Climate Change

1. Climate Change

2. Introduction • Climate is always changing. • Evidence shows that climate has changed in past, and nothing suggest that it will not continue to change. • Climate is the average weather, including seasonal extremes and variations, either locally, regionally, or across the globe. • Climate is controlled by the long-term balance of energy of the Earth and its atmosphere. Winds and ocean currents redistribute heat over the surface of the Earth. Natural events cause changes in climate. Human activities can also change the climate. What is different?

3. Outline • Investigate how the global climate has changed (Monday) • Examine some theories on why it has changed (Wednesday)

4. Earth’s Changing Climate • Not only is the Earth’s climate always changing, but a mere 18,000 years ago the earth was in the grip of a cold spell. • Some scientists believe we are still in an Ice Age, but in the comparatively warmer part of it. • Presently, glaciers cover only about 10% of Earth’s land surface; most of this ice is in the Greenland and Antarctic ice sheets.

5. Determining Past Climates How do climatologists gain information about the past? Scientist examine and piece together all the available evidence to understand what the climate was like in the past. • Core samples taken from ocean floor sediments • Ice cores from glaciers in Antarctica and Greenland • Oxygen-isotope ratio of marine organisms shells • Annual growth of rings of trees

6. Sea Surface Isotherms August 18,000 years ago August today

7. Examine Temperature Records of Earth’s Surface

8. Climate Through the Ages • Throughout much of the Earth’s history, global climate was probably between 8-15C warmer than it is today. • During most of this time, polar regions were free of ice. • These comparatively warm condition, however, were interrupted by several periods of glaciation. • Geologic evidence suggests that glacial period occurred approximately • 700 million years ago (mya) • 300 mya • 2 mya (most recent – Pleistocene epoch, Ice Age)

9. Climatic Conditions Leading up to Pleistocene • About 65 mya, Earth was warmer than it is now; ice caps did not exist. • Beginning about 55 mya, Earth entered a long cooling trend. • After millions of years, polar ice appears. • As average temperatures continued to lower, ice grew thicker • About 10 mya, a deep blanket of ice covered Antarctica • Snow and ice began to accumulate in high mountain valleys of Northern Hemisphere, and alpine glaciers soon appeared. • About 2 mya, continental glaciers appeared in Northern Hemisphere,, marking the beginning of Pleistocene Epoch. • Pleistocene was not a period of continuous glaciation, but a time when glaciers alternatively advanced and retreated (melted back) over large portions of North America and Europe. • Between glacial advances were warmer periods called Interglacial periods, which lasted for 10,000 years or more.

10. Pleistocene • Most recent North American glaciers reached their maximum thickness and extent about 18,000 years ago (ya) • At this time, sea level was perhaps 125m (395ft) lower than it is now. • Lower sea level exposed vast areas of land. • Bering land bridge (a strip of land that connected Siberia and Alaska) allowed human migration from Asia to North America. • Ice began to retreat about 14,000 ya as surface temperatures slowly rose. • Then, about 11,000 ya, average temperature suddenly dropped and northeastern North America and Northern Europe reverted back to glacial conditions. • About 1000 years later, cold spell (known as the Younger-Dryas) ended abruptly, and by 8000 ya continental ice sheets over North America were gone.

11. Holocene Epoch • From about 6000 – 5000 ya, climate was probably 1 warmer than at present. • Represents warmest of current interglacial period – mid-Holocene maximum (climatic optimum) • About 5000 ya, a cooling trend set in, during which extensive alpine glaciers returned, but not continental ice sheets.

12. Climate During Last 1000 Years • About 1000 ya, Northern Hemisphere was relatively warm and dry. • Sometime around 1200 AD, mild climate of western Europe began to show extreme variations. • For several hundred years, climate grew stormy • Both great floods and droughts occurred • Around 1400-1550, climate moderated • However, starting in middle 1550s, average temperature began to drop. Cooling trend known as Little Ice Age continued for almost 300 years. • “The year without a summer” - 1816

13. Climate During Last 100 Years • In late 1800s, average global temperatures began to rise. • From 1900 until 1940, average temperature of lower atmosphere rose nearly 0.5C. • Following the warmer period, Earth began to cool slightly over next 25 years. • In late 1960s and 1970s, cooling trend ended over most of Northern Hemisphere. • During 1970s and into 1980s, average yearly temperatures showed considerable fluctuation from year to year and from region to region, with overall trend pointing to warming. • Warming trend continued into 1990s, with the 8 hottest years of this century since 1979. Over the last 100 years the Earth’s surface has warmed by about 0.6C. - Warming is not uniform, as Northern Hemisphere has warmed less than Southern Hemisphere - U.S. has experienced little warming compared to rest of world. - Moreover, most of warming has occurred at night. When urban warming taken into account and improved SST information incorporated into data, warming over past 100 years measures between 0.3-0.6C. During past century, Earth has been in a warming trend.

14. Temperature Increase Significant? • A global increase in temperature between 0.3-0.6C may seem very small, but from global temperature figures, we can see that global temperatures varied no more than 1.5C during the past 10,000 years. • Consequently, an increase of 0.6C becomes significant when compared with temperatures changes over thousands of years.

