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  1. CHAPTER 18 Global Climate Change

  2. An introduction to global climate change • On August 29, 2005, Hurricane Katrina landed east of New Orleans • The costliest ($134 billion in damage) storm ever • The deadliest storm (killing 1,800) since 1928 • Leaving mountains of debris, ruined homes and lives • A month later, Hurricane Rita hit Louisiana and Texas • 2005 had a record 27 named storms • 2007 had 15 and 2008 had 18 named storms • There is a link between hurricanes and global warming • Warmer oceans create humid air, leading to hurricanes

  3. Number of category 4 and 5 hurricanes

  4. The atmosphere • Atmosphere: a collection of gases held by gravity around the Earth • Troposphere: the lowest level • Gases moderate the flow of energy to Earth • Gases are involved with biogeochemical cycling of elements • Ranges from 5 to 10 miles thick • Contains almost all the water vapor and clouds • Gets colder with altitude • Mixed air allows pollutants to reach the top of the layer

  5. Other atmospheric layers • Tropopause: caps the troposphere • Air shifts from cooling with height and begins to warm • Stratosphere: temperature increases with altitude • To 40 miles above Earth’s surface • Ozone (O3) absorbs the Sun’s high-energy radiation • Due to little mixing and no precipitation, substances remain for long periods of time • Mesosphere and thermosphere: declining ozone levels • Only small amounts of oxygen and nitrogen

  6. Structure and temperature of the atmosphere

  7. Weather • Weather: day-to-day variations in temperature, air pressure, wind, humidity, precipitation • Climate: the result of long-term regional weather patterns • Meteorology: the study of the atmosphere (weather and climate) • The atmosphere-ocean-land system is a huge weather engine • Driven by the Sun and affected by Earth’s rotation and tilt • Solar energy is reflected (29%) or absorbed by Earth • Absorbed energy heats the ocean, land, and atmosphere • Evaporation, convection, and reradiation of infrared energy release energy

  8. Solar-energy balance

  9. Flowing air • Some energy released from Earth goes to the atmosphere • Warmer air expands and rises, creating vertical air currents • Convection currents: large-scale vertical air movement • Horizontal air currents (wind) result from sinking cool air replacing rising warm air • Hadley cell: combination of rising warm air and sinking cool air • Creates regions of high rainfall (equator), deserts, and trade winds (horizontal winds)

  10. Convection • Convection currents: bring day-to-day weather changes • They move from west to east • Solar-heated rising air creates atmospheric high pressure • Leaving behind lower pressure closer to Earth • Moist high-pressure air cools (through reradiation and condensation) • Flowing horizontally to sinking cool, dry air regions of lower pressure • The air is warmed and creates a region of higher pressure • Differences in air pressure lead to airflows (winds)

  11. A convection cell

  12. Jet streams • Larger-scale air movements of Hadley cells are influenced by Earth’s rotation from west to east • Creating trade winds over oceans and the west-to-east flow of weather • Jet streams: rivers of air created higher in the troposphere from Earth’s rotation and air-pressure gradients • Move faster than 300 mph • Meander considerably • Can steer major air masses in the lower troposphere

  13. Put together … • Fronts: boundaries where air masses of different temperatures and pressures meet • Regions of rapid weather change • Other movements of air masses due to different pressures and temperatures: hurricanes, typhoons, tornadoes • Monsoons: major seasonal airflows • A reversal of previous wind patterns • Created by major differences in cooling and heating between oceans and continents • India’s summer monsoons bring rains and floods

  14. Climate is … • Climate: the general patterns of weather that characterize different regions of the world • Climate results from all the combined elements of • General atmospheric circulation patterns and precipitation • Wind and weather systems • Rotation and tilt of Earth, which creates seasons

  15. Climate change science • Biomes: reflect plant, animal, and microbe adaptations to the prevailing weather patterns (climate) of a region • Humans can adjust to almost any climate • But other organisms can’t • A major change in the climate represents a serious threat to the structure and function of existing ecosystems • Because humans depend on ecosystems for vital goods and services, we need functioning ecosystems

  16. Synopsis of global climate change • In 2007, scientists from the Intergovernmental Panel on Climate Change (IPCC) sifted through thousands of studies and published the Fourth Assessment Report (AR4) • The report concluded that warming of the climate is unequivocal • The atmosphere and oceans are warmer • Sea levels are rising and glaciers are melting • There are more extreme weather events

