Earth System - II
(ECS202)
Lecture 6
Dr Neena Joseph Mani
Earth & Climate Science
Change in solar radiation at the top of the atmosphere depends on the
Sun-Earth geometry and is affected by three factors,
(i) Ellipticity of the Earths orbit around the Sun, (ii) Obliquity of the
Ecliptic and (iii) Wobble of the axis
Spectral analysis reveals the
presence of cycles within
complex climate signals
-------Normally snow deposits in winter and melts away in summer. If the summer
insolation is less (mild summer), the snowpack will not melt completely and more
snowfall in winter will accumulate on top of it. Over a time span of thousands of
years it will grow to thick ice sheets which will spread to lower latitudes.
Continental ice sheets mass balance
They grow in regions where the overall rate
of snow and ice accumulation equals or
exceeds the overall rate of ice loss or
ablation.
At higher temperatures, ice accumulation is
limited by the fact that more of the precipitation
falls as rain.
At extremely low temperatures, all the
precipitation is snow, but cold air carries very
little water vapor that rates of ice accumulation
are low.
Ablation of ice accelerates rapidly when X axis: Annual (full year) mean
temperatures warm. Melting begins at mean temperature
annual temperatures above –10°C, equivalent
to summer temperatures above 0°C
Glacial – Climate Feedbacks
Positive feedback amplifies the growth of icesheets
Orbital changes and Ice sheets
Milankovitch suggested that the critical factor for Northern Hemisphere
continental glaciation was the amount of summertime insolation at high
northern latitudes.
Low summer insolation occurs when
▪ Earth’s orbital tilt is small and its poles
are pointed less directly at the sun.
▪ the northern summer solstice occurs with
Earth farthest from the Sun (aphelion)
▪ and when the orbit is highly eccentric
(further increasing the Earth-Sun
distance).
Opposite orbital configurations would result
in stronger summer insolation.
The Glacial – Interglacial cycles seem to correspond to the 100 kyr
eccentricity solar forcing. Why not the Precession and Obliquity
forcing?
Eccentricity variations cause changes in the annually averaged amount of
solar radiation reaching the Earth, whereas precession and obliquity
variations do not.
But, changes in total insolation due to changes in eccentricity is only
about 0.2%.
On the other hand, obliquity and precession cycles, can bring about 10-
12% change in seasonal insolation.
When Earth's eccentricity is nearly zero, there is no difference between the
perihelion distance and the aphelion distance from the Sun, so it does not matter
when summer or winter occurs.
When the eccentricity is large, Northern Hemisphere glaciation is especially favored
when precession causes Northern Hemisphere summer to occur at aphelion (less
seasonal contrast in Northern Hemisphere)
The Glacial – Interglacial cycles seem to correspond to the 100 kyr
eccentricity solar forcing. Why not the Precession and Obliquity
forcing?
Reason: More than the direct forcing, the importance of eccentricity is evidently
more indirect. Eccentricity modulates the insolation changes associated
with the precession band, as can be seen in the envelope of variation for
precession.
The combined effects of eccentricity and precession cause the distance from the Earth to
the Sun to vary by season, primarily at a cycle of 23,000 years, with significant amplitude
modulations at 100,000 year periodicity.
The Glacial – Interglacial cycles seem to correspond to the 100 kyr
eccentricity solar forcing. Why not the Precession and Obliquity
forcing?
The combination of the various orbital forcings causes Earth's climate
system to oscillate between warmer and cooler states. High eccentricity
increases the amplitude of the variations on precession cycles and thus is
more likely to be associated with transitions from interglacial to glacial
states
At present we are at low eccentricity, and the eccentricity will be decreasing to
a minimum near zero in about 30,000 years from now. With eccentricity so low,
the unusually cold winters needed to initiate Northern Hemisphere ice-sheet
growth will not occur. Thus, climatologists predict that the present interglacial
will be long-lived (at least 1.5-2.5 precession cycles).
The first continuous and detailed δ 18 O
record of the entire 2.75 Myr of northern
hemisphere glacial history was compiled in
the late 1980s by isotopic analysis of benthic
foraminifera from the North Atlantic Ocean
Ocean sediments contain two key
indicators of past glaciations:
(1) ice-rafted debris, a mixture of coarse
and fine sediments delivered to the
ocean by melting icebergs that calve
from ice sheet margins
(2) δ 18 O records from the shells of
foraminifera, which provide a
quantitative measure of the combined
effects of changes in ice volume and
temperature of ocean water.
Climate at the shorter time scales are embedded in slower
changes at longer time scales
The oldest record of climate from sediment cores indicate that prior to 100 Myr no
ice sheets were present even in the polar regions. The Earths temperature exhibits
a slow variability in hundreds of million year timescale which is associated with
plate tectonic processes, weathering and carbon cycle.
The rate of sea floor spreading has slowed down over the past 100 Myr, which has
reduced the concentration of CO2 in the atmosphere and hence cooled the planet.
Superimposed on this tectonic scale climate change are orbital scale variability
which causes advance and retreat of ice sheets –glacial-interglacial cycles of
periodicity of tens to hundreds of thousands of years.
There are even shorter timescale climatic signals that lasted a few thousand years
overriding the orbital scale variability – related to variability of CO2 concentrations
and internal variability of the ocean atmosphere system.
