Impact of the Atlantic Circulation Change on Terrestrial Ecosystems
As discussed above, changes in the ocean circulation affect temperature and precipitation worldwide. This has a direct impact on terrestrial ecosystems for which these two climate factors exert primary control on distribution of terrestrial vegetation and their productivity. Paleoclimate records reveal strong and rapid reorganizations of terrestrial ecosystems in response to climate changes. For example, Figure 3 shows a pollen record from southern Italy during the last glacial cycle. The record reveals numerous excursions apparently coeval with the changes recorded in Greenland, with an abrupt increase of woody pollen during Greenland warm events and its almost complete disappearance of during cold periods. Detailed analysis of different pollen species in this record indicates numerous transitions between forest and cold steep ecosystems during the glacial age. Similar changes in methane concentration shown in Figure 3 indicate abrupt and dramatic changes in the area and climate conditions of tropical and boreal wetlands synchronous with abrupt climate changes recorded in Greenland.
A strong response of terrestrial ecosystems to the reorganizations of the ocean circulation is also supported by results of modeling experiments performed both for present-day and glacial climate conditions. It was shown that a complete shutdown of the Atlantic thermohaline circulation would cause a pronounce impact on the distribution of ecosystems and their net primary production in different parts of the Earth. In the boreal latitudes of the Northern Hemisphere, the primary effect of change in the ocean circulation is a strong winter cooling and a reduction of the length of the growing season which results in a southward retreat of boreal forest area and shrinking of the area of temperate forest. In more southern locations, where the total amount of precipitation is the primary limiting factor, a southward shift of ITCZ and a weakening of summer monsoon lead to an expansion of Sahara desert, shrinking of the area of evergreen forest, and pronounced decline in productivity in certain areas. It was shown that if collapse of the Atlantic thermohaline circulation will occur under present-day climate conditions, it will result in substantial reduction of natural ecosystem biomass and net primary production. It would also have a dramatic impact on agricultural production, especially in the Western Europe and in some tropical areas.
Under the glacial climate conditions, in spite of the fact that a large portion of the most sensitive boreal zone was covered by the ice sheets, changes in the Atlantic thermohaline circulation still cause a considerable impact on ecosystem, their biomass, as well as soil carbon storage. Modeling results indicate that a reduction of terrestrial biomass might contribute to CO2 rises observed during Heinrich events, when the coldest climate conditions prevailed over the North Atlantic due to the collapse of the Atlantic thermohaline circulations.
The ocean circulation plays an important role in the ocean carbon cycle and affects marine biota. The most straightforward way of this influence is via changes in the horizontal and vertical transport of nutrients, which are the limiting factor for marine biota productivity over most of the globe. The highest productivity in the ocean apart from shelf areas is observed in the regions of strong vertical upwelling and the areas of deep convection. Both these mechanisms bring nutrients from the ocean interior to the surface layer, which is usually extremely depleted with nutrients. While strong upwelling in coastal and equatorial regions is caused primarily by surface wind, the areas of deep mixing and upwelling in the ocean interior are closely related to the thermohaline ocean circulation. This is why, changes in the ocean circulation directly, via changes in upwelling and mixing, and indirectly, via changes in surface wind, can affect marine ecosystems.
Paleoclimate data revealed strong variations in productivity in many locations, primarily in the North Atlantic, but also in Arabian Sea and coastal Pacific
Ocean areas, apparently synchronous with abrupt climate changes recorded in Greenland and attributed to the reorganizations of the ocean circulation. At the same time, paleoclimate data show an enhanced productivity in Iberian and North African coastal areas during cold events associated with enhanced coastal upwelling in these regions. Model simulations confirm that shutdown of the Atlantic thermohaline circulation can cause a strong and widespread decline of the marine ecosystem productivity. In particular, in the northern part of the North Atlantic, the collapse of the Atlantic thermohaline circulation leads to a decrease of planktonic biomass by factor of 2 while the globally averaged export production decreases by 20%. The main cause for such strong reduction of the North Atlantic plankton stock is a shoaling of wintertime mixed layer which reduces supply of nutrients from relatively nutrient-rich intermediate water masses to the nutrient-depleted surface ocean layer. For the rest of the globe, changes in marine biota production are caused by changes in upwelling and surface winds. Sea ice extension and surface cooling can also affect ocean productivity in the high latitudes. Modeling experiments also show that the response time of the marine ecosystem to changes in the Atlantic circulation is the shortest in the North Atlantic, while in the Pacific and Indian oceans the response time is order of centuries.
Even though a probability of a complete shutdown of the Atlantic thermohaline circulation in the future remains uncertain, a weakening of this circulation and a shoaling of mixed layer due to surface warming and freshening is the robust feature of majority of climate model simulations. This will inevitably lead to a decline of North Alantic plankton stock, which, in turn, will have a serious consequence for the fishery in these, currently relatively productive areas and, eventually, affects the food supply to the growing population of the planet.
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