Climate History Tour
Through Time
Earth’s Climate History
During its long history, Earth has experienced wide variations in climate from Ice Ages to
Hothouse. Fortunately for us, the range has never been large enough to exterminate or
incinerate life. What has caused the climate variations? There are several culprits. First,
is Solar Radiation (assuming Earth’s distance to the Sun has not changed). Theory
indicates that the Sun’s Irradiance has gradually been increasing over its long history
and was about 25 to 30% lower some 4.5 billion years ago. If there have been short
term variations, we know little about that. Today, solar irradiance varies by about 0.1%
and peaks when the Sunspot Cycle is at its maximum.
But if the young Sun was weaker, why wasn’t the young Earth encased in Ice? This is
called the Faint Young Sun Paradox. The Second Major Cause of Climate variations, is
due to changes in atmospheric composition via the Greenhouse Effect. Climate can vary
widely if the amount of Greenhouse gases varies. And there is plentiful evidence that the
amount of Greenhouse gases, most notably CO2 and CH4, have varied widely.
The third cause of Climate Change is due to changes in Earth’s geography, via Plate
Tectonics and Continental Drift. Land that moves away from the Equator gets drier at
first as it approaches the Subtropics and then markedly colder as it approaches the
Poles. The continents move so slowly that these changes take tens of millions of years.
The fourth cause of major climate change results from cyclic variations in Earth’s Orbit
and occurs on the time scale of tens to hundreds of thousands of years. This alters the
distribution of solar radiation over the seasons and with latitude.
Fifth is the impact of albedo. Lighter surfaces reflect more light and make climate cooler.
Permo-Carboniferous Ice Age
Major climate changes on Earth have occurred as continents moved from the equator
to the poles or vice – versa. Climate is warm and rainy near the equator where the Sun
is high in the sky and coldest near the poles where the Sun is low in the sky. Climate is
dry in the subtropical latitudes (between about 20° and 30° latitude) where the air
sinks. Therefore, tropical rainforests are located near the equator, most deserts in the
subtropics and ice sheets near the poles.
A great Ice Age occurred during Carboniferous and Permian Periods – from about 320
to 240 MBP - long before the Age of Dinosaurs. At the time the continents were largely
clustered together in a giant supercontinent called Pangaea. For the most part this
landmass was located much further south than today. Antarctica was close to where it
is now, centered over the South Pole, but South America, Africa, India and Australia
were all attached to it, and therefore much further south and nearer the Pole than now.
Late in the Devonian, another event combined with Continental Drift to start the PermoCarboniferous Ice Age. Giant forests spread over much of the land. During the
Carboniferous, wholesale burial of fallen trees in swamps accumulated to form the
world’s greatest coal deposits and remove vast quantities of CO2 from the atmosphere.
This weakened the Greenhouse Effect and led to the Ice Age.
The long Ice Age finally ended at the end of the Permian. A great outpouring of lava
and CO2 from rifts (and resulting forest and coal fires) in China (260 MBP) and Siberia
(251 MBP) produced a great warming that melted all the ice. It also ended deep ocean
circulation, depriving the oceans of O2, which led to the greatest mass extinction in
Earth history. This Extinction paved the way for the Mesozoic Age of Dinosaurs.
http://en.wikipedia.org/wiki/Paleoclimate
Ice Sheets and their Movement During the Carboniferous and Permian
Plate Tectonics
Animation of the
Moving Continents
over the past 200
million years
Europe
Greenland
Asia and
Siberia
North America
Subtropical Desert
China
Tropical Rainforest
Subtropical Desert
Arabia
South America
Africa
Polar Ice Cap
Antarctica
Madagascar
India
Australia
A map of the world early in the Permian, about 300 million years ago. Much land in the South
Hemisphere was covered by glaciers, which spread northward. Coal was produced in Equatorial
rainforests and in Temperate forests during the warmer "Interglacial" periods. Deserts occurred
in Canada, Scandinavia, and the Sahara, which were located between about 20 and 30° latitude.
Mid-Cretaceous Climate and Geography: A Hothouse
Mid-Cretaceous Hothouse
During the Mid-Cretaceous, the tropics were about 5ºC warmer than today but the high
latitudes were much warmer. This was not a time of major mountain building, so erosion
and burial of carbon was reduced. Rapid sea floor spreading increased volcanism,
raising CO2 levels to 4 - 10 times higher than now and widening the mid ocean ridges so
that the sea became shallower. This plus the absence of ice sheets raised sea level so
high that much land including the Great Plains was inundated and transformed into
broad, shallow seas such as the Tethys. The low albedo and high heat storage of the
widespread waters added to the warming, especially in winter. Dinosaurs migrated far
south to the polar regions of Antarctica and Australia or Canada. Tropical plants grew
and alligators cruised around Greenland and tropical reefs extended to at least 40º
latitude. Currents were weak in the warm oceans, so oxygen rich water did not sink, and
oxygen-poor black muds and shales
rich in organic matter accumulated.
