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Transcript
Understanding Past Climate Change
UNSW
- Past Climate is known as Paleo-climate, experts known as paleoclimatologists.
- How do we detect past climatic change?
- Understanding some specific events and causes
- 100-600 million years ago (dinosaurs and ‘greenhouse’ earth)
- 450 000 years ago (good detection from ice-cores)
- 18 000 years ago (last glacial event on earth) to present
Measuring Paleoclimatic Records: 1) Ocean Sedimen
UNSW
- Deep ocean is generally a quiet place with relatively continuous
deposition, and it yields climate records of higher quality than most
records from land, where water, ice and wind are active agents of
erosion.
Measuring Paleoclimatic Records: 2) Glacial Ice
UNSW
- Glacial ice: Annual deposition of snow can pile up continuous sequences of
ice.
- Ice core records can date back over 800,000 years in Antarctica and 100,000
years in Greenland.
Measuring Paleoclimatic Records: 3) Corals
UNSW
- Corals in clear sunlit waters at tropical and subtropical
latitudes form annual bands of calcite (CaCO3) that hold
geochemical information about climate. Corals live up to
hundreds of years.
Measuring Paleoclimatic Records: 4) Tree Rings
UNSW
- Tree rings have useful information of the last tens to hundreds of years.
-The annual layers of outer soft wood turn into harder wood.
How to tell when volcanoes erupted back in time
UNSW
- In Greenland ice core,
where deposition of snow is
rapid, annual layering may
remain visible 10,000 years
back. In Antarctica, where
snow accumulation rates
are small, annual layering
may not even occur at ice
surface.
- The further back one needs
time horizons, or known
events in the past that
leave a mark. For example
volcanoes will leave an
acidic layer that provides
an exact time, if the
volcanic eruption is known.
Methods for Detecting Past Climate
UNSW
Geologic Timeline
UNSW
The further we go back the less
certain is our ability to detect
and understand past climatic
change
Atmospheric CO2 Concentrations Over Geologic Time
UNSW
Long Term Climate Change on Earth
UNSW
• Alternated between: greenhouse
eras (times when no ice sheets
are present) without boiling its
oceans & lakes and icehouse
eras (times when ice sheets are
present) without ever freezing
solid.
Sun’s Intensity Has Increased
UNSW
•Energy output of the
Sun has increased by
25-30% over last 4.6
billion years, yet the
Earth has remained
hospitable to life.
•Solar output is the
result of the fusion of
hydrogen atoms to
form helium.
Atmospheric Composition Over Geologic Time
UNSW
• Stronger greenhouse and
weaker solar radiation in
the early earth, compared
to stronger radiation and
weaker greenhouse in
modern earth
Oxygen
UNSW
•
Why does present-day oxygen sit at 20%? This is not a trivial
question since significantly lower or higher levels would be
damaging to life. If we had < 15% oxygen, fires would not burn,
yet at > 25% oxygen, even wet organic matter would burn freely.
Species Evolution
UNSW
Climate Evolution and Plate Tectonics
UNSW
When was our climate a ‘hothouse’?
UNSW
Relationship Between CO2 and Temperature
UNSW
UNSW
Temperature 100 million years ago compared to
today
•Ice-free and average temperatures were about 10degC warmer
•High Latitudes were much warmer 100 million years ago than
today
What would cause the high CO2 during the dinosaur age?
UNSW
1) 175 million years ago Pangaea began to break apart into 6
continents similar as today with huge tectonic shifts causing
volcanism. Volcanism was extensive because of faster sea
floor spreading. Volcanoes spew out a lot of CO2
2) Area of continents was
smaller since higher sea
levels covered 20% of
continents. CO2 removal
from weathering on land was
dampened
100 Million
Years Ago
Sea level changes and past climate
UNSW
NB: Sea-levels and previous ‘greenhouse’ climates
UNSW
One of the most important drivers of sea
level
Dinosaurs Prospered 100 million years ago in earths ‘hothouse’
UNSW
Australian warm-blooded dinosaurs-in the Dinosaur Cove area
under the polar weather conditions that prevailed during the Early
Cretaceous (100 - 125 million years ago).
