1. The Earth's Changing Climate Chapter 16

2. Climate of the earth has always changed over time. Currently evidence shows that the earth is warming, this can be from natural occurrences but also from human (anthropogenic) sources. Climate Change

3. Early humans evolved in ice ages. As little as 18,000 years ago, the earth was covered with huge ice sheets, covering much of North America and Europe. Over the last 2.5 million years, it is thought that the glaciers advanced and retreated repeatedly, leaving geologic evidence in rocks as well as evidence in ice cores. Ice Ages

4. Figure16-2 p459 Extent of glaciation about 18,000 years ago over (a) North America and over (b) western Europe.

5. Figure16-3 p459 If all the ice locked up in glaciers and ice sheets today were to melt, estimates are that this coastal area of south Florida would be under 65 m (213 ft) of water. Even a relatively small one- meter rise in sea level would threaten half of the world’s population with rising seas. Sea level may rise one meter or more by the end of this century due to the rapid melting of ice in Greenland and Antarctica.

6. Techniques for reconstructing past climates: –Geological evidence from glaciers Striations in rocks can show glaciers retreating, as they scrape the rocks –Fossil pollen from ancient plants –CLIMAP Climate: long-range investigation mapping and prediction, multi-university research project Reconstructing Past Climates

8. Study of ocean sediments with shells of organisms that live in narrow temperature ranges –Oxygen isotopes- 16 O vs. 18 O – 18 O is heavier and tends stay in ocean, not to evaporate or become locked into glaciers –Shells have higher 18 O concentrations in colder climates Reconstructing Past Climates

9. Ice cores –Oxygen isotopes in ice cores –Colder air has higher concentration of 16 O in snow –Bubbles of air can have carbon dioxide, methane, and other greenhouse gases that can show the atmosphere composition –Sulfuric acid can show volcanic eruptions –Ice cores can be dated to provide timelines –Isotopes can show solar activity Reconstructing Past Climates

10. Reconstruction past climates from ice cores

11. Dendrochronology –Study of tree rings –Changes in thickness of ring can show climatic changes –Frost rings in cold periods –Stresses from temperature and moisture Reconstructing Past Climates

12. Figure16-5 p460

13. Other methods: –Lake sediments –Study of pollen –Corals –Stalactites in caves –Hydrogen in ice cores Reconstructing Past Climates

14. Warm conditions for most of earth’s history have been interrupted periodically by ice ages. The most recent ice age began 2.5 million years ago- the Pleistocene epoch, called the Ice Age. Climate Throughout the Ages

15. 65mya, earth was warmer with no ice caps. Earth started cooling around 55 mya. 10 mya, ice began covering the Antarctic. 2.5 mya, glaciers appeared in the Northern Hemisphere, beginning the Pleistocene epoch. 18,000 years ago, glaciers in N. America reached maximum thickness. Ice ages can have interglacial periods, warmer periods lasting up to 10,000 years in between glacial advances. History of Ice Age

16. 18,000 years ago: average temperatures in Greenland were 10°C lower than present, and sea level could have been 120m lower than it is now. Bering land bridge connecting Siberia to Alaska allowed humans and animals to migrate to North America. History of Ice Age

17. 14,000 y.a., warm spell called the Bolling- Allerod period occurred, until 12,700y.a. when it suddenly became glacial again, called the Younger Dryas. Over the next 10,000 years, the earth cooled and warmed periodically, warming during mid-Holocene maximum, and cooling over the Little Ice Age (cold period in Europe during the 15 th -19 th centuries)

18. Figure16-6 p461 Relative air temperature variations (warmer and cooler periods) during the past 18,000 years.

19. Figure16-7 p462 The average temperature variations over the Northern Hemisphere for the last 1000 years relative to the 1961 to 1990 average (zero line). Yearly temperature data from tree rings, corals, ice cores, and historical records are shown in blue. Yearly temperature data from thermometers are in red.

20. Modern temperatures –20 th century was the largest increase in temperature of any century during the past 1000 years –0.6ºC raise in global temperature during the 20 th century (1ºF), called global warming. –In the 21 st century, may be around 2ºC (3.6ºF) –For comparison, global temperatures have not varied more than 2ºC over the past 10,000 years. Climate Throughout the Ages

21. Why does earth’s climate change? External reasons: 1.Change in incoming radiation 2.Change in composition of the atmosphere 3.Change in earth’s surface Also internal changes: 1.Ocean and air circulation patterns that can redistribute energy. These mechanisms can be interrelated. Climate Change Causes

