Heat

Observed Climate Changes and Impacts
Figure 10: Greenland surface melt measurements from three
satellites on July 8 (left panel) and July 12 (right panel), 2012.
Figure 11: Observed changes in ocean acidity (pH) compared to
concentration of carbon dioxide dissolved in seawater (p CO 2
) alongside
the atmospheric CO 2
record from 1956. A decrease in pH indicates an
increase in acidity.
Source: NOAA 2012, PMEL Carbon Program.
Source: NASA 2012.
The Greenland ice sheet’s increasing vulnerability to warming is
apparent in the trends and events reported here—the rapid growth
in melt area observed since the 1970s and the record surface melt
in early July 2012.
Ocean Acidification
The oceans play a major role as one of the Earth´s large CO 2
sinks.
As atmospheric CO 2
rises, the oceans absorb additional CO 2
in an
attempt to restore the balance between uptake and release at the
oceans’ surface. They have taken up approximately 25 percent of
anthropogenic CO 2
emissions in the period 2000–06 (Canadell et al.
2007). This directly impacts ocean biogeochemistry as CO 2
reacts
with water to eventually form a weak acid, resulting in what has
been termed “ocean acidification.” Indeed, such changes have been
observed in waters across the globe. For the period 1750–1994, a
decrease in surface pH 9 of 0.1 pH has been calculated (Figure 11),
which corresponds to a 30 percent increase in the concentration
of the hydrogen ion (H + ) in seawater (Raven 2005). Observed
increases in ocean acidity are more pronounced at higher latitudes
than in the tropics or subtropics (Bindoff et al. 2007).
Acidification of the world’s oceans because of increasing
atmospheric CO 2
concentration is, thus, one of the most tangible
consequences of CO 2
emissions and rising CO 2
concentration.
Ocean acidification is occurring and will continue to occur, in
the context of warming and a decrease in dissolved oxygen in the
world’s oceans. In the geological past, such observed changes
in pH have often been associated with large-scale extinction
events (Honisch et al. 2012). These changes in pH are projected
to increase in the future. The rate of changes in overall ocean
biogeochemistry currently observed and projected appears to
be unparalleled in Earth history (Caldeira and Wickett 2003;
Honisch et al. 2012).
Critically, the reaction of CO 2
with seawater reduces the
availability of carbonate ions that are used by various marine
biota for skeleton and shell formation in the form of calcium
carbonate (CaCO 3
). Surface waters are typically supersaturated
with aragonite (a mineral form of CaCO 3
), favoring the formation
of shells and skeletons. If saturation levels are below a value
of 1.0, the water is corrosive to pure aragonite and unprotected
aragonite shells (Feely, Sabine, Hernandez-Ayon, Ianson, and
Hales 2008). Because of anthropogenic CO 2
emissions, the levels
at which waters become undersaturated with respect to aragonite
have become shallower when compared to preindustrial levels.
Aragonite saturation depths have been calculated to be 100 to 200
m shallower in the Arabian Sea and Bay of Bengal, while in the
Pacific they are between 30 and 80 m shallower south of 38°S
and between 30 and 100 m north of 3°N (Feely et al. 2004). In
upwelling areas, which are often biologically highly productive,
undersaturation levels have been observed to be shallow enough
for corrosive waters to be upwelled intermittently to the surface.
9 Measure of acidity. Decreasing pH indicates increasing acidity and is on a logarithmic
scale; hence a small change in pH represents quite a large physical change.
11