Unit3 Lecture2

ES 1111: 

ES 1111 Chapter 4: Evidence of Climate Change

Looking into the Past: 

Looking into the Past Must be careful with past information – there could be explanations other than climate change! In 17th & 18th centuries, the river Thames in London froze in winter Much colder winters? Other explanation: River was able to freeze more readily back then? (1) Old London bridge acted to slow river flow down (2) Lack of embankments back then (river wider) (3) Lack of waste heat from industrial plants Weather records show that winters back then were 1º C cooler

Constructing Past Climates: 

Constructing Past Climates Proxy measurements must be used to construct a picture of past climate Plate tectonics (folding, faulting) and erosion complicates the picture Much of the record is confused or destroyed (90-99% eroded) Little remains to tell us about the climate over the first 90% of Earth’s lifetime Don’t know arrangement of continents and oceans Don’t know atmospheric composition precisely First sedimentary rocks currently observed: 3,700 million years old First signs of life: 3,800 million years ago (provide little evidence of climate – bacteria)

Ice Age: 

Ice Age Times in Earth’s history when ice covered a large part of the Earth’s surface First proposed by Swiss naturalist Louis Agassiz “Erratics” – rocks found in unusual areas Agassiz found blocks of granite transported 100 kilometers from Alps

From 2.7 to 1.8 Billion Years Ago: 

From 2.7 to 1.8 Billion Years Ago “Snowball Earth”: Widespread, global glacial conditions Evidence of glaciers in parts of Africa located near the Equator at the time 3 discrete glaciations found in Wyoming between 2.5 and 2.2 billion years ago Glacial rocks from this period are then covered by carbonates and magma

Sudden Shift in Climate: 

Sudden Shift in Climate Catastrophic event caused increase in CO2 Earth remained free of ice caps for 1 billion years Snowball Earth appeared again – several ice ages observed following warm-up At end of Precambrian – dramatic evolutionary changes in species found “Cambrian explosion” – after last ice age in this period, acceleration in evolution observed

The Big Five: 

The Big Five Five clearly established events of mass extinction Over last 600 million years, 99.9% of all species have died Now: 1 million species, 75% insects

100 Million Years Ago: 

100 Million Years Ago Warmest climate period supported by proxy data 6º C to 12º C warmer than present day Configuration of continents played a role: seaway at Equator Today – circumpolar ocean current around Antarctica makes Antarctica colder than the Arctic

Changes in Earth’s Surface: 

Changes in Earth’s Surface Figure 4.7, Page 83

End of the Mesozoic: 

End of the Mesozoic Sudden cooling observed One of the “Big Five” Dinosaurs extinct

Pleistocene: 

Pleistocene Repeated ice ages – 32% of Earth covered by ice Evidence of 7 glacial periods Each glacial period occurs every 100,000 years Each glacial period had substantial fluctuations in climate – from extreme cold to near interglacial warmth. Fluctuation in climate dominated by cycles of 21,000, 41,000, and 100,000 years. From sediment/ice core data – changes can be sudden!

Heinrich Layers: 

Heinrich Layers Heinrich layers – sediment layers found in ocean cores thought to be produced by debris carried out to sea by icebergs (melting, then deposition) Vital question – how influx of freshwater can alter deepwater circulation and therefore climate Similar to the scenario in “The Day After Tomorrow”

End of Pleistocene: 

End of Pleistocene Last ice age reached greatest extent about 18,000 years ago Ice sheet 3 kilometers thick as far south as Great Lakes Total ice volume of 84-98,000,000 km3 (30 million today) Sea level lower by 90-120 meters Global average temperatures 5º C colder than now Dramatic warming started 15,000 years ago Rapid change in circulation pattern seen (dust) Conclusion: Climate is capable of sudden, large shifts

The Holocene: 

The Holocene Recent 10,000 years Warm, stable interglacial period Extraordinary quiet phase compared to earlier eras

6,000 Years Ago: 

6,000 Years Ago Laurentide ice sheet disappeared Peak in post-glacial warming Temperatures 2-3º C warmer than now Ancient trees found farther north than trees exist today

5,500 Years Ago: 

5,500 Years Ago Cooler and Drier conditions Historical records can be used now Desiccation of Sahara – decline in rain 4,000 years ago Tree line retreats southward Mountain glaciers growing

9th and 10th Centuries: 

9th and 10th Centuries Warming in Europe and North Atlantic Vikings settle Greenland Limited geographical coverage of records makes it difficult to say global climate was warmer

16th to 19th Centuries: 

16th to 19th Centuries Cooler period Firmer foundation of evidence Best known example of climate variability in recorded history Glaciers expanded Cool summers, severe winters Not a period of sustained cold “Little Ice Age” concentrated in winter half

Since Late-19th Century: 

Since Late-19th Century Global warming Evidence comes primarily from instruments Annual global surface temperatures have risen 0.62º C – slightly greater in Southern Hemisphere Warmest years in the record have all fallen in 1990s Warmest 20-year periods: 1925-1944, 1978-1997 Slight cooling in-between these years

Global Warming: 

Global Warming Warming seems to be due to decrease in areas affected by exceptionally cold temperatures To a lesser extent, global warming is due to increases in exceptionally warm temperatures In recent decades – night minimum temperatures have increased more rapidly than daytime maximum temperatures Diurnal temperature range has decreased 0.08º C per decade Most pronounced warming – northern continents Marked cooling – NW Atlantic Ocean, lesser amount in north central Pacific Changes appear most clearly in winter Warming trend of 40s in higher latitudes of northern hemisphere – southern warmed less

Consequences of Global Warming: 

Consequences of Global Warming Rise in global temperatures are only part of the concern In terms of economic impact, increases in extreme weather events would be the most damaging consequence Climatologists have concluded for recent decades thus far: No consistent trend in interannual temperature variability No consistent pattern for rainfall variability No pattern for intense rainfall or extratropical cyclones Decrease in tropical cyclones in north Atlantic (elsewhere not as reliable)

Climate Change: 

Climate Change No shortage of evidence of climate change in every time scale Evidence shows climate can change suddenly and rapidly naturally Scale of these changes had major impacts the world over Complete picture of the size of these changes, and where/when they took place is not possible (uncertainty increases as we go back in time) More questions remain than answers Accept anthropogenic global warming as real until new evidence/data becomes available?