The Ocean's Role in Global Change: Progress of Major Research Programs (1994)

To understand the ocean system, it is necessary to describe and quantify the interconnections among its biology, chemistry, geology, and physics. Such an understanding does not currently exist; however, recent technological advances and ongoing observational and process-based research projects are beginning to provide detailed knowledge of oceanic conditions and processes. The geological record of oceanic and atmospheric conditions provides crucial information for constructing predictive models and interpreting data collected by the programs described in the previous section. The geological record cannot be understood, however, without a solid understanding of the processes by which ocean sediments and crust are changed by contact with seawater, including biological, chemical, and physical processes that transform dissolved and particulate material in the ocean before and after burial in the seafloor.

Even with a complete understanding of today's ocean, current climate models and our ability to predict future climatic changes would still be limited. To understand past, present, and future climate change fully, we must understand the long-term, natural variability of the ocean-climate system; the sensitivity of the ocean/climate system to change; and the reliability of models to predict future climate change. The geological record, in particular the paleoceanographic record, which details the development of the ocean system

over time, is the sole source of much of the information needed to answer these questions.

Past oceanic conditions are discernable from the paleoceanographic record, deep-sea sediments, ice cores, shallow deposits from lakes and bays, and corals. Methods have been developed to collect and analyze data from these sources and to use these data to determine ocean and climate system change. Specifically, scientists attempt to reconstruct past temperature conditions in the ocean and atmosphere, chemical composition and circulation patterns of the ocean and atmosphere, changes in sea-level, precipitation and evaporation, and biological productivity. These data provide a historical record of past environmental conditions over a range of time scales, putting present oceanic conditions into perspective.

There are several potential causes and effects of climate changes. Understanding the potential causes of climate fluctuations, including astronomic forces (e.g. the Milankovich cycle caused by Earth orbit variations), interactions within the ocean-atmosphere system, or tectonism and the opening and closing of ocean basins and connecting waterways, is essential for climate prediction. The geological record provides high-resolution data that may explain the processes acting to control and change the ocean-climate system. For example, past sea-level changes are attributed to several causes. However, geological data suggest that the rise and fall was primarily due to the modification of the ocean basins by tectonic activity. Additional cause-and-effect relationships need to be investigated, including the influence of the mid-ocean ridge system on ocean chemistry and circulation, the contributions of minerals on and in the seafloor (e.g. methane hydrates) to the ocean-climate system, and the relationship between atmospheric temperature and sea-level changes. This knowledge may allow scientists to quantify the effect that anthropogenic influences may have on climate and to determine whether human alterations can be controlled or reversed.

Continued progress in computer technology has improved scientists' ability to analyze data and to construct models of ocean-atmosphere and global climate systems. For example, the 1991–92 El Niño event, which brought a devastating drought to southern Africa and severe rains to southern California, Texas, and other parts of the world, was accurately predicted nearly 2 years in advance. Unfortunately, many models cannot predict events with such accuracy. The paleoceanographic record provides the unique opportunity to test the reliability

of predictive mathematical models. The ability of models intended to describe the modern ocean to "hindcast" past ocean conditions accurately will provide confidence in their capacity to predict future oceanographic and climatic conditions. Hindcasting has already been used to test early atmospheric models. Paleoceanographers and modelers of the Climate Long Range Investigation Mapping and Planning (CLIMAP) program and Cooperative Holocene Mapping Project (COHMAP) tested the ability of atmospheric models to simulate past climates by utilizing data on predicted conditions from the glacial period that occurred 18,000 years ago. Similar tests can be performed on coupled ocean-atmosphere system models, which often lag in development behind atmospheric models. The reliability of climate predictions will almost certainly play a significant role in global environmental policy. The four programs described below focus on research programs designed to understand past climatic and oceanic conditions and the mechanisms that drive changes in these conditions.

