Effects of Climate Change and Ocean Acidification on Living Marine Resources
Scott Doney, Senior Scientist
Marine Chemistry & Geochemistry Department
Woods Hole Oceanographic Institution
Good morning Madame Chair, Ranking Member Snowe and members of the Subcommittee. Thank you for giving me the opportunity to speak with you today on global climate change, ocean acidification and the resulting impacts on fisheries and living marine resources. My name is Scott Doney, and I am a Senior Scientist at the Woods Hole Oceanographic Institution in Woods Hole MA. My research focuses on interactions among climate, the ocean and global carbon cycles, and marine ecosystems. I have published more than 90 peer-reviewed scientific journal articles and book chapters on these and related subjects. I serve on the U.S. Carbon Cycle Science Program Scientific Steering Group and the U.S. Community Climate System Model Scientific Steering Committee, and I am chair of the U.S. Ocean Carbon and Climate Change Scientific Steering Group and the U.S. Ocean Carbon and Biogeochemistry Scientific Steering Committee.
For today’s hearing, you have asked me to discuss the mechanisms by which greenhouse gases impact the ocean, coastal environment, and living marine resources, gaps in our current scientific understanding, and implications for resource management including adaptation and mitigation strategies. My comments are based on a broad scientific consensus as represented in the current scientific literature and in community assessments such as the 2007 Intergovernmental Panel on Climate Change (IPCC) reports (IPCC, 2007a; 2007b; 2007c).
Over the past two centuries, human activities have resulted in dramatic increases in atmospheric carbon dioxide and other greenhouse gases. There is broad scientific consensus that these excess greenhouse gases are altering our planet’s climate and acidifying the ocean. These findings are confirmed by real-world observations and supported by theory and numerical models. Climate change and acidification trends will accelerate over the next several decades unless there is deliberate action to curb greenhouse emissions. Rising atmospheric carbon dioxide and climate change produce upper-ocean warming, sea-ice retreat, sea-level rise, ocean acidification, altered freshwater distributions, and maybe even stronger storms.
Growing evidence suggests that these human-driven climate change and acidification will strongly impact ocean ecosystems as well. Further pressure will be put on living marine resources, such as fisheries and coral reefs that we depend upon for food, tourism and other economic and aesthetic benefits. We have an opportunity now to limit the negative impact of climate change and acidification in the future. This will require a comprehensive ocean management strategy that incorporates scientific understanding of climate change and acidification from the start. This strategy will also require a balance between adaptation to climate change and acidification that are unavoidable, and mitigation to reduce the rise in greenhouse gases and resulting impacts.
Greenhouse Gases and Climate Change
At the most basic level, the balance between incoming sunlight and outgoing infrared radiation (i.e., heat) determines Earth’s climate. The greenhouse gas carbon dioxide (CO2) plays a key role by absorbing infrared radiation and thus trapping heat near the Earth’s surface much like a blanket. Other trace greenhouse gases such as methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs) are also important to warming, equivalent to about half of that from carbon dioxide, because molecule for molecule they absorb more infrared radiation than carbon dioxide. Other factors involved in human-driven climate change include aerosols and land vegetation.
Over the last two centuries, atmospheric carbon dioxide has increased by more than 30%, from 280 to 380 ppm (part per million) by 2007. The main source is fossil-fuel combustion with contributions from cement production, agriculture and deforestation. Many economic and climate models predict atmospheric carbon dioxide values as high as 700 to 1000 ppm, about triple preindustrial levels, by the end of the twenty-first century. The Earth has not experienced carbon dioxide levels that high for the past several million years. Other trace greenhouse gas levels are growing as well due to land-use, agriculture and industrial practices. These greenhouse gases persist in the atmosphere for years to decades, meaning that they will remain and accumulate in the atmosphere, impacting the global climate for a long time to come. In contrast, aerosols in the lower atmosphere are removed on time-scales of a few days to weeks, and their climatic impacts, mostly cooling, are concentrated near their sources.
