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Weather, Climate & Society
ATMO 336
Natural Climate Variability
Perspective: Time Scales
4.6 b.y.
2.1 b.y.
13.7 b.y.
3.5 b.y.
65 m.y.
21 s
Avg. human life span=0.15 s
http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-301Fall-2006/LectureNotes/index.htm
Perspective: Astronomic Distance Scales
13.7 Ga (+/- 1%)
1 lt.y.=1016 m
http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-301Fall-2006/LectureNotes/index.htm
http://en.wikipedia.org/wiki/Earth
What is Climate Change?
• Climate change - A significant shift in the mean
state and event frequency of the atmosphere.
• Climate change is a normal component of the
Earth’s natural variability.
• Climate change occurs on all time and space scales.
• It was stated that “a plethora of evidence exists that
indicates the climate of the Earth has changed.”
What is that evidence?
How do we know what we know?
Determining the Past Climate
Paleoclimatology - the study of past climates.
• Past 100-200 years (weather observations)
• Must use indirect climate measures, proxies, to
examine further into the past. Some proxies:
- Tree rings (1,000+ years before present BP)
- Trapped pollen (10,000+ years BP)
- Glacial ice cores (100,000+ years BP)
- Ocean sediment cores (1 Million+ years BP)
- Geology (1 Billion+ years BP)
Detecting Change With Proxies
Scientifically, the best way to detect change is to
directly measure it.
Unfortunately for the timescales of interest in climate
science, we were not always able (interested in?) to
measure quantities such as temperature, precipitation,
wind speed, direction, greenhouse gas levels, etc.
But, obviously we want to know what these properties
were and how they changed in the past to test our
understanding of how climate changes.
The study of past climate is known as paleoclimate
science.
Courtesy J. Thornton, U Wash
Unlocking “Stored” Climate Change
• Modern Instrument
Record
• Tree Rings
• Ice Cores
• Sediment cores
• Rock formations/types
Record: 1000 ~ Present day
“Length” of growing season
Good versus stressed years
Major fires
Courtesy J. Thornton, U Wash
Unlocking “Stored” Climate Change
• Modern Instrument
Record
• Tree Rings
• Ice Cores
• Sediment cores
• Rock formations/types
Record: ~ 1Mya to ~ 20th cen
Inert gases (CO2, CH4, N2O,…)
Particulates (soot, ash, etc)
Temperature??
Courtesy J. Thornton, U Wash
Unlocking “Stored” Climate Change
• Modern Instrument Record
• Tree Rings
• Ice Cores
• Sediment cores
• Rock formations/types
Courtesy J. Thornton, U Wash
Record: ~ 200 Mya
Microfossils (ocean T),
Volcanic glass
Organic detritus
Magnetic pole location
Unlocking “Stored” Climate Change
• Modern
Instrument Record
• Tree Rings
• Ice Cores
• Sediment Cores
• Rock formations/types
Banded Iron Formations
BIFs tied up oceanic O2
Prevented atmospheric O2
Date no later than ~2 GYA
Record: ~ 4.5 Gya
Geologic formation
Geochemical analysis
Magnetic pole
Continental Location
Fossil record
Detecting Change With Proxies
Another property/qty that is a function of
property of interest.
Think approximate
The measured property is a PROXY for the one of interest.
Courtesy J. Thornton, U Wash
Water Cycle – Water Isotope T Proxy
18O/16O
18O/16O
18O/16O
lower
18O/16O
lower still
low
high
Courtesy J. Thornton, U Wash
Vostok Ice Core Record
Last Ice Age
During last ice age
(18,000 years ago)
Temps 6oC colder
CO2 levels 30% lower
CH4 levels 50% lower
(Sea level was higher)
than pre-industrial
interglacial values
T based on water
isotope proxy
Courtesy J. Thornton, U Wash
O18 analysis of ocean sediments can be
used to construct past temperatures
Cold
Warm
http://en.wikipedia.org/wiki/%CE%9418O
600 Million Years of Climate
The past climate of the Earth can be deduced “by mapping
the distribution of ancient coals, desert deposits, tropical
soils, salt deposits, glacial material, as well as the distribution
of plants and animals that are sensitive to climate, such as
alligators, palm trees & mangrove swamps.”
