by Carol Anne Clayson
A significant area of uncertainty in climate science and one of the biggest limitations on our ability to predict the timing, location and impacts of climate change is our limited understanding of ocean processes and their interactions with the atmosphere, land, and ice systems.
Any serious effort to address climate change and mitigate its impacts must include support and investment in more ocean research. Understanding how much heat and carbon the ocean absorbs is vital to understanding sea level rise and predicting how much, how fast, and where the atmospheric temperature will change.
Climate models can only make calculations based upon our current scientific understanding of how these complex systems function. While the climate models include what is known today about the ocean and its influence on climate, there are still many gaps in our knowledge. We fill these gaps with our best scientific assumptions, but these gaps and the assumptions we use to fill them are a major reason that there are still significant differences among the various climate models related to projected climate impacts and their timing. What we need are more and better observations – data – that enhance our scientific knowledge and that we can use to improve our models and reduce the uncertainties in our climate forecasts.
These comments explain the scientific importance of the ocean to our climate system, some of the significant gaps in our knowledge, and how filling those gaps would enable us to better address climate change going forward. We owe it to ourselves to make sure we get the information we need to develop informed and effective solutions.
Relationship Between Oceans and Climate
It is often said that the ocean is the flywheel of the Earth’s climate. Just as the flywheel in an engine absorbs and releases energy to stabilize engine speed, the ocean absorbs and releases thermal energy, stabilizing the Earth’s climate. The ocean reacts more slowly than the atmosphere to changes in heat but also stores this heat and releases it over longer time periods. As such, the ocean plays a significant role in moderating the weather and global climate.
Since the poles absorb much less solar radiation than the equatorial region, there is a surplus of energy in the equatorial region and a deficit in the poles. The ocean circulation carries the majority of the heat out of the tropics, and the further movement of heat to the poles is mainly carried out by the atmospheric circulation. This uneven heating and consequent transport of heat by the ocean and the atmosphere sets the basic outlines of the Earth’s weather and climate.
Evaporation from the ocean supplies 90% of the water vapor that eventually becomes rain and snow, and this evaporation is a source of heat to the atmosphere. The ocean helps to determine the location of clouds and precipitation by influencing the atmospheric motions that move water across the globe. The evaporative heating occurs in regions of warm water, such as the equatorial region, and masses of warm and buoyant air are formed, causing the air to rise. As the warm air rises, the water vapor in the air condenses, releasing heat to the atmosphere. This heating provides the energy to drive an important feature of the atmospheric circulation, the cell of air that moves warm air from the tropics to the mid- latitudes. These circulation patterns set the main regions of rain and arid conditions across the globe, and the resulting surface winds drive the ocean currents, which act to redistribute the heat within the ocean, setting the regions of warmer and cooler sea surface temperatures. Sea surface temperatures in turn then help set the atmospheric circulation.
The ocean has its own circulation patterns, set in part by the wind patterns. The oceanic circulation is also a function of the density structure of the water masses comprising the ocean basins. The density at the surface of the ocean is affected by the amount of freshwater coming in through precipitation and freshwater going out by evaporation. In addition, the density at the surface is affected by the amount of heat coming and going out between the ocean and the atmosphere. When the density of the surface water is increased, typically by cooling of the surface water by heat loss to the atmosphere and/or making the surface water so salty through evaporation or by sea ice growth, the surface water sinks and forms deep water, connecting the surface through to the deepest layers.
The movement of heat by the ocean at the surface from the equatorial regions to the poles occurs as part of this large-scale circulation. A component of this circulation in the North Atlantic basin is called the Atlantic Meridional Overturning Circulation (AMOC), and the surface expression of this is the Gulf Stream. The Gulf Stream is of crucial importance in defining the climate of Europe, and for this reason the AMOC is a major ocean process of study for oceanographers.
In addition, the ocean plays an important role in the storage and release of CO2. The ocean is a large reservoir of carbon. In the case of CO2, there is, on average across the globe, a net uptake of airborne carbon through the surface of the ocean. How much CO2 the surface ocean can take up is dependent on the temperature of the surface layer, and how much of the carbon is then mixed to deeper water. Increases in heat and CO2 in the ocean leads to a reduction in pH and carbonate saturation rate, a process called “ocean acidification”.
