A quote from Arthur C. Clarke gets it right:
|"How inappropriate to call this planet Earth when clearly it is Ocean"|
--Nature, v. 344, p 102, 1990.
New evidence for the essential role of the oceans in climate is coming out of
the recent World Ocean Circulation Experiment (WOCE), supported by the National
Science Foundation. A globe-spanning set of ship-based observations in the '90s
revealed that the depths of the ocean had warmed significantly since previous
observations in the '50s. In fact, about half the "missing" greenhouse
warming has been found in the ocean. It was missing because models had projected
a larger increase than had been observed. It now appears this was because they
had not properly accounted for the capacity of the oceans to store large
quantities of heat on short timescales. In fact, it is easy to calculate that if
all of the extra heat due to the greenhouse change in the radiation balance were
to be deposited in the deep ocean, it would take 240 years for it to rise 1o C.
Thus, monitoring the ocean's patterns of heat storage is absolutely essential
for understanding global warming, yet we have no system for such observations.
But the oceans do more than simply delay global warming. Research over the past twenty years has brought a growing appreciation of how the slow movement of warm and cold patches of ocean water can affect our weather for months at a time. The alternating influence of El Nino and La Nina are now well known to the public and are rashly blamed for any type of unusual weather. These 3-5 year period disruptions in weather patterns are caused by the movement of warm water in the tropical Pacific, and are now predictable up to a year in advance because of a special monitoring network of ocean buoys maintained there. The influence of El Nino on US weather is well publicized, but it actually explains only a small part of the variation in temperature and rainfall over the United States. Some other natural ocean climate cycles known as the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation (NAO) can explain much more of the variability in winter-time weather than El Nino. (Figure 1.). The NAO in particular has much more impact on the eastern half of the United States than El Nino.
|Wintertime Potential Predictability|
Exciting new findings suggest that the ocean controls the timescale of the NAO, thus holding out the hope that these weather patterns will be predictable when sufficient ocean observations become available.
Recent research indicates that the NAO's changes in atmospheric pressure patterns over the Atlantic are linked to the slow variation in water temperatures, as the ocean currents rearrange the warm and cold ocean patterns that serve to guide the atmosphere in its preferred modes of oscillation. Only the ocean has the long-term memory to provide the decadal time scales observed in the NAO. An understanding of these natural modes of climate variation is essential for accurate predictions of the regional trends in US climate. That the two models examined in the Climate Assessment report should differ so widely in prediction of future US precipitation is no surprise. Models are only a repository for what we think we know, and an understanding of the important oceanic phenomena such as PDO and NAO has not yet been achieved. In order to understand these phenomena we need to observe the motion of the deep warm and cold patches that give the ocean its multi-decadal memory, and we need to sustain those observations through a few cycles of the oscillations. In contrast to the 1,200 records of US land temperature used to examine climate trends in the report, we have only three sites with anything like a continuous deep record in all of the North Atlantic! For these few sites with rather short records, an observation once a month is often the best we have. This observation system is woefully inadequate. It is obvious that the ocean is the long-term memory of the Earth's climate system yet we persist in ignoring it. Some think it sufficient to look at the surface of the ocean with a satellite and try to model the interior. However, satellites can tell us nothing about the deep interior temperatures that influence winter-time climate.
The Water Cycle and Thermohaline circulation
Also, satellites can tell us nothing about the salt content of the ocean, which reflects the workings of the water cycle. There is an increasing attention to the importance of the water cycle in global change; for most communities drought or flood are more pressing challenges than a few degrees of warming. However, there has been little recognition that most of the water cycle occurs over the oceans. It would take a diversion of only 1% of the rainfall falling on the Atlantic to double the discharge of the Mississippi River. Water travels quickly through the atmosphere, spending only about 10 days on a short ride from one spot to another. Water molecules spend thousands of years on the slow return flow in the ocean. But the process of water leaving the surface of the ocean, and thereby changing its salt content and density, drives an interior flow many times larger than the flux of water due to evaporation and precipitation alone. This "thermohaline circulation" is a key element of the climate system, as it is responsible for most of the ocean's heat transport from equator to pole. When salty water gives up its heat to the atmosphere, it can become dense enough to sink to the bottom of the ocean, thereby keeping making room for more warm water to come north for cooling. The North Atlantic is the saltiest ocean and the most active site for such "deep convection". However, if it becomes too fresh from rainfall the surface waters cannot sink and the flow of warm water stops.
