Download Climate Drivers

Climate Drivers
Global Change Ecology
Botany 275
External and Internal
Drivers of Climate
External Drivers
-Sunspot Cycles
-Orbital Variations
Internal Drivers
-Plate Tectonics
-Volcanic Activity
-Albedo
-Greenhouse Effect
Climate forcing mechanisms
Mechanism
1. Solar Forcing
Solar intensity (sunspots)
Orbital Variations
2. Plate Tectonics
Time period
(10 s to 100 s of years)
(Thousands of years)
(Millions of years)
Mountain building, continent locations
3. Albedo
4. Aerosols
(all time scales)
(1-10 years)
Volcanoes, pollution
5. Greenhouse Effect
(all time scales)
CO2, Methane, Water vapor
6. Land use
(1 to 100 s of years)
Climate Drivers
Incoming solar energy
Radiative
Forcing of Climate Change
Measured
in units of watts per square meter (watts/m2)
Average solar radiation reaching the earth at the top of
the atmosphere=1370 watts/m2
This equates to 343 watts/m2 when distributed
uniformly over the earth s surface.
For reference: A doubling of CO2 from pre-industrial
level of 280 ppmv to 560 ppmv results in radiative
forcing of about 4 watts/m2,
Estimated solar irradiance variations
1750-2000
One way to measure
solar intensity is from
satellite observations,
which are available only
since the late 1970 s.
These show solar
variations of about 0.2
watts/m2. This graph
shows estimates of
changes in solar output
since 1750.
Figure 2.17
Figure 2.16
Sunspots
Number of sunspots
Sunspot Cycles
Maunder
Climate Drivers
Orbital Variations or Milankovitch
Cycles
Milutin Milankovitch 1879-1958
http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/
Current Eccentricity
Variation in Axial Obliquity, 40,000 year cycle
Tilt of the axis
http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/
Precession of the equinoxes, ~19,000 to 23,000 year
Cycle: Direction of tilt Wobbling top
http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/
Orbital Variations:
Milankovitch Cycles
Orbital Eccentricity. Shape of the
Earth s orbit (cycles ~100,000
years) – changes the distance
between the Earth and Sun
Axial tilt (cycles over 41,000
years) – changes noon day Sun
elevation and daylength
Precession of the equinoxes
(cycles over 19,000-23,000 years)
– changes when winter and
summer occur on Earth.
Insolation at 65 degrees north latitude
from the present to 1 million years ago
Berger 1991
Vostok time series and insolation
Atm
Temp
Question:
1.  How do Milankovitch cycles (e.g.,
changes in orbital parameters) lead
to the onset and termination of ice
ages? (e.g., warm summers & cold winters,
cool summers and warm winters, etc.)
2.  Propose a sequence of events or
processes considering that the
overall change in insolation is small
(e.g., ~0.25 watts/m2), but that the
ice ages are global.
GISP2
(N. Atlantic SSTs)
-35
per mille
VSMOW
Methane
700
-45
ppbv
275
ppmv
CO2
400
Sea-Level
(Ice Volume)
100
m
175
50
warm
0
July (45 deg. N)
Insolation
W/m2
cold
-50
25
20
January (45 deg. N)
15
10
5
0
Calendarof
ka years ago
Thousands
GISP2
(N. Atlantic SSTs)
-35
per mille
VSMOW
Methane
700
-45
ppbv
275
ppmv
CO2
400
100
Sea-Level
(Ice Volume)
Small ice sheets
m
175
50
July (45 deg. N)
0
Large ice sheets
Insolation
W/m2
January (45 deg. N)
-50
25
20
15
10
5
0
Calendarof
ka years ago
Thousands
GISP2
(N. Atlantic SSTs)
-35
per mille
VSMOW
Methane
700
-45
ppbv
275
ppmv
CO2 (from ice cores)
High CO2
400
Sea-Level
(Ice Volume)
100
m
Low CO2
175
50
July (45 deg. N)
0
Insolation
W/m2
January (45 deg. N)
-50
25
20
15
10
5
0
Calendarof
ka years ago
Thousands
GISP2
(Oxygen isotopesMeasures air
temperature)
warm
-35
per mille
VSMOW
Methane
cold
700
-45
ppbv
275
ppmv
CO2 (from ice cores)
400
Sea-Level
(Ice Volume)
100
m
175
50
July (45 deg. N)
0
Insolation
W/m2
January (45 deg. N)
-50
25
20
15
10
5
0
Calendarof
ka years ago
Thousands
Milankovitch Theory of Ice Ages
The Milankovitch (1941) theory of the ice ages assumes that
summer insolation anomalies at high latitudes in the Northern
Hemisphere (NH) drive the ice ages: minimum summer
insolation allows snow and ice accumulated in the cold
season to survive, while maximum summer insolation tends
to melt the ice sheets.