The ocean has moderated the total CO2 in the atmosphere, with measurements indicating that the atmosphere would have 55 parts per million more CO2 were it not for the uptake of CO2 by the ocean. Roughly half of all the anthropogenic carbon released into the atmosphere between 1800 and 1994 is currently stored in the ocean. It should be noted that the uptake of CO2 by the ocean is decreasing, possibly due to fact that CO2 solubility in the ocean decreases as temperature increases.
The ocean plays a key role in natural climate variability. A particular feature of our climate system is the ubiquity of naturally recurring patterns of variability, called oscillations. One of the most commonly-known atmosphere-ocean natural climate events is the El Niño/Southern Oscillation (ENSO). The warm phase, El Niño, occurs approximately every two to seven years and typically lasts between nine months and two years. The impacts of the changes in rainfall and temperature on the U.S. due to the 1982-1983 El Niño were significant enough that a new moored buoy array (the tropical ocean global atmosphere (TOGA) tropical atmosphere ocean (TAO) array) was put into place in the tropical Pacific Ocean beginning in 1984, with completion in 1994. This array, and data from other ocean sensors, can be used to track changes in the sea surface temperatures that can help identify developing El Niño and La Niña events. This information has improved forecasters’ ability to predict ENSO events and the related changes in temperature and rainfall. The TAO data is also helping to refine our understanding of ENSO, which will help in predictions of ENSO variability over longer time scales.
The ocean is also a driver of longer-term multi-decadal natural climate variability, such as the Pacific Decadal Oscillation (PDO). The ocean can play a role in more abrupt climate changes (changes occurring over relatively short time scales) as well. An example is the abrupt end to the millennial-long very cold climate conditions that existed up to roughly 12000 years ago. The transition to this cold climate occurred in a decade or less and is notable for causing the extinction of nearly three-quarters of the large mammals in North America. The abrupt climate changes during the last ice age have been linked to large variations in the AMOC, caused by changes in the amount of deep water formation, perhaps as a result of changes in sea ice. These types of abrupt climate changes are of significant concern because their key characteristic is that they may in the future occur faster than expected or for which we may be planning. In any case, the ocean clearly has a significant influence over short-term and longer-term natural variability in the climate system that affects weather patterns across the globe.
Key Uncertainties Remain
This section highlights just a few of the many key uncertainties that remain in our understanding of the ocean and its interaction with the atmosphere, land, and ice. These uncertainties significantly affect our ability to understand past and current climate variability and our ability to use this understanding to predict future climate variability.
Lack of Data, Scientists and Priority
While much has been learned by oceanographers and climate scientists about the processes in the ocean and their effect on the climate in the last decades, understanding is hampered by a significant lack of key data. The ocean remains highly under sampled, from the surface exchanges between the ocean and atmosphere all the way down to the deepest parts of the ocean. Many of the processes, and especially those that relate to interactions between the ocean and the atmosphere, are simply not well understood due to both a lack of data and the relatively small number of researchers funded to work in these areas. Similarly, relatively few direct observations exist of the ocean deeper than a mile down, and this hampers our ability to understand how the deep ocean stores and exchanges heat, salt, and carbon with the upper ocean and how it transports these properties around the globe.
It is unsurprising that these observational gaps exist, given the relative difficulty and high cost of getting scientists and instruments to remote ocean regions. Satellites can only “see” the surface of the ocean, below which scientifically-based inferences must be made in many cases to relate observed surface properties to deeper ocean aspects such as circulation and temperature structure.
Air-Sea Exchanges
Exchanges between the ocean and atmosphere of heat, moisture, momentum, and gasses are rarely directly measured. This is especially true under extreme conditions like high wind speeds, including hurricanes, and in difficult to reach locations, such as the Southern Ocean, where very few direct observations
have ever been made Instead, we estimate the exchanges by measuring the relatively easier components of the lower atmosphere (such as wind speed, humidity, and temperature), which still have significant uncertainties, and estimating the exchange from these variables.
Open questions remain about the influence of waves, sea spray, bubbles, and other properties of the air-sea interface on these exchanges, particularly under extreme conditions. Quantitative understanding of how the ocean and atmosphere exchange heat, moisture, and momentum requires better observations and models of these processes.
