Climate variability and climate change impacts on the marine environment and on its living resources and ecosystems are issues that require sound and unbiased research, and translation of research results into advice for policymakers and information for the general public. While we can begin to identify and monitor some of these impacts, many questions remain about how ocean processes will change in the future and what effects these changes may have on the ocean environment.

Ocean and Coastal Circulation Patterns

The natural variability of the oceans in the climate system is substantial, and with our current level of scientific understanding and modeling ability, only partially predictable. As a major reservoir and transporter of heat in the climate system, the ocean and its interactions with the atmosphere are at the root of natural modes of interannual climate variability such as the El Niņo-Southern Oscillation, and decadal patterns of variability such as the North Atlantic Oscillation (NAO) and Pacific Decadal Oscillation (PDO). Their impacts are felt through changes in rainfall and storminess patterns, and can yield substantial regional changes in sea level. Changing patterns of circulation also bring shifts in marine ecosystems and fisheries. What are the current limits of our predictability and how can they be improved? How will these natural modes of variability change as the climate changes?

The ocean thermohaline circulation transports heat poleward, particularly strongly in the North Atlantic, and evidence suggests that it in past climates it has been seriously perturbed by injection of fresh water from melting land-based ice, preventing the wintertime formation of deep water. How important is this northward heat transport in keeping Europe warm? Could a substantial change in the thermohaline circulation happen again, and what effect would it have? Are we effectively monitoring this circulation?

The current ability to model sea ice dynamics is limited, but critical to predictions of the climate system due to the amplifying effect of the ice-albedo feedback in polar regions. With the warming climate, some scientists predict a summertime ice-free Arctic before 2100. How can we improve sea ice models? How will the predicted changes impact Arctic and Antarctic marine ecosystems?

Sea Level Rise and Coastal Erosion

The IPCC Fourth Assessment Report (2007) reports that global sea level is expected to rise between 18 and 59 cm by the end of this century, not accounting for changes in ice flows in Antarctica and Greenland, which could boost that figure. Recent scientific results confirm that rates of sea level rise have been accelerating in the 20th century. Local rates of sea level change depend not only on the overall global warming and ice melt, but on regional changes in ocean and wind circulation patterns. With strong growth in coastal populations worldwide, sea level rise has strong and direct impacts on low-lying areas through increased coastal flooding and erosion, contamination of groundwater supplies, and increased vulnerability to storm surges. What are the major sources of scientific uncertainty and how can they be reduced? How can predictions best be used for planning and coastal management?

Natural Hazards

Hurricanes (typhoons) draw energy from ocean heat, and their intensity is strongly dependent on the upper ocean heat content along their track. Predictions of future climate change suggest that on average, hurricane intensity will grow. The average track of mid-latitude winter storms is also affected by the natural variability of the climate system and large-scale patterns such as the NAO. Such storms and their associated coastal storm surges are the major ocean-related natural hazard that will vary with the changing climate. How can we improve the long-term predictions? And how can these predictions best be incorporated into planning?

Carbon Sources and Sinks (CO2 and CH4)

Since the beginning of the industrial revolution, the oceans have taken up approximately 48% of fossil-fuel CO2 emissions, greatly reducing the impact on climate. However, the geographic distribution of this uptake and its drivers are poorly understood. Can we continue to rely on the oceans to take up CO2 at the same rate, or will the rate change as environmental conditions change? What are the impacts of these changes on uptake rates and on ocean biogeochemistry?

Methane (CH4), which has a stronger radiative forcing potential than that of CO2, exists in the marine environment in the form of methane hydrates, which are crystalline solids of methane gas trapped in a frozen cage of six water molecules. These hydrates exist in quantities exceeding all known fossil-fuel reserves, and occur primarily under continental shelf sediments and in Arctic permafrost. There is geological evidence that suggests massive releases of methane hydrates have been associated with periods of global warming in the past. Such events could trigger undersea landslides and tsunamis, as well as release massive amounts of methane to the atmosphere. What are the mechanisms that control gas hydrate stability? What is the risk of such a large-scale release in the future under warmer conditions?

Ocean Acidification

As a result of ocean uptake of anthropogenic CO2, the pH of the oceans is decreasing (e.g., becoming more acidic). By the end of this century, if concentrations of atmospheric CO2 continue unabated, we may expect to see changes in ocean pH that are three times greater and 100 times faster than those experienced during transitions from glacial to interglacial periods. Such large changes in ocean pH have probably not been experienced on the planet for the past 21 million years. As a result of a more acidic ocean, marine calcification rates will decrease, affecting growth and reproduction rates of organisms that use calcium carbonate to construct their shells and skeletons (including calcareous phytoplankton and corals). By the middle of this century, the estimated reduction in calcification rates may lead to a situation where we are losing more coral reef area to erosion than can be rebuilt through new calcification by the organisms. What are the likely scenarios for reduced pH and calcification rates by the middle and end of this century? What affect will this have on marine ecosystems and biogeochemistry of the oceans? How can we monitor these ecosystems for signs of damage?

Fisheries and Ecosystem Impacts

Marine organisms will be influenced by changes in circulation, ventilation, and stratification through changes in temperature, light, and nutrient supply. Alterations of any of these drivers may lead to changes in species abundance and composition, possibly leading to large-scale regime shifts and species migrations. Such changes will affect marine organisms higher up on the food chain in ways that are not yet fully understood. Naturally-occurring climate phenomena, such as ENSO and NAO, have significant impacts on marine ecosystems and fisheries, but these links remain poorly understood. Habitat loss, resulting from sea level rise, and invasion by non-native species will also perturb marine ecosystems, including marine mammals and sea birds, affecting the health and biodiversity of marine ecosystems. What are the main drivers and impacts of climate variability on marine ecosystems? How can we improve understanding and predictability of impacts on ecosystems of natural climate phenomena such as El Nino and NAO? Can we define "acceptable" levels of change and critical breaking points for climate effects on marine ecosystems?