Current warming and likely future impacts - Climate Change: Biological and Human Aspects - page 352

salmon ( Oncorhynchus ), mackerel (family Scombridae), herring ( Clupea ), cod (fam-
ily Gadidae) and baleen whales (Cetacea of the suborder Mysticeti). In the Ross
Sea, for example, the prominent sub-polar pteropod, Limacina helicina , sometimes
replaces krill as the dominant zooplankton. Indeed, in this sea pteropods account for
the majority of the carbonate and organic carbon-export flux. If pteropods cannot
grow their shells then they are unlikely to survive. If carbonate and organic carbon
fluxes are altered then this could also conceivably have climatic repercussions.
Calcite is another form of calcium carbonate that is a little more stable than arag-
onite. According to the ocean-chemistry models that the researchers developed from
the IS92a scenario, calcite undersaturation will lag that for aragonite by some 50-100
years. Species that use the calcite form of calcium carbonate include Foraminifera
and coccolithophorids. Moreover, some Antarctic and Arctic species use magnesium
calcite, which can be more soluble than aragonite. These include sea urchins (echino-
derms). Although all these species may survive in warmer latitudes, nearer the poles
ecosystems are likely to be severely disrupted. Further, should acidification continue
well beyond this century then oceanic refugia for vulnerable species will become
more scarce and the likelihood of an extinction event more probable. Finally, the
IPCC 2007 assessment notes that the net effect on the biological cycling of increased
ocean acidity is not well understood.

6.6.8 Climatethresholds

Sudden jumps in the global climate, or regional conditions and processes, at first
might seem surprising given that the IPCC forecasts depict gradual change. Yet the
IPCC do warn of surprises and sudden 6 jumps can take place as the biosphere system,
or components thereof, crosses a critical transition. In such circumstances the Earth
system, or components thereof, can be said to have passed a critical threshold and
entered a new state. A good example of a climate threshold at the global scale and
associated critical transition is that of those between the current Quaternary Earth's
glacial and interglacial states (see Chapter 4); the last of these occurred before
10 000 years ago when in a few thousand years the Earth system flipped from the
glacial mode and crossed the climate threshold into the Holocene interglacial mode
(section 4.6.3) that we have today.
The phenomena of critical transitions may seem new but readers will undoubtedly
have come across them without realising many times before. For example, supply
heat at a steady rate to water and the temperature rises in a near-linear way. But
continue to supply heat and suddenly the system flips to a new state: the water boils.
Indeed, at the point near boiling it takes extra energy (latent heat) to get the water to
actually vaporise. The water has reached a critical threshold and is transitioning to a
new state.

6

'Sudden' might be a year in which a region's seasonal rains, which had failed periodically, fail and then
do not return in subsequent years. It might be just a few decades within a century of warming regarding
the disappearance of end-summer Arctic ice. Sudden might also be in the geological sense of a few
centuries or even a few thousand years (a very brief time, geologically speaking) but involve a major
change to the entire global system.