Greenhouse gases dominate over other human-driven climate perturbations, and the increased heating translates into changes in climate properties such as surface temperature, rainfall, sea-level and storm frequency and strength. The climate change resulting from an increase in greenhouse gases can be amplified by other climate processes. For example, ocean warming leads to a large retreat in Arctic sea-ice, which further strengthens warming because the dark water surface can then absorb more sunlight than the highly reflective ice. The largest unknowns at present arise from cloud dynamics. Numerical model climate projections for this century show global mean surface temperature increasing, with a range of +1.1 to 6.4°C (+2.0 to 11.5°F) above late 20th century levels. This large temperature range is somewhat misleading as a significant fraction of the variation depends on human behavior, specifically how much carbon dioxide and other gases we emit to the atmosphere in the future. The lowest temperature projections occur only when emissions are reduced sharply over the next few decades.
The largest projected temperature changes are concentrated over the continents and at higher latitudes during the winter season, but some level of warming will occur globally, over the ocean, and year-round. Sea-level is estimated to rise due to thermal warming and melting glaciers and ice sheets by an additional +0.18 to 0.59 m (+0.6 to 1.9 feet) by 2100. Many simulations suggest a general strengthening of the water cycle, with increased precipitation in the tropics and high latitudes, drier conditions in the subtropics, and an increased frequency of extreme droughts and floods. Other common features of a warmer climate are more El Niño-like conditions in the Equatorial Pacific, a melt back of polar sea-ice and glaciers, and a slowdown in the formation of ocean deep water at high latitudes.
The Changing Ocean Environment
Global warming should be called ocean warming, as more than 80% of the added heat resides in the ocean. Clear alterations to the ocean have already been detected from observations. The magnitude and patterns of these changes are consistent with an attribution to human activities and not explained by natural variability alone. Global average land and ocean surface temperatures increased at a rate of about 0.2°C/decade over the last few decades (Hansen et al., 2006), and ocean temperatures down to 3000 m (10,000 feet) depth are also on the rise. Averages rates of sea-level rise over the last several decades were 1.8±0.5 mm/y, with an even larger rate (3.1±0.7 mm/y) over the most recent decade. Higher precipitation rates are observed at mid to high latitude and lower rates in the tropics and subtropics. Corresponding changes have been measured in surface water salinities. One of the most striking trends is the decline in Arctic sea-ice extent, particularly over the summer. September Arctic ice-cover from 2002-2006 was 18% lower than pre-1980 ice-cover (http://www.arctic.noaa.gov/detect/ice-seaice.shtml, and some models predict near ice-free conditions by 2040. Recent studies of the Greenland ice sheet highlight an alarming increase in surface melting over the summer, and percolation of that melt water to the base of the ice sheet where the melt-water could lubricate ice flow and potentially greatly accelerate ice loss and sea-level rise. These new findings have not been full incorporated into projected sea-level rise estimates, which thus may be underestimated.
Over half of human carbon dioxide emissions to the atmosphere are absorbed by the ocean and land biospheres (Sarmiento and Gruber, 2002), and the excess carbon absorbed by the ocean results in increased ocean acidity. The physical and chemical mechanisms by which this occurs are well understood. Once carbon dioxide enters the ocean, it combines with water to form carbonic acid and a series of acid-base products, resulting in a lowering of pH values. The amount and distribution of human-generated carbon in the oceans are well determined from an international ocean survey conducted in the late 1980s and early 1990s (Sabine et al., 2004). The rate of ocean carbon uptake is controlled by ocean circulation. Most of the excess carbon is found in the upper few hundred meters of the ocean (upper 1200 feet) and in high-latitude regions, where cold dense waters sink into the deep ocean. Surface water pH values have already dropped by about 0.1 pH units from preindustrial levels and are expected to drop by an additional 0.14-0.35 units by the end of the 21st century (Orr et al., 2005).