How good are climate predictions? This question can only be answered by checking predictions against the actual course of events. To understand the full spectrum of possible responses of the Earth's systems to disturbances from human activities, the geological record must be consulted. The most comprehensive long-term record of global climatic and oceanic change comes from the sediments of the world's ocean. The Earth's paleoceanography is reconstructed through the analysis of deep-sea cores collected from around the world by the Ocean Drilling Program (ODP). Information extracted from ODP cores makes it possible to reconstruct past climatic and oceanic conditions, and, more importantly, to begin to understand the mechanisms that drive these changes. A few recent accomplishments of the program are listed in Box 10.

The Ocean Drilling Program is an international program that uses the products from the 470-foot drilling vessel JOIDES Resolution to answer questions about past and present processes of the Earth. Drilling is based on suggestions and proposals from the entire scientific community. The program provides core samples and data from downhole experiments in the ocean basins, as well as facilities for the study of these samples and data—on the ship and on the shore.

The deep-sea record contains the critical evidence for large-amplitude, short-term climate change. The study of cores from different oceanographic regimes related to physical, chemical, and biological gradients across latitudes and depth is the most efficient method to capture the response of the ocean's systems to global climatic change. Since 1988, one of the ODP's major operational goals has been to obtain continuous sedimentary sections from key places in the world oceans. Partial depth transects have been collected in the western and northern Pacific Ocean, the Tyrrhenian Sea, the eastern equatorial Atlantic, the Peru Margin, the Maud Rise, the Subantarctic, the Indian Ocean, the tropical Pacific, the Santa Barbara Basin, and soon the tropical Atlantic, the subarctic Atlantic, and the Norwegian Sea. Other proposed targets include the California Current, the Walvis Ridge, the western equatorial Atlantic, and the high Arctic.

As a result of these studies, it is known that biotic, geochemical, and sedimentary cycles that correlate with periodic changes in the Earth's orbit are pervasive in the marine record of past climates. Orbital signatures may vary spatially and evolve through time, but they have major implications for mechanisms of climate change. The Neogene (past 24 million years) is the period when present-day oceanic conditions became established, and, based on ODP studies, conceptual frameworks have begun to emerge to explain Neogene climate change. However, the details of competing hypotheses—and their global implications—remain to be tested. Testing competing theories will require recovery of sediments over a wider range of geography and depth; sediments accumulating at very high rates, such as on continental margins and in enclosed ocean basins; and sediments from high latitudes.

The ODP uses deep-sea cores to study how the oceans respond to Earth's changing climate. Key foci include: (1) understanding how changes in ocean circulation, ocean chemistry, and biological fluxes influenced climate and atmospheric PCO₂ during the Neogene; (2) determining causes of anomalously warm global climates; (3) detecting changes in hemispheric thermal gradients and circulation intensity; (4) determining causes of the initiation of continental glaciation; and (5) discovering the mechanisms for forming carbon-rich and/or anoxic sediments. Results of ODP studies have raised serious questions that challenge modern climate theory. For example, the ODP has recovered convincing evidence for the existence of warm salty bottom water sources in the ancient oceans—this poses new questions about sources, distribution, and mechanisms of watermass formation that remain unanswered and will require additional study.

During the past 5 years, oceanographers have become more aware of the critical role of biological productivity of the oceans in influencing global climate. Drilling in high-production areas such as the Peru and Oman margins, the subantarctic Atlantic, northwest Africa, and the equatorial Pacific and Atlantic has greatly enlarged the distribution of samples suitable for productivity studies. Because of ODP advances in analyzing past oceanic productivity through the recovery of complete sedimentary sections, it is now possible to express sedimentary data in terms of actual fluxes through time. Concurrently, under the auspices of other oceanographic programs such as JGOFS, the role of biological productivity on ocean chemistry and climate is becoming better understood. In combination, these advances have led to significant improvements in our ability to model paleoceanographic and paleoclimatic conditions.

