Understanding past climate change

1. Understanding past climate change • Overview of past climate change • A 4.6 billion year story! • How do we detect past climate changes? • Sources of data and their limitations • Past changes and what they tell us about the Earth’s climate: • Dinosaurs and “Hothouse Earth” • Ice ages and glacial cycles • The Last Glacial Maximum • The current interglacial and the rise of human civilisation • Past climate is known as palaeoclimate, and the scientists as palaeoclimatologists

2. The past 100 years

3. The past 2000 years

4. Past 542 million years

5. The Geologic Clock

6. The Geologic Timeline The further back in time we go, the less certain is our ability to detect and understand past climatic change.

7. How do we detect past changes?

8. The instrumental record • The instrumental record comprises data from the following sources: • Manual weather stations • Automatic weather stations • Satellites

9. The historical record The Little Ice Age: Frost fairs on the River Thames in London

10. The historical record The Mediaeval Warm Period: Viking settlements in Greenland

11. The historical record Even wine can tell us a story!

12. Tree rings • Tree rings can provide information about temperature and precipitation changes over the last tens to thousands of years. • The annual layers of outer soft wood turn into harder wood.

13. Ice cores • 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.

14. Coral reefs • 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.

15. Ocean sediments • The deep ocean is generally a quiet place with relatively continuous deposition. • It yields climate records of higher quality than most records from land, where water, ice and wind are active agents of erosion.

16. Proxies • 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. • There are two types of proxies: • Physical proxies e.g. • Ice cores • Boreholes • Lake and ocean sediments • Biological proxies e.g. • Tree rings • Coral reefs • Pollen • Plankton

17. Plankton • Plankton are most useful as proxies 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.

18. How do we detect cold periods? Page 67 of textbook • Most oxygen atoms are the isotope O16. • However, around one in 500 atoms are the isotope O18. • O18 is heavier than O16, and will evaporate less readily. • During a warm period, the relative amount of O18 in the ocean will increase, as more of the O16 is evaporating • Conversely, O18 is preferentially removed by precipitation and snowfall. Colder Warmer

19. The early history of the Earth

20. The Sun’s intensity has increased The Sun’s intensity has increased by around 40% over the Earth’s lifetime, and yet the planet has remained hospitable to life.

21. The composition of the atmosphere • Early Earth: • stronger greenhouse, weaker solar radiation • Modern Earth: • weaker greenhouse, stronger solar radiation

22. The atmosphere and life on Earth • Since life first evolved, the atmosphere and life on Earth have been influencing each other. • Some of the earliest life was cyanobacteria (blue-green algae) which can still be found today.

23. Oxygen and the atmosphere • The concentration of oxygen in the atmosphere has gradually increased over time.

24. The evolution of the Earth’s climate • The Earth’s climate has alternated between: • greenhouse eras, when no ice sheets were present, without boiling its oceans and lakes. • icehouse eras, when ice sheets were present, without ever freezing solid.

25. 800 million years ago: Snowball Earth? • There is some evidence that the tropics were glaciated around 800 million years ago. • This has led to a hypothesis that the Earth was nearly frozen at that time. • This is called the Snowball Earth hypothesis, and is unproven and controversial.

26. How might Snowball Earth have happened? -50degC

27. The dinosaurs and Hothouse Earth

28. The evolution of life on Earth

29. Atmospheric carbon dioxide concentration

30. What caused the high CO2 concentrations? 1) 175 million years ago, Pangaea began to break apart into six continents similar to today’s. The extensive tectonic activity caused widespread volcanism. Volcanoes spew out a lot of carbon dioxide. • 2) The area of continents was smaller, as 20% of them were covered by higher sea levels. CO2 removal from weathering on land was dampened. 100 Million Years Ago

31. Relationship between CO2 and temperature

32. How do we know CO2 is important?

33. Relationship between CO2 and temperature

34. The climate 100 million years ago • Shows us what a high-CO2 world can look like. • The Earth was about 10oC warmer. • High latitudes were much warmer, and were completely free of ice.

35. Sea level changes and past climate

36. Cosmic rays and global climate

37. Dinosaurs prospered in “Hothouse Earth” 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).

38. The extinction of the dinosaurs • 10km asteroid hit Earth 65 million years ago, gouging a 180 km crater in North America. • Explosion equivalent to four times the energy of all currently-existing nuclear weapons. • Wiped out the dinosaurs.

39. The extinction of the dinosaurs • 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. Dust and soot injected into stratosphere takes months to years to settle back to the Earth’s surface. • Temperatures rapidly dropped 3-5oC. • Global-scale extinction of some 70% of species. • It took 1000 years for the climate system to recover.

40. Climate evolution and plate tectonics

41. Drivers of sea level changes

42. Glacial cycles

43. Glacial cycles • The current ice age began around 2.5 million years ago. • Since then, the world has seen cycles of glaciation. Ice sheets have advanced and retreated on a 41,000 year cycle and then on a 100,000 year cycle. • Glacials are when ice sheets advance; interglacials are when ice sheets retreat. • The Earth is currently in an interglacial.

44. Glacial cycles Why is there a distinct 100,000 year cycle?

45. Milankovic 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 changes in the Earth’s orbit. Milutin Milankovic (1879-1958)

46. Milankovic cycles • Earth's orbit is not perfectly circular, but rather is elliptical. • The Earth's orbital parameters vary over time because of gravitational attractions among Earth, the Moon, the Sun and other planets. • These attractions cause three variations: 1. Earth's angle of tilt. 2. Eccentricity of orbit around the sun. 3. Position of the solstices and equinoxes.

47. Obliquity • The tilt of the Earth's axis varies between 22.2°and 24.5°, with a period of 41,000 years. • The angle of tilt affects the amount of solar radiation received at each amplitude. • Increased tilt amplifies seasonal differences, decreased tilt reduces seasonal differences.

48. Eccentricity • The eccentricity of the Earth’s orbit varies between 0.005 and 0.0607, with periods of 100,000 years and 413,000 years.

49. Precession • The “wobbling” motion of the Earth’s axis is called axial precession. • One cycle takes 25,700 years.

50. Milankovic cycles and solar radiation The Milankovic cycles affect the amount of solar radiation received during summer at 65oN.