Climate Change and Ocean Acidification Impacts on Marine Ecosystems
Climate change and ocean acidification will exacerbate other human influences on fisheries and marine ecosystems such as over-fishing, habitat destruction, pollution, excess nutrients, and invasive species. Thermal effects arise both directly, via effects of elevated temperature and lower pH on individual organisms, and indirectly via changes to the ecosystems on which they depend for food and habitat. Acidification harms shell-forming plants and animals including surface and deep-water corals, many plankton, pteropods (marine snails), mollusks (clams, oysters), and lobsters (Orr et al., 2005). Many of these organisms provide critical habitat and/or food sources for other organisms. Emerging evidence suggests that larval and juvenile fish may also be susceptible to pH changes. Marine life has survived large climate and acidification variations in the past, but the projected rates of climate change and ocean acidification over the next century are much faster than experienced by the planet in the past except for rare, catastrophic events in the geological record.
One concern is that climate change will alter the rates and patterns of ocean productivity. Small, photosynthetic phytoplankton grow in the well-illuminated upper ocean, forming the base of the marine food web, supporting the fish stocks we harvest, and underlying the biogeochemical cycling of carbon and many other key elements in the sea. Phytoplankton growth depends upon temperature and the availability of light and nutrients, including nitrogen, phosphorus, silicon and iron. Most of the nutrient supply to the surface ocean comes from the mixing and upwelling of cold, nutrient rich water from below. An exception is iron, which has an important additional source from mineral dust swept off the desert regions of the continents and transported off-shore from coastal ocean sediments. The geographic distribution of phytoplankton and biological productivity is determined largely by ocean circulation and upwelling, with the highest levels found along the Equator, in temperate and polar latitudes and along the western boundaries of continents.
Key climate-plankton linkages arise through changes in nutrient supply and ocean mixed layer depths, which affect the light availability to surface phytoplankton. In the tropics and mid-latitudes, there is limited vertical mixing because the water column is stabilized by thermal stratification; i.e., light, warm waters overlie dense, cold waters. In these areas, surface nutrients are typically low, which directly limits phytoplankton growth. Climate warming will likely further inhibit mixing, reducing the upward nutrient supply and thus lowering biological productivity. The nutrient-driven productivity declines even with warmer temperatures, which promote faster growth. At higher latitudes, phytoplankton often have access to abundant nutrients but are limited by a lack of sunlight. In these areas, warming and reduced mixed layer depths can increase productivity.
A synthesis of climate-change simulations shows broad patterns with declining low-latitude productivity, somewhat elevated high-latitude productivity, and pole-ward migration of marine ecosystem boundaries as the oceans warm; simulated global productivity increased by up to 8.0% (Sarmiento et al., 2004). While not definitive proof of future trends, similar relationships of ocean stratification and productivity have been observed in year to year variability of satellite ocean color data, a proxy for surface phytoplankton (Beherenfeld et al., 2006); satellite data for 1997-2005 from GeoEYE and NASA’s Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) show that phytoplankton declined in the tropics and subtropics during warm phases of the El Niño-Southern Oscillation (ENSO) marked by higher sea surface temperatures and ocean stratification. Ecosystem dynamics are complex and non-linear, however, and new and unexpected phenomena may arise as the planet enters a new warmer and unexplored climate state. Ocean nitrogen fixation, for example, is concentrated in warm, nutrient poor surface waters, and it may increase under future more stratified conditions, enhancing overall productivity.
Changes in total biological productivity are only part of the story, as most human fisheries exploit particular marine species, not overall productivity. The distributions and population sizes of individual species are more sensitive to warming and altered ocean circulation than total productivity. Temperature effects arise through altered organism physiology and ecological changes in food supplies and predators. Warming and shifts in seasonal temperature patterns will disrupt predator-prey interactions; this is especially important for survival of juvenile fish, which often hatch at a particular time of year and depend up on immediate, abundant source of prey. Temperature changes will also alter the spread of diseases and parasites in both natural ecosystems and marine aquaculture. Warming impacts will interact and perhaps exacerbate other problems including over-fishing and habitat destruction.
