1

Introduction

Humans inhabit only one planet, and the climate of that planet is changing. There is a strong scientific consensus that human-induced increases in greenhouse gases, especially carbon dioxide (CO₂), are driving more rapid and profound changes to climate than at any time since the advent of human societies. These human-driven, or anthropogenic, changes are known to be well beyond the range of natural fluctuations that occur on a variety of timescales. They are observed in the Earth’s atmosphere, on land, in the ocean, and in the cryosphere, and are detectable in global and regional averages of many important climate variables, such as increasing temperatures in the ocean and atmosphere.

What will future generations need to know to understand the Earth’s climate system? This is the fundamental question underlying the interest in sustaining critical observing systems. Regular and consistent collection of environmental observations over decades to centuries provides a record of how the climate is changing and of its range of fluctuations and variability. Sustained observations are also necessary to collect the critical data used to validate, calibrate, and refine climate models that provide insights about future events. Improved climate models will help to ensure the best possible answers to questions about future weather patterns (e.g., drought, heat waves, tropical storm strength and frequency, and agricultural growing seasons), about regional shifts in average climate conditions, and about other aspects of a changing climate that will impact society (e.g., rate and extent of sea-level rise, ocean acidification, species loss, and occurrence of floods and droughts). Sustained observations of environmental variables are thus essential to advance understanding of the state of the climate system now and in the future.

This report focuses on the observational needs for the ocean component of

the Earth’s climate system. The ocean covers about 70 percent of the Earth’s surface and acts as its primary reservoir of heat and carbon, absorbing over 90 percent of the surplus heat (Rhein et al., 2013), about 30 percent of the CO₂ emissions associated with human activities (Ciais et al., 2013), and it receives close to 100 percent of fresh water from melting land ice, which contributes directly to global sea-level rise. Heat and CO₂ are absorbed at the ocean’s surface and, from there, transported throughout the ocean depths along complex pathways. Although exchange across the ocean’s turbulent surface boundary layer can happen rapidly, in hours or days, and significant exchange of water between the boundary layer and the stratified main thermocline occurs over timescales of years to decades, deep water takes many decades to millennia to return to the surface, acting as long-term storage for heat and CO₂ and thereby lessening the near-term impacts of climate change (Ciais et al., 2013). As a result, even if CO₂ emissions stopped tomorrow, surface heat and CO₂ concentration would continue to change for centuries due to ocean processes, contributing to prolonged deviations from preindustrial climate conditions (Zickfeld et al., 2017). Because changes in heat, carbon, fresh water, and other properties of the ocean that interact with climate typically occur over such long timescales, long-term, sustained observing (over decades and longer) is required to fully document and understand the climate system, to detect and attribute changes driven by human activities, and to predict how the climate system will likely behave in the future.

THE NEED FOR OBSERVATIONS AND MODELS

The argument has been made that we have passed from the Holocene into the Anthropocene, the first epoch wherein the human enterprise is a demonstrable force majeure shaping the planet (Crutzen and Stoermer, 2000). In the Anthropocene, understanding the dynamics of our planet’s climate system acquires new and profound value to enable societies and commerce to adapt to and predict the changes that lie ahead. Observations play a foundational role in documenting change, improving understanding of the climate system, and facilitating future climate predictions and scenario developments. With the knowledge gained through these observations and their analysis, informed decisions can be made about mitigating greenhouse gas emissions or responding and adapting to impacts of climate change relevant to national security, the economy, and society.

Climate models are the best available tools for providing insight into possible scenarios of the future climate system. These models are tested, calibrated, and improved through information gained from observational records. Models incorporate our mechanistic understanding of how the climate system operates to extrapolate into future climate regimes, beyond what has been observed in the past or present. As our understanding of the many interacting phenomena on our planet improves, and as computing power grows, climate models increasingly go well beyond just simulating purely physical interactions to comprehensively in-