22. Positive feedback mechanisms: an increase in heating is reinforced by other processes. Water vapor-greenhouse gas feedback (+) –As earth heats, more water evaporates, and water is a greenhouse gas which leads to more warming, which leads to more evaporation. –If left undisturbed, all water would evaporate. This would be called a runaway greenhouse effect. Feedback Mechanisms

23. Snow-albedo feedback –As snow and ice melt, the albedo of the surface is reduced. This allows more solar energy to reach the surface, which heats it up and raises the temperature, melting more ice, which reduces albedo more and heats up more. –In cooling planet, ice increases and reflects more heat back, cools more, makes more ice. This is called a runaway ice age. Feedback Mechanisms

24. Negative feedback mechanisms: weaken the interactions among variables rather than reinforce them. Infrared radiation (-): warming planet emits more infrared radiation, cooling the surface. Chemical weathering CO 2 feedback (-): carbon dioxide is removed from the atmosphere as silicate minerals decompose in presence of moisture. Generally happens on warmer planet because of more evaporation/precipitation Climate Change Caused by Natural Events

25. In polar regions snow reflects much of the sun’s energy back to space.

26. If the air temperature were to gradually increase, some of the snow would melt, less sunlight would be reflected, and more sunlight would reach the ground, warming it more quickly.

27. Figure16-9c p465 The warm surface would enhance the snow melt which, in turn, would accelerate the rise in temperature.

28. –Theory of plate tectonics –As continents move, ocean circulation changes, latitude changes –Volcanic activity venting CO 2, earlier earth may have had more activity –Mountain interaction with airflow and ocean currents Climate Change Caused by Natural Events

29. Figure16-11 p467

30. –Milankovitch Theory: Variations in the earth’s orbit influence climate change, including: 1.Eccentricity: changes in the shape of the earth’s orbit around the sun 2.Precession of the earth’s axis of rotation, or wobbling. 3.Obliquity: changes in the tilt of the earth’s axis. Climate Change Caused by Natural Events

31. Eccentricity- changes in the shape of the earth’s orbit around the sun. Every 100,000 years goes from being elliptical to circular, The greater the eccentricity, greater variation in solar energy (seasons) Currently low eccentricity, circular, closest in January and farthest in July, changes the distribution of heat energy. Climate Change Caused by Natural Events

32. Figure16-12 p468 For the earth’s orbit to stretch from nearly a circular (solid line) to an elliptical orbit (dashed line) and back again takes nearly 100,000 years.

33. Precession- as the earth rotates on its axis, it wobbles like a spinning top. Occurs in a cycle of 23,000 years. Presently, earth is closer to sun in January and farther in July, but will be reversed in 11,000 years. Obliquity – changes in tilt of the earth’s orbit. Every 41,000 years the tilt varies. Climate Change Caused by Natural Events

34. Figure16-13 p468 Precession: Like a spinning top, the earth’s axis of rotation slowly moves and traces out the path of a cone in space.

35. Figure16-14 p469 Obliquity: The earth currently revolves around the sun while tilted on its axis by an angle of 23 1⁄2 °. During a period of 41,000 years, this angle of tilt ranges from about 22° to 24 1⁄2 °.

36. According to Milankovitch cycles, high latitudes of the Northern Hemisphere should trend towards cooler summers over high latitudes at the present time. Scientists have found strong evidence that climate changes over the past several hundred thousand years have followed these cycles. Milankovitch Cycles

37. Greenhouse gases such as CO 2 and methane can counteract global cooling from Milankovitch cycles. Other factors include: –Dust and aerosols in the atmosphere –Reflectivity of ice sheets –Concentration of other greenhouse gases –Changing characteristics of clouds –Rebounding of land from ice Milankovitch Cycles

38. Variations in Solar Output –Sunspots- magnetic storms on the sun that cause the generation of more radiation, occurring in cycles of 11 years. –Emits around 0.1% more energy during maximum –Accounts for small climatic changes over decades and centuries. Climate Change Caused by Natural Events

39. Atmospheric Particles –Aerosols = natural + anthropogenic –Volcanic eruptions: fine particles of ash and dust are ejected into the atmosphere. –Greatest impact from volcanoes is thought to be from sulfur gas, which combines with water vapor making visible sulfuric acid particles, forming a thick haze. –The haze can stay for several years, reflecting the sun’s energy, and cooling the earth. Climate Change Caused by Natural Events

40. Figure16-17 p471

41. Volcanic eruptions: Mount Pinatubo in Philippines in 1991 –20 million tons of sulfur dioxide –In 1992, mean surface temperature had decreased by 0.5°C –Can also increase lower stratosphere temperature by absorbing radiant energy from sun and earth, can create stratospheric winds. Climate Change Caused by Natural Events

42. Figure16-18 p471 Changes in average global air temperature from 1990 to 1992.