The ODP is interested in improving the understanding of global sea-level history and the causes of sea-level change, since few aspects of global change are unrelated to sea-level change. However, knowledge about global sea-level change is still primitive. It has become clear that glacially-induced sea-level changes of hundreds of meters have occurred during the past 30 million years, but sea-level changes recorded in rocks during the preceding 500 million years have not been sufficiently studied. Clearly, we must understand the mechanisms of sea-level change before we will be able to predict the course of sea-level change over the next few decades, or even over millennia. The ODP has made significant progress toward understanding the mechanisms and timing of global sea-level change by implementing focused drilling programs on mid-oceanic islands (atolls) and continental margins.

The scientific community interested in ODP's contribution to global change studies is growing rapidly; liaisons are being established between the ODP and other geological global change programs such as the U.S. Marine Earth System History (MESH) program and its international counterpart MAGES, which is associated with PANASH (Paleoclimate of the Northern and Southern Hemispheres) of the IGBP Past Global Climate (PAGES) program. All of these programs focus on the causes and consequences of global climate change during the past 500,000 years, but MESH also focuses on the warm climates of the Pliocene and the early Eocene. To study the long-term evolution of climate and

environmental variability under different climatic conditions will undoubtedly require continued scientific drilling by the ODP. These studies will lead to a better understanding of the conditions in the ancient oceans and of climatic changes through time and, in turn, to a fuller comprehension of the continuing evolution of Earth's environment.

Funding for ODP is provided by NSF together with contributions from international partners including the Canada/Australia Consortium, the European Science Foundation, the Federal Republic of Germany, France, Japan, and the United Kingdom. Joint Oceanographic Institutions, Inc. (JOI) is the prime contractor. JOI subcontracts to Texas A&M University, which leases, operates, and staffs the drillship JOIDES Resolution and maintains facilities for storage and study of cores. Lamont-Doherty Earth Observatory of Columbia University is the contractor responsible for downhole logging measurements.

The sediments at the bottom of the ocean record in detail and over a long period of time (more than 100 million years) the history of our planet. This record is dominated by climate changes, including those induced by changes in ocean circulation, glaciation, impacts of celestial bodies (asteroids and comets) on Earth, volcanic eruptions, and plate tectonics. ODP is essential to deriving Earth history information from the deep-sea sediments and rocks. Without the capacity to drill and sample in the deep ocean, this important branch of Earth Science would not exist.

The Ridge Inter-Disciplinary Global Experiments (RIDGE) initiative is unique among the many programs studying the role of the ocean in global change because it addresses the volcanic and tectonic cycles, which are driven by the terrestrial energy source rather than the sun. Significant amounts of heat and chemicals are exchanged between the ocean and the solid Earth via the global mid-ocean ridge system. Much of this exchange is accomplished through hydrothermal activity, which results from the circulation of seawater through the oceanic crust. During this process, seawater changes composition as it circulates through, and reacts with, the young lavas. As it heats up, it becomes buoyant, resulting in the emanation of hot, acidic fluids on the seafloor. These hot springs are associated with very unusual, diverse biological communities that are sustained by chemosynthesis. The fluxes associated with this activity have an important, but as yet unquantified, impact on ocean circulation, heat transport, seawater chemistry, ocean ecosystems, and ocean-atmosphere interactions, all of which may in turn affect global climate.

RIDGE is designed to understand the causes and to predict the consequences of mantle-driven physical, chemical, and biological fluxes within the global spreading center system. Through integrated observational, experimental, and theoretical studies, it seeks to determine the primary processes that have shaped the physical evolution of our planet and to describe long-term temporal variations that may have modified the past climate of the Earth. For example, geological evidence indicates that early in Earth's history there was a cooler sun but high atmospheric carbon dioxide concentrations were critical in keeping the Earth warm. It is therefore of fundamental importance to understand the rate of release of carbon dioxide during mid-ocean ridge volcanism, and the factors controlling its variability, in order to determine its role in the global carbon cycle and in climate change.

