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NATS 101
LectureNatural Climate Variability
Artist’s rendition of snowball earth, 650 Mya
Perspective: Time Scales
13.7 Gya
4.6 Gya 2.1 Gya
3.5 Gya
65 Mya
Avg. human life span=0.15 s
21 s
http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-301Fall-2006/LectureNotes/index.htm
1 ly = 1016 mly=light year
13.7 Ga (+/- 1%)
http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-301Fall-2006/LectureNotes/index.htm
Perspective: Astronomic Space Scales
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.• We know that “a plethora of evidence exists that
indicates the climate of the Earth has changed.” What is that evidence? What is that evidence? How do we know what we know?How do we know what we know?
Causes of Climate Change
AstronomicalAstronomical
SurfaceSurface
CompositionComposition
Detecting Change With Proxies
Scientifically, the best way to detect change is to directly measure it. Data available last 100-200 yrs.
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
Detecting Change With Proxies
Another property/qty that is a function of property of interest.
The measured property is a PROXY for the one of interest.
Think approximate
Courtesy J. Thornton, U Wash
Record: 1000 ~ Present day“Length” of growing seasonGood versus stressed yearsMajor fires
Unlocking “Stored” Climate Change
• Modern Instrument Record
• Tree Rings
• Ice Cores
• Sediment cores
• Rock formations/typesCourtesy J. Thornton, U Wash
Record: ~ 1Mya to 20th centInert gases (CO2, CH4, N2O,…)Particulates (soot, ash, etc)Temperature??
Unlocking “Stored” Climate Change
• Modern Instrument Record
• Tree Rings
• Ice Cores
• Sediment cores
• Rock formations/typesCourtesy J. Thornton, U Wash
Record: ~ 200 MyaMicrofossils (ocean T),Volcanic glassOrganic detritusMagnetic pole location
Unlocking “Stored” Climate Change
• Modern Instrument Record
• Tree Rings
• Ice Cores
• Sediment cores
• Rock formations/types
Courtesy J. Thornton, U Wash
Unlocking “Stored” Climate Change
• Modern Instrument Record
• Tree Rings
• Ice Cores
• Sediment Cores
• Rock formations/types
Record: ~ 4.5 GyaGeologic formation Geochemical analysisMagnetic poleContinental LocationFossil record
Banded Iron Formations BIFs tied up oceanic O2 Prevented atmospheric O2 Date no later than ~2 GYA
18O/16O low
18O/16O high
18O/16O lower18O/16O lower still
Water Cycle – Water Isotope T Proxy
Courtesy J. Thornton, U Wash
T based on water isotope proxy
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
Courtesy J. Thornton, U Wash
Vostok Ice Core Record
T and GHG correlatedCausality?
O18 analysis of ocean sediments can be used to construct past
temperatures
http://en.wikipedia.org/wiki/%CE%9418O
Warm
Cold
600 Million Years of Climate
http://www.scotese.com/climate.htm
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.”
542 Mya
65 Mya
490 Mya
200 Mya
145 Mya
251 Mya
299 Mya
417 Mya
359 Mya
444 Mya
1.8 Mya
540 Mya of Climate Change from O18
http://en.wikipedia.org/wiki/Oxygen_isotope_ratio_cycle
http://www.snowballearth.org/images/geologic_column.gif
Snowball Earth!
http://nai.nasa.gov/newsletter/03182005/snowball.jpg
Snowball Earth: Some Evidence
dropstones
http://www-eps.harvard.edu/people/faculty/hoffman/Snowball-fig11.jpg
Basic physics are understood: Runaway ice-albedo feedback
How does earth thaw? CO2?
cap carbonates
http://si.wikipedia.org/wiki/%E0%B6%BB%E0%B7%96%E0%B6%B4%E0%B6%BA:Oxygen_atmosphere.png
Life is responsible for the “recent” rise of oxygen
Use of sunlight for metabolism. Oxygen is a byproduct.
Use of oxygen for metabolism. Water and CO2 are byproducts.
Multicell organisms
Cambrian explosion
Atmospheric constituents have changed radically through the ages
http://www.ozh2o.com/atmos.jpg
Dimmer Sun Brighter Sun
Long-Term Climate Change
250 million years ago, the world’s landmasses were joined together and formed a super continent termed Pangea.