15. Climate Change • Earth’s climate is not totally understood. • Many theories attempt to explain the changing climate, but no single theory alone can satisfactorily account for all the climate variations in the geologic past. • Why hasn’t the riddle of a fluctuating climate been completely solved? Intricate interrelationships of the Earth-atmosphere system

16. Feedback Mechanisms Positive feedback Initial increase/decrease is reinforced by other processes • Water-vapor temperature rise • Snow-albedo Negative feedback Tend to weaken the interactions among variables rather than reinforce them

17. Water Vapor-Temperature Rise Feedback Mechanism Temperature slowly rises  water from oceans rapidly evaporate into warmer air  increased quantity of water vapor  absorbs more of Earth’s infrared energy  raises air temperature even more  further increases evaporation rates Positive feedback mechanism

18. Snow-Albedo Feedback Increase surface air temperature  snow and ice melt  reduce albedo  all more energy absorbed at surface  raise surface temperature Positive feedback mechanism

19. Negative Feedback Mechanism As the surface warms:  more water evaporates from oceans  global low cloudiness increases  low clouds reflect incoming sunlight  less solar energy to heat the surface  warming slows

20. Current Theories of Climate Change External Causes of Climate Change • Variations in Solar Output • Variations in Earth’s Orbit Internal Causes of Climate Change • Theory of Plate Tectonics • Atmospheric Particles • Carbon Dioxide (CO2)

21. Variations in Solar Output • Radiometers aboard satellites suggest that Sun’s energy output varies considerably more than once thought. • Sun’s energy appears to change slightly with sunspot activity • Occur in cycles, with number and size reaching a maximum approximately every 11 years. • During periods of maximum sunspots, sun emits more energy than during periods of sunspot minimums

22. Theory of Plate Tectonics • Earth’s outer sheet is composed of huge plates that fit together like pieces of a jigsaw puzzle. Plates slide over a partially molten zone below them, move in relation to one another • Rate of motion is extremely slow, only a few centimeters per year • Now existing continents were at one time joined together in a single huge continent which broke apart. • Plate tectonics help to explain past climates. For example: • Glacial features found near sea level in Africa suggest area underwent period of glaciation hundred of million years ago. Ice sheets formed when African land mass was located at a much higher latitude. • Fossil remains of tropical vegetation found under layers of ice in polar regions today.

23. Variations in Earth’s Orbit Milankovitch Theory – named for astronomer Milutin Milankovitch who first proposed idea in 1930s As the Earth orbits the sun, three separate cyclic movements combine to produce variations in amount of solar energy that falls on the Earth • Eccentricity • Precession • Obliquity

24. Eccentricity • Changes in shape of Earth’s orbit as it revolves about the Sun • Earth’s orbit changes from being elliptical to being nearly circular • 100,000 years to go from less elliptical to more elliptical and back again • Greater the eccentricity of orbit, greater the variation in solar energy received at the top of the atmosphere between Earth’s closest and farthest approach to Sun

25. Precession • As the Earth rotates on its axis, it wobbles like a spinning top. • Change in orientation of Earth axis directly alters timing and intensity of seasons. • Occurs in a cycle of about 23,000 years.

26. Obliquity • Relates to changes in tilt of the Earth as it orbits the sun. • Cycle takes about 41,000 years to complete

27. Milvankovitch Cycles • Combine to produce variations in solar radiation received at the Earth’s surface. • In 1970s, scientists found strong evidence in deep-ocean sediments that variations in climate during past several 100,000 years were closely associated with these cycles. For example: Studies conclude that during the past 800,000 years, ice sheets have peaked about every 100,000 years which corresponds naturally to variations in Earth’s eccentricity. Superimposed on this are smaller ice advances that show up at intervals of 41,000 and 23,000 years. It appears that eccentricity is the forcing factor as it appears to control the severity of climatic variation. But orbital changes alone are probably not totally responsible for ice buildup and retreat.

28. Aerosols • Tiny liquid and solid particles that enter the atmosphere from both anthropogenic (human induced) and natural sources • Factory and automobile emissions • Agricultural burning • Wildfires • Effect is complex and depends on particle’s size, shape, color and vertical distribution above the surface Tropospheric aerosols • Absorb sunlight and infrared radiation from Earth’s surface, hence warming the air • Reflect and scatter incoming sunlight back to space which reduces amount of shortwave energy reach surface, causing a cooling during daytime. • At night, absorption and re-emission of longwave infrared radiation produce a net warming of surface air. • Serve as condensation nuclei, potential for altering physical characteristics of clouds Sulfate aerosols reflect incoming sunlight which tends to lower Earth’s surface temperature during day. Stratospheric Aerosols • Volcanic eruptions eject fine particles of ash and dust into stratosphere • Absorb and reflect back to space sun’s incoming energy • Effect cause warming in stratosphere and cooling of global surface air temperature.

29. Carbon Dioxide (CO2) • Greenhouse gas that strongly absorbs infrared radiation • Plays major role in warming lower atmosphere • Increasing steadily in atmosphere • Burning of fossil fuels • Deforestation – burning and replaced with less efficient plans Estimates Annual average - 355 parts per million in 1993 Forecasted to double within next century Climate model experiments predict that doubling CO2 will result in a global warming of surface air temperature between about 1.5 and 4.5C over next 50 years. Uncertainties in our understanding of climate and models.