Physical and chemical changes in the ocean over basin-wide zones and decadal or longer time-scales: perspectives on current and future conditions - Ocean Sustainability in the 21st Century - page 93

3.5 Biogeochemistry and atmospheric greenhouse gas levels
The ocean has a key role in global biogeochemical cycles, and the documented changes dis-
cussed above can affect the marine biogeochemical cycles of, for example, carbon, oxygen,
and nutrients. The ocean has always been a large reservoir for carbon dioxide, but follow-
ing the Industrial Revolution the net uptake (to 2011) is estimated to be ~157 GtC, 1 which
has led to a gradual acidification of the ocean. The fraction of carbon dioxide emissions
from fossil fuel burning and cement production that has be taken up by the ocean has de-
creased from ~(48 ± 9)% during 1800-1994, to ~(34 ± 6)% during 1990-1999, to ~(30 ±
7)% during the decade 2002-2011. 2 Prior to 1750, atmospheric CO 2 concentrations were
260-280 ppm for about 10,000 years. During recent ice ages, peak levels were 180 ppm,
during the warmest interglacial peaks, nearly 300 ppm, and during the Pliocene at about 3
million years ago, may have been around 400 ppm. In 2011 it passed 390 ppm (Le Quéré
et al ., 2012 ) . The estimated oceanic mean uptake per decade has been increasing: from 1.5
± 0.5 GtC/yr in the 1960s, to 2.5 ± 0.5 GtC/yr for 2002-2011 (Le Quéré et al ., 2012 ). The
tropical ocean is out-gassing CO 2 . The extra-tropical Northern Hemisphere Ocean is a net
sink and the Southern Ocean is a large sink, although there are (controversial) recent indic-
ations that it is nearing saturation (e.g. Le Quéré et al ., 2008 ) . The highest latitudes appear
to be essentially neutral. The warming of the ocean results in release of additional carbon
dioxide due to reducing solubility - a positive feedback.
A more sluggish ocean circulation and increased density stratification due to surface
warming, both expected in a warmer climate, will slow down the vertical transport of car-
bon, alkalinity, and nutrients as well as the replenishment of subsurface waters that have not
been in contact with the atmosphere. This provides a positive feedback to the atmospheric
GHG concentration. Changes in the ocean circulation can also affect the regional circulation
of shelf and coastal seas, leading to either an increased export of nutrients plus carbon from
shallow seas into the open ocean, or to increased upwelling of nutrients plus carbon onto
the shelf areas and coastal seas. The physical bottleneck feedback mechanism dominates the