The buried carbon taken from the
atmosphere helped cool the climate
late in the Cretaceous, but now
provides the world with much of its
oil and gas. The warm waters also
helped deposit in Europe’s shallow
seas the huge chalk deposits that
gave the Cretaceous its name.
http://www.soest.hawaii.edu/GG/FACULTY/POPP/Lecture9.ppt
Cenozoic Climate Overview
Zachos, et al. Trends, Rhythms in Cenozoic Climate, Science 292, pp 686-693 (2004)
The Cenozoic – Recent Ice Ages
A long period of global cooling preceded the recent Ice Ages. A permanent ice sheet
covered part of Antarctica by 34 MBP and ultimately covered East Antarctica by 11
MBP. Ice cover in West Antarctica experienced large oscillations from perhaps 5.5 to
2.8 MBP. Frigid Antarctica chilled the nearby waters which sunk to fill the deep oceans.
Deep ocean water cooled from about 15ºC to about 3ºC. This cooling set the stage for
the North Hemisphere ice sheets which began growing by 2.8 MBP. Ice Ages then
became somewhat periodic events, timed to cycles in Earth’s Orbit (see
ATM_CLIM_CYCLE.) Before 0.8 MBP they matched the 41,000 year cycle of obliquity.
Since then, there have been about 8 major ice ages that have more or less matched
the 100,000 year cycle of eccentricity. At the peak of these Ice Ages, ice sheets
between 2 and 3 km deep covered Canada, parts of the northern United States and
large sections of north Europe and Asia. Sea level dipped to  120 m lower than now. T
fell by less than 5ºC in the tropics but more than 10 or 15ºC in regions covered by ice.
Rapid and irregular swings of T occurred between warm and ice age conditions, often
within a few years. Warming and melting after each Ice Age was so rapid that
incredible flooding occurred and sea level rose up to 30 mm yr-1.
During Ice Age Peaks, the regime of Polar Climate expanded. This squeezed all other
climate zones toward the Equator. The subtropical deserts expanded and encroached
on much smaller tropical rain forests. In the frigid world, most climates were drier, but
because the jet stream and storm belt were centered over the Western United States,
precipitation and lake levels rose while temperature fell. The snow line lowered so that
many mountain glaciers expanded in the Himalayas, Alps, Andes, Rockies, etc.
Scenic Variety on the Antarctic Ice Sheet
Dome C (elevation 3222 m), one of the coldest places on
Earth. T hardly rises above −25°C in summer and can
fall below −80°C in winter. Annual average T = −54.5°C.
Today, despite the
spectre of Global
Warming, there is
still much ice
on Earth.
Year Round Ice Cover: Today
But there was a
time when ice
covered much
more of the
Earth.
Year Round Ice Cover: 20 KBP
http://www.museum.state.il.us/exhibits/ice_ages/index.html
Melting of the North American Ice Sheet
Postglacial Warming and the Younger Dryas Cold Spell
As the world began to emerge from the ice age, warming and melting accelerated to a
breathtaking pace about 15,000 years ago. Sea level rose more than 1 m (3.3 ft) per century on
average, with bursts above 4 m (13.3 ft) per century. As the climate warmed, snowfall actually
increased on the ice caps. Most other parts of the world also experienced wetter conditions. Lake
Bonneville (now the Great Salt Lake) expanded rapidly and by 15,000 years ago, was ten times
larger and 825 feet (250 m) higher than now. At that point it filled the Great Basin and
overflowed into the Snake River. At the same time, other lakes in the western United States such
as Lake Lahonton in Nevada also expanded and overflowed.
Around 14,500 years ago, colder and drier conditions returned. For the fifth time in 30,000 years,
huge masses of ice calved off the eastern end of the North American ice sheet around Labrador,
freshening the Atlantic and diverting its warm currents from the Arctic. But the ice sheets were
soon retreating again. By about 12,500 years ago almost half of Scandinavia was free of ice.
Then, a second event in North America gave the ice age its last hurrah.
Several enormous lakes were impounded at the southern fringe of the retreating North American
ice sheet. At first, overflow from these lakes poured down the Mississippi River into the Gulf of
Mexico. Around 12,500 years ago the ice sheet retreated far enough to reopen the St. Lawrence
River and the huge postglacial lakes quickly drained into the North Atlantic. This diverted the
North Atlantic’s warm currents from the Arctic once again and set off a 1000-year long cold, dry
spell called the Younger Dryas. As sea ice spread southwards, Greenland returned to a deep
freeze and the Scandinavian ice sheet advanced to the southern tip of Sweden. Water levels at
Lakes Magadi and Natron in Africa’s East Rift Valley, which had risen about 60 m (200 ft) in the
wake of the ice age, fell by 50 m (167 ft) and remained low throughout the Younger Dryas.
