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Natural Causes of Climate Change
Climate change theory needs to explain (1) on set
of ice age through geologic time (long period of
warmth interrupted by cooler period); and (2).
account for warming and cooling period that
occur within an ice age.
Key is the flows of energy in and out of the
system and the ways in which the energy is
exchanged within the earth-ocean-atmosphere
system
Basic Concepts: energy budget of the earthatmosphere system
Albedo: percentage of solar radiation
reflected by a surface
The Greenhouse Effect and Atmospheric
Warming
• Greenhouse effect: greenhouse gases (CO2,
H2O, N2O, O3, CH4, CFCs) absorb energy of
longwave radiation and re-radiate some
energy back to the earth’s surface to keep it
warm
• Without greenhouse gases, the earth’s
surface air temperature would be much
colder
• If the amount of greenhouse gases increase,
the surface air temperature will increase
(global warming)
Energy Budget
by Latitude
Figure 4.13
Short-term changes (within thousands of years)
1. Variation in solar irradiance
(a)
the sun’s surface temperature fell 11C in a single year of 1977-the normal temperature is
5438C);
(b)
the brightness of the sun faded in 1980s, so irradiance of the sun decline 0.07% from 198184. (computer simulation shows irradiance drop of 1-2% would bring about little ice age
condition on the earth)
2. Sun spots: dark, circular areas over the sun’s outer surface or photosphere. In these areas, sun’s
temperature is 1400C lower than surrounding areas. The number of them varies from 5-6 to
100 spots. An unusually high numbers of sun spots formed in a 200-year period around
1180A.D. No sunspots occurred between 1645-1715 (little ice age). It appears that sun spots
also follows cycle of 11-year, 22-year, 33-year. This is believed to be related to sun’s surface
magnetic field, which reverse its polarity every 22 years.
3. Variation in atmospheric dust
Changes in transparency of the atmosphere result from changes in dust content, cloud cover, and
ozone content of the upper atmosphere. A reduction in radiation absorbed by the earth by as
little as 1% can produce a change in surface temperatures by as much as 1.2C to 1.5C.
Volcanic eruption increases stratospheric temperatures decreases in earth’s surface temperatures
(SO2).
One hypothesis: during active times of earth-building forces, continued volcanic activity over long
periods would have an extended effect. Once glaciations was initiated, the role of volcanic
dust would be secondary to changing surface albedo.
Between 1645 and 1715 no sunspots appeared for years at a time. This
was called the Maunder Minimum, during the Little Ice Age. On the
other hand, an unusually high number of sunspots occurred in the 200
year period around 1180. This later period was called the Midieval
Warm Period, when Greenland was colonized.
Short-term climate change (continue)
4. Human induced changes in earth’s surface
(a) Use of fire result in deforestation of large areas of the world
(b) Farming. 50% of central Europe was converted from forest to farmland over
the last 1000 years
(c ) desertification (India; Africa, and South America). Misuse of marginal lands
(d) Urbanization
(e) raw material extraction; energy generation; and other processes that
significantly altered the face of the earth
Long-term Climate Changes
1. Earth-sun relationships-Milankovitch Theory
(a)
the obliquity of the ecliptic: the angle of axis revolves around the sun (is 23.5°) varies
1.5° about the mean of 23.1° during a cycle of 41,000 years
Obliquity of 0, equal length of day and night, lack of seasonal change; Obliquity of 54, great
extremes in the length of summer and winter days and nights.
Earth-sun relationship (continue)
(b) Earth’s orbital
eccentricity
Eccentricity (Le/α)
(comparing the path to
that of a true circle;
smaller the number and
more circular). Current
is 0.017; it ranged from
0.001 to 0.054. It
influences the amount of
solar radiation
intercepted by the earth
and modifies the dates
at which the solstice and
equinoxes occur.
Continue
(c ) precession of the equinoxes
Days on which the earth reaches at perihelion and aphelion change over time. In 10,000
years, date of perihelion will pass to the Northern Hemisphere summer season. The
cycle then is about 20,000 years.
Milutin Milankovitch (Yugoslavian scientist) derived values going back thousands of
years-Milankovitch cycles. Based on this cycle, the global cooling which started some
6000 years ago will continue.
Long-term Climate
Change, Continue
2. Redistribution of continents
Plat Tectonic theory suggests the
present position of the world’s
land masses are but a transitory
location in the long-term
evolution of the continents and
oceans.
The primary requirement for the
formation of great ice caps is
the polar location of continents;
the role of mountain building
and continental uplift increases
the potential for ice caps;
3. Variation in the oceans
Changes in SSTs linked to
changing circulation
patterns and weather
anomalies
(a) Drops in sea level would
increase heights of the
continents and enlarge
land masses in areas
(b) Heat storage
mechanisms; ocean is
less variable in
temperature; change in
water temperature affect
world climate. It also
changes in salinity,
evaporation rate and
relative solar penetration
(c) Mobile medium: ocean
currents transport heat.
Basic concepts: differential heating of land and
water
Land–Water Heating Differences
•
•
•
•
•
•
Evaporation
Transparency
Specific heat: the amount of heat required to increase 1 gram of
substance by 1°C. (water has the highest specific heat than anything else
on the earth-5 times higher than earth’s rock materials).
Movement
Ocean currents and sea-surface temperatures
Marine vs. continental effects
4. Extraterrestrial impacts
Large objects from space have struck the earth, altered climate tremendously
for short period of time to 1000 years
Cretaceous-Tertiary geologic period (65 millions years ago), an event
eliminated over half of all species of organisms living on the earth at the
time. Firestorms generated from scattering of molten meteorite and gases
(a) removed O2 and added CO2; (b) dust clouds blocked most the sun for
month (huge cloud of soot mass of material blown into the atmosphere as
the object struck was 100 times the mass of the meteorites)-halting or
greatly reducing photosynthesis and had lethal effect on marine organisms;
(c) earth’s surface temperature cools; (d) precipitation tuned into acid rain;
(d) ozone in stratosphere disappeared for sometime; (e) largest animals
were eliminated due to fire related shortage of food supply; (f) Temperature
rebounded to levels higher than before the impact afterwards, much of the
earth’s biota was killed or could not adjust to the changes; (g) changed
seasonal variability of climate due to reduced seasonal CO2 variation; (h)
the Milankovitch cycles were greatly strengthened by the event (took
hundred thousands years to back to previous levels)
5. Other theories
Earth’s climate result from a spectrum of causal elements.