UNSW
Impact Winter - the most abrupt climate change of the
past
•10km asteroid hit earth 65 million years ago gauging a180
km crater in North America
• Explosion equivalent to 4 times the energy of all currently
existing nuclear weapons
•Wiped out the dinosaurs and referred to as Impact Winter
Enhanced Global Dimming
UNSW
-
-
-
Water and rock were instantly vaporized by heating
causing global wildfires that sent a thick layer of
soot into the atmosphere.
Dust and soot blocked 90% of the incoming solar
radiation. It takes months to years for dust and
soot injected to stratosphere to settle back to the
Earth surface.
Temperatures dropped 3-5degC, very rapidly
Global-scale extinction of some 70% of species
The asteroid impact was a short term event (decades
to hundreds of years) - while the climate restored itself
after 1000 years.
The Last 450 000 years from Ice-cores
UNSW
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
The Earth is currently in an Interglacial Period
UNSW
 Last Glacial Maximum was
18,000 years ago and Global
temperature was
approximately 5degC colder
than now
 The last ice age ended 11,000
years ago.
Why is there a distinct
100 000 year cycle
between glacial
events?
UNSW
Long-Term Changes in Earth's Orbit Milankovitch Cycles
•For most of its life, Earth has been largely free of permanent (year-round) ice. It
is a warm planet,punctuated by perhaps seven relatively brief ice ages.
•Oscillations in temperature and ice cover are called glacial/interglacial cycles.
•Long term climate oscillations are mainly determined by earths orbital
changes
• Earth's orbit is not perfectly circular, but rather is elliptical.
• The Earth's orbital parameters are not fixed over long intervals of time
because of gravitational attractions among Earth, its moon, the Sun, and
other planets and their moons.
They cause 3 variations:
1. Earth's angle of tilt,
2. Eccentricity of orbit around the sun,
3. Position of the solstices and equinoxes around the elliptical orbit.
Obliquity
UNSW
• Angle of Earth's tilt varies
between 22.2° and 24.5° with
a period of 41,000 years.
These variations are mainly
caused by the gravitational
pull of Jupiter.
• Changes in tilt cause longterm variations in seasonal
solar insolation received on
Earth, with the largest
changes at high latitudes.
• Increased tilt amplifies
seasonal differences,
decreased tilt reduces
Eccentricity
UNSW
Difference of orbit from a perfect circle
• Eccentricity has varied over time
between 0.005 and 0.0607 with
periods of 100,000 years and
413,000 years.
Precession
UNSW
• Wobbling motion of
Earth, called axial
precession caused by
the gravitational pull of
the sun and moon on
the Earth’s equatorial
bulge.
•
25 700 year cycles
Milankowitch Theory - Summary
UNSW
Milankovitch Amplification
UNSW
Climate Proxy
UNSW
•Proxy refers to a substitute for an actual climate measurement.
•Need to understand how a measurement or observation can be related to a
climate variable.
2 types of proxies:
1. Biotic proxies
2. Geological-geochemical proxies
Biotic Climate Proxy
UNSW
•
Plankton are most useful as biotic climate
proxies (substitutes) because they are widely
distributed: plankton live in all oceans
•
Populations of plankton in different areas
tend to be dominated by a small number of species
with well-defined climate preferences.
In the ocean, use shell-forming animals and plant
plankton for climate reconstruction:
•CaCO3 (calcite) shells are formed by sand-sized
planktic foraminifera and clay-sized coccoliths.
•SiO2 (opal) shells include silt-sized diatoms and
sand-sized radiolaria.
How do we detect cold periods and glacial ice?
