Climate Change in the last 1000 years
Major (?) Events
•1000 – 1300: Medieval Warm Period
•1400 – 1850: Little Ice Age
•1850 – present: Significant warming
First 2 events are not
necessarily global,
but the recent
warming definitely is
The Little Ice Age
•Most significant in the North Atlantic and Europe
•Not a “true Ice Age” – no major ice sheets developed,
but locally glaciers advanced
The Little Ice Age - Baffin Island
•Lichen – plants that grow on and
“digest” rocks
•Slow growers, but long lived
•Don’t mind the cold, but they
must have light
•Can be dated using 14C
•Halo on Baffin Island rocks show
that old lichens grew (MWP) and
then died (LIA), now growing
again
•Conclusion, semi permanent
snow/ice covered the area during
the LIA
The Little Ice Age - Greenland
•Norse settlements on the S.
and SE coasts from ~800until
1400
The Little Ice Age - Iceland
•Fisherman have kept detailed records of ice
conditions for over 1000 years
The Little Ice Age - Europe
•During the LIA farming zones were
forced to move southerward
•Alpine glaciers moved down valley
and destroyed farms and homes
•Lakes and rivers are depicted in
paintings as frozen over, but never
do so in modern times
LIA in other parts of the world: 1
Mountain Glacier Ice Cores:
δ18O from Greenland and Antarctica are excellent sources of climate
information at high latitudes. We also need proxies from low latitude.
•Quelccaya ice cap, Peru: δ18O “kind of” indicates a cold period the 16th through
19th centuries with warming in the 20th century. Original cores from 70’s & 80’s;
A return expedition in the 90’s found that the ice cap is now melting!
•For 500 years this glacier accumulated layers, now it is melting????
•Dundee ice cap, Tibet: δ18O similarly “kind of” shows post-medieval cooling and
recent warming.
•20th century warming is most dramatic in 12,000 years.
•Although these mid-latitude glaciers sort of “see”Little Ice Age cooling, it is not
well recorded in either Greenland or Antarctic ice.
•So the Little Ice Age does not appear to be a truly global event.
•What all cores show is dramatic 20th century warming – A GLOBAL EVENT.
LIA in other parts of the world: 2
Tree Rings
•Trees have annual growth rings.
•Width/density/color depends on temperature and rainfall,
especially in spring/early summer.
•Trees near the edge of their geographic range are most
susceptible to variation in climate.
•CO2 fertilization in recent years may effect interpretation
•Arctic & Central Asian studies: Tree ring studies from around the
arctic and also tree ring studies from Mongolia indicate generally
cooler temperatures but with intervals of warm during the 17th
through 19th centuries.
•Southern hemisphere studies: Tree ring studies in Tasmania show
no sign of a Little Ice Age.
•So again, the Little Ice Age appears not to be a global event.
Records from the Ocean
•Coral show growth rings just like trees
•δ18O in corals and ocean sediments reflects
ocean temperatures
•Problem – changes in circulation – El Nino
•El Nino – reversal in equatorial flow from the
normal east to west to west to east
•Normally warm and wet in northern
Australia. Cooler, drier and upwelling in
western South America
•Upwelling means good fishing
•El Nino - reverse all this: SA becomes warm
and raining with bad fishing and increase in
diseases
•El Nino affects areas far beyond the tropical
Pacific – Wetter CA and maybe few Atlantic
Hurricanes.
•Bottomline – Ocean temperature changes may
not reflect global temperature changes, just
circulation changes
Human Records
•Bloom time of the trees and flowers
•First and last frost dates
•Sea ice conditions in high latitudes
•Artist depictions
•etc
•These are not as good as thermometers,
barometers, and anemometers, but if it all you
have…….
Reconstructing temperature trends
Various Northern Hemisphere proxies (S. Hemi – too little data to really
reconstruct)
Observations:
•Error range is LARGE
•Gradual cooling between 1000 AD and 1800 AD
•They all data sets mark a dramatic rise in the last 200 years
Causes of Climate Change in the last
1000 years: 1
•Orbital forcing
•Summer insolation has been decreasing in N.
Hemisphere
•Can account for 20 – 30% of cooling
•Millennial Scale Bipolar Seesaw
•Seemed important during glacial times, but we are
in an interglacial
•Maybe it could account for N. Hemi cooling, but
not enough S. Hemi data to compare
Causes of Climate Change in
the last 1000 years: 2
Sun Spots – Relatively cool places on the
Sun’s surface
•11 year cycle
•Occasional periods low activity
•At best could explain 10%
Causes of Climate Change in the last
1000 years: 3
•Volcanic Eruptions
•Cause global cooling due to dust blocking the sun
•Certainly have weather effects, and maybe climate
effect, if lots of them erupt in the same region over a
span of years
Causes of Climate Change in the last 1000
years: Greenhouse Gases
•Ice Cores indicate that CO2 has
varied during the last 1000 years
•However, dating of bubbles in ice
this you is difficult due to
circulation of air in young snow
•Slightly higher levels early in the
millennium
•Falling to a low point by the 1600
– 1800’s
•Controls of this variation
•Humans? – When people
died, CO2 decreased
Summary of Climate Change in the last
1000 years