The Oligocene to the Quaternary: climate and biology - Climate Change: Biological and Human Aspects - page 141

(a)

(b)

90 ° 80 ° 70 ° 60 ° 50 ° 40 ° W

Carlaco basin

Trade winds

10 °

Weakened
north-east trade wind

September-ITCZ

September-ITCZ

ODP 942C

0 °

March-ITCZ

GeoB core
3104-1

Brazil

0 °

Atlantic
Ocean

10

°

Salar de Uyuni
Lake Titicaca

Brazil

FORTALEZA

20

° RECIFE

Sao Francisco River

Trade winds

10 °

30 °

SALVADOR

Pacific Ocean

March-ITCZ

Atlantic Ocean

40 °

Prevailing wind direction
5 00 k m

20 °

Polar
advections

50 ° S

RIO DE JANEIRO

Strengthened
south-east trade wind

Amazon
rainforest

Atlantic
rainforest

Caatinga Cerrado/Savanna

200 km

50 ° 40 °

Fig. 4.9 TheIntertropicalConvergenceZone(ITCZ)andkeyareasofSouthAmericantropicalforest.Wetperiods

coincidentwithHeinricheventsenabledtheAmazonandAtlanticforeststoperiodicallyjoinupinthedepthof
glacials.ReproducedwithpermissionfromWangetal.(2004).

carried by water in a cave but also sometimes associated with hot springs). Layers
in the deposits were accurately dated using uranium and thorium isotopes and this
showed periods of time when there was considerable rain (pluvials) and other peri-
ods when it was dry. They found that these pluvials coincided with Heinrich events.
During these pluvials the two tropical rainforests, Amazonian and Atlantic, were able
to merge. This merging of the forests enabled species (and gene) transfer between the
two zones (Figure 4.9).
Changes in the Broecker thermohaline conveyor are not just important in that it
helps explain the drunken walk of global climate change during the last glacial but,
as we shall see later (Chapter 6), it is of relevance to present-day concerns about
anthropogenic climate change.
The second major factor affecting temperatures during a glacial arises from the
concentration of greenhouse gases. As noted, methane effectively tracks glacial-
interglacial temperature largely due to the global extent of wetlands. These decline
as the planet cools and the atmosphere becomes less moist. Atmospheric moisture
(water vapour) itself is a greenhouse gas and so a cooler, hence drier, atmosphere holds
less moisture and also exerts less greenhouse forcing of climate. Both atmospheric
moisture and the extent of wetlands (affecting methane release into the atmosphere)
are further examples of positive-feedback phenomena, noted in section 1.5.
Another source of methane comes from methane hydrates (or methane clathrates as
they are sometimes known) that exist in considerable quantities in permafrosts and in