ern Hemisphere summer. About 10,000 years ago the amount
of solar radiation during the summer was approximately 8
percent higher than it is today, because the Northern Hemi-
sphere summer solstice occurred at the point when the earth
is closest to the sun rather than at the point when the earth is
farthest from the sun (as is the case today).
EFFECT OF THE INTERTROPICAL CONVERGENCE
ZONE ON AFRICAN CLIMATE
Th e rainfall that is driven by increased solar radiation (and
temperatures) is associated with a zone near the equator
called the Intertropical Convergence Zone. Africa’s global
position during the past 10,000 years (straddling the equator)
places much of the continent between the northern Tropic of
Cancer and the southern Tropic of Capricorn, fi rmly within
the Intertropical Convergence Zone, the intersection between
two of the earth’s major surface wind systems: the southern
and northern trade winds. Th ese two major winds systems
are known as the Southeast Trade Winds (also referred to as
the southern monsoon winds), which travel from west to east
and pick up moisture as they move over the Atlantic Ocean,
and the Northeast Trade Winds, which travel down from the
northwest across the northern Atlantic Ocean. Th e intersec-
tion of these wind systems is associated with a wide swath of
low pressure that is driven by solar heating. In the zone in
which these trade winds come together, the low pressure lift s
the moist air around the equator, which results in abundant
rainfall when the moist air cools as it is lift ed higher in the
atmosphere and is forced to release moisture. Th e rain associ-
ated with the Intertropical Convergence Zone falls over Af-
rica near the equator as the winds move east across the Sahel,
just north of the equator and just south of the Sahara.
Th e location of the low pressure that is associated with
the Intertropical Convergence Zone corresponds approxi-
mately to the latitude at which sunlight falls perpendicularly
to the land’s surface (when the sun is directly overhead, a
time also known as the equinox). Since the latitude at which
the sun strikes perpendicularly changes seasonally, so does
the positioning of the Intertropical Convergence Zone and
therefore the region of Africa that receives monsoonal rain-
fall. In modern times in the Northern Hemisphere summer,
the latitude at which sunlight strikes the earth perpendicu-
larly is at or around the Tropic of Cancer (23.5 degrees north
of the equator); conversely, the low pressure associated with
the intersection of the trade winds is found at or around the
Tropic of Capricorn (23.5 degrees south of the equator) in the
Southern Hemisphere summer. Th e north-south oscillation
of the Intertropical Convergence Zone is primarily driven by
the tilt of the earth and the seasons. Since this zone and the
low pressure and rains associated with it move north to south
twice in a year, the regions surrounding the equator in Africa
experience two rainy seasons each year.
Since the swath of the Intertropical Convergence Zone
and the intensity of its rainfall over Africa have varied over
time, the landscape in this area has also varied. Relative dry-
ness or wetness shaped the ecological landscape, supporting
expansion of forest areas (which require more moist condi-
tions) in the areas of wetter periods and expansion of more
drought-resistant open vegetation, such as grasslands or des-
ert ecosystems, during drier periods. Th e border between
the Sahara (arid desert) and the Sahel (semiarid savanna and
grasslands) has shift ed north and south over time as a result
of either more or less rainfall, which is associated with more
or less solar radiation reaching the earth.IMPACT OF THE HUMID PERIOD ON
AFRICAN ECOSYSTEMS
About 10,000 years ago, increased moisture from the Atlantic
monsoonal rains (partly attributed to increased solar radia-
tion in the Intertropical Convergence Zone) created tropical
rainfall conditions in parts of the Sahel and Sahara in north-
ern Africa. Some estimates place rainfall up to 50 percent
higher in selected areas of Africa compared with that in mod-
ern times. Evidence shows a northward movement of vegeta-
tion from the Sahel into the Sahara and an expansion and
thickening of forests, such as the Zambezi woodlands and the
area around Lake Victoria (East Africa) during this humid
period. Areas of the Sahara were even fl ooded at times. Th e
rain forests of Ghana expanded from 9,000 to 15,000 years ago
(a ft er having completely dried out during the last ice age).
Lake-bottom sediments used for researching past cli-
mate change show that lake levels were high in central and
eastern Africa, including those of water bodies in Kenya’s
Rift Valley and the upper part of the White Nile River, which
originates from Lake Victoria (in East Africa), and the Ni-
ger River, inner Niger delta, the Senegal rivers (all in West
Africa). Lake Chad, which during high times borders pres-
ent-day Niger, Chad, Nigeria, and Cameroon (western Af-
rica), occupies a very shallow basin, allowing for enormous
variability in the area covered by water with changing depth.
Th is lake is believed to have covered approximately 186,400
to 248,550 square miles 6,000 to 12,000 years ago (compared
with 885 square miles in 2003). During this humid period ar-
eas of today’s more arid Sahara were more densely populated,
as people (and animals) moved out of the fl ooded areas of the
inner Sahel into the more moderately wet and hospitable re-
gions of the outer Sahel and farther north in the present-day
Sahara regions.
Pollen samples from ice or sediment cores provide valu-
able evidence of plant type and volume and can paint a pic-
ture of vegetative cover and rainfall for a particular moment
in history. Such samples from sediments in the dry desert re-
gion of eastern Libya have confi rmed the presence of Ty p h a
and Cyperaceae (wetland plant families) about 9,700 years
ago. Paleoclimatologists use the properties of these and other
plants and animals from the present day to draw conclusions
about past climate. For example, we know from present-day
species of Ty p h a and Cyperaceae that they are wetland plants;
however, little is known about how ancient vegetation (those
species that do not exist today) resisted drought or fl ooding.climate and geography: Africa 237