric warming. But oceanographers plumbing the ocean depths have been unable to say who was right, because records of deep-ocean temperature have been too spotty to pick out clear trends.

Now, on page 2225 of this issue of Science, physical oceanographers rummaging through piles of neglected data report that they have turned up millions of old, deep-ocean temperature measurements, enough to draw up oceanic fever charts that confirm the climate models' predicted ocean warming.

"We've shown that a large part of the 'missing warming' has occurred in the ocean," says oceanographer Sydney Levitus, the lead author of the paper. "The whole-Earth system has gone into a relatively warm state."

The international data search-and-rescue effort "adds credibility to the belief that most of the warming in the 20th century is anthropogenic," says climate modeler Jerry D. Mahlman of the National Oceanic and Atmospheric Administration's (NOAA's) Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey. It also suggests that past greenhouse gas emissions guarantee more global warming ahead and that the climate system may be more sensitive to greenhouse gases than some had thought.

How could millions of valuable oceanographic measurements go missing for decades? Oceanographers have never had the orchestrated, worldwide network of routine observations that meteorologists enjoy. Instead, 40 or 50 years ago, ocean temperature profiles made by dropping a temperature sensor down through the sea might end up handwritten on paper, captured in a photograph, or recorded in analog form on magnetic tape. Everything from mold to mice was devouring the data. That's why, under the auspices of the United Nations-sponsored Global Oceanographic Data Archeology and Rescue project, data archaeologists like Levitus have spent the past 7 years seeking out ocean temperature data around the world and digitizing them for archiving on modern media.

After adding 2 million profiles of ocean temperature to the previously archived 3 million profiles, Levitus and his NOAA colleagues in Silver Spring, Maryland, could see a clear change. Between 1955 and 1995, the world ocean--the Pacific, Atlantic, and Indian basins combined--warmed an average of 0.06ºC between the surface and 3000 meters. That's about 20 1022 joules of heat added in 40 years, roughly the same amount the oceans of the Southern Hemisphere gain and lose each year with the change of seasons. Half the warming occurred in the upper 300 meters, half below. The warming wasn't steady, though; heat content rose from a low point in the 1950s, peaked in the late '70s, dropped in the '80s, and rose to a higher peak in the '90s. All three ocean basins followed much the same pattern.

These rescued data have oceanographers excited. "I've never seen anything like this before," says physical oceanographer Peter Rhines of the University of Washington, Seattle. "What surprises me is how much [of the warming] is in the deepwater." The newly retrieved data "show how active the [deep-ocean] system is," says oceanographer James Carton of the University of Maryland, College Park, "and how it's a part of the climate system on short time scales."

The friskiness of the whole-ocean system came as a surprise as well. "There's striking variability from decade to decade," says Rhines. That the heat content tends to rise and fall in concert across all three ocean basins, in both the north and the south, is "quite amazing," he adds. Meteorologists and oceanographers are increasingly recognizing that the atmosphere connects ocean basins (Science, 10 July 1998, p. 157), but as to what could be coordinating global swings in heat content, "I really don't know," says Rhines.

The most immediate reward for retrieving so much data from the oceanographers' attic seems to be more confidence in climate models. The increased heat content of the world ocean is roughly what climate models have predicted. "That's another validation of the models," says climatologist Tom Wigley of the National Center for Atmospheric Research in Boulder, Colorado.

As the models implied, rising ocean temperatures have delayed part of the surface warming, says climate modeler James Hansen of NASA's Goddard Institute for Space Studies in New York City, but that can't continue indefinitely. Even if rising concentrations of greenhouse gases could be stabilized tomorrow, Hansen says, gases that have already accumulated will push surface temperatures up another half-degree or so.

The ocean-induced delay in global warming also suggests to some climatologists that future temperature increases will be toward the top end of the models' range of predictions. Mainstream climatologists have long estimated that a doubling of greenhouse gases, expected by the end of the 21st century, would eventually warm the world between 1.5º and 4.5ºC. Some greenhouse contrarians have put that number at 1ºC or even less. Now, the ocean-warming data "imply that climate sensitivity is not at the low end of the spectrum," says Hansen. He, Wigley, and some others now lean toward a climate sensitivity of about 3ºC or a bit higher. But as climatologist Christopher Folland of the Hadley Center for Climate Prediction in Bracknell, United Kingdom, notes, the considerable variability in ocean heat content from decade to decade means scientists will still be hard pressed to find a precise number for climate sensitivity.