Food-web interactions are often complicated, and we should expect that some species will suffer under climate change while others will benefit. Broadly speaking though, warm-water species are expected to shift poleward, which already appears to be occurring in some fisheries (Brander, 2006). Biological transitions, however, may be abrupt rather than smooth. Large-scale regime shifts have been observed in response to past natural variability. Regime shifts involve wholesale reorganizations of biological food-webs and can have large consequences from plankton to fish, marine mammals and sea-birds. Thus, rather subtle climate changes or ocean acidification may have the potential to disrupt commercially important species for either fisheries or tourism. Decadal time-scale regime shifts have been documented in the North Pacific, and in the Southern Ocean observations show a large-scale replacement of krill, a food source for mammals and penguin, by gelatinous zooplankton called salps.
A number of other factors also need to be considered. Species that spend part of their life-cycle in coastal waters will be impacted by degradation of near-shore nursery environments, such as mangrove forests, marshes and estuaries, because of sea-level rise, pollution and habitat destruction. Rainfall and river flow perturbations will alter coastal freshwater currents, affecting the transport of eggs and larvae. Some of the largest fisheries around the world, for example off Peru and west coast of Africa, occur because of wind-driven coastal upwelling, which may be sensitive to climate change. Warming will reduce gas solubility and thus increases the likelihood of low oxygen or anoxia events already seen in some estuaries and coastal regions, such as off the Mississippi river in the Gulf of Mexico.
Knowledge Gaps and Ocean Research Priorities
Accurate projections of climate change and ocean acidification impacts on living marine resources hinge on several key questions: 1) how will greenhouse gas and aerosol emissions and atmospheric composition evolve in the future? 2) how sensitive are regional-scale ocean physics and chemistry to these changes in atmospheric composition? and 3) how will individual species and whole-ocean ecosystems respond? Fossil fuels are deeply intertwined in the modern global economy, and carbon dioxide emissions depend upon changing social and economic factors that are not well known: global population, per capita energy use, technological development, national and international policy decisions, and deliberate climate mitigation efforts. Future projections of atmospheric carbon dioxide levels are also relatively sensitive to assumptions about the behavior of land and ocean carbon sinks, which are expected to change due to saturation effects and responses to the modified physical climate (Fung et al., 2005). Climate change on local and regional scales is more relevant for people and ecosystems than global trends. While progress is being made, improved and better-validated regional ocean climate forecasts remain a major need for future research.
Even when predictions about the physical environment are well known, significant knowledge gaps exist about ocean ecology, hindering the creation of the skillful forecasts needed to guide ocean management decisions. While not precluding taking action now to address climate change and ocean acidification, better scientific understanding will help refine ocean management in the long-term. Several elements need to be pursued in parallel: improved on-going monitoring of ocean climate and biological trends; laboratory and field process studies to quantify biological climate sensitivities; historical and paleoclimate studies of past climate events; and incorporation of the resulting scientific insights into an improved hierarchy of numerical ocean models from species to ecosystems.
Rapid advances in in-situ sensors and autonomous platforms, such as moorings, floats and gliders, are revolutionizing ocean measurements, and ocean observing networks are being constructed for coastal and open ocean regions (e.g., Gulf of Maine Ocean Observing System http://www.gomoos.org/; Pacific Coast Ocean Observing System; National Science Foundation Ocean Observing Initiative http://www.ooi.org). The number of historical, multi-decadal ocean time series is limited, but their scientific utility is almost unrivalled. Federal commitment is needed for continued, long-term investment in ocean monitoring and enhanced coordination across observing networks.