cluding the wide range of biogeochemical and ecological interactions that govern the evolution of the Earth system as a whole. There are many aspects of climate models that can be informed by existing and future observations: the mean state of the ocean (e.g., temperature and salinity, circulation patterns, sea level, and pH), the magnitude of the seasonal and diurnal cycles, the climate response to aerosols in the atmosphere from volcanic events, and the observed changes since the advent of systematic observational systems. Some models already do a reasonable job of capturing many well-observed climate patterns and natural modes of variability, including seasonal and diurnal cycles. Through data assimilation, models can formally maximize the information that can be gained from available observations about present anomalies as a starting point for skillful projections out to decadal timescales. There are important emergent properties of the climate system that will require sustained and additional long-term observations to improve climate models, including the overall climate sensitivity (i.e., how much the global mean temperature changes for a given change in net absorption of solar radiation, known as radiative forcing); the efficiency of heat and CO₂ uptake by the ocean; the rate of ocean acidification; changes in evaporation and precipitation; and the sensitivity of mass loss from polar ice sheets, mountain glaciers, and ice caps to temperature changes in the ocean and atmosphere. It is only in comparison of climate models with sustained high-quality observations of the evolving state of the climate system, including the oceans, that models can be evaluated, calibrated, and improved.

THREE IMPORTANT BUDGETS FOR CLIMATE

As understanding of and skill in predicting climate variability improves, careful consideration of the needs for sustained observations and sufficiency of existing programs is warranted. An important test of our understanding and of the sufficiency of current observations is to ask whether we can reconcile the inputs to, exchanges among, and storage of energy or specific elements within the components of the Earth system: the ocean, land, atmosphere, and cryosphere. In other words, can we create a balance sheet that accounts for all contributions to this budget? For example, does the sum of the measured heat increases of the atmosphere, land, and ocean correspond to the net energy input from solar radiation? The net global imbalance of energy is a very small residual between incoming solar and outgoing longwave radiation. Temperature measurements in the ocean allow for a more accurate estimate of accumulated heat than is possible for net atmospheric radiation. With sufficient observations, it should be possible to close the heat budget, within the accuracy and sampling of the observations, and account for the observed change in radiative forcing. Comparison of the inputs, exchanges, and storage parameters between a model and observations allows for characterization of the accuracy of the representation of the key processes used

to close the budget within the model, which is critical to then projecting future changes.

This report focuses on three distinct global budgets as a way to illustrate the importance of sustained ocean observations for climate: heat, carbon, and fresh water. These budgets were selected for their ability to inform climate model projections, to detect and attribute changes within the climate system, and for the fundamental role the ocean plays in each. They are also well recognized as priority areas within the international ocean observing community, as discussed later in this report. Quantifying and closing these three budgets requires global ocean observing systems that provide ongoing, calibrated measurements to monitor short- and long-term changes indicative of the evolving state of the Earth’s climate. Observations and subsequent improvements to the processes built into climate models based on these observations also allow for more informed societal and policy responses to a changing climate. For example, the heat budget is central to understanding the delayed warming of the Earth’s surface temperatures due to the ocean’s heat uptake and storage (Winton et al., 2010; Fyfe et al., 2016). Similarly, a thorough understanding of the carbon budget can be used to help predict future atmospheric CO₂ concentrations under various greenhouse gas emission scenarios. Balancing the carbon budget requires observations of the amount and rate of absorption of atmospheric CO₂ by processes in the ocean and on land. This information can then be used to predict how much CO₂ will be absorbed by the ocean versus remain in the atmosphere and can help guide future emissions policy to reduce the extent of future warming. It also informs the rate at which ocean acidification increases.

The fresh water budget is important for understanding changes in ocean salinity. Salinity and temperature determine sea water density, which sets the vertical stratification of the ocean. Generally, temperature is the dominant driver of this stratification with warmest water at the surface and coldest in the deep ocean, but salinity can be an important determinant when fresh water inputs are large, such as in regions of high rainfall and river outflow, in polar regions where sea ice melts, and in cold waters where salinity dominates sea water density. Low surface salinity makes the surface waters more buoyant, increasing the stability of stratification and decreasing vertical mixing, which reduces the exchange of heat and carbon with the deep ocean. A low-salinity surface “lens” allows for higher sea surface temperature (at low latitudes) and lower sea surface temperature (at high latitudes), and influences the local air-sea exchange of heat.