Getting better numbers for ocean heat content remains a top priority for oceanographers. "There's still a vast amount of data out there that needs digitizing," says Folland. And for future numbers, an international effort called Argo, now under way, will create an ocean-spanning network of 3000 free-floating instrument packages. Linked by satellites, the Argo drifters will create a "weather map" of the ocean down to 1500 meters. At least future oceanographers won't have to rummage through the data detritus of their predecessors to see what the ocean is up to.

World's Oceans Warming Up, Could Trigger Large Climate Changes

By Cat Lazaroff

WASHINGTON, DC, March 24, 2000 (ENS) - The oceans of the world have warmed substantially during the past 40 years, the National Oceanic and Atmospheric Administration announced Thursday. NOAA researchers suggest that much of the heat from global warming may have been stored in the oceans, reducing atmospheric temperature increases but leading to potentially huge climate changes in the near future.

Researchers from NOAA's Ocean Climate Laboratory in Silver Spring, Maryland examined three major ocean basins - the Atlantic, Indian and Pacific.

They found the greatest warming has occurred in the upper 300 meters (975 feet) of the ocean waters. This level has warmed an average of 0.56 degrees Fahrenheit. The water in the upper 3,000 meters (9,750 feet) of the world's oceans has warmed on average by 0.11 degrees Fahrenheit.

These findings represent the first time scientists have quantified temperature changes in all of the world's oceans from the surface to a depth of 3,000 meters.

"Since the 1970s, temperatures at the earth's surface have warmed, Arctic sea ice has decreased in thickness, and now we know that the average temperature of the world's oceans has increased during this same time period," said NOAA Administrator D. James Baker.

The ocean and atmosphere interact in complex ways to produce Earth's climate. Owing to its large mass, the ocean acts as the memory of the earth's climate system and can store heat for decades or longer.

As a result, it might become possible some day for scientists to use ocean temperature measurements to forecast the earth's climate decades in advance, the researchers said.

"It is possible that ocean heat content may be an early indicator of the warming of surface, air and sea surface temperatures more than a decade in advance," said Sydney Levitus, who heads NOAA's Ocean Climate Laboratory.

"For example, we found that the increase in subsurface ocean temperatures preceded the observed warming of surface air and sea surface temperatures, which began in the 1970s," Levitus said.

"Our results support climate modeling predictions that show increasing atmospheric greenhouse gases will have a relatively large warming influence on the earth's atmosphere," Levitus warned.

"One criticism of the models is that they predict more warming of the atmosphere than has been actually observed. Climate modelers have suggested that this ‘missing warming' was probably to be found in the world ocean. The results of our study lend credence to this scenario," he explained.

The scientists determined their findings by using data - 5.1 million temperature profiles - from sources around the world, to quantify the variability of the heat content (mean temperature) of the world’s oceans from the surface through 3000 meter depth for the period 1948 to 1996.

The researchers looked at temperature changes in the Atlantic, Indian and Pacific oceans.

"In each ocean basin substantial temperature changes are occurring at much deeper depths than we previously thought. This is just one more piece of the puzzle to understanding the variability of the earth's climate system," said Baker.

The Pacific and Atlantic Oceans have been warming since the 1950s, while the Indian Ocean has warmed since the 1960s. The similar warming patterns of the Pacific and Indian Oceans suggest that the same phenomena is causing the changes to occur in both oceans.

The world ocean warming is likely due to a combination of natural variability, such as the Pacific Decadal Oscillation, and human induced effects, the researchers say. The scientists, led by Levitus, report their findings in today’s issue of the journal "Science," in an article titled "Warming of the World Ocean."

The NOAA report was made possible in part by an international ocean data management project headed by Levitus that has added more than two million historical temperature profiles to electronic archives during the past seven years.

"International cooperation in building the global ocean databases required for understanding the role of the ocean as part of the earth's climate system has been excellent," said Levitus.

Contributions of subsurface ocean temperature data have come from all countries that make oceanographic measurements including the United States, Russia, the United Kingdom, Germany, France, Canada, Australia, and Japan.

Nearly all of the data were gathered by research ships, naval ships, buoys, and merchant ships. Some merchant ships deploy instruments that measure the temperature of the upper ocean as participants in voluntary programs.

