Changes to major ocean currents

Changes to Major Ocean Currents

          Our oceans function because of the cycling of currents. These current systems are vital to the ecosystems and food chains in our oceans. Recent literature suggests that these current systems may be slowed as the planet warms, triggering massive changes in community dynamics in the oceans. Some suggest that these changes could be catastrophic for many ecosystems, as well as the people that depend on them. The purpose of this page is to inform and discuss on the current literature (and debate) concerning oceanic current models in response to global warming. As in my other tabs, I will discuss the evidence from the research, followed by a bit about the consequences to humans and biota on planet Earth.

A map showing the major oceanic currents of our planet. 

Research

          In the 1970s there began a major shift in climate that affected the Pacific Ocean surface temperatures and currents. Several water flows were seen to be affected, including the North Pacific, the Subarctic Pacific, the Kurishio-Oyashio current, the California Current, and the Peru Current. (Hayward, 1997). The most studied of these systems was the California Current, due to its proximity to a major area of scientific study. This current saw a rise in sea surface temperature (SST), sea level (which is a measure of calculating circulation), and an increase in water cycling from bottom to top (upwelling index). These ecosystems may be an indicator of what’s to come if our planet continues to warm.

The main (albeit somewhat complicated) hypothesis is this: human induced increases in CO₂ have warmed the atmosphere, thereby warming ocean surface temperatures, and henceforth increasing oceanic hydrological cycles (top to bottom swelling), which then weakens oceanic circulation. The reason this weakening effect is felt is due to the density differences in sea water as the planet warms. We know today that winds in the atmosphere are the primary driver of oceanic currents, and many climate models predict that ocean currents will diminish in magnitude as the planet warms.  But there are also scientists working on describing the possibility of an increase in current due to global warming (Toggweiler and Russel, 2008).  These scientists have been filtering through the known data surrounding the most recent global warming event: the end of the last ice age, 18,000 years ago.

During the melt-off of the last great ice age similar phenomenon to the oceanic weakening current hypothesis were occurring; warming planet, warming oceans, and greater upwelling events. However, Toggweiler and colleagues propose that changes in mid-level atmosphere created stronger winds that acted to increase ocean circulations. This could mean that our ocean’s currents could actually become more robust as the planet warms. It is still not completely predictable as to whether the changes in atmospheric winds will be enough to compensate for the increase in overturn and thereby be able to distribute absorbed heat throughout the ocean, minimizing impacts on currents. We have approximately 200 years to find out (Toggweiler and Russel, 2008). 

Consequences

          The consequences to oceanic current shifts could be severe. Many species depend on migration along oceanic currents as an integral part of their natural history. For example, in the Bering Sea, Walleye Pollock (Theragra chalcogramma) depend on oceanic currents to transport their offspring away from adult populations. This is helpful to the species because the adults will actually feed on their young. So a decrease in oceanic currents will cause an increase in intra-species cannibalism, and therefore a decrease in offspring survivorship (Walther et al. 2002). Other examples include zooplankton, who have seen sharp decreases in numbers in many areas since the 1970s, which may be correlated with oceanic current changes. As stated elsewhere on this website, phyto and zooplankton are the base of the oceanic food chains. If we lose them, we will see catastrophic losses in marine biodiversity.  One further example is the diminishing California kelp forests since the 1970s, which may also have associations to decreasing oceanic currents (Hayward, 1997).  A final example is the significant decline in Sardines (Sardinops sagax) and anchovy (Engraulis mordax) populations, both of which are on the bottom rungs of the food chain,  in the California basin. Current populations were measured against historic populations for 2000 years using anoxic sediment layers.

          The consequences to marine life will also have direct influence on human life, in dramatic ways. Much of the world’s humans depend on the oceans for food, and if the oceanic currents collapse we could end up with massive food shortages. As suggested above, we may not know for at least 200 years as to whether the changes in global temperature will cause long term decreases in oceanic current systems, but I don’t want to wait until it’s too late to find out.

Will increases in windy weather overcome the ocean's slowing currents due to CO2 emissions?