Principal indicators of past climates - Climate Change: Biological and Human Aspects - page 54

14

12

10

8

6

4

2

0

70 60 50 40 30 20 10 0

↑ ↑ ↑ ↑ ↑

First grasses, opening
of N. Atlantic and separation
of Australia/Antarctic

Initial uplift of Spread of C 4 Main uplift of Quaternary

Tibetan plateau plants Tibetan plateau glaciations

Millions of years ago

Fig. 2.3 Approximate(andwithcoarseresolution)Cenozoic-presentglobaltemperaturetrendscomparedto1990,as

estimatedfrom 18 OanalysisofcompositebenthicforamrecordsfromtheAtlanticDeepSeaDrillingProgram
sites.BasedondatafromMilleretal.(1987)andsimpliiedfromWillisandNiklas(2004)and
IntergovernmentalPanelonClimateChange(2007).Indicatedaresomeofthefactorsafectingtheglobal
climate.GrassesandC 4 plantsenabledenhancedphotosyntheticsequestrationofatmosphericcarbon.The
raisingoftheTibetanplateaunotonlyafectedthecontinentalAsianclimatebutalsocreatedaconsiderable
bodyofmid-latitudeicethatafectedplanetaryalbedo.(Milankovitchefectsoccurontooshortatimescaleto
bededucedinthiswayandresolvedoverthisperiodoftime.)

interglacial, and especially in glacials with their substantive ice caps. This applies
to all the Quaternary period of the past 2 million years. As noted, 18 O-containing
water molecules require more energy to evaporate from the oceans than 16 O water.
The principal thing here is that as the lighter 16 O evaporates preferentially from the
sea it tends to leave behind 18 O and so the sea becomes enriched in 18 Oastheice
caps grow. This is reflected in 18 O in forams and corals. Consequently, whereas 18 O
may be a reasonable palaeothermometer between the Eocene and Cretaceous (when
there were no ice caps), when there are ice caps this isotopic partitioning between
marine and terrestrial ice environments comes to the fore. At such times 18 Oinmarine
calcite is an indicator of terrestrial ice and, hence, global sea levels (as opposed to
temperature). The higher the concentration of marine calcite 18 O the greater the extent
of terrestrial ice and the lower the sea level. (As terrestrial ice caps - rich in lighter
isotopes - grow, accounting for more water in the biosphere, so the amount left for
the oceans decreases.) Intuition suggests that this indicator of ice volume is sort of
a palaeothermometer in itself, as one might reasonably suspect that when the planet
has large ice caps it must be cooler than when it does not. But this is not strictly
true. Ice caps do not come and go overnight. We see substantial ice caps today in
Antarctica and Greenland, on which snow falls even now in the middle of a warm
interglacial (even if around the margins of these caps there is melting). So, at times
when our planet (or in all probability any other Earth-like planet) does have ice caps,