Understanding the role of the ocean in climate change and making 10 year climate forecasts will soon be greatly enhanced by observations planned as part of an emerging international Global Ocean Observing System.

Meanwhile, a recently completed study of climate over the past 100 years suggests that interactions between the atmosphere, ocean and sea ice systems may have played a prominent role in the global warming of the early 20th century, NOAA scientists say.

Using climate models run on high performance supercomputers, scientists at NOAA's Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, conducted six experiments to explore possible causes for the warming in the first half of the century. Their findings were also published in today’s issue of "Science."

They linked warming in the early part of the century with a combination of ingredients, including increasing concentrations of greenhouse gas and sulfate aerosols.

The supercomputers turned up strong evidence that warming in the latter part of the 20th century was due in large part to human generated greenhouse gases.

"The fact that all experiments capture the warming from 1970 on is indicative of a robust response of the climate model to increasing concentrations of greenhouse gases," said Thomas Knutson, a research meteorologist.

Researchers Find Ocean Temperature Rising, Even in the Depths

By William K. Stevens, The New York Times, March 24, 2000

An important piece of the global-warming picture has come into clearer focus with a confirmation by scientists that the world's oceans have soaked up much of the warming of the last four decades, delaying its full effect on the atmosphere and thus on climate.

The warming of the deep oceans had long been predicted, and the consequent delaying effect long thought to exist.

But until now the ocean's heat absorption had not been definitively demonstrated, and its magnitude had not been determined.

The finding, by scientists at the National Oceanographic Data Center in Silver Spring, Md., is based on an analysis of 5.1 million measurements, by instruments around the world, of the top two miles of ocean waters from the mid-1950's to the mid-1990's.

The analysis, the first on a global scale, is being published in the March 24, 2000 issue of the journal Science.

As the earth warms, from either natural or human causes, or both, not all the extra heat goes immediately into the atmosphere, where its effect on climate is most direct.

Much of it is absorbed by the oceans, which store it for years or decades before releasing it.

This means that to whatever extent the planet is being warmed by emissions of greenhouse gases like carbon dioxide, which are produced by the burning of coal, oil and natural gas, only part of that heating has materialized so far at and above the earth's surface.

Some experts believe that about half the greenhouse warming is still in the oceanic pipeline and will inevitably percolate to the air in the decades just ahead.

The average surface temperature of the globe has risen by about 1 degree Fahrenheit over the last 100 years. Over the last 25 years, the rate of surface warming has accelerated, amounting to the equivalent of about 3.5 degrees a century.

By comparison, the world is 5 to 9 degrees warmer now than in the depths of the last ice age, 18,000 to 20,000 years ago.

Scientists generally agree that it is unclear how much of the warming is attributable to greenhouse gases and how much to natural causes; many think both are involved.

The new study shows that the average warming of the seas over the 40-year study period amounted to about one-tenth of a degree Fahrenheit for the top 1.9 miles of ocean water as a whole, and more than half a degree in about the top 1,000 feet.

It is possible that the ocean may now be giving up to the atmosphere some of the heat it stored in the early part of the study period, but this has not been established, said Sydney Levitus, the chief author of the study. He is the director of the Ocean Climate Laboratory, part of the data center at Silver Spring, which in turn is part of the National Oceanic and Atmospheric Administration.

Likewise, Mr. Levitus said, it is possible but not established that more frequent appearances of the phenomenon known as El Niño, a semi-periodic warming of the eastern tropical Pacific that disrupts weather around the world, are related to the generally warming ocean.

The magnitude of the oceanic warming surprised some experts. One, Dr. Peter Rhines, an oceanographer and atmospheric scientist at the University of Washington in Seattle, said it appeared roughly equivalent to the amount of heat stored by the oceans as a result of seasonal heating in a typical year.

"That makes it a big number," he said.

Dr. James E. Hansen, a climate expert at the NASA Goddard Institute for Space Studies in New York, said the finding was important because, "in my opinion, the rate of ocean heat storage is the most fundamental number for our understanding of long-term climate change."

Three years ago, Dr. Hansen and colleagues used a computer model to calculate the amount of warming that should have been produced up till then by external influences on the climate system like greenhouse gases and solar radiation.

They found that because of the storage of heat in the ocean, only about half the surface warming should have appeared.

Mr. Levitus and his fellow researchers say in their paper that their findings support the Hansen conclusion.