Then, in an abrupt turnaround, the climate warmed over a twenty year period from about 11,660
to 11,640 BP, and the Younger Dryas came to an end. Warming and melting continued for the
next 1500 years, after which global temperatures approached today’s values.
In the past 10,000 years most climate variations have been small compared to the wild ice-age
gyrations. But one more cold, dry episode that lasted about 400 years swept across the middle
latitudes about 8200 years ago, likely when some remaining ice age waters impounded in North
American lakes were released. At this time the Black Sea was separated from the ocean and
partly evaporated. When warming resumed, so did the rising of sea level. About 7600 years ago
water from the Mediterranean Sea poured into the Black Sea, causing an inundation that may
have given birth to the Biblical account of Noah’s flood.
The Altithermal and Wet Subtropics
On the Tassili plateau in the center of the Sahara desert, ancient paintings and etchings on the
rock walls of people herding cattle, raising grain and hunting hippopotamus from canoes speak of
much wetter times. Human testimony is not the only evidence that the subtropical deserts were
once much wetter. Fossils of fish abound, ancient beach terraces stand well above the level of
today’s shriveled or dried out desert lakes, and radar has probed beneath the shifting sands to
reveal buried drainage patterns of forgotten streams and rivers.
For most of the time from the end of the Younger Dryas until about 5000 years, the Sahara and
much of today’s subtropical deserts were fertile grasslands, which averaged about 4 inches of rain
in each of the summer months. A difference in the Earth’s orbit was the cause. Now, we are
closest to the sun on January 4, but 11,000 years ago we were closest at the beginning of July, and
the summer sunshine was 7% more intense than it is today. Ironically, as hot as the Sahara is
today, it is not quite hot enough for the air to rise and produce adequate rain, but up to about 5000
years ago it was. Urged by the stronger summer sun, the tropical rain belt and the summer
monsoon of Africa and Asia wandered further north than they do today, watering lands it has
since abandoned.
But by 5000 years ago the Earth did not approach the sun until after the summer monsoon had
ended. The monsoon weakened and the tropical rainbelt no longer moved quite as far north.
Rains began to fail across the Sahara, which began reverting to desert. Lake levels fell
precipitously and many lakes and smaller rivers dried up. The dessication forced most Saharan
residents to flee to places like Egypt with a guaranteed water supply from large rivers. It is no
coincidence that in the drying subtropical millenium from about 5000 to 4000 years ago Egypt
consolidated into a nation, the Akkadians and Sumerians clustered along the fertile floodplain of
the Tigris and Euprates rivers, and the highly urbanized and sophisticated Harappan civilization
sprang up on the fertile Indus River basin.
As the drying continued, droughts became more frequent and severe. A severe drought about
2200 BC may have caused the sudden collapse of the Akkadian Empire. A few centuries later the
Harappan civilization disappeared without any apparent violence other than drought and a change
of the course of the Indus River. The Bible also records how famine drove first Abraham and then
the sons of Jacob to Egypt for grain. By the time of Christ, perihelion had advanced to early
December, far too late to rouse summer rains. From the Atlas Mountains of Morocco to the Indus
River the desert had conquered.
The wetter period up to 5000 years ago was also the warmest time during the past 125,000 years.
The world of the early civilizations grew more than 1°C warmer than it is today. The tree line
across Europe and in the western United States was almost 200 m (660 ft) higher than now.
An Atlas of Ice Age Earth
http://www.esd.ornl.gov/projects/qen/nerc.html#maps
The Horned Goddess
Petroglyph
Aouanrhet, Tassili, Central
Sahara Desert ca 3500 BC
This painting describes a
wetter time in the middle of the
Sahara Desert from about
9000 to 5500 BP when wheat
was grown, cattle were raised,
and rain fell (note rainbow).
This wet period occurred
because the summer sun was
strong enough to make air in
the Sahara rise to produce
rain.
The recent, dramatic increase of CO2 is not matched by anything in the last
several million years on Earth. This has caused a similar sharp rise in global T.
Changes of CO2 and CH4 over the past 20,000 years
When did human impact on climate begin? Deforestation from the Agricultural Revolution
changed albedo and released CO2 and CH4 in the atmosphere, increasing the Greenhouse
Effect and raising global T. Did this prevent the Ice Age from returning?
