CLIMATE CHANGE

THE GREAT DEBATESession 6

HOLOCENE CLIMATE CHANGE

• The Holocene is generally taken to begin at about 12,000 BP, following the end of the Younger Dryas cold period we discussed last week

• The Holocene epoch has been a period of climatic stability compared with the Last Glacial that preceded it

• Nevertheless, even minor changes have had major impacts on human activities, both in prehistoric and historic times

The Younger Dryas was followed by rapid climatewarming with relatively stable temperatures duringthe last 10,000 years

HOLOCENE CHANGES

• The warmer temperatures caused rapid melting of the continental ice sheets in the Northern Hemisphere and rapid retreat of mountain glaciers elsewhere

• This led to a rapid rise in sea level until it reached its present level about 6,500 years ago. This had a big impact on coastal areas due to inundation and changing coastal currents causing localized climate changes

• Globally precipitation was much higher than it had been during the Last Glacial

SEA LEVEL CHANGES

• They caused many continental areas to be separated as islands

• Tasmania and New Guinea were separated from the Australian mainland

• Britain and Ireland were separated from Europe

• The land bridge between East Asia and Alaska was flooded

• Populations of humans, plants and animals became isolated

BLACK SEA CATASTROPHY• During the Last Glacial the Black Sea

had been cut off from the Mediterranean and had become a fresh-water lake – the Euxine Lake

• About 7500 years ago the rising Medi-terranean flooded back through the Bosporus into the lake causing a very rapid rise in water levels (80 meters) creating the Black Sea

• Human populations around the lake were displaced and the event probably gave rise to the story of Noah’s flood

HOLOCENE MAXIMUM• Minor climate changes are not

necessarily in phase between the two hemispheres

• In the Northern Hemisphere a warm period lasted from approx. 8000 to 4000 years ago. The Arctic was up to 3°C warmer than today with evidence from Greenland, Iceland and Canada

• The Sahara had a much wetter climate with rock paintings of animals and people found in areas that are now desert and have been for the last 4000 years

TASMANIAN HOLOCENE CLIMATE CHANGES

• In eastern Tasmania the climate was mild and wet during the early Holocene with evergreen beech (myrtle) growing in the Derwent Valley

• From 4500 to 3000 years ago the climate was dry and windy, fires were frequent and fossil dunes were being re-activated by the wind

MIDDLE EAST

• Early civilizations in the middle East were based on irrigation agriculture and their fortunes were strongly affected by periods of drought associated with climate cooling. Early cities in the Indus valley were abandoned about 4000 years ago

• Millennia later the Mayan Empire in Central America also reliant on irrigation agriculture collapsed in about 900 AD

ROMAN WARMING

• The Mediterranean climate warmed between 250 BC and 450 AD and gave rise to the flowering of the Roman Empire. Grapes were drown in Britain and olive trees in the Rhine Valley.

• The climate was wetter as well as warmer and agriculture thrived. North Africa became a breadbasket for the Romans

THE DARK AGES

• It was a time of sudden cooling that lasted from 530 to 900 AD. It caused food shortages which led to famine, war and disease

• The Black Sea froze in 800, 801 and 829 AD and ice formed on the Nile River

• There were large scale migrations of people. By 590 AD an estimated 25 million people in Europe had died of the plague

MEDIEVAL WARMING

• This occurred from 900 to 1300 AD. Summers were warmer and longer and crops were plentiful. The amount of agricultural land increased and extended to higher altitudes and latitudes.

• In the Middle East the Muslim Empire reached its zenith and the Vikings prospered in Scandinavia and colonized Iceland and Greenland and even reached North America

LITTLE ICE AGE• It lasted from 1300 to 1850 AD. It is a

misnomer because it was not an ice age or even a glacial but simply another period of unusually cool climatic conditions. Glaciers advanced all over the world.

• The Vikings were cut off from their colonies in Greenland and Iceland by sea ice and the Greenland population died out between 1420 and 1500 AD. Iceland’s population suffered famine

Little Ice Agetemperaturefluctuations(estimated) in Iceland from 950 to 1980 AD

LITTLE ICE AGE• After a brief warming during the first

half of the 16th Century, the second phase from 1550 to 1850 AD was even colder than the first phase.

• Temperatures in Europe were lower than at any other time since the Last Glacial. Much agricultural land at higher latitudes and higher altitudes was abandoned.

• Glacial ice in the Alps reached its maximum extent in the first half of the 19th Century

Holland in the Little Ice Age – a 17th CenturyPainting by Hendrik Averkamp

Little Ice Age winter scene in Holland paintedby Van der Keer. Source: Brian John in “TheWinters of the World”

Most recent temperature reconstruction is indark blue, the oldest is in red

RECENT WARMING• Since 1850 world climates have

warmed significantly and most glaciers are retreating rapidly

• The climate is now similar to what it was in the Roman and Medieval warm periods but probably not quite as warm as it was in the early Holocene

• The warming is generally blamed on the increase in greenhouse gases in the atmosphere but we must be aware that the recent warming is not unprecedented

Global Warming from 1880 to 2003

Athabasca Glacier in retreat, Canadian Rockies

Mueller Glacier in retreat, New Zealand

Retreating Hooker Glacier and meltwater lake