These budgets also provide critical information about sea-level rise. The heat budget provides estimates of rates of thermosteric sea-level rise, the rise in sea level caused by the expansion of ocean water as it absorbs increasing amounts of heat (Church et al., 2013). Increases in the net fresh water input to the ocean occur when rising temperatures cause significant melting or dynamic instabilities of land ice, which also contributes to sea-level rise and may be the dominant contribution to sea-level rise in the future (Church et al., 2013). Closure of the

sea-level budget requires additional estimates of the water mass transfer from land to ocean, which are provided by satellite gravity and cryosphere observations. Coordinated satellite and in situ observations are needed to understand risks to coastal communities and infrastructure in the United States and the low-lying regions worldwide. Our current understanding of the sea-level budget has also benefited from some redundancy in the observing system components (sea surface height by satellite altimetry, thermosteric expansion from Argo floats, and ocean mass by satellite gravity), which is fundamental for characterizing the drivers of sea-level change as well as their uncertainties.

Though only three global ocean budgets and their associated observations are detailed in this report, there are other key long-term ocean observations that are important for monitoring and understanding the effects of both short- and long-term changes in the oceans. These include distributions of dissolved oxygen and nutrients (which can directly impact marine ecosystems) and documentation of changes in marine ecosystems. Like observations for climate, the value of the observational record for detecting significant changes and distinguishing between natural variability and human-induced change is dependent upon the continuity of the observations over the long term.

THE OCEAN CLIMATE OBSERVING SYSTEM

Observations of the ocean developed as a result of growth in technical capability combined with increased understanding of the need to observe the ocean following World War II. Initially, sampling of the ocean was exploratory, with limited observation of the global patterns and variability of subsurface ocean properties. An incomplete and hence biased view of temperature, salinity, and density distributions was pieced together from regional surveys and a few more extensive efforts, such as the German survey of the Atlantic Ocean in the 1920s, the International Geophysical Year survey of the Atlantic Ocean in 1957-1958, and surveys of the Pacific and Indian Oceans in the 1960s. Except for measurements at isolated mid-ocean weather stations in the Northern Hemisphere to support early transoceanic aircraft flights, basin-scale surveys did not include regularly repeated measurements. Instead, regular observing was mostly confined to coastal regions where there were concerns about fisheries (e.g., U.S. West Coast) or navigational hazards such as sea ice. In 1982, an unusually strong El Niño climate anomaly, an event that occurs as part of the natural El Niño–Southern Oscillation (ENSO) climate phenomenon, went undetected until it was fully developed because of insufficient in situ observing capacity in the tropical Pacific (McPhaden et al., 1998). This strong El Niño had enormous ecological and economic impacts around the Pacific Rim, including erosion and flooding in agricultural and residential areas and declines in fish and seabird populations, due to rainfall and temperature extremes (see, e.g., Valle et al., 1987; Arntz and Tarazona, 1990; Storlazzi et al., 2000).

To ensure that events like this “El Niño surprise” would not be repeated,

more systematic ocean observing began to be implemented. The World Climate Research Programme (WCRP) mounted a major project, Tropical Ocean Global Atmosphere (TOGA, 1985-1994), including installation of a permanent ENSO Observing System (ENSO OS) in the tropical Pacific Ocean. TOGA drove major advances in understanding tropical climate variability and the ENSO OS enabled seasonal forecasting of ENSO and its regional impacts. In the same era, the growing realization of the global ocean’s central role in climate, including the massive oceanic storage of heat and its transport by ocean circulation, provided the impetus for the WCRP’s World Ocean Circulation Experiment (WOCE). WOCE included an intensive global ocean survey made by research vessels between 1991 and 1997, as well as satellite and in situ observations of the time-varying circulation. At this time, in addition to the physical observations carried out under TOGA and WOCE (which also had a geochemical component, including a global survey of ocean carbon, nutrients, and oxygen), interest in the oceanic role in the carbon cycle gave rise to the Joint Global Ocean Flux Study (JGOFS), whose objective was to quantify and understand the time-varying exchange of carbon between the atmosphere, ocean, seafloor, and continents. In addition to their observational achievements, TOGA and WOCE catalyzed the development of a new generation of autonomous instrumentation that continues to revolutionize ocean observation today.

