Lecture 1: Introduction + Chapter 1: overview of climate science
Climate = broad composite of the average conditions of a region, measured by its temperature,
amount of rainfall/snowfall and other factors. Weather fluctuates in hours, days, weeks or a few
months.
Climate forcings:
- Tectonic processes: internal heat. Continents movement, uplift and basins. Millions of years.
- Change in Earth’s orbit around the Sun: alter amount of solar radiation received on Earth by
seas and latitude. Occur over tens to hundreds of thousands of years.
- Change in the strength of the Sun: also affect amount of solar radiation arriving on Earth. The
strength of the Sun has slowly increased throughout Earth’s existence.
- Anthropogenic forcing: unintended by-product of agriculture, industrial and other human
activities through additions of CO2 and other greenhouse gasses, sulphate particle and soot.
A slow warming between 300 and 100 million years ago was followed by a gradual cooling during the
last 100 Myr. This led to the appearance of Antarctic ice sheet and later to the northern hemisphere
ice sheets (which advanced and retreated due to shorter isolations). Since 1 Ma big ice.
,Lecture 1: Introduction + chapter 2: Earth’s Climate System Today
Climate system exists of solar radiation budgets,
hydrological cycles, climate zones and weather
systems, and main components of the systems. Earth
receives and absorbs more heat in the tropics than the
poles and the climate system works to compensate for
this imbalance by transferring energy from low to high
latitudes.
Albedo reflects radiation (white reflects much, black
reflects little). Albedo increases in NHS in winter
because of increased snow cover over land and more
extensive sea ice and in the SHS in winter because of
more extensive sea ice. In the southern hemisphere
there are no continents at the boreal latitudes, which
causes the asymmetry in seasonal albedo change.
There is an unequal heating of the Earth Surface, because of
the earth’s rotation etc. There is more heat at the equator
which is converted to the poles. This unequal heating leads
to circulations and different cells. This causes global
atmospheric circulations, which effect monsoons in the low
to mid latitudes.
- Summer monsoon: in-and-up flow of moist air that produces precipitation. The land surface
heats up fast and the ocean absorbs the heat and warms up slowly.
- Winter monsoon: reverse. In winter, the Sun’s radiation is weaker, and land surfaces cool by
back radiation. Land surfaces cool faster and more intensely than the oceans.
The uppermost layer of the ocean is heated by solar radiation and
float on top of the colder, denser deep ocean. There is surface
circulation by winds due to Coriolis. Two thermoclines exist: (1) a
deeper permanent portion that is maintained throughout the year
and (2) a shallower portion that changes as a result of seasonal
heating by the Sun
- North Atlantic deep water: high salinity in dry tropics and becomes denser so it sinks.
- Antarctic bottom water: colder and denser water mass fills up deep Pacific and Indian
oceans.
- Antarctic intermediate water: warmer and less dense that North Atlantic deep water
- Mediterranean overflow water: winter chilling of surface water with very much salt.
Cryosphere: ice as a component of its own. It is a source of long atmospheric records and a player in
asymmetry of climate cycles and global sea-level change. There are mountain glaciers and
continental ice sheets.
The biosphere affects the climate system by albedo, evapotranspiration, carbon storage (biomass
takes in CO2 and deltaic C trapping), vegetation (roughness to wind fields) and soil protection (dust
particles). The biosphere responds to climate change with some lag. On land there are tree rings,
pollen vegetation history, soils volcanoes and loess.
,Lecture 2: Milankovitch forcings + chapter 8
Changes in Earth’s orbit alter the amount of solar radiation received by latitude and by season (=
insolation). High latitude ice sheet response in the Northern hemisphere:
- small ice (progressive effect of glaciations in 41 ka regular cycles > 1 Ma)
- big ice (revolution towards 100 ka saw-tooth cycles in the last 1 Ma).
Milankovitch influences when the northern hemisphere or southern hemisphere has its summer
close to the sun. Obliquity is most important cycle.
1. Obliquity (tilt): variation of 22.2 to 24.5 degrees and cycles of 41.000 years. This affects the
seasonality and higher latitudes. It enhances differences between summer-winter insolation
(weak or strong seasonality) and the effect of variations increase towards the poles. Changes
the seasonality for both the N and H hemisphere.
2. Precession (wobble): strong: 23.000-year cycle and weaker 19.000-year cycle. This affects the
low-mid latitudes (monsoons, sapropels). It determines which season is enhanced/reduced
(seasonal timing) and has opposite effects for N and S hemispheres.