On shorter time scales, the distribution of hydrothermal activity along the mid-ocean ridge system, its episodicity and variability, play important roles in regulating ocean chemistry and circulation—two of the critical factors in predicting global climate change. Within RIDGE, there are several research initiatives that are investigating these factors through reconnaissance surveys of previously unexplored parts of the global ridge system, development of response strategies to the detection of events on mid-ocean ridges, and detailed measurements of temporal variations at a single ''observatory'' site.

Given the significant exchange of heat and mass between the solid Earth and the ocean, studies of the mid-ocean ridges are an integral part of the study of the ocean-atmosphere system and of global change. The RIDGE Initiative has been formally identified as an element of the U.S. Global Change Research Program, with funding from NSF. In addition, NOAA, the Navy, and the U.S. Geological Survey (USGS) have programs focusing on the dynamic processes along mid-ocean ridges. The RIDGE Initiative has recently been expanded internationally (InterRIDGE), and several countries have developed their own equivalent national research efforts. InterRIDGE has a working group sponsored by SCOR.

Since 1989, when the Initial Science Plan for the RIDGE program was published, progress has been made in several of the research areas identified as initial foci of the program (Box 11). One long-term goal of the program is to obtain sufficiently detailed spatial and temporal definitions of the global midocean ridge system to construct quantitative, testable models of how this system works. These models could then be used to predict the impact of variability in

ridge processes on global climate change. An essential element of this strategy is the identification of key variables that affect the crustal accretion process. The RIDGE program has focused initially on the interplay of spreading rate and magma supply as two first-order variables. With the goal of collecting two comprehensive, comparable datasets—one in a fast-spreading and one in a slowspreading regime—field programs began in 1991 to collect data on the slowmoving Mid-Atlantic Ridge. During Phase I of the French-American Ridge Atlantic (FARA) project (1991–93), the purpose of the studies was to define the overall architecture of the plate boundary and to describe its first-order tectonic, bathymetric, petrologic and hydrothermal characteristics. This program has been successful and has resulted in virtually continuous and routine geophysical coverage of the axial zone between 15°N and 40°N, together with densely spaced rock samples for geochemical studies. In addition, cruises have prospected for hydrothermal plumes using towed vehicles carrying physical and chemical sensors and combined conductivity-temperature-depth (CTD) and rosette sampling. This effort has led to the discovery of two new hydrothermal sites, one on shallow crust near the Azores, and the other just south of the Atlantis Fracture Zone, thereby doubling the number of known active hydrothermal sites on the Mid-Atlantic Ridge. During Phase II of the FARA project (1993–95), more focused studies will concentrate either on smaller areas identified to be of particular interest, such as the new hydrothermal sites, or on specific experiments, such as detailed seismic investigations.

One of the major challenges for the RIDGE program is to develop practical and reliable methods for detecting, locating, and responding to transient ridge crest phenomena, such as volcanic eruptions, tectonic activity, or catastrophic releases of heat or chemical mass into the water column (Box 11). Monitoring of subaerial eruptions in Hawaii and Iceland has led to major advances in understanding how these systems work. Observation of an actively spreading portion of the mid-ocean ridge system will result in a better understanding of the sequence, duration, and type of activity and of the interrelationships among the volcanic, tectonic, hydrothermal, and biological processes involved in the creation of new crust.

An important accomplishment of RIDGE over the last few years has been the monitoring of a site on the East Pacific Rise crest where a very young volcanic eruption was serendipitously discovered in 1991 during submersible operations in the area. The eruption was associated with abundant and widespread venting of hydrothermal fluids. Mineral deposits and biological

communities typically seen at such vents had not yet developed; however, white bacterial mats covered the fresh, glassy lava flows, and the overlying water was filled with white bacterial matter. A return to the site in 1992 found dramatic changes, with venting more focused at localized high-temperature vents, changes in the temperature and chemistry of the fluids, less bacteria, and a diverse biological community, including tubeworms up to 30 cm in length. This area will continue to be revisited over the next couple of years to observe the changes that occur as the lava cools and the hydrothermal system matures. However, it is clear from the rapid changes observed in the first year following the eruption that early detection and rapid response are critical in studying the first stages in the formation of the oceanic crust.