The ocean climate observing system as it stands today is described in the remainder of this report. The system has continued to evolve and grow as platform and sensor technology has improved and as the needs for understanding the climate and budgets have been better identified and prioritized. The dominant components of the ocean observing system are the in situ elements, including profiling floats, ocean gliders, global drifters, moorings, tide gauges, data buoys, and ship-based observations. These elements collect the data needed to better quantify components of the three budgets. Satellites remotely collect complementary data on limited parameters such as sea surface height, sea surface temperature, surface wind stress, ocean mass, and, recently, sea surface salinity. Although satellites are only usable for some types of data collection, the integration of in situ and satellite observations through formal synthesis or data assimilation allows for detailed knowledge of specific aspects of the climate system, greater spatial coverage than is available for many in situ measurements alone, and intercalibration between observing platforms. The need to periodically replace platforms as well as the introduction of new, more capable and effective observing methods are anticipated, but overlapping of methods and intercalibration will be required to document the comparability and quality of new methods and thus ensure the continuity and quality of observational records. In addition, ongoing research on and development of new sensors and observing methods will go from a pilot stage to maturity and routine deployment in order to measure variables and quantify processes not yet routinely observed and to address new scientific challenges. Altogether, this ocean climate observing system provides information critical for understanding

the current and future state of the climate, improving predictability of natural climate cycles, and making decisions with respect to mitigating and adapting to adverse changes arising from long-term, anthropogenic climate change.

THE END-TO-END SYSTEM

The successful collection of sustained observations in the ocean is built on a foundation spanning a wide range of activities and actors. As illustrated in Figure 1.1, the deployment of a given component of the observing network is built on engineering, operations, data management, and information products that are supported through planning and governance from international and regional coordination entities and by national agencies. Planning and governance activities include the proposals and the administration of the proposals that generate the funding support within the government agencies; administration of observing system operations in federal, state, or academic research organizations; development and documentation of performance metrics; coordination between partners within the United States and internationally; and representation of the U.S. component of the observing system as part of globally coordinated and multiplatform arrays of ocean observing elements. Engineering activities address platform design and improvement, including life extension and reliability, sensor selection and testing, power needs, and data telemetry and platform tracking. Operations include construction and procurement, predeployment testing, finding ships for deployment, deploying floats, and monitoring performance in the field. Observational scientists and technical staff deploy moored instrumentation at fixed sites, sample

from research ships, or use research or volunteer observing ships to deploy instrument platforms that either drift on the sea surface, make underway measurements, or take profiles within the water column. Data management involves acquisition, processing, and calibration of the raw data by the observing system operators, quality control, and storage of raw and processed data in national and international data centers. The data are then used by the wider community of researchers, extending beyond those involved in the operations of the observing system itself. Data assimilation plays an important role in the synthesis of the diverse but disparate observational data streams and heterogeneous sampling patterns. Insights gained from both observing operations and data use result in ongoing improvements to data collection, distribution, and instrumentation which contribute to continuous evolution of the observing system over time.

An example of this end-to-end system is the global network of Argo profiling floats, which includes participants from a wide range of communities. More than 23 countries now contribute to the global Argo program infrastructure, and participate in international coordinating efforts to standardize priorities and performance metrics. In the United States, five laboratories (federal and academic institutions) operate the Argo program with funding from the federal government; each of these five groups is led by a principal investigator who makes the day-to-day and multiyear decisions directly relevant to that laboratory’s operations including equipment, deployment, and data processing. Data processing for Argo floats, which measure temperature, sea water conductivity (an indicator of salinity), and pressure, includes real-time preparation of temperature and salinity profiles as a function of pressure from the temperature and conductivity data with initial automated data quality screening, and also the delayed-mode preparation of fully quality-controlled temperature and salinity profiles drawing on additional sources, such as shipboard profiles, for float calibration. Data are provided through an Argo-specific data archive and forwarded to national ocean data archives. Analysis activities are undertaken by students, postdocs, and principal investigators, many of whom are not within a group that deploys the floats; their experience with the data often feeds back into improvement in operations or data quality. This example illustrates the need for a wide range of engineering, field operations, technical, and scientific talent that is well coordinated to ensure success of the end-to-end system and the importance of adequate financial resources to support sustained ocean observing.

The ocean observing system is largely supported by the federal government in the United States. Agencies coordinate their investments in ocean observing and in ocean science broadly through interagency bodies. The federal government identifies priorities for the ocean observing investments and participates in international structures where global priorities are developed. However, as described above, implementation of the end-to-end system relies on wide expertise from a broad range of actors from research institutions, private industry, and nonprofits.