1.8 Mya
65 Mya
145 Mya
200 Mya
251 Mya
299 Mya
359 Mya
417 Mya
444 Mya
490 Mya
542 Mya
http://www.scotese.com/climate.htm
540 Mya of Climate Change from O18
http://en.wikipedia.org/wiki/Oxygen_isotope_ratio_cycle
Snowball Earth!
http://www.snowballearth.org/images/geologic_column.gif
Snowball Earth: Some Evidence
dropstones
http://www-eps.harvard.edu/people/faculty/hoffman/Snowball-fig11.jpg
http://nai.nasa.gov/newsletter/03182005/snowball.jpg
Basic physics are understood:
Runaway ice-albedo feedback
How does earth thaw? CO2?
Simple climate models exhibit hysteresis
between ice-no ice states
Delayed response between CO2 forcing and rapid
jumps between ice free and snowball earth.
http://www.snowballearth.org/slides/Ch7-6.gif
Snowball cycles show freeze-thaw-hothouse
http://www.snowballearth.org/images/snwbltvst.gif
Is snowball earth a mechanism for jump
starting evolutionary leaps?
Metazoa and plants appear after last episode
http://www.snowballearth.org/slides/Ch7-6.gif
Life is responsible for the “recent” rise of oxygen
Cambrian explosion
Multicell organisms
Use of oxygen for metabolism.
Water and CO2 are byproducts.
Use of sunlight for metabolism.
Oxygen is a byproduct.
http://si.wikipedia.org/wiki/%E0%B6%BB%E0%B7%96%E0%B6%B4%E0%B6%BA:Oxygen_atmosphere.png
Atmospheric constituents have changed
radically through the ages
http://www.ozh2o.com/atmos.jpg
Long-Term Climate Change
NA
E-A
SA Af
180 M BP
India
Aus
Ant
NA
E-A
Af India
SA
Today
Aus
Ant
Ahrens, Fig 13.6
250 million years ago, the world’s landmasses were joined
together and formed a super continent termed Pangea.
As today’s continents drifted apart, they moved into
different latitude bands.
This altered prevailing winds and ocean currents.
Long-Term Climate Change
• Circumpolar seaway
leads to large latitudinal
temperature gradient.
Current S.H. Situation
• Circumequatorial
seaway leads to small
latitudinal temperature
gradient.
Situation 50 MYa
http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html
Long-Term Climate Change
• Circumpolar ocean
current formed around
Antarctica 40-55 MY ago
once Antarctica and
Australia separated.
• This prevented warm air
from warmer latitudes to
penetrate into
Antarctica.
• Absence of warm air
accelerated growth of
the Antarctic ice sheet.
http://www.ace.mmu.ac.uk/eae/Climate_Change/Older/Continental_Drift.html
Most Recent Ice Age
Aguado and Burt, Fig 16-4
Extend of continental glaciers 18,000 years BP.
Sea level was 100-125 m lower than present.
Bering land bridge between Siberia and Alaska.
http://en.wikipedia.org/wiki/Image:Recent_Sea_Level_Rise.png
http://earthobservatory.nasa.gov/Study/BorealMigration/Images/pleistocene_modern.jpg
SST 18,000 years BP
18,000 BP
Ahrens, Fig 13.2
Much cooler over the North Atlantic Ocean.
Ocean currents were undoubtedly different.
North Atlantic Drift was probably much weaker.
Today
Temperatures since the last Ice Age…
the time that humans have flourished
Glacial
advance
Glacial retreat
Apline
advance
Rapid melt
Ahrens, Fig 13.3
Rapid warming occurred at end of Younger-Dryas period.
Ice cores indicate that Ice Age conditions ended in 3 years!
Climate affects human societies
Viking colonization
in Greenland
Viking settlements
lost in Greenland
Ahrens, Fig 13.4
Temperatures for Europe during the last 1200 years.
Evidence of Climate Change
0.6oC warming
past century
Ahrens, Fig 13.5
Surface temperatures based on meteorological observations.
Is the warming of the past century due to human activities?
Controversial “Hockey Stick”
Key Points: Climate Change
• Proxy data are used to infer the past climate.
• Data show that the Earth’s Climate
Has changed in the past
Is changing now
And will continue to change
• Key question is determining whether recent
changes are due to natural causes or man.
Key Points: Climate Change
• The climate system is very complex.
Contains hundreds of feedback mechanisms
Feedbacks are not completely understood.
• Three general climate change mechanisms:
Astronomical
Atmospheric composition
Earth’s surface