1. Chapter 13: The Earth’s Changing Climate
Climate change
Possible causes of climatic change
Global warming

2. The Earth’s Changing Climate
18,000 years ago
The earth’s climate is
always changing
18,000 years ago the
sea level was so low
that the English
Channel didn’t
even exist, and Bering land bridge did exist

3. Determining Past Climates
1) fossil evidence
2) Ocean floor sediment oxygen isotope ratio (of oxygen 18 over oxygen 16)
Isotopes are atoms whose nuclei have the same number of protons but different numbers of neutrons.
This ratio is higher during a colder climate (as the oceans contain less water)
3) dendrochronology- tree ring growth depends on temperature and precipitation over certain regions
Q: Which university established the dendrochronology field first?
a) U. of Arizona; b) Harvard; c) Stanford; d) Yale

4. Determining Past Climates
sediment cores: ocean, lake
ice cores: Antarctic, Greenland
borehole temperature profiles
Dating calcium carbonate layers of stalactites in caves
As the global warming debate has intensified in recent years, many methods of reconstructing past climates have undergone close scrutiny
Q: What is the main uncertainty of these proxy data in representing temperature?
a) relation is not linear
b) non-temperature effects are not easy to separate
c) correlation is not causality
d) all of them

5. Climate During Past 1000 Years and since 1860
Relative to
“Hockey Stick” graph
The global warming of the
past 100 years has not been constant
Q: is the temperature decrease from contradictory with global warming?
a) yes; b) no
Relative to

6. Climate Change and Feedback Mechanisms
Feedbacks cause climate changes to be either amplified (positive feedback) or reduced (negative feedback).
Q: The water vapor-greenhouse feedback is?
a) positive feedback; b) negative feedback
c) unknown
Q: Why is the snow-albedo feedback positive?
A: increasing T decreases snow cover; this decreases surface albedo and hence increases surface solar absorption; leading to further increasing T
Q: why is cloud feedback overall negative?
A: low clouds are negative by reflecting solar radiation; high clouds are positive just like greenhouse gases; overall low clouds are more dominant, leading to overall negative feedback

7. Possible Causes of Climate Change
external: changes in incoming solar radiation
external (e.g., volcano) or human (e.g., CO2): changes in the composition of the atmosphere
external (e.g., mountain uplifting) or human (e.g., land use in urban and agricultural areas): changes in the earth’s surface
Emissions of CO2 and other greenhouse gases are by no means the only way to change the climate.

8. Climate Change and Atmospheric Particles (decadal-century)
Q: why does volcano eruptions, rich in sulfur, cool the atmosphere in general?
A: sulfur aerosols reflect solar radiation; they also modify clouds by increasing the cloud’s reflectivity
Sulfate aerosols cool the climate and therefore counteract global warming to some extent.

9. Climate Change and Variations in Solar Output (decadal-century)
Sunspots are huge magnetic storms on the sun that show up as cooler (darker) regions on the sun’s surface
Q: why does solar radiation increases with the increase of sunspot numbers?
A: sunspots are cooler; but the surrounding margins of sunspots are brighter (i.e., hotter); overall effect is to increase solar radiation

10. Climate Change and Variations in the Earth’s Orbit (10K-100K years)
Milankovitch theory: - eccentricity: geometry of orbit (100K yr) - obliquity: tilt angle of axis (41K yr)
- precession: direction of axis (23K yr)
Min: 22o
Max: 24.5o

11. Climate Change, Plate Tectonics, and Mountain-Building (100M yr)
theory of plate tectonics
Landmasses 150M years ago Today
Earth’s land mass in 650 million years:

12. Q: what are the external causes of climate change?
a) solar radiation change; b) air composition change;
c) earth’s land mass change; d) all of them
Q: How do major volcanoes change surface temperature?
decrease; b) no change; c) increase
Q: Since the sunspot number is below average in 1986, the solar radiation that year is
a) below average; b) normal; c) above average

13. Warming is Unequivocal
Rising atmospheric temperature
Rising sea level
Reductions in NH snow cover
And oceans..
And upper atmosphere….

14. Human and Natural Drivers of Climate Change
CO2 is causing the bulk of the forcing
On average, it lives > 100 years in the atmosphere and therefore affects climate over long time scales.

15. Natural versus human-caused temperature increase
Climate models are needed:
aerosols; cloud; precipitation; land processes; ….

16. Understanding and Attributing Climate Change
Anthropogenic warming is likely discernible on all inhabited continents
Observed
Expected for all forcings
Natural forcing only

17. What’s in the pipeline and what could come
Warming will increase if GHG increase. If GHG were kept fixed at current levels, a committed 0.6°C of further warming would be expected by More warming would accompany more emission.
B1 and A1B have similar population projections – ca 7 billion at A2 population more than double at 15 billion. GDP/capita is 47k$, 75k$, 16k$ for B1, A1B, A2.
Relative to

18. A1B is a typical “business as usual” ( ) scenario: Global mean warming 2.8oC; Much of land area warms by ~3.5oC Arctic warms by ~7oC; would be less for less emission
Relative to Relative to