STUDY TASK AND APPROACH

This study committee (“the committee”) has been charged with considering processes for identifying priority ocean observations that will improve understanding of the Earth’s climate processes and the challenges associated with sustaining these observations over long time frames (see Statement of Task in Box 1.1). The committee determined that the existing international bodies that coordinate observing activities among nations have developed detailed, robust, and ongoing processes for identifying and developing sampling specifications for the highest priority ocean climate observations. Rather than duplicating these

efforts, the committee has focused its attention on “challenges to maintaining long-term observations and suggest[ing] avenues for potential improvement.” The importance of long-term ocean observations has been widely recognized in the scientific community, but sustaining these measurements has been difficult, in part due to unpredictable funding streams under the annual cycle of government appropriations, tight budgets, and competing priorities for funding within government agencies (Baker et al., 2007; Wunsch et al., 2013). This report focuses primarily on the in situ elements of the observing system. Remote sensing using satellites is coordinated across nations and subject to planning and coordination in the United States under the decadal surveys conducted by the National Aeronautics and Space Administration (https://decadal.gsfc.nasa.gov). To date, in situ ocean observing has not been the subject of such coordination within the United States, and the committee effort reflects their determination that the greatest need for improved coordination to sustained ocean observing is associated with the in situ effort. Additionally, the in situ ocean observing system is a technically challenging enterprise, with observing platforms deployed in remote locations far from land and for long time spans in order to collect the data that improve our understanding of the global climate. As well, the in situ system is one that will evolve further, as capabilities increase to transition new observing methods to maturity and as new scientific challenges in understanding climate need to be addressed by additional observations.

A major activity of the committee was the convening of a 2-day information-gathering workshop, during which members of the U.S. and International Ocean observing community described the global ocean observing system and its evolution and the processes in place for prioritizing ocean observing investments. Participants in the discussions included researchers and decision makers from the U.S. government, academic institutions, and international coordination groups. The agenda and panelists from the workshop can be found in Appendix B.

The committee evaluated the strengths and needs of the existing elements of the global observing system and its ability to measure climate variability, with thematic emphasis on the three budgets described earlier in this chapter: heat, carbon, and fresh water. The committee determined that sustained and improved measurements of heat, carbon, and fresh water system components will continue to provide and strengthen crucial information in understanding climate and significantly improve modeling capabilities, while also serving as the basis for findings that could be applied more broadly in the context of other important observations. The committee used global sea-level rise as an example of a climate-dependent property that requires global ocean observations and has great societal impact. Although the focus is on variables related to these budgets, the committee does not intend to suggest that these three budgets are the only important variables to measure for long-term climate trends. The committee also recognizes that changes in climate can have serious consequences for biological processes and that variables related to changing ecosystems and observations of

these impacts are critical, but not the focus of this report. Instead, improvements in the ability to model the heat, carbon, and fresh water budgets may allow for predictions of future changes to the Earth’s ecosystems.

A system designed for carrying out long-term climate observations will provide benefits for other near-term areas of science, commerce, and human safety. This is acknowledged as an important aspect of the system by the committee (discussed further in Chapter 2). However, for the purposes of their task to describe priority observations for understanding the Earth’s future climate, the committee did not find it appropriate to weight the benefits of any observing system or variable by its near-term benefits.

This report focuses primarily on open ocean observing system elements that contribute to the fundamental understanding of long term trends in the Earth’s climate system. Most coastal ocean observing platforms have been developed to address applications with important short-term societal benefits. For assessing shorter term climate variability and climate impacts, coastal regions play an important role, but with the exception of sea level these observations are less critical for understanding long-term climate trends. The budgets of heat, fresh water, and carbon that are foci of this report are dominated by the vast volume of the global open ocean. Although it might be ideal to include coastal observations at some point, because these systems were developed at a more local level to address shorter term societal needs (such as early detection of algal blooms), they are distinct from the system of global observations, and less critical to the concerns detailed in this report.

In the remaining chapters of this report, the committee highlights the importance of an ocean climate observing system (Chapter 2), and describes the strengths and challenges of existing international frameworks (Chapter 3) and the associated U.S. contributions to the ocean observing system (Chapter 4). The findings and conclusions from the committee for improvements to overcome challenges and explore potential new models for sustaining ocean observations are detailed in Chapter 5. Information presented in Chapter 5 may also inform a possible second phase of this project, which would consist of a workshop to explore innovative new methods and partnerships for supporting sustained ocean observations.

This page intentionally left blank.