1. Weather, Climate & Society ATMO 336 Natural Climate Variability

2. Perspective: Time Scales
4.6 b.y.
2.1 b.y.
13.7 b.y.
3.5 b.y.
Origin of the Universe--Jan. 1
Origin of our galaxy--Jan 24
Solar system origin--Sept. 9
Earth Solidifies--Sept. 14
Life on Earth--Sept. 30
Sexual reproduction advent--Nov. 25
Oxygen atmosphere--Dec. 1
Cambrian explosion (600 mil years ago when most complex organisms appear, fish, trilobites)--Dec. 17
Land plants & insects--Dec. 19, 20
First amphibians--Dec. 22
First reptiles & trees--Dec. 23
First dinosaurs--Dec. 25
KT impact, mammal age, birds--10:00 AM Dec. 30
First primates--Dec. 30
Australopithicenes (Lucy, etc.)--10:00 PM Dec. 31
Homo habilis--11:25 PM Dec. 31
Homo erectus--11:40 PM Dec. 31
Early Homo sapiens--11:50 PM Dec. 31
Neanderthal man--11:57 PM Dec. 31
Cro-Magnon man--11:58:38 PM
Homo sapiens sapiens--11:58:57 PM Dec. 31
Human history--11:59:39 PM
Ancient Greeks to now--last five seconds
Average human life span seconds
65 m.y.
21 s
Avg. human life span=0.15 s

3. Perspective: Astronomic Distance Scales
13.7 Ga (+/- 1%)
1 lt.y.=1016 m

5. 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?

6. Determining the Past Climate
Paleoclimatology - the study of past climates.
Past 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)

7. 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

8. Unlocking “Stored” Climate Change
Record: 1000 ~ Present day
“Length” of growing season
Good versus stressed years
Major fires
Modern Instrument
Record
Tree Rings
Ice Cores
Sediment cores
Rock formations/types
Courtesy J. Thornton, U Wash

9. Unlocking “Stored” Climate Change
Record: ~ 1Mya to ~ 20th cent
Inert gases (CO2, CH4, N2O,…)
Particulates (soot, ash, etc)
Temperature??
Modern Instrument
Record
Tree Rings
Ice Cores
Sediment cores
Rock formations/types
Courtesy J. Thornton, U Wash

10. Unlocking “Stored” Climate Change
Record: ~ 200 Mya
Microfossils (ocean T),
Volcanic glass
Organic detritus
Magnetic pole location
Modern Instrument Record
Tree Rings
Ice Cores
Sediment cores
Rock formations/types
Courtesy J. Thornton, U Wash

11. 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

12. 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

13. Water Cycle – Water Isotope T Proxy
18O/16O lower still
18O/16O lower
18O/16O low
18O/16O high
Courtesy J. Thornton, U Wash

14. Courtesy J. Thornton, U Wash
Vostok Ice Core Record
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
Last Ice Age
T based on water isotope proxy
Courtesy J. Thornton, U Wash

15. O18 analysis of ocean sediments can be used to construct past temperatures
Cold
Warm

16. 600 Million Years of Climate
542 Mya
490 Mya
444 Mya
417 Mya
359 Mya
299 Mya
251 Mya
200 Mya
145 Mya
65 Mya
1.8 Mya
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.”

17. 540 Mya of Climate Change from O18

18. Snowball Earth!

19. Snowball Earth: Some Evidence
dropstones
Basic physics are understood: Runaway ice-albedo feedback
How does earth thaw? CO2?

20. Simple climate models exhibit hysteresis between ice-no ice states
Delayed response between CO2 forcing and rapid jumps between ice free and snowball earth.

21. Snowball cycles show freeze-thaw-hothouse

22. Is snowball earth a mechanism for jump starting evolutionary leaps?
Metazoa and plants appear after last episode

23. 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.

24. Atmospheric constituents have changed radically through the ages

25. Long-Term Climate ChangeNA
E-A Af SA
India
Ant
Aus
180 M BP
Today
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.

26. 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

27. Long-Term Climate Change
Circumpolar ocean current formed around Antarctica 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.

28. Most Recent Ice Age Extend of continental glaciers 18,000 years BP.
Aguado and Burt, Fig 16-4
Extend of continental glaciers 18,000 years BP.
Sea level was m lower than present.
Bering land bridge between Siberia and Alaska.

30. http://earthobservatory. nasa

31. SST 18,000 years BP Much cooler over the North Atlantic Ocean.
Today
Ahrens, Fig 13.2
Much cooler over the North Atlantic Ocean.
Ocean currents were undoubtedly different.
North Atlantic Drift was probably much weaker.

32. Temperatures since the last Ice Age… the time that humans have flourished
Glacial advance
Apline advance
Glacial retreat
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!

33. Climate affects human societies
Viking settlements lost in Greenland
Viking colonization in Greenland
Ahrens, Fig 13.4
Temperatures for Europe during the last 1200 years.

34. 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?

35. Controversial “Hockey Stick”

36. Causes of Climate Change
Atmospheric Composition - Anything that changes the radiative properties of the atmosphere (volcanic aerosols, carbon dioxide).
Astronomical - Anything that alters the amount or distribution of solar energy intercepted by the Earth (solar variations, orbital variations).
Earth’s Surface - Anything that alters the flow of energy at the Earth's surface or changes its distribution (desertification, continental drift).

37. Causes of Climate Change
Astronomical
Composition
Surface

38. 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.

39. Milankovitch Theory of Ice Ages
Ice ages occur when summers are cool to melt less snow.
Partially agrees with observations, but many questions unanswered.
What caused the onset of the first Ice Age?

40. Milankovitch Theory
Change in daily solar radiation at top of atmosphere at June solstice
Changes as large as ~15% occur

41. Complexity of Climate System
The climate system involves numerous, interrelated components.

42. Closer Look at Climate System

43. Climate Feedback Mechanisms

44. Positive and Negative Feedbacks
Assume that the Earth is warming.
- Warming leads to more evaporation from oceans, which increases water vapor in atmosphere.
-More water vapor increases absorption of IR, which strengthens the greenhouse effect.
-This raises temperatures further, which leads to more evaporation, more water vapor, warming…
“Runaway Greenhouse Effect”
Positive Feedback Mechanism

45. Positive and Negative Feedbacks
Again assume that the Earth is warming.
- Suppose as the atmosphere warms and moistens, more low clouds form.
- More low clouds reflect more solar radiation, which decreases solar heating at the surface.
- This slows the warming, which would counteract a runaway greenhouse effect on Earth.
Negative Feedback Mechanism

46. Positive and Negative Feedbacks
Atmosphere has a numerous checks and balances that counteract climate changes.
All feedback mechanisms…
Operate simultaneously.
Work in both directions.
The dominant effect is difficult to predict.
Cause and effect is very difficult to prove at the “beyond a shadow of a doubt” level.

47. 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.

48. 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