Progress is being made in early detection of ridge events. In 1991, NOAA's VENTS program began continuously recording hydrophone data from a network of permanent deep-ocean hydrophone array (SOSUS) operated by the U.S. Navy and located in the northeast Pacific. These sophisticated sensing devices are designed to monitor sound in the SOFAR (sound fixing and ranging) channel and are ideally distributed for monitoring and locating activity along the spreading centers off the coasts of Washington and Oregon. Initially, these data were recorded and sent to NOAA for identification and location of specific events. However, in June 1993, a system was installed that allowed real-time monitoring of the Juan de Fuca Ridge and, within 4 days, a burst of seismic activity was observed near Axial Seamount. A Canadian survey ship in the area confirmed the presence of hydrothermal plume signals. Since then, studies have continued with a short response cruise in late 1993. This new capability for ridge event detection will be available to the scientific community in the near future and will almost certainly lead to new and exciting discoveries in the North Atlantic.

Over the next few years, the RIDGE Initiative will see the implementation of several major field programs and experiments. Estimation of the energy and chemical fluxes associated with ridge processes is vital to assessing their impact on global climate change. However, these estimates require characterization of the tectonic structure, geochemistry, biology, and energy fluxes of the midocean ridge on a global scale—a goal possible only in the context of a concerted international effort coordinated through InterRIDGE. The general approach will be to identify 1,000–2,000-km-long sections of the ridge that are located in the least well-characterized portions of the oceans. In the next 2 years, RIDGE

field programs will be conducted along the Southeast Indian Ridge in the vicinity of the Australia-Antarctic Discordance as part of this international effort.

Another important, but poorly understood, aspect of the spreading process is melting beneath the ridge axis and the mechanism of melt migration through the oceanic lithosphere to the seafloor. Since 1989, RIDGE scientists have been designing a field experiment aimed at investigating the size and geometry of the melting region, mantle flow, and melt migration beneath the mid-ocean ridge. This project, the Mantle Electromagnetic and Tomography (MELT) experiment, will involve deployment of up to 40 ocean bottom seismometers to record the effects of near-ridge structure on signals from distant earthquakes and up to 20 electromagnetic instruments to determine the conductivity structure of the crust and uppermost mantle for time periods of up to a year. An experiment of this scale will require detailed coordination of many groups within the United States and utilization of almost the entire inventory of these specialized instruments. The location chosen for this study is the East Pacific Rise south of the Garrett Fracture Zone—one of the fastest spreading portions of the mid-ocean ridge system—and it is expected that sea-going operations will commence in 1995.

One important long-term goal of the RIDGE program is to understand interactions among ridge crest processes on time scales of seconds to decades. This requires time-series observations of magmatic, volcanic, tectonic, and hydrothermal phenomena at selected sites along the global ridge system. A start has been made with the response effort on the East Pacific Rise that was discussed above. However, a second, high priority approach of the RIDGE program is the initiation of a seafloor observatory using instrumentation capable of long-term monitoring of volcano-hydrothermal systems. The site selected for this effort is the Cleft Segment on the Juan de Fuca Ridge, and a number of instruments and moorings have already been deployed in this region to look at spatial and temporal variability in volcanic, tectonic, and hydrothermal processes. It is expected that, over the next few years, a coordinated effort will result in the simultaneous measurement and monitoring of a number of key variables (e.g., ground deformation, microearthquake activity, flow and chemical composition of vent fluids, distribution of biological organisms) that will allow estimation of energy and mass fluxes from an individual ridge segment over a short time scale. This will be an important step in determining the role of these systems in regulating ocean chemistry and circulation—two critical factors in predicting global climate change.