In a similar vein, satellite measurements provide an unprecedented view of the temporal variations in ocean climate and ecology. The ocean is vast, and the limited number of research ships move at about the speed of a bicycle, too slow to map the ocean routinely on ocean basin to global scales. By contrast, a satellite can observe the entire globe, at least the cloud free areas, in a few days. The detection of gradual climate-change trends is challenging, and the on-going availability of high-quality, climate data records is not assured during the transition of many satellite ocean measurements from NASA research to the NOAA/DOD operational NPOESS program. For example, the present NASA satellite ocean color sensors, needed to determine ocean plankton, are nearing the end of their service life, and the replacement sensors on NPOESS may not be adequate for the climate community. Further, refocusing of NASA priorities away from earth science may dramatically limit or full preclude new ocean satellite missions need to characterize ocean climate and biological dynamics.
We need to know if there are climatic tipping points or thresholds beyond which climate change may induce rapid and dramatic regime shifts in ocean ecosystems. Many current scientific studies examine climate sensitivities of species in isolation; the next step involves examining responses of species populations, communities of multiple interacting species, and entire ecosystems to realistic size perturbations. Experiments on plankton and benthic communities can be conducted under relatively controlled conditions in mesocosms (large enclosed volumes such as aquarium or floating bags deployed at sea) or by deliberate open-water perturbations studies. Both approaches will benefit from further directed technological developments. Larger mobile species require different approaches such as using past climate events as analogues for human-driven climate change. Biology models are pivotal to ocean management. They are being improved progressively by incorporating new information from laboratory and field experiments and by comparing model forecasts with real-world data. It is often as important to identify where the models do poorly as where they do well because research can then be focused on resolving these model errors.
Climate Adaptation, Mitigation, and Ocean Management
Given the potential for significant negative impacts of climate change and ocean acidification on living marine resources, we need to develop comprehensive local, national and international ocean management strategies that fully incorporate climate change and acidification trends and uncertainties. The strategies should follow a precautionary approach that accounts for the fact that ocean biological thresholds are unknown. The strategies should include improved scientific information for decision support, adaptation to reduce negative climate change and acidification impacts, and mitigation to decrease the magnitude of future climate change and acidification.
Currently the United States and other countries invest significant resources in monitoring the ocean and improving scientific understanding on many of the physical, chemical and biological processes relevant to climate change and acidification. However, this wealth of data and information is typically not in a form that is easily accessible by ocean resource managers and other stakeholders, ranging from private citizens and small-businesses to large corporations, NGOs and national governments. For example, even state-of-the-art climate projections typically resolve climate patterns at relatively coarse spatial resolutions and include either relatively simple ocean biology or no ocean biology at all. In contrast, decision makers need information tailored to specific local fisheries and ecosystems. The national climate modeling centers should be encouraged to create on a routine basis targeted ocean biological-physical forecasts on seasonal to decadal time-scales, building on nested regional models, probabilistic and ensemble modeling of uncertainties, and downscaling methods developed for related applications (e.g., agriculture, water-resources). The utility of such forecasts and their uncertainties will be maximized if stakeholders are involved in their design from the onset and if the model results are translated into more accessible electronic forms that are widely distributed to the public.
A second challenge is to create more adaptive ocean management strategies that emphasize complete and transparent discussion on the risks and uncertainties from climate change and ocean acidification. Some amount of climate change and acidification is unavoidable because of past greenhouse emissions, and even under relatively optimistic scenarios for the future, substantial further ocean impacts should be expected at least through mid-century and beyond. Decisions will need to be made in the face of uncertainty, relying on for example the precautionary principle to limit future risk. Climate change trends are growing in magnitude, but will still be gradual compared with natural interannual variability; management policies must include both types of variations and uncertainties. Empirical approaches developed from historical data cannot be used in isolation because climate change will shift the baseline for ocean biological systems. Serious efforts should be directed at reducing other human factors such as overfishing and habitat destruction to allow more time ecosystems and social systems to adapt. Mechanisms such as marine reserves, that protect specified geographical locations, need to account for the fact that ecosystem boundaries will shift under climate change. Procedures also need to be in place to monitor over time the effectiveness of ocean conservation and management policies, and that information and improved future climate forecasts should be used to modify and adapt management approaches.