Reforestation may have been one of the causes of the Little Ice Age. The Spaniards brought
diseases to the New World that reduced the population by 90 to 99%. That destroyed
agricultural societies and allowed the forests to grow back, reducing atmospheric CO2. Solar
irradiance was also lower as indicated by the absence of sunspots.
Global Temperature for the Last 1000 Years
A relatively warm period of Medieval Warmth was followed by a cold period called the
Little Ice Age. Since the decade of 1810 – 1820, and especially since 1980, the world
has experienced significant warming in conjunction with the increase of CO2 and CH4.
GLOBAL DIMMING
Global Dimming, the reduction
of solar irradiance at the Earth’s
surface due to the high albedo
of human-produced aerosols
has acted to cool climate. But
as coal burning has become
cleaner, soot has been reduced
(except in China) so that there
is now less dimming. Ironically,
this has increased the rate of
global warming.
Now look at China’s haze
and polluted waters
MODIS AQUA 4/7/2007
Over the past century, global T has risen 0.8C on average – about 0.35C from 1900 to
1940, followed by a decrease of 0.1C from 1940 to 1970 and since then a further
increase of 0.55C. Nightly minimum T has increased more sharply, so that the daily
range of T has decreased. T over land has increased somewhat more than over the
oceans, and especially at higher latitudes. T in the lowest 10 km has decreased even
more than at the surface but T in the lower stratosphere has decreased.
Accompanying observed increases in surface T, have been decreases in the length of
river and lake ice seasons and glacial mass and extent, melting of the Greenland Ice
Sheet, decreases of snow cover in many Northern Hemisphere regions, and of both ice
thickness and extent in the Arctic, especially in spring and summer; the oceans are
warming; and sea level is rising.
Relative humidity seems to be about the same, so that the temperature increase is
accompanied by an increase in the amount of water vapor in the atmosphere. No
noticeable changes have taken place in total global precipitation, but the distribution of
precipitation has changed. It appears to be concentrated in more intense storm events
and the mid-latitude storm belt has migrated poleward as the realm of polar air has
shrunk. This means that places on the poleward margins of the subtropical deserts have
dried while higher latitudes have become wetter and even snowier. Drought has become
more prevalent because while temperature and hence potential evaporation has
increased precipitation has not.
Several natural oscillations of atmospheric and oceanic temperatures and circulation
including El Niño, and the North Pacific and North Atlantic Oscillations are superimposed
on the secular trends and reduce our degree of certainty regarding the global changes.
Sea Level Rise and Polar Sea Ice
Over the past several decades the sea ice covering the Arctic Ocean has been melting.
It has thinned from an average thickness of 3 m to about 2 m and its area, particularly in
Summer has decreased markedly so that the famous Northwest Passage has finally
opened. The summer of 2008 has had the most dramatic melting by far. This greatly
exceeded any prediction and shocked the scientific community.
One of the gravest consequences of global warming is the rise of Sea Level. Sea level
has been rising, and at an increasing rate. The present rate is just over 3 mm per year.
Although sea level does not change when floating sea ice melts, sea level rises
when water gets warmer (because it expands) and when ice perched on land
melts. About half the rise of sea level is due to thermal expansion and half to melting of
ice sheets. The total possible sea level rise from melting of the Greenland and Antarctic
Ice Sheets is 64 m. Increasing T by 1C causes water to expand by  0.008%. When this
occurs through the  4000 m depth of the ocean, it will raise sea level by  0.32 m.
While floating sea ice does not raise sea level when it melts, it may indirectly allow a rise
of sea level because it may no longer hold back perched ice from sliding into the Sea.
One concern is that the melt water of the perched ice sheets, which forms shallow lakes
on the ice surface will lubricate the ice and allow large chunks to slide into the sea. Many
of these lakes (that form in summer) suddenly disappear by draining when they melt
their way clear through to the bottom of the ice sheet. No cataclysmic breakoff event has
occurred in the past century, but it remains a grave concern because of the sudden rise
of sea level such an event would produce.
Into the Future: Predicted Temperature Changes
NASA GISS Climate Model Simulation for 2xCO2.
http://data.giss.nasa.gov/efficacy/#table1
Computer Simulation of Melting of Greenland Ice Sheet with 4
times Preindustrial Level of Atmospheric CO2.
Here is a prediction of the almost unthinkable. If we continue rampantly burning fossil
fuels, CO2 and global temperatures will increase enough to melt the Greenland Ice
Sheet, although it will take almost 2000 years. That will submerge many places now just
barely above sea level. And melting in Antarctica will raise sea level even more.
So, as sea level rises, watch Florida disappear beneath the waves.
What will rising sea
level do to Florida?
00.0 m
10.0 m
20.0 m
At the present rate of sea
level rise (3 mm/yr) this
would take 10,000 years
30.0 m