As continents drifted apart to their present configuration, they moved into different latitude bands.
This altered prevailing winds and ocean currents.
NAE-A
AfSAIndia
NAIndiaAf
SA
E-A
AntAus
Ant
Aus
180 M BP Today Ahrens, Fig 13.6
Long-Term Climate Change• Circumpolar ocean
current formed around Antarctica 40-55 MY ago as Antarctica and Australia separated.
• Kept warm air from low latitudes from reaching 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
Extend of continental glaciers 18,000 years BP.
Sea level was 100-125 m lower than present.
Bering land bridge between Siberia and Alaska.
Aguado and Burt, Fig 16-4
http://en.wikipedia.org/wiki/Image:Recent_Sea_Level_Rise.png
SST 18,000 years BP
Much cooler over the North Atlantic Ocean.
Ocean currents were undoubtedly different.
North Atlantic Drift was probably much weaker.
18,000 BP TodayAhrens, Fig 13.2
Milankovitch Theory of Ice Ages• Attempts to explain ice
ages by variations in orbital parameters
• Three cycles:
Eccentricity (100,000 yrs)
Tilt (41,000 yrs)
Precession (23,000 yrs)• Changes the latitudinal
and seasonal distributions of solar radiation.
MilankovitchTheory
Change in daily solar radiation at top of atmosphere at June solstice
Changes as large as ~15% occur
Milankovitch Theory of Ice Ages• Ice ages occur
if summers are cool and less snow melts.
• Partially agrees with observations, but many questions unanswered.What caused the onset of the first Ice Age?
http://en.wikipedia.org/wiki/Milankovitch_cycles
Temperatures since the last Ice Age… the time that humans have
flourished
Younger-DryasRapid cooling at the start occurred over a period of 1,000 yrs. The abrupt warming at the end occurred in only a few yrs!
Glacial retreat Rapid melt
Glacial advance
Apline advance
Ahrens, Fig 13.3
FRESH WATER FRESH WATER
TO NORTH TO NORTH ATLANTICATLANTIC
As the ice sheets were retreating, a giant prehistoric lake formed called Lake Agassiz.The lake eventually broke through the ice sheet and drained into the North Atlantic.The freshening of the sea water shut down the Gulf Stream and the heat transport toward Europe, what happened then??
What caused the Younger Dryas?
Climate affects human societies
Temperatures for Europe during the last 1200 years.
Viking settlements lost in GreenlandViking colonization
in Greenland
Ahrens, Fig 13.4
Evidence of Climate Change
Surface temperatures based on meteorological observations.Is the warming of the past century due to human activities?
0.6oC warming past century
Ahrens, Fig 13.5
Anthropogenic warming?
Complexity of Climate System
The climate system involves numerous, interrelated components
Feedback Mechanisms
Examples of feedbacks in the Examples of feedbacks in the climate system with global climate system with global
warmingwarmingPositive feedback = Enhances the warming
Negative feedback = Mitigates the warming
IF YOU HAVE THE FOLLOWING:
MORE WATER VAPOR: Positive feedback because it’s a greenhouse gas.
LESS SEA ICE: Positive feedback because more solar radiation is absorbed instead of reflected.
MORE CLOUDS: Feedback can be positive or negative depending on the type of clouds. THIS IS STILL A BIG UNKNOWN…
HiGH CLOUDS: Positive feedbackLOW CLOUDS: Negative feedback
MORE AEROSOLS: Negative feedback because more solar radiation is reflected.
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
• There has been 1F warming during the past century, half of which has occurred during the past 30 years.
Key Points: Climate Change
• The climate system is very complex.
Contains hundreds of feedback mechanisms.
Feedbacks are not completely understood.
Biosphere-Atmosphere interactions.
Key Points: Climate Change
• Three general climate change mechanisms:1) Astronomical2) Atmospheric composition3) Earth’s surface
• Humans modifying 2 and 3 on global scale.• Are recent climate and weather changes
due to natural causes or human activities?Next Time
Assignment for Next Lecture Anthropogenic Climate Change
• Reading- Ahrens
4th: 373-399
5th: 383-409
• Homework11- D2L
4th-Pg. 399: 14.12, 15, 16, 19
5th-Pg. 412: 14.12, 15, 16, 19
Do Not Submit D2L