Once heat or CO2 is deposited in the upper ocean, the eventual extent to which the temperature of concentration changes is largely dependent on ocean dynamics and mixing between the upper and lower ocean, processes which themselves are not fully understood. This mixing in the ocean sets the limits of the amount of anthropogenic CO2 that the ocean can uptake over decadal time scales. On centennial time scales, basic scientific principles dictate that the ocean CO2 amount will equilibrate with atmosphere CO2, but an open question remains as to the rate at which this ocean uptake will occur, and our understanding of some key processes that control the carbon distributions in the ocean is still quite limited.
Ocean Currents and Circulation
The importance of understanding how mixing redistributes heat between the upper and the deeper ocean has been underscored in recent years, in light of hypotheses about deep ocean storage of heat. It is hypothesized that an increase in heat transfer from the surface to the deep ocean may be a key component of the pause in the increase of the Earth’s mean near- surface temperature. Some of this increased downward mixing may be a result of a significant strengthening of the Pacific trade winds, which in turn appears to be related to changes in the atmospheric circulation, possibly associated with changes in the PDO and the Atlantic circulation, although the actual mechanisms by which this might be occurring are currently unknown. None of the current climate models have captured this increase in Pacific trade winds, indicating that our understanding of the relevant processes is incomplete. This lack of understanding in turns leads to an inability to accurately reproduce ENSO, PDO, and other types of natural variability in the climate models, thus making it difficult to accurately predict how this natural variability may change in the future.
Ocean circulation has also changed significantly over time, such as during abrupt climate changes, when the North Atlantic region cooled but the southern Hemisphere warmed. Since the ocean circulation is a key feature of how the ocean stores heat and carbon, and since it is so variable, we need to know not only the present variability but how and why it has changed in the past and how it might change in the future. Some of this variability is due to changes in the heat transport associated with changes in how much deep water is formed. A remaining question is what types of reorganization of the ocean circulation may happen in the future and how will this impact the atmosphere. For instance, there remain long-standing questions about the importance of the main processes that drive the AMOC as well as whether we can in fact predict changes in the AMOC. Our uncertainties about these processes mean that different types of ocean climate models have very different sensitivities to changes in greenhouse gases and produce sea surface temperature changes that can even differ between increases and decreases over time.
Ocean Temperature Trends and Impacts
A key dataset for understanding climate variability is the thermal energy of the ocean at all levels. This information is crucial to expanding our understanding of past, present, and future climate variability. It is difficult to overestimate the importance of this kind of data for understanding the present and predicting the future of human- induced changes. Increases in upper ocean heat content constitute a significant fraction of the heat storage that has occurred over the past 5 decades; one estimate is that the ocean warming accounts for roughly 90% of the total of Earth’s heat storage. As the ocean heat content increases, sea level rises due to expansion of the ocean water. This is a critical issue, as two-thirds of the world’s largest cities are at least partially in regions that are less than 30 feet above the current sea level, and issues associated with changes in sea level will profoundly impact this population.
Compared to the relatively longer and more global observations of atmospheric temperature changes, however, our time series and global coverage of temperature below the ocean surface is much shorter and much less globally complete, and a number of gaps still remain in our knowledge of heat changes, particularly in the deeper ocean. The first moderately usable ocean temperatures were isolated observations in the 1870’s. Since that time, the upper 700 m heat content has warmed substantially on the global scale, which we can most reliably observe since 1980. There has also been an increase over the upper 2000 m (roughly one mile), which scientists have only been able to fairly reliably measure since 2005.
The observations down to 2000 m were made possible when a new observing system called the Argo program achieved a more global reach. Argo is a data collection system consisting of many floats cycling up and down in the water column measuring ocean temperature and salinity. However, significant data gaps remain, particularly below 2000 m, which is nearly unmeasured.
