Ocean Behavior

None of the 2006 reports are of the scale of those in 2005 (e.g., with the startling information about the decline in the volume of water circulating in the eastern North Atlantic Ocean as reported by Bryden et al. in Nature in December 2005 or the radical change in ocean acidity as indicated in the 2005 report of the UK Royal Society). Nonetheless, they indicate an ongoing and significant set of observed and projected changes.

Church and White document sea level rise over the 20th century based on data from tide-gauge records from 1950-2000. The authors find that sea level rise rapidly accelerated after 1993 when compared with other periods during the century. Earlier analyses of sea level rise had failed to detect such an acceleration.

Implications: This research suggests that if the new, higher rates of sea level rise continue, ocean levels would be 28 to 34 centimeters higher than 1990 levels by 2100, which is consistent with moderate warming scenarios. However, the results also suggest a non-linear behavior -- which, if carried forward into the future, could imply higher than anticipated increases, with concomitant impacts to coastal ecosystems and low-lying populations. The dynamic behavior of ice sheets, as well as the processes that might lead to it, are currently not incorporated into IPCC estimates of sea level rise. If such dynamic effects are included, the full range of possible sea level rise, including all uncertainties, is likely to be even greater than anticipated by the IPCC.

The thermohaline circulation is a critical component of ocean circulation, acting as a conveyer belt bringing colder, polar deepwater to the equator and warmer surface waters to the North Atlantic. Rennermalm et al. demonstrate that river runoff from the Arctic plays a critical role in determining the circulation’s strength. The authors used a model developed by the University of Victoria and examined the sensitivity of the circulation to varying levels of river discharge. An inverse relationship between the strength of the thermohaline circulation and Arctic river runoff was found (the higher the runoff, the weaker the circulation).

Implications: Warming temperatures in the Arctic are leading to ice melt and increased river runoff. Between 1936 and 1999, river discharge increased by 7 percent and between 1957 and 2004, the thermohaline circulation, measured in the North Atlantic, had apparently slowed by 30 percent (although more recently, there has been some suggestion that such shifts mark fluctuations rather than an overall slowing). While other factors contribute to the circulation mechanism, the authors suggest that river runoff is a major factor affecting the strength of this ocean conveyer belt. The weakening of the thermohaline circulation, the authors suggest, could eventually lead to its collapse, which would impact the transfer of warm waters to the North Atlantic and have larger consequences for regional temperature and precipitation (particularly in northern Europe, which has been warm despite its latitude).

Peterson et al. quantify all freshwater inputs to the Arctic and North Atlantic to assess whether the freshening -- or reduction in salinity -- of the North Atlantic can be attributed to these sources instead of ocean mixing and circulation processes. They find that over the last 50 years 2000 cubic kilometers of freshwater was discharged from glacial melt; 15,000 cubic kilometers from sea ice loss; and 20,000 cubic kilometers from excess river flow and precipitation -- a total that matches the measured ocean freshening. They hypothesize that as freshwater accumulates in the Arctic Ocean, it is likely to spread southward.

Implications: Freshwater inputs and their spread to other latitudes are likely to impact ocean and atmospheric cycles, which could have large implications for regional temperature and moisture. The inputs analyzed by Peterson et al. have been demonstrated by other research to largely result from human-induced warming. If freshwater contributions continue unabated, the impacts to ocean behavior will likely be amplified. Some models have projected that a freshening of the oceans will slow or even stop the ocean's conveyer belt, or thermohaline circulation. Changes in ocean salinity could also shift entire coastal ecosystems as nutrient fluxes and phytoplankton and zooplankton production is altered.

The Pliocene epoch (which ended approximately 2 million years ago) had a climate similar to our own (although somewhat warmer). However, although the carbon dioxide concentration was similar to today's, sea levels were roughly 25 meters above current levels, and polar temperatures were substantially higher. Federov and colleagues explain the "Pliocene paradox" in a recent publication in Science magazine, finding that a permanent El Niño changed cloud distribution and extent, causing a change in the local albedo. The authors suggest that the Pliocene climate could return if the increases in the carbon dioxide concentration continue and that this would lead to greater precipitation and temperatures at high latitudes and a deepening of the layer between surface and deep ocean waters.

Implications: The IPCC's Third Assessment Report suggests sea level rise would be a modest 9-88 cm with projected climate changes over the next 100 years. However, if Fedorov et al.'s analysis is correct, a more radical shift could occur --with the potential for constant El Niño conditions, massive ice melt causing 25 meters of sea level rise above today’s levels, and with drastic implications for low-lying ecosystems and peoples.

Vecchi et al. find that since the mid-19th century, the Walker Circulation (a component of tropical atmospheric circulation which cycles across the equatorial region of the Pacific Ocean) has weakened. The scientists attribute the weakening to human-induced climate forcing and suggest that the weakening may increase to roughly 10 percent by 2100. As ocean surface water warms, water vapor increases at a faster rate than precipitation, which impacts atmospheric circulation.

Implications: When the Walker Circulation is weakened, upwelling of nutrient-rich waters around the equator is dampened. Without these biologically rich waters, marine ecosystems could face a major transformation and a substantial loss of productivity. These changes would come on top of the already observed reductions in productivity measured in the Millennium Ecosystem Assessment of 2005, largely due to human activity in the oceans and along coastal areas.