3. Eccentricity (shape orbital ellipsoid): distance varies between 153 and 168 x 10^6 km. It has a
cycle of 100.000 year and 413.000 years. The eccentricity is the modulator over long term. It
modulates effects of precession (even stronger monsoons) and enhances/reduces
seasonality (pacing of ice build-up). It produces just 0.2% radiation difference on its own.
Earth-Sun distance
The different graphs can be added to create a total graph of the changes. This can be
done to create changes over time in insolation. So, then the different time periods
can be recognized. It can also work the other way around, you can for example look at
the sapropels in sediment and see if they belong to a 20/40/100 thousand year cycle,
so which type of forcing caused the sapropels to form.
60-65 degrees north is often taken because:
- that is the place where land accumulates snow during winter
- albedo-feedback sensitive region
- It is where past major ice sheets have formed because snow was stored long
enough at periods of low irradiance + albedo feedback
- Latitude where ice sheets melt away at the end of glacial, when irradiation is
on the increase
There is a difference between the northern and the southern hemisphere because of the angle of the
axis of the Earth with relation to the sun (ap and perihelium). When it is summer in the southern
hemisphere, the Earth is closer to the sun (so warmer) than when it is summer in
the northern hemisphere.
Every 41 ka, ice sheets grew, after which they melt again. The ice sheets are
domes with 1 km thickness in the centre. In the 100 ka cycles, the ice sheets
grew bigger with a thickness of 2.5 km in the centre. They grew bigger because
the ice survived. Winds go from North America towards Europe, which is
amplified in ice ages because of a large gradient between poles and the equator.
Ocean-recorder volume of ice sheets. Ocean record of continental ice voluwe
shows a slow cooling trends tarting in Neogene, interglacials modest cooling and
glacials dip deeper and deeper. For very long time there were 41 ka cycles. The
last 100 cycles were 100 ka.
, Lecture 3: Northern Hemisphere Glaciations Conceptual Models + chapter 10
Ocean record of continental ice volume:
- Slow cooling trend starting in Neogene
- Interglacials modest cooling
- Glacials dip deeper and deeper
There are two ice cores: the South Pole (Antartic) which goes back 800 ka and Greenland on the
Northern hemisphere which goes back to the last ice age, so around 100 ka.
You can look at the Earth in two ways: you can look at expectations (calculate pression, obliquity and
eccentricity and add them up) or start with a geological record e.g. ice core or marine core and strip
it down to the components (=frequency analaysis) and you can also get the different components
(pression, obliquity and eccentricity). These give however different results, so there is a problem.
Milankovitch alone does not directly explain ice ages around 1 Ma, because:
- Orbital forcing is always on, ice ages differ over time: the severity and
length of glacial cycles changed.
- Orbital forcing has opposite effects on NH and SH, while climate change
occurred on both hemispheres
- Variations in received radiation (“insolation’’) theoretically result in very
small T (1-4°C), so amplification is needed.
- Rapid terminations of glacial periods (“cycle asymmetry”) cannot be
explained by orbital variations alone. When it would be orbital forcing, it
would be symmetric.
- Climate on Earth is not determined by just external controls: Earth
system shows internal response via feedbacks. Earth climate responses
lag the orbital forcing a bit.
Glaciers are smaller and can be found in mountains, they end at lower elevation. There is an
equilibrium between the zone of accumulation and ablation (melting). Ice sheets are on continental
scale (like Antarctica and Greenland). They create their own elevation (nice ice core in the middle)
and have calving as ablation. Ice sheets do not only have elevation, but they also have mass which
causes a depression in the land surface so the ice mass can grow higher (glacial isostasy), which takes
long to response (so there is a lag). Ice sheets have a lagged response to summer isolation. It will
continue its process when there is a maximum or minimum and will later respond (maximum
summer insolation = ice melting, minimum summer insolation = ice growing).
There is a feedback loop. The higher the ice sheet surface elevation, the colder it is, there is more
snow and ice summer survival and there will be more ice sheet surface elevation (positive feedback),
which is cancelled out because if a ice sheet becomes too high, snow cannot accumulate anymore.
There is also an albedo effect, the ice sheet whiteness causes more reflection of incoming radiation,
which causes snow and ice summer survival, which causes even more ice sheet whiteness (positive
feedback). This causes the feedbacks to be complicated. Mountain ice are black ice, so they melt
quicker.
There is a half cycle to 41 k (irridation minimum), then the ice sheet builds
up height and lastly, there is hysteresis effect in response half cycle back to
minimum.