Records from ocean sediments of the fossils of marine life indicate that this
has happened many times in the past, with dramatic consequences for climate over
a large area. The most recent event was about 12,000 years ago, when the
freshwater from melting glaciers shut down the thermohaline circulation in the
North Atlantic. This had dramatic consequences for the North Hemisphere,
returning much of it to glacial conditions for 1000 years. The data indicate
that this happened rapidly, in only a decade or two. Some models predict that
such abrupt climate change could happen again as the water cycle intensifies
with future global warming. However, such transitions in the thermohaline
circulation have been shown to depend on the rate of interior mixing in the
ocean, and we know that this is incorrectly treated in the present generation of
In fact, oceanographers have many complaints about how poorly climate models simulate the ocean. Because of computer limitations, they must treat it as a very viscous fluid, more like lava or concrete than water. Such models fail to simulate the real ocean's changes in deep temperatures. We know that the "sub-grid-scale" parameterizations for mixing processes are incorrect, reflecting none of the observed spatial variations or differences between heat and salt. This mixing drives the interior flows in the ocean. We know that the processes by which ocean currents give up their momentum are incorrectly treated. And these are not problems that will quickly yield to increased spatial and temporal resolution in the computer models. Even if computer power continues to increase by an order of magnitude every 6 years, it will be over 160 years (1) before models have the resolution necessary to simulate the smallest ocean mixing processes! Society cannot afford to wait that long. We will not come to an understanding of climate by more computational cycles of models with incorrect physics. We require a systematic study of the sub-grid-scale processes in the ocean. This is noticeably lacking in our current Global Change Research Program.
While we have in place a system for monitoring El Nino, we have no such ability to observe the motions of thermal anomalies in the mid- and high latitude oceans. Nor do we monitor the salt content of ocean currents, to determine the potential for deep convection or to help understand the vast water cycle over the oceans. But new technology, the vertically profiling ARGO float (Figure 4.), promises to give us the data we need to begin to understand this largest component of the global water cycle. These are like weather balloons for the ocean, drifting at depth for 10 days then rising to the surface to report profiles of temperature and salinity to a satellite. They then resubmerge for another 10 day drift, a cycle to be repeated 150 times or more. The distance traveled between surfacings provides a measure of the currents at the depth of the drift. The ARGO program (http://www.argo.ucsd.edu/) is an international plan to maintain a global distribution of ~3000 floats as a core element of a Global Ocean Observing System (Figure 5.). Other parts of the system involve fixed sites with moored buoys and underwater profilers that record temperature and salinity all the way to the bottom of the ocean. These new technologies will give us the data we need to begin to decipher the complex climate phenomena we know to be operating in the ocean. Science is the process of testing ideas against observations, and failure to make the observations is an abandonment of the scientific process.
|Annual Average Surface Salinity (Levitus '92) with potential S-PALACE deployment positions|
What can Congress do?
Policy makers would like climate scientists to produce firm predictions. However, they must always remember that science is the process of testing ideas against facts and access to quantitative data is essential to the process. The ocean is a crucial element of the climate system, yet its "subgrid-scale" processes are too poorly understood and its basic structure too poorly monitored, to provide much confidence in the details of present day predictions. The National Climate Assessment Report is a good faith effort to assess the effects of global warming on US climate; the regional disagreements of the two available models are to be expected, given our poor understanding of the ocean. Global warming due to the effect of greenhouse gases on the radiation balance is as certain as the law of gravity, but the issues of how rapidly heat is sequestered in the oceans, its impact on the water cycle, and the important regional variations in climate, remain very challenging research questions.
Climate prediction is a hard problem, but appears to be tractable. An abundance of evidence indicates that the key to long-term prediction is in the workings of the ocean, which has 99.9% of the heat capacity of Earth's fluids. It is the heart of the climate "beast", the atmosphere its rapidly waving tail, with only 0.1% of the heat capacity. Let us get to the heart of the matter, with an unprecedented new look at the ocean. We have the technical capabilities. The cost is modest. The payoff is large. The society that understands long-term climate variations will realize tremendous economic benefits with improved predictions of energy demand, water resources and natural hazards, and it will make wiser decisions on issues affecting the habitability of the planet, such as greenhouse gas abatement.
(1) It will take a factor of 108 improvement in 2 horizontal dimensions (100 km to 1 mm, the salt dissipation scale), a factor of 106 in the vertical dimension (~10 levels to 107) and ~105 in time (fraction of a day to fraction of a second); an overall need for an increase in computational power of ~1027. With an order of magnitude increase in computer speed every 6 years, it will take 162 years to get adequate resolution in computer models of the ocean.
Raymond W. Schmitt is a senior scientist of the Department of Physical Oceanography at Woods Hole Oceanographic Institution.