UNSWUnderstanding Past Climate Change

- Past Climate is known as Paleo-climate, experts known as paleo-climatologists.

- 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

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- 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: 1) Ocean Sediments

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- 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: 2) Glacial Ice

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- 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: 3) Corals

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- Tree rings have useful information of the last tens to hundreds of years.

-The annual layers of outer soft wood turn into harder wood.

Measuring Paleoclimatic Records: 4) Tree Rings

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- 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.

How to tell when volcanoes erupted back in time

UNSWMethods for Detecting Past Climate

UNSWGeologic Timeline

The further we go back the less certain is our ability to detect and understand past climatic change

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Atmospheric CO2 Concentrations Over Geologic Time

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• 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.

Long Term Climate Change on Earth

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Sun’s Intensity Has Increased

•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.

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Atmospheric Composition Over Geologic Time

• Stronger greenhouse and weaker solar radiation in the early earth, compared to stronger radiation and weaker greenhouse in modern earth

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Oxygen

• 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.

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Species Evolution

UNSWClimate Evolution and Plate Tectonics

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When was our climate a ‘hothouse’?

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Relationship Between CO2 and Temperature

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•Ice-free and average temperatures were about 10degC warmer

•High Latitudes were much warmer 100 million years ago than today

Temperature 100 million years ago compared to today

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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

What would cause the high CO2 during the

dinosaur age?

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

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Sea level changes and past climate

NB: Sea-levels and previous ‘greenhouse’ climates

UNSWOne of the most important drivers of sea level

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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).

Dinosaurs Prospered 100 million years ago in earths ‘hothouse’

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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

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- 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.

Enhanced Global Dimming

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QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

The Last 450 000 years from Ice-cores

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The Earth is currently in an Interglacial Period

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?

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Long-Term Changes in Earth's Orbit - Milankovitch Cycles

•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.

•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

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Obliquity

• 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 long-term variations in seasonal solar insolation received on Earth, with the largest changes at high latitudes.

• Increased tilt amplifies seasonal differences, decreased tilt reduces seasonal differences.

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Eccentricity

•Eccentricity has varied over time between 0.005 and 0.0607 with periods of 100,000 years and 413,000 years.

• Difference of orbit from a perfect circle

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Precession

• Wobbling motion of Earth, called axial precession caused by the gravitational pull ofthe sun and moon on the Earth’s equatorial bulge.

• 25 700 year cycles

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Milankowitch Theory - Summary

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Milankovitch Amplification

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•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

Climate Proxy

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• 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.

Biotic Climate Proxy

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How do we detect cold periods and glacial ice?

•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 clues about global temperatures and glacial ice volumes.

Warmer

Colder

Page 67 of text-book

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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 thata land bridge existed between Siberia and Alaska.

Last Glacial Maximum

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Last Glacial Maximum

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Last Glacial Event (18000 years ago)

Caused by a temperature drop

of only 4-6C

Huge climatic shifts

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Ice-ages and Human Migration

• Human civilization migrated north as the last ice-age retreated

•What does the future hold for mass migrations?

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•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

Earths Glacial Events

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Ice Ages

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•Warming began about 15,000 years ago, interrupted about4,000 years later by the Younger Dryas, a time whencolder conditions returned for about 1,000 years.

•10 000 years ago another period of abrupt warmingbegan bringing climate into the present interglacial.

Last 18 000 years

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Younger Dryas and the Ocean Link

•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

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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’

The Last 1000 Years - Houghton, pg 64

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Sunspot activity and climate change

• 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.

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Sunspot Activity and The Little Ice-age

• 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

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Summary: How Does Climate Change on Long Time-scales?

1.Orbital changes - known as Milankovitch Cycles2.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 activity5.Climate feedbacks like the ice-albedo feedback

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- 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 800 million years ago?

UNSWSnowball Earth?

-50degC