Module 4

1. Module 4 Changes in Climate

2. Global Warming? • Climate change • The pattern(s) of variation in climate (temperature, precipitation) over various periods of time • Global warming • The anticipated/apprehended increase in mean global temperature associated with the increase of greenhouse gases in the contemporary atmosphere

4. Historic Seal Level Rise

5. Looking back in time

6. A longer view

7. A much longer view

8. Instrumental Records • The earliest records of temperature measured by thermometers are from western Europe beginning in the late 17th and early 18th centuries. • The network of temperature collection stations increased over time and by the early 20th century, records were being collected in almost all regions, except for polar regions where collections began in the 1940s and 1950s.

9. Proxy Data Glacier ice cores

10. Other historical evidence

11. The last 1000 years • Recent reconstruction of NH air temperature based on tree-ring, ice core, coral and historical evidence.

12. Vostok Ice Core (Antarctica)

13. So … different scales of time

14. Glacial Cycles • By ~2.5 myr ago, temperatures began alternating by 4-10oC every 40,000 to 100,000 years (quasi-periodic cycles). • Extensive glacial ice began to occur during the cold phases • Eventually, massive ice sheets formed over the land areas of the NH during glacial periods. • During each glaciation, the sea level on Earth was reduced by 100-130 metres.

15. The last Ice Age

16. Vostok Ice Core (Antarctica)

17. Using oxygen isotope ratios Isotopes: atoms of different mass number (# neutrons in nucleus) When water evaporates, O18 is preferentially left behind, and O16 is preferentially stored in glacier ice Low values of the ratio correspond to a cold climate

18. The past 1 myr

19. The Ins and Outs of Ice Ages • The detailed, long term records of oxygen isotope data reveal that the climate tends to drift slowly into glacial conditions, while interglacials tend to develop suddenly in the space of a few hundred years.

20. The past 1 myr

21. Back 2.5 myr Should read ocean cores

22. Why are there glacial cycles? • The last 700,000 years are marked by wider swings in temperature that indicate a large shift in the amount of land ice present. • On the geologic time scale, the Earth seems to be getting cooler. • There is good evidence that variations in the Earth’s solar orbit are linked to glacial cycles.

23. Orbital forcing • The theory that large scale climate changes (glacials/interglacials) are due to the variations in precession, eccentricity and obliquity of the Earth’s solar orbit that affects the amount of solar radiation received at the surface of the Earth. • Attributed to Milankovitch

24. Orbital attributes • The Earth has three fundamental orbital attributes: • Changes in the tilt of the axis of rotation (termed the obliquity) • Changes in the shape of the elliptical orbit around the sun (termed eccentricity) • Changes in the date of the Earth’s closest approach to the Sun (termed precession of the equinox)

26. The tilt of the Earth’s axis varies

28. Periods of variation • The tilt of the Earth’s axis varies over a period of about 41,000 years • The cycle of orbital eccentricity is 90,000 to 100,000 years • The precession cycle of the equinox is about 23,000 years

29. Milankovitch Cycles

30. Back 2.5 myr Should read ocean cores

31. Milankovitch Cycles in the record

32. The previous figure indicates a dominant period in glacial variations of 41,000 years. There are other significant cycles at 96,000 480,000 (and 23,000) years. These correspond to the cycles (and harmonics) in the Earth’s orbital variations.

33. Global Annual Temperature Trends: 1901 - 1990 Source: Watson 2000

34. GlobalPrecipitation Trends (% per decade) 1900 - 1994 •= increasing, •= decreasing Source: Watson 2001

35. Extreme Precipitation Events in the U.S.