UNSW
Page 67 of
text-book
•Normal oxygen contains 8 protons, 8 neutrons (O16) a small
fraction (one in a thousand) of oxygen atoms contain 8 protons,
10 neutrons (O18) - this is an isotope of oxygen
•O18 is heavier than O16 - will evaporate less readily than O16
•Hence, during a warm period, the relative amount of O18 will
increase in the ocean waters since more of the O16 is
evaporating
•Conversely, O18 is preferentially removed by precipitation and
snowfall.
•Hence, looking at the ratio of O16 to O18 in the past can give
Colder
Warmer
Last Glacial Maximum
UNSW
In the depths of the last glaciation, around 20,000 years ago,
land ice covered much more area as seen in the map above.
Sea level was about 120 m lower than it is now, so that
a land bridge existed between Siberia and Alaska.
Last Glacial Maximum
UNSW
UNSW
Last Glacial Event (18000 years
ago)
Caused by a temperature drop
of only 4-6C
Huge climatic shifts
Ice-ages and Human Migration
UNSW
• Human civilization migrated north as the last ice-age
retreated
•What does the future hold for mass migrations?
Earths Glacial Events
UNSW
•Throughout much of earth's history global climate was 8°C-15°C warmer than
todays climate
•Polar regions ice free
•Warm climate was periodically interrupted by periods of glaciation
700
Projected
)
2100
600
Ice Ages
550
500
450
400
Current
350
300
250
200
400,000
300,000
200,000
100,000
Years Before Present
0
150
CO2 Concentration
650
Last 18 000 years
UNSW
•Warming began about 15,000 years ago, interrupted about
4,000 years later by the Younger Dryas, a time when
colder conditions returned for about 1,000 years.
•10 000 years ago another period of abrupt warming
began bringing climate into the present interglacial.
Younger Dryas and the Ocean Link
UNSW
•Large volumes of melt-water were deposited into the North Atlantic from
North American thawing glacial ice
•This freshwater shut-down the North Atlantic thermohaline circulation,
resulting in a massive cooling during the Younger Dryas
UNSW
The Last 1000 Years - Houghton, pg
64
Medieval Warm
Period
The Little Ice-age
• There is evidence that the period a.d. 900–1200 was warm in the North Atlantic.
This Medieval Warm Period,coincides with the Viking settlement of Greenland.
• The so-called Little Ice Age, from 1400 to 1850,was a cold period for western
Europe as alpine glaciers advanced and temperatures fell by about 0.5 to 1°C.
•CO2 could not be the cuase for these variations - rather solar radiation variations
and volcanic activity causing increased ‘global dimming’ appears to also contribute to
the Little ice Age’
Sunspot activity and climate change
UNSW
•
•
A sunspot is a region on the Sun's surface (photosphere) that is marked by a
lower temperature than its surroundings and intense magnetic activity, which
inhibits convection, forming areas of low surface temperature
Since sunspots are dark it is natural to assume that more sunspots means less
solar radiation. However, the surrounding areas are brighter and the overall
effect is that more sunspots means a brighter sun.
UNSW
•
Sunspot Activity and The Little Iceage
Solar luminosity is lower during periods of low sunspot activity. It is widely believed that
the low solar activity during the Maunder Minimum and earlier periods may be among
the principle causes of the Little Ice Age
UNSW
Summary: How Does Climate Change on
Long Time-scales?
1. Orbital changes - known as Milankovitch Cycles
2. Asteroid Impact (which wiped out the Dinosaurs)
3. Volcano Impact through enhanced ‘Global Dimming’
4. Greenhouse gas changes (CO2 and CH4) through oceans, land
and volcanic activity
5. Climate feedbacks like the ice-albedo feedback
Snowball Earth 800 million years ago?
UNSW
-
Some evidence suggests that glaciated continents at that time were in the
tropics. This supposition has led to a hypothesis that the Earth was nearly
frozen at that time.
-
Called the snowball earth hypothesis (unproven and controversial).
Alternative interpretation is that these older glaciations between 850 - 550
Myr ago occurred when glaciated continents were outsides the tropics so that
Earth need never have been close to a frozen state.
Snowball Earth?
UNSW
-50degC