Many aspects of the research conducted within RIDGE provide fundamental knowledge necessary for the nation's technical and economic development. For example, study of submarine hydrothermal systems is leading to a new understanding of how ore deposits form and may lead to new methods of exploration. The thermophilic bacteria that inhabit submarine vents are providing enzymes that have spawned a new area of biotechnology allowing genetic chemistry to be conducted at boiling temperatures. Currently a $600 million per year industry, the market for these enzymes is expected to grow dramatically in the next decade. The seafloor mapping and deep submergence technology developed in ridge-related research is already a billion dollar business being widely used in marine-related industries ranging from petroleum exploration to fisheries.

The goal of the RIDGE program is to understand the geology, physics, chemistry, and biology of processes occurring along the global mid-ocean ridge system. The mid-ocean ridge is the largest continuous geological feature on the planet and it is the surface expression of convective processes occurring within the Earth's mantle. Volcanism along the mid-ocean ridge creates the oceanic crust, which forms 60 percent of the Earth's surface. These volcanic and related hydrothermal processes play a critical role in controlling the thermal evolution of the planet, regulating the chemistry of seawater, and providing the energy source to support a diverse and unique biological community. Progress has been made in several research areas including obtaining detailed spatial and temporal information of the ridge system and the detection and monitoring of ridge crest phenomena (Box 11).

To address national and international concern over the prospect of environmental changes due to human activities and to provide relevant information on global climate change to the government and the research communities, the Geologic Division of the U.S. Geological Survey conducts research in paleoclimate and current climatic processes. The Global Change and Climate History Program focuses on five primary research elements: paleoclimate, cold regions, arid and semi-arid regions, biogeochemical dynamics, and volcano emissions.

The role of the ocean in global climate change is addressed primarily in three research areas: (1) Pliocene climate reconstructions, (2) Arctic paleoceanography, and (3) integration of marine and terrestrial records of climate change in western North America. Accomplishments and future plans in these research areas are presented in Box 12.

As part of a multidisciplinary study to map environmental conditions during the mid-Pliocene warm intervals (2.5–3.5 million years ago) and to document variability during the Pliocene, an effort is under way to derive geological proxy measures for oceanographic properties such as sea-surface temperature and deep-ocean circulation. The results from this study, combined with those from a

coordinated effort studying vegetation in terrestrial sites, will be used to constrain and validate the results of general circulation model experiments to "hindcast" warm intervals of the Pliocene.

To document the history of the Arctic Ocean during the Late Neogene (past 5 million years), another multidisciplinary study is in progress. The study includes seismic profiling, stratigraphic and sedimentological observations, and detailed climate history research using micropaleontology and stable-isotope analysis of cores recovered from the Arctic basin.

To improve predictions of future climates, a study is under way to understand the climate history of the eastern North Pacific Ocean and the adjacent land areas of the western United States over the past 130,000 years. A primary objective of the study is to provide detailed correlations of marine and terrestrial climate records along a transect extending from the western interior of the United States to the eastern Pacific Ocean. A full range of paleontological, sedimentological, geochemical, isotopic, and chronological studies and techniques are being used to reconstruct the climate history of the region.

The results from these specialized studies will add to a more complete understanding of the world ocean and of the potential changes that it may undergo in the future. The USGS effort is integrated with other federal agencies through the U.S. Global Change Research Program, coordinated by the Committee for Earth and Environmental Sciences. Studies in the Arctic involve formal cooperation with the Geological Survey of Canada.

The objective of the Marine Aspects of Earth System History (MESH) program is to understand long-term natural variation in global environments, which are recorded in the ocean's geologic record. The 10-year program focuses on the dynamics of the coupled ocean-climate system, including sensitivity to change over a range of time scales, responses to external forcing, and internal variability. Through generation and analysis of data on the Earth's response to various types of forcing (both internal and external to the climate system) and experimentation with climate models, critical environmental feedback mechanisms, such as the role of ocean chemistry and the greenhouse effect, will be better quantified. This will yield improved understanding of global change processes, and better models for predicting future environmental changes on the scale of tens to thousands of years. This information in turn will be important to policy discussions about global change, including society's attempts to adapt to, or mitigate the effects of, future change.