Still, Mr. Levitus said the cause of the oceanic warming was not clear, although "I believe personally that some of it is due to greenhouse gases."

Some scientists believe that natural factors like recurring oscillations in ocean surface temperature in various parts of the world may play a role in the last century's warming. For example, studies by Dr. Gerard Bond of Columbia University's Lamont Doherty Earth Observatory found that the climate of the North Atlantic region, at least, had alternated between cooler and warmer every 1,500 years, more or less.

The world may be entering one of the natural warming cycles now, say Dr. Bond and Dr. Charles D. Keeling, a climate expert at the Scripps Institution of Oceanography in San Diego.

In a study published this week in the online edition of Proceedings of the National Academy of Sciences, Dr. Keeling suggested that a natural fluctuation in ocean tides over hundreds of years might contribute to these long-term cycles of warming and cooling.

Other possible causes have also been suggested.

Oceans may hold answer to global warming riddle, scientists say

By H. Josef Hebert, Associated Press, The Boston Globe, March, 24, 2000

WASHINGTON - Scientists have discovered a significant, surprising warming of the world's oceans over the past 40 years, providing new evidence that computer models might be on target when they predict the Earth's warming.

The broad study of temperature data from the oceans, dating to the 1950s, shows average temperatures have increased more than expected - about half a degree Fahrenheit closer to the surface, and one-tenth of a degree even at depths of up to 10,000 feet.

The findings, reported by scientists at the National Oceanic and Atmospheric Administration, also might explain a major puzzle in the global warming debate: why computer models have shown more significant warming than actual temperature data.

Global warming skeptics contend that if the computer models exaggerate warming that already has occurred, they should not be trusted to predict future warming. The models have shown higher temperatures than those found in surface and atmospheric readings. But now, the ocean data may explain the difference, scientists said.

In the administration study, scientists for the first time have quantified temperature changes in the world's three major ocean basins.

''We've known the oceans could absorb heat, transport it to subsurface depths, and isolate it from the atmosphere. Now we see evidence that this is happening,'' said Sydney Levitus, chief of the agency's Ocean Climate Laboratory and principal author of the study.

Levitus and fellow scientists examined temperature data from more than 5 million readings at various depths in the Pacific, Atlantic, and Indian oceans, from 1948 to 1996.

They found the Pacific and Atlantic oceans have been warming since the mid-1950s, and the Indian Ocean since the early 1960s, according to the study published today in the journal Science.

The greatest warming occurred from the surface to a depth of about 900 feet, where the average heat content increased by 0.56 degrees Fahrenheit. Water as far down as 10,000 feet was found to have gained on average 0.11 degrees.

''This is one of the surprising things. We've found half of the warming occurred below 1,000 feet,'' Levitus said. ''It brings the climate debate to a new level.''

This story ran on page A06 of the Boston Globe on 3/24/2000.
©
Copyright 2000 Globe Newspaper Company.

 

Warming of the World Ocean

Science, March 23, 2000

Sydney Levitus, * John I. Antonov, Timothy P. Boyer, Cathy Stephens

We quantify the interannual-to-decadal variability of the heat content (mean temperature) of the world ocean from the surface through 3000-meter depth for the period 1948 to 1998. The heat content of the world ocean increased by ~2 × 1023 joules between the mid-1950s and mid-1990s, representing a volume mean warming of 0.06°C. This corresponds to a warming rate of 0.3 watt per meter squared (per unit area of Earth's surface). Substantial changes in heat content occurred in the 300- to 1000-meter layers of each ocean and in depths greater than 1000 meters of the North Atlantic. The global volume mean temperature increase for the 0- to 300-meter layer was 0.31°C, corresponding to an increase in heat content for this layer of ~1023 joules between the mid-1950s and mid-1990s. The Atlantic and Pacific Oceans have undergone a net warming since the 1950s and the Indian Ocean has warmed since the mid-1960s, although the warming is not monotonic.

National Oceanographic Data Center/National Oceanic and Atmospheric Association (NODC/NOAA), E/OC5, 1315 East West Highway, Silver Spring, MD, 20910, USA.
* To whom correspondence should be addressed. E-mail:
slevitus@nodc.noaa.gov

The Intergovernmental Program on Climate Change (1), the World Climate Research Program CLIVAR (2), and the U.S. National Research Council (3) have identified the role of the ocean as being critical to understanding the variability of Earth's climate system. Physically we expect this to be so because of the high density and specific heat of seawater. Water can store and transport large amounts of heat.