The third challenge is to pursue climate mitigation approaches that limit the emissions of carbon dioxide and other greenhouse gases to the atmosphere or that remove fossil-fuel carbon dioxide that is already in the atmosphere. Stabilizing future atmospheric carbon dioxide at moderate levels to minimize climate change impacts will require a mix of approaches, and no single mechanism will solve the entire problem. Emissions of carbon dioxide can be reduced through energy conservation and transition to alternative, non-fossil fuel based energy sources (wind, solar, nuclear, biofuels). Attention also needs to be placed in the near-term on limiting other greenhouse gases such as chlorofluorocarbons, which may provide additional time to tackle the more challenging issues associated with carbon. Progress is being made on approaches that would remove carbon dioxide at power plants so that it can be sequestered in subsurface geological reservoirs (e.g., old oil and gas fields, salt domes).
Mitigation approaches have also been proposed using ocean biology, but these methods should only be pursued if critical questions are resolved on their effectiveness and environmental consequences. Biological mitigation strategies are based on the fact that plants and some marine microbes naturally convert carbon dioxide into organic matter during photosynthesis. Enhancing biological carbon removal can reduce atmospheric carbon dioxide if the additional organic matter is stored away from the atmosphere for multiple decades to a century or longer. The deep-ocean is one such reservoir because it exchanges only slowly with the surface and atmosphere. Thus one potential mitigation method would be to fertilize the surface ocean phytoplankton so that they produce and export more organic carbon into the deep ocean. In many areas of the ocean, phytoplankton grow is limited by the trace element iron, which is very low in surface waters away from continents and dust sources. About a dozen scientific experiments have been conducted successfully showing that adding iron to the surface ocean causes a phytoplankton bloom and temporary drawdown in surface water carbon dioxide. But there remain outstanding scientific questions about whether iron resulted in any enhanced long-term carbon storage in the ocean.
As with any other mitigation approach on land or in the sea, the scientific and policy communities need to work closely to assure that the following questions are answered for large-scale commercial ocean fertilization. Is the method effective in removing carbon from the atmosphere, can the removal be validated, and how long will it remain sequestered? Could the method result in unintended consequences such as enhanced emissions of other, more powerful greenhouse gases (in the case of iron fertilization potentially nitrous oxide and perhaps methane)? What are the broad ecological consequences, and could carbon mitigation efforts conflict with maintaining living marine resources and fisheries? Systematic approaches to verify effectiveness and environmental impacts need to be put in place to assure a level playing field for commercial mitigation and carbon credit trading systems.
Over the past two centuries, human activities have resulted in the build up in the atmosphere of excess carbon dioxide, other greenhouse gases and aerosols. There is now significant evidence that these changes in atmospheric composition are altering the planet’s climate. Human-driven climate change is expected to accelerate over the next several decades, leading to extensive global warming, sea-ice retreat, sea-level rise, ocean acidification, and alterations in the freshwater cycle. As the reality of climate change is becoming clearer, the emphasis shifts toward understanding the impact of these climate perturbations on society and on natural and managed ecosystems.
Marine fisheries and ocean ecosystems are susceptible to global warming and ocean acidification. While ocean biological responses will vary from region to region, some broad trends can be identified including poleward shifts in warm-water species and reduced formation of calcium carbonate by corals and other shell-forming plants and animals. For fisheries, climate change impacts will interact and perhaps exacerbate other problems including over-fishing and habitat destruction. Management strategies are needed balancing adaptation to an evolving climate and mitigation to reduce the magnitude of future climate change and atmospheric carbon dioxide growth. Decision support tools should be developed for marine resource managers that incorporate the emerging scientific understanding on climate change, focusing on impacts over the next several decades. Systematic testing is required on the effectiveness and environmental consequences of climate mitigation approaches, such as deliberate iron fertilization, designed to sequester additional carbon in the ocean.
Thank you for giving me this opportunity to address this Subcommittee, and I look forward to answering your questions.
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