Global warming and the oceans

Warming of ocean is three dimensional process

Historical trend has been rising sea levels

Thermal expansion of ocean also contributes significantly to sea level rise

Thermal expansion likely to persist for

centuries

Although global trend is for rising sea levels, variability does occur

Some areas of the ocean have risen more than others

Predictions of sea level rise also dependent upon emissions scenarios

Upper limits to sea level rise

• High level of uncertainty for upper range of sea level rise

• Uncertainty arises because dynamics of melting ice are poorly known

• Sea levels could rise as much as 1-1.5 meters by 2100 according to some models

• The last time the polar regions were significantly warmer than present for an extended period (about 125,000 years ago), sea levels were 4-6 meters higher

Meridional overturning circulation (MOC)

Return to glacial conditions in Europe?

• Slow down of MOC known as Dansgaard-Oeschger events

• Precedents in past (most recently 8200 years ago)• Sequence of events

– Increased melting in Greenland– Higher precipitation and runoff into North Atlantic– Freshening of northern Atlantic and Arctic Ocean– Sinking of cold salty waters stops– North Atlantic drift slows transport of warm water– Europe no longer warmed by this current and turns cold

Return to glacial conditions in Europe?

Agulhas Current may be contributing more warm water to North Atlantic drift and

thereby strengthen MOC

Coral bleaching

• Caused by prolonged high sea surface temperatures. At high temps:– Zooxanthellae (photosynthetic algae) in coral

decrease production of photosynthate for coral animal• These changes result in the expulsion of

zooxanthellae from coral polyps• Corals deprived of color and food, decline and

death

bleaching

Ocean acidification• As CO2 is absorbed, the ocean’s

pH decreases, resulting in what is known as ocean acidification.

• Slightly lower pH in the ocean will prevent many organisms from secreting carbonate out of sea water

• Carbonate is the substance that comprises the skeletons and shells for a variety of marine organisms

Ocean acidification

• Occurred in past (Paleocene-Eocene Thermal Maximum)

• Areas where acidic waters form naturally will be the first location to exhibit acidification effects

• Cold bottom water is rich in CO2 and lower in pH

• Where it upwells it can bring low pH waters closer to the surface

Temperature determines CO2 absorbtion, pH of sea water, and availability of carbonate for organisms

Cold ocean water in Pacific causes lower pH in ocean water. The lack of this deep cold water current on the Atlantic side and the overall warmer temperatures create higher pH.

• Color key– White: coral skeleton– Medium gray: pores within coral

skeleton– Dark gray: inorganic precipitation of

aragonite, the form of carbonate in the skeleton of corals

• From A to C shows decreasing formation of aragonite and an overall lower amount of interskeletal cementation

Other organisms impacted by acidification

• Coralline algae• Mollusks (bivalves and gastropods)• Echinoderms: sea cucumbers, starfish and sea

urchins• Sponges (those with carbonate spicules)• Foraminifera and coccolithophores (have

tests, shells, of calcium carbonate.• Pteropods

Scanning electron microscopy (SEM) photographs of coccolithophores under different CO2 concentrations. A – C represent low CO2 conditionsD – F represent enriched CO2 conditions