Simpson (4) conducted the first study of Earth's heat balance which concluded that the Earth system is not in local radiative balance, and therefore transport of heat from the tropics to the poles is required for the Earth system to be in global radiative balance. Identifying the mechanisms by which heat is transported from the tropics to the poles is one of the central problems of climate research. In addition, Rossby (5) drew attention to the fact that because of its large specific heat capacity and mass, the world ocean could store large amounts of heat and remove this heat from direct contact with the atmosphere for long periods of time. The results of these studies are the subject of this research article.

Until recently, little work has been done in systematically identifying ocean subsurface temperature variability on basin and global scales, in large part due to the lack of data [recent studies include (6-8)]. The first step in examining the role of the ocean in climate change is to construct the appropriate databases and analysis fields that can be used to describe ocean variability. About 25 years ago, ship-of-opportunity programs were initiated to provide measurements of subsurface upper ocean temperature. Before the initiation of these programs, subsurface oceanographic data were not reported in real time, as is the case with much meteorological data. During the past 10 years, projects have been initiated (9) that have resulted in a large increase in the amount of historical upper ocean thermal data available to examine the interannual variability of the upper ocean. Using these data, yearly, objectively analyzed, gridded analyses of the existing data were prepared and distributed (7) for individual years for the period 1960 to 1990. We have used the recently published World Ocean Database 1998 (10-13) to prepare yearly and year-season objectively analyzed temperature anomaly fields. Detailed information about the temperature data used in this study can be found in this series. Computation of the anomaly fields was similar to our earlier work (7), but some procedures were changed (7).

To estimate changes in heat content at depths greater than 300 m, we prepared objective analyses of running 5-year composites of all historical oceanographic observations of temperature for the period 1948 to 1996 at standard depth levels from the surface through 3000-m depth using the procedures described above. Constructing composites of deep-ocean data by multiyear periods is necessary due to the lack of deep-ocean observations. Most of the data from the deep ocean are from research expeditions. The amount of data at intermediate and deep depths decreases as we go back further in time.

Temporal Variability of Upper Ocean Heat Content

Figure 1 shows the variability of yearly heat content anomalies in the upper 300 m for 1948 to 1998 for individual ocean basins defined using the Equator as a boundary. Each yearly estimate includes the standard error of the mean anomaly value for each year plotted as a vertical bar. The anomaly fields for the Atlantic and Indian oceans, for both the entire basins and Northern and Southern Hemisphere basins of each ocean, show a positive correlation. In each basin before the mid-1970s, temperatures were nearly all relatively cool, whereas after the mid-1970s these oceans are in a warm state. The year of largest yearly mean temperature and heat content for the North Atlantic is 1998. In 1998 heat content reaches a value of ~4 × 1022 J, equivalent to a volume mean temperature anomaly of 0.37°C. [Expanded versions of Figs. 1 and 4 with volume mean temperature scales as well as heat content scale and similar time series for heat content integrated through 1000-m depth can be viewed at Science Online (14) as Web figures 1 to 3.]

Both Pacific Ocean basins exhibit quasi-bidecadal changes in upper ocean heat content, with the two basins positively correlated. During 1997 the Pacific achieved its maximum heat content. A decadal-scale oscillation in North Pacific sea surface temperature (Pacific Decadal Oscillation) has been identified (15, 16), but it is not clear if the variability we observe in Pacific Ocean heat content is correlated with this phenomenon or whether there are additional phenomena that contribute to the observed heat content variability.

In order to place our results in perspective, we compare the range of upper ocean heat content with the range of the climatological annual cycle of heat content for the Northern Hemisphere and world ocean computed as described by (8) but using a more complete oceanographic database (10-13). There is relatively little contribution to the climatological range of heat content from depths below 300 m. Our results indicate that the decadal variability of the upper ocean heat content in each basin is a significant percentage of the range of the annual cycle for each basin. For example, the climatological range of heat content for the North Atlantic is about 5.6 × 1022 J, and the interdecadal range of heat content is about 3.8 × 1022 J.

