Page 48 - (DK) Ocean - The Definitive Visual Guide

46 OCEAN GEOLOGY

Currents, Continents,

and Climate
Along with the atmosphere, the oceans are the means by
which heat is redistributed around the Earth. Most energy
arriving from the Sun is absorbed as heat near the Equator. It is
then redistributed to colder regions. About 40 per cent of the heat
reaching the poles from the Equator comes via ocean currents.
The pattern of circulation in the oceans therefore has a large
influence on the Earth’s climate (see pp.66–67). As continents,
oceans, and currents have shifted through geological time, major SNOWBALL EARTH
climate changes have occurred. Conversely, warmer and colder During snowball events,
periods affect sea level and the extent of seas. There is even global glaciation would
speculation that the ocean froze to a depth of 2,000m (6,500ft) have left only the peaks
of the highest mountains
in places during a series of “snowball” events 775–635 million years free of ice, as is the case
ago, and possibly earlier, each event lasting up to 15 million years. today in Antarctica.

MESOZOIC CURRENTS Greenhouse to Icehouse
100 million years ago,
ocean currents flowed During the Mesozoic Era (252–65 million years ago) the climate was warmer than it
through a continuous is today, with a more even temperature distribution and no polar ice caps. Ocean
seaway from the Tethys
Ocean in the east, currents freely flowed around the Equator, absorbing energy as they went, and carried
through what is now the heat to higher latitudes. The transition from this
Mediterranean, the “greenhouse” climate to today’s cooler “icehouse” is
Central Atlantic between due to shifts in ocean currents following the breakup
North and South
America, and into the of Gondwana. When the other continents moved
Pacific in the west. north, the Antarctic was surrounded by the Circum-
polar Current, blocking heat flow from the Equator.
TODAY’S CIRCULATION Equatorial flow between the oceans finally stopped
Today, equatorial ocean currents are when the Isthmus of Panama closed 5–3 million
blocked by landmasses, and the South
Circumpolar Current is the strongest years ago. Antarctica now lies over the South Pole,
current, blocking heat flow to the South allowing snow to accumulate into a thick ice cap,
Pole. The polar regions are colder. which reflects energy rather than absorbing it.
Beringia English Channel Gulf of Yellow Sea
land bridge land bridge Persia dry dry
Siberian LAST GLACIAL (21,500 YEARS AGO)
Greenland Ice Sheet
Ice Sheet Earth’s climate swings between ice ages and
Scandinavian warmer periods over cycles lasting 100,000
Laurentide
Cordilleran Ice Sheet years or more. Within ice ages, there are
Ice Sheet Ice Sheet
colder periods called glacials and warmer
periods called interglacials. During glacials
(the last of which peaked 21,500 years ago),
the world’s ice-sheets expand, lowering
global sea levels and revealing land bridges.

Sunda Sahul
Patagonian sea ice
Ice Sheet land bridge land bridge
Antarctic Ice Sheet

INTRODUCTION