WEEK 4: Climate History of Earth and Modern Climate Change
From Glacial to Interglacial
• Last 800,000 years have seen switch between interglacials and glacials, but last 400,000
years have been when this 100,000 year cycle most apparent
o Last interglacial 100,000 years ago when temperatures warmer than average
o Last glacial maximum likely one of the colder periods, with more ice
• As time goes on, we have more diverse proxy records, so have reasonable picture of
transition we saw
• Last glacial maximum (LGM) 20,000-25,000 years ago, change of
~4°C in global temperature
o Mid Holocene period sticks out due to relative stability
o We have records from LGM but don’t have a lot of them
o Over 10,000 years we have more records available, but large
decline before that
▪ So magnitude of changes observed over this period still
a bit of a work in progress
▪ Important seasonal component -> Certain proxies respond to summer vs
winter temperatures
o Called LGM because larger continental ice sheets were all at their maximums at
some point during this period
o At its maximum, the Laurentide ice sheet (22,000 BP) covered most of Canada
o Parts of Alaska and Yukon were ice free
o Sea level was lower by 100 metres and there was a land bridge connecting Alaska
and Russia (Beringia)
▪ Huge numbers of animals and humans gathered
▪ Allowed First Peoples to find their way across
• Modern world vs LGM
o Between combination of snow cover and ice, was pretty widespread across all of
Canada in LGM whereas now we only get annually averaged sea ice and some
caps throughout the Canadian north
o If you compare July air temperatures from today to then, ice surface of LGM
would have dramatically changed climate and ice sheets around so widespread,
much cooler conditions extending much further south
o If we visualize amount of ocean temperature between now and then, see largest
changes at high latitudes, especially at north Atlantic basin
▪ Just because we had widespread ice doesn’t mean temperatures at
equatorial zone were dramatically different
o Vegetation cover -> Currently have tundra in northern, boreal across most Canada
▪ During LGM pretty much no vegetation, but saw them developing a lot
south of where the ice sheets developed (in the US)
▪ Spruce pollen early on, moved to oak pollen recently
• Impacted atmospheric circulation with 2-3km thick ice sheets in some areas, shifting lots
of patterns across north
• Had similar amount of radiation coming in during that time as we do today, saw lower
carbon dioxide levels
• Having ice in northern hemisphere meant drawdown of carbon dioxide because of
regional cooling and cooling of oceans, up-took more carbon and methane, helped
transfer signal from northern to southern hemisphere
• Saw expanded ice in mountain environments and lower tree lines in southern hemisphere
• Younger Dryas
o Within what might have been as short as a single human lifetime, a huge swatch
of the earth fell back into an ice age
o Temperatures in some places dropped as much as 10°C and they stayed that way
+1,000 years
o Laurentide ice sheet (up to 3km thick) stretched all the way from Arctic to New
York -> Ice melted, collected in Lake Missoula, and Lake Agassiz
▪ At one point, Agassiz might have been size of black sea and covered parts
of North Dakota, Minnesota, Manitoba, Ontario, Saskatchewan
▪ Due to heat, dams in lake started to weaken -> Lake started to drain,
sending what may have been over 9,000 cubic km of water into ocean
• One study estimated if it took entire year to drain, flood would
have moved average of 300,000 cubic meters of water per second,
making flow more powerful than Amazon
o Atlantic Meridional Overturning Circulation (AMOC)
▪ Work like conveyor belt in ocean -> In tropics, water from depths rises to
surface and is warmed up
• Warm water moves northward in currents like Gulf Stream
• As it makes its way to the north, water loses some of its heat,
keeping a lot of places on the way warm
▪ Eventually cools down so much that some of it ends up frozen in ice,
leaving behind cold, saltier water, which is dense
• That water sinks to seafloor, heads south again, cycle repeats
o When current stopped, heat stopped, triggering cooling
▪ Huge swaths of northern hemisphere turned back into ice-age setting
▪ Onset associated with disappearance of Clovis culture
o AMOC still around and while there isn’t a single, giant lake pouting into it all at
once, scientists becoming increasingly worried the collective runoff from melting
ice will cause similar slowdown
• The Holocene (Interglacial Period)
o Key reason to look at last 10,000 years because we would’ve received most
sunlight associated with Milankovitch cycles, but have really seen a decline
o Last 12,000 years ago to present, climate has been fairly stable ±1°C
▪ Warmest period about 7,000 to 10,000 years ago (Holocene Thermal
Maximum – HTM)
▪ Coldest temperatures globally about 1-1.5°C colder than present, about
200 years ago during “Little Ice Age”
o Global average shows less variability than northern marine or areas in northern
Atlantic sea temperatures, or North America and Europe
▪ Fairly stable at low latitudes, relatively more change at high latitudes
o 8,000 years ago when receiving beak of sun, still had substantial body of ice
within interior of eastern Canada, especially over Hudson’s bay
▪ 7,000 years ago, more breakup over Hudson’s Bay, Laurentide ice sheet
broken up into 3 major nodes
▪ 5,000 years ago last remnants of Laurentide ice sheet in continental
Canada
o Quebec and Labrador really strong geomorphic impact of retreating ice sheet
o As consequence of ice melting, all kinds of glacial lakes
▪ Labrador and Quebec border, shorelines displayed quite clearly
o Last mammoth died only 3,5000 years ago in Russia
o Over time, modern sea levels have dramatically risen compared to drop that
occurred when Laurentide ice sheet present (100-120 metres less than what we
have today)
o Lots of shallow water areas today that would’ve been land at the time the sheet
was present
o Different areas experienced peak warmth with lots of variability, despite average
being fairly stable
▪ North Atlantic basin had really warm peaks that coincide with late period
o During HTM in Agassiz, had pretty warm temperature (above normal) but there’s
been long-term decline in temperatures since then, followed by really large spike
▪ Even though periods where it was really warm, really starting to get back
closer to those time periods
▪ Recent warming past decades at high latitude ice caps comparable to
things 6,000-8,000 years ago
• Highlights how unprecedented recent changes are
The Last Millennium
• Long period of relative stability from 0 to about 1000 years ago, then cool
period, then rapid warming
• Global rates of change – average trend lines over 50-year period with
black lines, pretty consistent until you get to the past 100 years where
there’s a rapid increase around x2-3 greater
• Northern hemisphere temperature change
o Region experienced a little bit more change compared to global
o Two relatively cool periods where they’re pretty much connected
o Most prominent warm change (Medieval Warm Period)
▪ Used widely as climate contrarians as evidence of previous
period of time where there was rapid warming and humans had
documentation -> Justify not understanding current science
▪ Also when there was viking expansion – westward expansion ended as a
result of changing climate leading to cooler period, not allowing for access
to resources
▪ Problem is when you look globally, you don’t see indication of this
• Can only see when you look specifically at northern hemisphere
• Dip in global graph coincides with Little Ice Age, especially prominent in north Atlantic
basin and Europe
o Occurred in different places at different times, but normally 1600 to 1580s were
relatively cooler, which led to expansion of glaciers in most alpine locations