Changes in Temperature at 1750-m Depth in the North Atlantic Ocean

Figure 2, A and B, shows the changes of temperature at a depth of 1750 m for 1970-74 minus 1955-59 (Fig. 2A) and for 1988-92 minus 1970-74 (Fig. 2B). The difference field for the two earlier periods shows that much of the North Atlantic was warming between these periods, with the exception of a region of cooling associated with the Mediterranean Outflow (17, 18). The difference field between the later two pentads demonstrates the opposite picture. The subarctic has cooled, with the magnitude of maximum changes exceeding 0.4°C in the Labrador Sea.

Parts of the midlatitudes and subtropical regions have also cooled substantially. Maximum warming is associated with the tongue of temperature associated with the Mediterranean Outflow. The changes in salinity at this depth (not shown) in both sets of pentadal differences are positively correlated with the changes in temperature, with the result that these changes in temperature and salinity are at least partially density compensating. Tests of statistical significance (Student's t test) have been performed on these difference fields, and we find (not shown) that the changes over most of the North Atlantic are statistically significant, as was found for the earlier pentadal differences (18). The observed changes are not small and can make an appreciable contribution to Earth's heat balance on decadal time scales, which we quantify in the next section.

Heat Storage of the North Atlantic

Figure 3, A and B, shows the heat storage (computed as the time derivative of heat content) for the 0 to 300 m (Fig. 3A) and 0 to 3000 m (Fig. 3B) layers of the North Atlantic between the 1970-74 and 1988-92 pentads (using the midpoints of the two pentads to compute the time difference between periods). This figure clearly indicates that maximum heat storage for this basin occurs at depths exceeding 300 m.

Cooling occurred throughout the subarctic gyre, with the maximum heat storage exceeding 6 W min the Labrador Sea. Warming occurred in the midlatitudes and subtropics, with values exceeding 8 W min the midlatitudes of the western North Atlantic. We have computed the contribution to the vertically integrated field shown in Fig. 3B from each 500-m layer of the North Atlantic. The cooling of the Labrador Sea is  from each 500-m-thick ocean layer down to 2500-m depth. The warming in the western midlatitudes is due to nearly equal contributions by the 0- to 500- and 500- to 1000-m layers, with some small contributions from deeper layers. The warming associated with the Mediterranean Outflow is mainly due to contributions from the 1000- to 2000-m layer.

 

Temporal Variability of Heat Content for the World Ocean

Figure 4 shows the heat content for 5-year running composites by individual basins integrated through 3000-m depth. Only the Atlantic exhibits a substantial contribution to these basin integrals below 1000-m depth. We present the distributions through 3000-m depth for consistency.

There is a consistent warming signal in each ocean basin, although the signals are not monotonic. The signals between the Northern and Southern Hemisphere basins of the Pacific and Indian oceans are positively correlated, suggesting the same basin-scale forcings. The temporal variability of the South Atlantic differs significantly from the North Atlantic, which is due to the deep convective processes that occur in the North Atlantic. Before the 1970s, heat content was generally negative. The Pacific and Atlantic oceans have been warming since the 1950s, and the Indian Ocean has warmed since the 1960s. The delayed warming of the Indian Ocean with respect to the other two oceans may be due to the sparsity of data in the Indian Ocean before 1960. The range of heat content for this series is on the order of 20 × 1022 J for the world ocean.

Discussion

Our results demonstrate that a large part of the world ocean has exhibited coherent changes of ocean heat content during the past 50 years, with the world ocean exhibiting a net warming. These results have implications for climate system research and monitoring efforts in several ways. We cannot partition the observed warming to an anthropogenic component or a component associated with natural variability. Modeling studies are required even to be able to attempt such a partition. However, our results support the findings of Hansen et al. (19), who concluded that a planetary radiative disequilibrium of about 0.5 to 0.7 W mexisted for the period 1979 to 1996 (with the Earth system gaining heat) and suggested that the "excess heat must primarily be accumulating in the ocean." Hansen et al. included estimates of the radiative forcings from volcanic aerosols, stratospheric ozone depletion, greenhouse gases, and solar variability. Such information is critical for studies attempting to identify anthropogenic changes in Earth's climate system. This is because coupled air-sea general circulation model experiments that are used to assess the effects of increasing carbon dioxide frequently begin integration with a sudden increase of atmospheric carbon dioxide (e.g., twice the present value) rather than the gradual buildup observed in nature. This is done to minimize computer time required for completion of the time integrations of these numerical experiments. Integration in this manner introduces what is known as a "cold start" error (20, 21).