The importance of ocean temperatures to global climate coupled with the paucity of data regarding ocean temperatures makes understanding the present and predicting the future of human-induced changes a challenging problem for oceanographers. Due to the sparseness of the dataset, many assumptions need to be made about what to use as a climatological reference, and how to include unsampled areas, which creates considerable uncertainty about the amount of ocean heat content change even over the upper mile over the past 50 years. Further, there is no coherent strategy for making measurements deeper than a mile on the types of spatial and temporal scales that would provide information about how the upper ocean and lower ocean communicate and would enable us to understand how much heat the ocean is likely to absorb in the future. Autonomous vehicles capable of making measurements down to roughly three miles are available, but they are few and funded entirely for individual research projects on small scales rather than at a level necessary for coordinated measurements of the entire deeper ocean.
The recent observed weakening in ocean uptake of CO2 could be a result of either human-induced activities or natural variability (or both). A warmer ocean reduces CO2 solubility, but it may also reduce deep water formation, which could also limit mixing of CO2 downwards. Both of these effects would act to weaken ocean uptake of CO2. However, a possible mitigating factor is the Southern Ocean, which might have higher uptake in the future. Specifically, if the current forecasts of strengthening winds are correct, it would lead to more cool water brought from below and an enhancement of CO2 uptake, possibly enough to change the global net uptake to increasing rather than decreasing. An important need is for data in such data-poor regions as the Southern Ocean in order to understand the processes that drive changes between the atmosphere and ocean. This understanding could help improve the models and reduce uncertainty over future climate scenarios.
Clearly the uncertainties in our understanding of surface processes, carbon uptake, and ocean circulation are all connected. Most of the processes involved in these connections are not well monitored nor understood, so it is not clear how the ability of the ocean to store anthropogenic carbon will be affected over the next few decades. Over a very long time scale, the ocean will eventually arrive at equilibrium with respect to the atmosphere, but the rate at which this will occur is extremely uncertain. Improved observations of these processes would provide a basis for reducing the uncertainty in our estimates, as would global strategies that provided resources for gathering together and providing observations that may now only be accessible to a few scientists.
Our understanding of the marine ecosystems and marine organisms, and the potential implications of climate changes on these systems, is also subject to significant knowledge gaps, even with the increase in understanding in the past decade. Significant uncertainties still exist about species interactions under ocean acidification, and the extent to which natural cycles may influence these interactions. Some of these uncertainties are due to the wide range of abilities of the global climate models to actually represent some of these natural oscillations such as ENSO and the PDO. There is a potential for these cyclical interactions between the ocean and atmosphere to either amplify or diminish the original change, especially between the physics and the biology of the ocean, and these interactions are not well understood or represented in our models. Similar interactions that drive changes in the cycling of carbon through the atmosphere and ocean are also occurring, with similar uncertainties.
Need for Ocean Research and Funding for Research
The best way to improve our understanding of the ocean and its crucial impact on the climate system will be through a combination of data collection, data analysis/understanding, and the transfer of this increased understanding to our global climate models. Current funding levels, however, are inadequate to reduce these uncertainties on a timely basis, and are in some cases inadequate to continue even the current rate of progress.
Funding for many aspects of ocean research has been declining over the last decade. As an example, the Argo drifter program was initiated in FY 2003, with a budget of roughly $9.8 million. This program is one of the successes of 21st century oceanography, with many of the new results that have been discussed here being a product of this greatly enhanced observational capability. As a result, Argo is frequently asked to include enhancements by the larger community, including expansion of coverage, additional sensors, and improved sampling of the near-surface layer. However, in nominal dollars (not adjusted for inflation), the U.S. funded Argo program has received flat funding since FY 2003 (FY 2013 funding was $9.65 million, all from NOAA). Adjusting for inflation, funding for the Argo program has actually decreased.
Funding cuts have also severely impacted the TAO buoy array that is essential for our ENSO early-warning system, putting it in even more critical condition. Data recoverage dropped to 40% earlier this year, due to the aging of the buoys and lack of money to keep the system at full strength.Prior to 2012, the total budget for the TAO project was $10-12 million, of which $6 million was budgeted for ship operations. After the ship’s retirement, NOAA has spent roughly $2-3 million to charter boats for servicing these buoys, but this has not been enough to keep the system at full strength. The NOAA spend plan for FY 2013 was $415 million for research and development (R&D) and $128 million for R&D equipment. It should be noted that NOAA conducts scientific research in areas such as climate, weather, air quality, and ocean and Great Lakes resources, and that the funds listed above are used to support all of these activities, not just ocean research. The level to which the NOAA funding is stretched thin is exemplified by the need to save a few million dollars that would otherwise have kept the TAO array at optimal strength.
