Glaciology
CLIMATE
A
Causes of Ice Ages - Hypothesis
Information about past climate is of outmost importance for mankind. Polar ice caps preserve
detailed records of climate.
1
What is an ice-age?
• Snow line up to 1000 m lower in Iceland.
• Air temperature 2–12◦C lower on Earth; lower closer to the poles.
• Weather changes globally.
• Climate zones, and therefore weather systems, move to the equator.
• Interchanging warm-and cold periods, not periodic, with warm periods lasting ∼10-30 kyr,
and cold periods lasting ∼100 kyr.
• Sea level is 120 m higher now than 20 kyr ago.
All theories of climate change are concerned with terms in the energy balance equation,
Q = Qi (1 − α) + Ii + Io + QH + QE ,
| {z } | {z } | {z }
A
A
i)
ii)
iii)
B
i)
ii)
C
i)
ii)
B
B
(1)
C
Changes in short wave radiation, Qi (1 − α)
Changes in the solar constant
Distance, tilt, eccentricity of earth’s path around the sun (Figs. 1-3)
Changes in α. Eruptions, cosmic dust, . . .
Changes in Ii (changes in Io are smaller).
Increasing CO2 → warmer,
Uncertain change in cloudiness.
Changes in QH + QE
changes in atmospheric circulation
changes in ocean circulation
Milankovitch Cycles and Glaciations
The three Milankovitch Cycles impact the seasonality and magnitude of solar energy around the
Earth, thus impacting contrasts between the seasons.
Eccentricity of the Earth’s orbit around the Sun reduces or increases the amount of radiation
received at the Earth’s surface in different seasons, Figure 1.
Today axial tilt is ∼23.5 degrees, which largely accounts for our seasons. Because of the periodic
variations of this angle the severity of the Earth’s seasons changes. With less axial tilt the Sun’s
solar radiation is more evenly distributed between winter and summer. Figure 2 show axial tilt.
Throstur Thorsteinsson
-1-
research.turdus.net
Glaciology
Earth
Sun
More elliptical
Earth orbit
Less elliptical
Figure 1: Eccentricity has a periodicity of about 100 kyr. Reduces or increases the amount of radiation
received at the Earth’s surface in different seasons.
Axis of rotation
radiation
21°
24°
Equator
Figure 2: Axial tilt has a periodicity of about 41 kyr.
One hypothesis for Earth’s reaction to a smaller degree of axial tilt is that it would promote the
growth of ice sheets. This response would be due to a warmer winter, in which warmer air would be
able to hold more moisture, and subsequently produce a greater amount of snowfall. In addition,
summer temperatures would be cooler, resulting in less melting of the winter’s accumulation. At
present, axial tilt is in the middle of its range.
Figure 3 shows precession. Due to this wobble a climatically significant alteration takes place.
When the axis is tilted towards Vega the positions of the Northern Hemisphere winter and summer
solstices will coincide with the aphelion and perihelion, respectively. This means that the Northern
Hemisphere will experience winter when the Earth is furthest from the Sun and summer when the
Earth is closest to the Sun. This coincidence will result in greater seasonal contrasts. At present,
the Earth is at perihelion very close to the winter solstice.
winter
Now
summer
5250
23°
winter
summer
10,500
Figure 3: Precession has a periodicity of about 23 kyr.
These variations are important because most of the landmass is in the Northern hemisphere.
Throstur Thorsteinsson
-2-
research.turdus.net
Glaciology
1
Greenhouse effect
CO2 traps heat (note the importance of H2 O). Short-wave radiation enters easily, but long-wave
radiation is trapped.
Does the precipitation increase?
What happens to the cloud cover?
Glaciers grow initially . . .
C
Warming
Figure 4 presents data showing that the northern hemisphere surface air temperature has increased
about 0.5 degrees C during the past century. Overall, the average air temperatures at the Earth’s
surface have increased. The red line is data from instrumental records, the blue line from proxy
data, and the black line is a 20-year “running average” to smooth out annual jumps and highlight
the overall trend. The running average is obtained by averaging the temperatures for a 20-year
period. The warming trend indicated in the graph is important because it provides physical evidence in support of the predictions of climate models. These models predicted that an increase
in atmospheric carbon dioxide (CO2) would cause a heightened greenhouse effect, which in turn
would cause a rise in global temperatures.
0.8
0.6
∆ T (°C)
0.4
0.2
0
−0.2
−0.4
−0.6
1000
1100
1200
1300
1400
1500
Year
1600
1700
1800
1900
2000
Figure 4: Northern hemisphere temperature change for the past 1000 years. The thick line is a 20year running average, and the thin lines are reconstructed temperature from proxy data (blue line) and
instrumental data (red line).
Figure 5 shows that average atmospheric carbon dioxide concentrations observed at Mauna
Loa, Hawaii have increased approximately 40 parts per million by volume (ppmv) since 1958. This
increase is believed to be largely because of human activity. (The current concentration of about
353 ppmv indicates that an alarming increase of 25 percent has occurred over the pre-industrial
concentration of 280 ppmv that existed some 200 year ago.) Superimposed on the long-term
increase in Figure 5 are the seasonal variations, due primarily to the withdrawal and production of
CO2 by the terrestrial boita, with minima during the Northern Hemisphere summers (when global
photosynthetic activity is greatest) and maxima 6 months later.
Throstur Thorsteinsson
-3-
research.turdus.net
Glaciology
350
340
2
CO (ppmv)
360
330
320
310
1960
1970
1980
Time (yr)
1990
Figure 5: CO2 concentration at Mauna Loa, Hawaii, since 1958.
Throstur Thorsteinsson
-4-
research.turdus.net