Hansen et al. 2007. Climate change and trace gases. Phil. Trans. R. Soc. A. 1925-1954.
Alternative Theory of Ice Ages
Hansen et al. 2007 suggest that spring is the critical season for
terminations, because the albedo feedback works via the large
change in absorbed sunlight that begins once the ice/snow
surface becomes wet, after which the surface albedo remains
low until thick fresh snow accumulates. A spring maximum of
insolation anomaly pushes the first melt earlier in the year,
without comparable shortening of autumn melt, thus abetting
ice sheet disintegration. And an increase of GHGs stretches
the melt season both earlier and later, while also increasing
midsummer melt.
Hansen et al. 2007. Climate change and trace gases. Phil. Trans. R. Soc. A. 1925-1954.
Figure 3. (a) Temperature, CO2, and sea level (SL), (b) late spring
(April,May,June) insolation at 60 degrees N and (c) late spring (October,
November, December) insolation at 75 degrees S.
Hansen et al. 2007. Climate change and trace gases. Phil. Trans. R. Soc. A. 1925-1954.
Could both theories contribute to
glacial cycles?
Perhaps the Milankovitch theory (summer/winter insolation)
could lead to the initial formation of ice sheets, but the Hansen
theory leads to the sudden termination.
Vostok Ice Core
Hansen et al. 2007. Climate change and trace gases. Phil. Trans. R. Soc. A. 1925-1954.
Question:
1.  The increase in temperature in the
Vostok ice core seems to preceed
CO2 rise by several hundred years.
How do you account for this? What
does this imply about the role of
CO2?
2.  How do would you respond to a
sceptic that uses this result to argue
that we therefore should not be
concerned about rising CO2 levels?
Question:
1.  How do would you respond to a
sceptic that argues that solar output
and sunspot activity control global
temperatures (and therefore we
should not be concerned about rising
CO2 levels) ?
Solar forces have affected
the climate system
1
Radiative forcing (W/m2)
0
-1
-2
-3
-4
1900
1950
2000
Figure SPM.2
Question:
What is the role of external forcing
likely to be in recent warming?
Question:
1. What seasonal distribution of
insolation do you expect to lead to
onset and end of ice ages?
(e.g., warm summers & cold winters, cool summers
and warm winters, etc.)
2. Why? Propose a sequence of events
or processes that explain and support
your answer to 1.
Variation in Orbital Eccentricity (~100,000 year cycle)
perihelion
aphelion
http://earthobservatory.nasa.gov/Library/Giants/Milankovitch/
SPM 3
External and Internal
Drivers of Climate
External
-Sunspot Cycles (Decades)
-Orbital Variations (Thousands of years)
Internal
-Plate Tectonics (Millions of years)
-Volcanic Activity (1-3 years)
-Albedo (All time scales)
-Greenhouse Effect (All time scales)
What is climate ?
• Climate is average
weather
and its variability
for a particular region
over a period of time
Climate is what we expect,
weather is what we get.
What is climate change?
• Climate change is a shift in climate relative to a
given reference time period
• It is caused by:
Natural factors
-Solar variability
-Volcanic dust levels
-Internal variability
-Geological change
Human factors
- Greenhouse gases
- Aerosols
-Ozone depletion
-Land use change
Proxy data also indicate that the recent warming is
likely unprecedented in at least the past millennium
Source: IPCC(2001)
Review
FAQ 6.1, Figure 1
What are the periodicities associated with each orbital
variation?
Questions:
1. How do you expect orbital eccentricity
and precision to interact to affect
seasonal insolation?
2. How about obliquity and precision?
3. Under what conditions might we
expect most seasonal variation in
insolation?
Record of oxygen isotopes in ocean sediment over
the last 800,000 years shows several glaciations
(The last glaciation was 18,000 years ago)
Warm (interglacial)
Thousands of Years Ago
Cold (glacial)