Global sea surface temperature time series (1) for the past 100 years show two distinct warming periods. The first occurred during the period 1920 to 1940 and was followed by a period of cooling; the second warming began during the 1970s. It is important to note that the increase in ocean heat content preceded the observed warming of sea surface temperature. It is not clear what physical mechanisms may be responsible for the observed increase in ocean heat content. The warming could be due to natural variability, anthropogenic effects, or more likely a combination of both. It may seem implausible that subsurface ocean warming preceded the observed global mean warming of surface air and sea surface temperature. This phenomenon is possible because the density of sea water is a function of salinity as well as temperature. Thus, relatively warm and salty water or cold and fresh water can reach subsurface depths from a relatively small region of the sea surface through the processes of convection and/or subduction and can then spread out and warm or freshen a much larger region such as an entire gyre or basin. This is clearly occurring in the North Atlantic Ocean by the mechanism of deep ocean convection (Fig. 2). Lazier (22) has documented the cooling and freshening of the deep Labrador Sea that began with the renewal of deep convection in the early 1970s. Dickson et al. (23) have related the renewal of convection in the Labrador Sea to the North Atlantic Oscillation (NAO) in sea-level pressure.

Nerem et al. (24) showed for the period 1993 to 1998 that a relative maximum in global mean sea level and sea surface temperature [based on TOPEX/Poseidon altimetric measurements and the Reynolds sea surface temperature analyses (25)] occurred at the beginning of 1998. This was associated with the occurrence of El Ñino. Global sea level began decreasing during the rest of 1998. Part of the reason for extreme values in North Atlantic heat content observed during 1998 may be related to the 1997 El Ñino, but additional analyses are required to understand the large increase in the North Atlantic heat content between 1997 and 1998. In addition, we emphasize that the extreme warmth of the world ocean during the mid-1990s was in part due to a multidecadal warming of the Atlantic and Indian oceans as well as a positive polarity in a possible bidecadal oscillation of Pacific Ocean heat content.

One possible link between the Northern Hemisphere oceans and the atmosphere may be found in recent research culminating in the publication by Thompson and Wallace (26). Their work indicates that the NAO may in fact be part of a hemispheric mode of sea-level pressure termed the Arctic Oscillation. These authors also relate changes at sea level associated with the NAO to changes at the 500-mb height of the atmosphere. Recently, other investigators have related changes in the Northern Hemispheric stratospheric circulation to tropospheric changes related to the NAO pattern (27-31). Dickson et al. (23) have correlated convection in the Labrador Sea with the polarity of the NAO. To the extent that these relations are found to be statistically significant, it may be that changes we observe in global ocean heat content may be related to the hemispheric and/or global modal variability of the atmosphere, from sea level through the stratosphere. Determining such possible links is a major part of understanding the mechanisms that govern the state of Earth's climate.

Our final point relates to the large change in Atlantic heat storage from depths exceeding 300 m. Because convection can result in mixing of water through the entire 2000-m depth of the water column in the Labrador Sea, changes in sea surface temperature may remain relatively small in this region despite a large heat flux from ocean to atmosphere. This flux is responsible for the large changes of heat content we have documented at 1750-m depth. This may be an important consideration when comparing the relative role of the tropics and high-latitude convective regions in effecting climate change, whether due to natural or anthropogenic causes.

REFERENCES AND NOTES

  1. Intergovernmental Program on Climate Change, Climate Change 1995: The Science of Climate Change, the Contribution of Working Group 1 to the Second Assessment Report of the Intergovernmental Panel on Climate Change (Cambridge Univ. Press, Cambridge, UK, 1996).
  2. CLIVAR, CLIVAR Science Plan, World Climate Research Programme, WCRP-89 (WMO/TD NO. 690), 1995.
  3. National Research Council, Global Environmental Change: Research Pathways for the Next Decade (National Academy Press, Washington, DC, 1999).
  4. G. C. Simpson, Mem. R. Meteorol. Soc. (1929), p. 53.
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  32. The construction of the analyses shown in this work was supported by grants from the NOAA Climate and Global Change program. Preparation of the databases used in this work was supported by the NOAA and NOAA/NASA Climate and Global Change programs and the NOAA ESDIM program. J.I.A. is a University Corporation for Atmospheric Research (UCAR) Project Scientist at NODC/NOAA.

8 November 1999; accepted 11 February 2000