The initial MESH Advisory Panel was convened in 1990 and proposed five broad programmatic themes that emphasized the marine geological aspects of global change. The report from this meeting served as the springboard for the panel to solicit white paper proposals from the ocean sciences community to address how the marine geologic record could be used to help understand the process and record of global change. At a workshop held in March 1993, the white papers were discussed, scientific priorities were defined, and a MESH Steering Committee was elected. The Steering Committee met in September 1993 to refine the results of the March meeting and to develop a MESH Program Plan.

The MESH Advisory Panel and Steering Committee identified and prioritized eight themes: (1) ocean geochemical dynamics and climate change, (2) climate sensitivity and variability at time scales of 103 to 105 years, (3) extreme warmth, (4) marine records of seasonal to millennial scale variability, (5) abrupt climate change, (6) polar cryospheric history and global climatic change, (7) marine records of continental climate change, and (8) biological response to climate change. In ranking the eight themes, it was recognized that significant overlap exists between the scientific objectives of individual themes. For example, knowledge of the state of the polar cryosphere is an essential part of understanding times of extreme warmth. Taking these overlapping priorities into account, MESH identified five program elements that address the highest

priorities of the paleoceanographic community. These highest priority research themes and their objectives are:

1. Ocean geochemical and climate change. To better understand how ocean circulation, ocean chemistry, and biologic fluxes have influenced the observed record of atmospheric CO₂ over the past 500 thousand years and the consequences of these changes to the ocean, atmosphere, and biosphere system.

2. Climate sensitivity and variability at time scales of 10³ to 10⁵ years. To study the response of the ocean to a known, strong, direct forcing (changes in the distribution of solar radiation and atmospheric CO₂ content) in order to identify processes that control variability in ocean circulation and chemistry; and to use paleoceanographic histories along with climate models to identify the sensitivity and character (i.e., quasi-linear to nonlinear internal feedbacks) of these climate related processes.

3. Extreme warmth. To better characterize episodes of extreme climatic conditions over the past 120 million years, including features of the ocean and atmospheric chemistry, with a focus on a selected set of short, stable intervals of warm climates; and to document changes in the climate system and develop models of these climate episodes in order to understand the origin of these extremes, identify the feedbacks within the climate system that maintained global climate in the extreme states, and articulate the mechanisms that brought an end to these extreme conditions.

4. Marine records of seasonal to millennial scale variability. To understand how the climate system operates on societal (seasonal to millennial) time scales, including the response of climate variability to past changes in boundary conditions; and to assess the abilities of predictive numerical models to simulate seasonal to millennial climate variability and sensitivity.

5. Abrupt climate change. To identify and characterize isolated events and transitions in the geologic record that can help in evaluating the processes and feedbacks that determine the degree of sensitivity and stability of the Earth system.

A Program Plan is being submitted by the Steering Committee for MESH. It represents the first step in implementing the scientific priorities defined in the MESH report, the solicited white papers from the community, and the discussions and report of the March 1993 meeting. MESH is partly organized under the scope of the International Geosphere-Biosphere Program's Past Global Climate (IGBP PAGES) program and is the U.S. component of the international program IGBP International Marine Global Changes Study (IMAGES). MESH will be funded as part of the NSF Ocean Sciences Program.

The objective of MESH is to understand long-term natural variations in global environments, which are recorded in the ocean's geologic record. The 10-year program focuses on dynamics of the coupled ocean-climate system, including sensitivity to change over a range of time scales, responses to external forcing, and internal variability. Through generation and analysis of data and experimentation with climate models, critical environmental feedback mechanisms such as the role of ocean chemistry and the greenhouse effect will be better quantified. This will yield improved understanding of global change processes, and thus better models to predict future environmental changes on the scale of decades to millennia. This information will be useful to policy discussions for global change, including society's attempts to adapt to future changes, or to mitigate its effects.