Wolfgang H. Berger

Scripps Institution of Oceanography

Wolfgang H. Berger 博士

スクリップス海洋研究所

San Diego, California

　

　　W. H. Berger, Scripps Institution of Oceanography, UCSD, San Diego, USA Fish are an important food source. Why are the fish abundant in some places and not in others? Will the productivity of the ocean change as a result of global warming?

　　To understand how global warming will change the productivity of the ocean, we must understand the basic processes involved in generating productivity. The point is, fish depend on the food chain for growth, and hence on the presence of nutrients and sunlight which is necessary for microscopic algae to grow. Vast expanses of the ocean, covered by a nutrient-poor warm water layer, are deserts with very modest production.

　　Production is concentrated where cold, nutrient-rich water comes to the surface and mixes with warm water. This happens above all near the world's coastlines, and especially along the shores where eastern boundary currents flow. Here we have upwelling driven by trade winds. These regions are among the richest fishing grounds.

　　I propose that global warming will decrease the productivity of the ocean. The reason is that, on the whole, the low-production warm-water layers will expand and interfere with the upwelling of cold water. Also, the trade-winds will tend to get weaker, which will decrease the intensity of upwelling and mixing along the eastern shores of ocean basins, and also result in a decrease of equatorial upwelling. None of these predictions are for certain; in fact, the climate experts are divided in their opinions on the matter. Why then do I think this is a fairly safe prediction?

　　The reason is that the ocean has a history. We can study that history, and from this we can infer how the real existing ocean reacts to climate change. This can produce insights that go well beyond simulating ocean behavior using complex computer programs (which are useful in their own right). The foremost instrument for the study of ocean history is the Ocean Drilling Program using the drilling ship JOIDES Resolution. This is a program which has its beginnings in the 1960s, and has been one of the most important Earth science projects for decades. Many nations participate and Japan has been an important partner in carrying the project. The productivity of the ocean can be reconstructed for the last 100 million years or so. Microfossils provide a means by which to do this reconstruction.

　　For understanding the response of the ocean to climate change, and its role in change, the last 2 million years are of great interest, because we had large excursions in climate conditions during this period, also referred to as the "ice ages". The ice age fluctuations may be considered experiments on the ocean-climate system, for us to study. The evidence from the sedimentary record indicates that productivity was low whenever the Earth warmed, and was high when it cooled or was cold (that is, when ice mass on Earth was large).

　　Where productivity is high, in the coastal-ocean upwelling areas, the seafloor is relatively shallow, and so some of the organic matter produced can easily reach the seafloor and become deposited and buried. Thus, the regions with the most organic matter within the seafloor are not far below the shelf break. In many of these regions, gas is produced within the sediment, through fermentation and decay of organic matter. The products are carbon dioxide and methane. These gases tend to be trapped below layers of "clathrates". Clathrates are water ice with a cage structure allowing the inclusion of gas molecules. Methane clathrates are stable are low temperatures and high pressures. They are quite abundant in the continental margins, from the tropics to the Arctic Sea. In some areas, methane is seen to escape from the seafloor, either through cracks in the layer of ice, or because the ice is weakened through sliding or melting.

　　Should a situation arise where, through warming of the upper ocean, layers of methane-bearing ice are weakened, there is a chance that the rate of escape of methane will greatly increase. This would be unfortunate, because methane is a powerful greenhouse gas (much more so than carbon dioxide). The large-scale addition of methane to the atmosphere would result in increased warming, which could then lead to further release of methane.

　　The nations of the world are arguing what to do about global warming. Some are arguing that the problem is ill-defined or not urgent. It is likely that by the end of the present century we shall have answers to these questions and we shall know how the ocean's productivity changes in response to rapid warming.