Of the $7.3 billion allocated for National Science Foundation in the President’s FY 2015 budget, roughly 2% will be available for investment in research on any aspect of the ocean, and of that, only a tiny fraction supports projects directly related to the ocean’s impact on climate change. The President’s FY 2015 budget also includes roughly $11.6 billion for R&D at NASA. In the recent past less than 4% of this amount has gone to Earth science research, which covers research on all aspects of the weather and climate system, including the land, atmosphere, and ocean. When one includes the total requests for other scientific R&D, such as that for National Institutes of Health at roughly $30 billion, the total government allocation for ocean research is under 1% of the government’s total R&D spend, which has been true during the 1980’s and 1990’s as well.
Our understanding of the ocean is central to our understanding of current and future changes to the Earth’s climate system. The current funding level is simply not enough to provide the level of information that the public, and the policy makers, need to make informed decisions. Perhaps the best summation of the current state of affairs was by Senator Mark Begich (D-AK), who in a 2013 hearing on the deep sea challenge facing the United States, asked hypothetically where we would be today if we had spent half as much money exploring the ocean as we have spent exploring space. The actual answer to this question is unknowable, but it seems safe to say that our understanding, observations, and models of the ocean and its interaction with the climate system would be much less uncertain.
The President’s 2013 Climate Action Plan acknowledges that “[s]cientific data and insights are essential to help government officials, communities, and businesses better understand and manage the risks associated with climate change.” However, because the ocean and climate are “inextricably linked,” the Administration cannot properly understand and manage those risks without funding and actively pursuing a better understanding of the ocean and its impacts on climate change.
This post is excerpted from remarks on understanding the role of the ocean in climate [EPA comments] that I submitted in response to the U.S. Environmental Protection Agency (EPA) request for comments on its document “Standards of Performance for Greenhouse Gas emissions from New Stationary Sources: Electric Utility Generating Units,” 79 Federal Register 1430, 1440 (Jan 8, 2014).
Biosketch. Carol Anne Clayson is Director of the Ocean and Climate Institute at the Woods Hole Oceanographic Institute and a Senior Scientist in the Department of Physical Oceanography at the Woods Hole Oceanographic Institution (WHOI). She has been tenured faculty at Florida State University and Purdue University, and while at FSU was the Director of the Geophysical Fluid Dynamics Institute. Her research covers the areas of air-sea interaction, satellite remote sensing, and ocean modeling, and she has received funding for her research from NASA, NOAA, the Office of Naval Research, and NSF. She is the recipient of an NSF CAREER award and the Office of Naval Research Young Investigator Award. She received a Presidential Early Career Award for Scientists and Engineers from President W. Clinton. She has served on multiple committees for the American Meteorological Society and the National Research Council, including the Board on Atmospheric Science and Climate. She has been a lead reviewer of the U.S. Climate Change Science Program Synthesis and Assessment Product. She currently serves on a number of national and international panels, including as chair of the World Climate Research Program GEWEX SeaFlux project, an international group of scientists working on improved estimations of air-sea turbulent heat fluxes from satellite. She is the inaugural leader of the WCRP Flux Task Team, and is currently a member of the CLIVAR Phenomena, Observation, and Synthesis Panel. She has served on the NASA Science Review team, as well as on the NASA Decadal Survey, in addition to currently serving on several NASA Science Teams. She also sits on external advisory boards for the University of Colorado Aerospace Engineering Sciences Department and the Los Alamos Laboratory Institute of Geophysical and Planetary Physics. Dr. Clayson received her B. S. degree in Physics and Astronomy from Brigham Young University in 1988, and her M.S. and Ph.D. degrees in Aerospace Engineering Sciences and the Program in Atmospheric and Oceanic Sciences from the University of Colorado, Boulder in 1990 and 1995.
JC note: This is an invited blog post. Carol Anne was one of my first Ph.D. graduates at the University of Colorado. I am obviously a very proud academic ‘mama.’ As with all guest posts, please keep your comments relevant and civil.
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