Chapter 19: Climate Change
From a perspective of millions of years, we are now in a cold era (ice age). During the
warmer periods, there was no ice in the poles! Sea level would be 213 ft higher than
now. The warmer eras were ~ 10°C warmer than now. (See Fig. 19.11)
Natural temperature variations are large over large timescales (5°C in 10,000 yrs) and
have a drastic affect on climate.
Measuring temperature variations:
1. Oxygen isotope method –
Stable isotopes of oxygen
Abundance
O 16
8
99.76%
O 17
8
0.038%
O 18
8
0.20%
Because O18 is heavier, it doesn’t evaporate as readily.
During cold eras,
ocean is low and has
increased O18
Glacier
Concentrated
O16
Ocean
Concentrated O18
2. Dendrochronology –
Look at tree rings to see temperature and precipitation history. Compare rings
hundreds of years ago in long–lived trees with rings of recent trees when climate
known.
3. Ocean sediment coresExistence of fossils of certain organisms can give temperature range.
Causes of Climate Change
•
Plate tectonics
Movement of oceanic plates cause volcanic eruptions (CO2, H2O, SO2) which can
influence climate. See fig 19.14 for eruption of Mt. Pinatubo. Influence is over
millions of years.
•
Variations in Earth’s orbit
Eccentricity – period ~ 100,000 yrs
Precession – period ~ 23,000 yrs
Obliquity – period ~ 41,000 yrs
(22° - 24.5°)
Milankovitch theory – supported by climate data
•
Variations in CO2 (fig 19.11) from analysis of trapped air bubbles.
Note: CO2 concentration now is 360 ppm! It was ~290 ppm 100 yrs ago
•
Atmospheric particles
Sulfates from pollution and phytoplankton - effect is to cool atmosphere
Volcanic eruptions (high in sulfur)(fig 19.14) – Little Ice Age correlates to acidic
ice uncovered from AD 1350 → 1700.
•
Variations in solar output
Sunspots – max every ~ 11 yrs
Magnetic cycle – 22 yr cycle
(Magnetic field reverses every ~11 yrs)
(See fig from paper – shown in class)
Almost no sunspots 1645 – 1715
Maunder minimum – corresponds to Little Ice Age.
CO2 and Global Warming
Now: CO2 is ~ 360ppm
This is expected to double within 100 yrs.
Most climate models predict doubling of CO2 will lead to warming of surface air of 1.5°
to 4.5°C (2.7 to 8.1°F).
Fig 19.18 – polar areas warm more than mid latitudes
Many scientists disagree even on the sign of changes because earth systems interact and
are complex.
For example:
Higher temperatures lead to more evaporation and more clouds.
Does increased cloudiness cause positive feedback (further warming) because
more IR radiation is absorbed and emitted back to earth.
Or
Negative feedback (cooling) because incident sunlight is reflected by the cloud
tops!
One problem is that many models predict the earth’s temperature should already have
increased by at least 1°C in the last 100 years, whereas the actual increase is roughly
0.5°C.
Consequences of global warming –
•
•
•
Rise in sea level of ~ 0.5 meter
More rain (globally)
Expanded forests and plant life
Page 525 – conclusions of IPCC – Intergovernmental Panel on Climate Change – read
these.