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UNSW Understanding Past Climate Change -Past Climate is known as Paleo-climate, experts known as paleo- climatologists. -How do we detect past climatic.

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Presentation on theme: "UNSW Understanding Past Climate Change -Past Climate is known as Paleo-climate, experts known as paleo- climatologists. -How do we detect past climatic."— Presentation transcript:

1 UNSW Understanding 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

2 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: 1) Ocean Sediments

3 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: 2) Glacial Ice

4 UNSW -Corals in clear sunlit waters at tropical and subtropical latitudes form annual bands of calcite (CaCO 3 ) that hold geochemical information about climate. Corals live up to hundreds of years. Measuring Paleoclimatic Records: 3) Corals

5 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. Measuring Paleoclimatic Records: 4) Tree Rings

6 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. How to tell when volcanoes erupted back in time

7 UNSW Methods for Detecting Past Climate

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

9 UNSW Atmospheric CO 2 Concentrations Over Geologic Time

10 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. Long Term Climate Change on Earth

11 UNSW 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.

12 UNSW 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

13 UNSW 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 25% oxygen, even wet organic matter would burn freely.

14 UNSW Species Evolution

15 UNSW Climate Evolution and Plate Tectonics

16 UNSW When was our climate a ‘hothouse’?

17 UNSW Relationship Between CO 2 and Temperature

18 UNSW 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

19 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 CO 2 What would cause the high CO 2 during the dinosaur age? 2) Area of continents was smaller since higher sea levels covered 20% of continents. CO 2 removal from weathering on land was dampened 100 Million Years Ago

20 UNSW Sea level changes and past climate NB: Sea-levels and previous ‘greenhouse’ climates

21 UNSW One of the most important drivers of sea level

22 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). Dinosaurs Prospered 100 million years ago in earths ‘hothouse’

23 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

24 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. Enhanced Global Dimming

25 UNSW The Last 450 000 years from Ice-cores

26 UNSW 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?

27 UNSW 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

28 UNSW 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.

29 UNSW 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

30 UNSW Precession 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

31 UNSW Milankowitch Theory - Summary

32 UNSW Milankovitch Amplification

33 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 Climate Proxy

34 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: CaCO 3 (calcite) shells are formed by sand-sized planktic foraminifera and clay-sized coccoliths. SiO 2 (opal) shells include silt-sized diatoms and sand-sized radiolaria. Biotic Climate Proxy

35 UNSW How do we detect cold periods and glacial ice? Normal oxygen contains 8 protons, 8 neutrons (O 16 ) a small fraction (one in a thousand) of oxygen atoms contain 8 protons, 10 neutrons (O 18 ) - this is an isotope of oxygen O 18 is heavier than O 16 - will evaporate less readily than O 16 Hence, during a warm period, the relative amount of O 18 will increase in the ocean waters since more of the O 16 is evaporating Conversely, O 18 is preferentially removed by precipitation and snowfall. Hence, looking at the ratio of O 16 to O 18 in the past can give clues about global temperatures and glacial ice volumes. WarmerColder Page 67 of text-book

36 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

37 UNSW Last Glacial Maximum

38 UNSW Last Glacial Event (18000 years ago) Caused by a temperature drop of only 4-6  C Huge climatic shifts

39 UNSW Ice-ages and Human Migration Human civilization migrated north as the last ice-age retreated What does the future hold for mass migrations?

40 UNSW How do we know CO 2 is important?

41 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 Earths Glacial Events

42 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. Last 18 000 years

43 UNSW 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

44 UNSW 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. CO 2 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

45 UNSW 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.

46 UNSW 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

47 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 (CO 2 and CH 4 ) through oceans, land and volcanic activity 5. Climate feedbacks like the ice-albedo feedback

48 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 800 million years ago?

49 UNSW Snowball Earth? -50degC

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