1.2 Long Term Climate Change

1.2 Long Term Climate Change

1. Medieval Warm Period

   • A time of warm climate in the North Atlantic region, lasting from about AD 800 - 1300
   • Vikings took advantage of ice-free seas to colonise Greenland and other outlying lands of the far north
   • Prolonged droughts affected many parts of the western United States and especially eastern California and the western Great Basin
   • The climate in equatorial east Africa has alternated between drier than today, and relatively wet
   • The drier climate took place during the MWP

2. The Little Ice Age

   • A time of cooler climate in most parts of the world. Although there is some disagreement about exactly when the little ice age started, records suggested the temperatures began cooling around 1250AD. The coldest time was during the 16th and 17th centuries. By 1850 the climate began to warm.
   • Temperatures were 1-1.5 degree celsius cooler than they were today.
   • Fur trappers reported that southern hudson bay remained frozen for about 3 weeks longer each spring.
   • Fishermen reported large amounts of sea ice floating in the North Atlantic.
   • Alpine glaciers grew larger. In some cases, the ice engulfed mountain villages.
   • Winters were longer and growing seasons shorter according to tree ring data and records of cherry tree flowering.
   • Limited crops and unhealthy livestock caused famine in areas of Northern and Eastern Europe. Unlike today, there was no way to transport food around the world to areas where crops had failed and people were hungry.
   • Farms and villages in Northern Europe were deserted because the farmers couldn't grow crops in the cooler climate
   • Wet weather caused disease that affected people, animals and crops including the bubonic plague (black death). This disease killed more than a third of Europeans.

3. Evidence

   • Greenland and Antarctic Ice Cores
     • Cores removed from ice sheets reveal layers going down through the ice. Each layer records a season of snowfall, buried and compressed by later falls.
     • The 3,200 metre East Antarctic ice core records the climate of the past 800,000 years.
     • Air bubbles trapped in the ice contain atmospheric carbon dioxide. Low concentrations of CO2 occur during glacial periods and high concentrations in interglacials.
   • Oxygen Isotope Record
     • Oxygen isotopes are sampled from ice cores and ocean sediment cores.
     • The ratio of O16 to O18 isotopes is a good indicator of past sea levels.
     • During glacial periods, O16 was evaporated more easily from the oceans which became enriched with O18.
     • Ice from glacial periods is enriched with O16.
   • Pollen Record
     • Pollen, extracted from sediment cores in peat bogs, records the ecology of the past.
     • Pollen sequences show that ecosystems have changed in the past in response to climate change.
     • In the UK, tundra ecosystems were present in the past glacial periods, whereas forest gradually colonised areas as interglacial periods developed.