  17. Annual mean global surface temperature anomalies

  18. The IPCC’s report • The report concluded that it is very likely (90% probability) that warming is caused by human factors • Increased greenhouse gases (GHGs) trap infrared radiation • GHGs come from burning fossil fuels • Along with deforestation • The major GHG: CO2 • Responses to climate change • Mitigation: reducing GHG emissions • Adaptation: adjusting to climate change

  19. Atmospheric carbon dioxide concentrations

  20. IPCC • Founded in 1988 by the UN Environmental Program and the World Meteorological Society to provide accurate and relevant information leading to understanding human-induced climate change • Working Group I: assesses scientific issues of climate change • Working Group II: evaluates impacts and adaptation to it • Working Group III: investigates ways to mitigate its effects • The AR4 report had over 2,000 experts from 154 countries • Risk assessment: is the climate changing? • Risk management: how do we adapt and mitigate effects?

  21. Third assessment • The IPCC’s 2001 report showed • Increasing information shows a warming world • Humans are changing the atmosphere, which will affect climate • We have increased confidence in models of future climate change • Stronger evidence that most recent warming is human-caused • Human influences will continue to change the atmosphere • Temperature and sea levels are rising • We need more information and understanding

  22. A Nobel Effort • AR4 produced Climate Change 2007: Synthesis Report • Contained key findings of the working groups • The 2007 Nobel Peace Prize went to the IPCC and former Vice President Al Gore: the leading advocate of the need to take action on climate change • For their efforts to disseminate knowledge about man-made climate change and to lay groundwork to counter it • Gore also was awarded the Academy Award for his film An Inconvenient Truth

  23. Climates in the past • It is harder to find evidence of climate change the further into the past we search • Records of temperature, precipitation, storms have been kept for only 100 years • Since 1880, especially since 1976, our climate warmed • Proxies: records providing information on climate • Using temperature, ice cover, precipitation, tree rings, pollen, landscapes, marine sediments, corals, etc. • Earth warmed from 1100 to 1300 A.D. • Little Ice Age: 1400–1850

  24. Ice cores • Analyzing ice cores from Greenland and the Antarctic shows global climate can change within decades • Uses CO2 and CH4 (methane) and isotopes of O and H • Climate oscillates between ice ages and warm periods • Ice ages tie up water in glaciers, lowering sea levels • 8 glacial periods occurred over the past 800,000 years • Ice ages have lower GHGs and temperatures • CO2 levels ranged between 150 and 280 ppm • Milanovitch cycles: climate oscillations due to Earth’s orbit • Periodic intervals of 100,000, 41,000, and 23,000 years

  25. Past climates, as determined from ice cores

  26. Rapid changes • Rapid climatic fluctuations are superimposed on the major oscillations during glaciation and warmer times • The Younger Dryas event: 11,700 years ago • Dryas: a genus of arctic flower • Arctic temperatures rose 7ºC in 50 years • Caused enormous impact on living systems • Warming was not caused by changing solar output

  27. Climate fluctuations

  28. Oceans and climate • Oceans play a dominant role in determining climate • They are a major source of water and heat • Evaporation: supplies water vapor to the atmosphere • Condensation: supplies heat to the atmosphere • Heat capacity: oceans absorb energy with heated water • Oceans convey heat through currents • Thermohaline circulation pattern: the effects of temperature and salinity on the density of seawater • This giant, complex conveyor belt moves water from the surface to deep oceans and back

  29. Thermohaline circulation • High-latitude North Atlantic ocean flows from the Gulf Stream north on the surface and is cooled by Arctic air • North Atlantic Deep Water (NADW): the cool water increases in density, so it sinks (up to 4,000 m) • The current spreads to Africa’s southern tip • It is joined by cold Antarctic waters • The two streams spread north into the Indian and Pacific Oceans as deep currents • The currents slow and warm and rise to the surface • Move back to the North Atlantic

  30. The oceanic conveyor system

  31. Thermohaline circulation affects climate • The movement of warm water toward the North Atlantic transfers enormous amounts of heat toward Europe, providing a much warmer than expected climate • The circulation pattern cycles over 1,000 years • It is vital to maintaining current climate conditions • In the past, the conveyor system has been interrupted • Abruptly changing the climate • Large amounts of fresh water lower water’s density • Preventing the sinking of surface waters • Slowing the northern movement of warmer, saltier water

  32. Heinrich events • Heinrich events: fresh water from melting icebergs from the polar ice cap dilutes salt water • Six times in the past 75,000 years • Diluted water doesn’t sink • The conveyor system is shifted southward to Bermuda (instead of Greenland) • The climate cools in a few decades • Return of the normal pattern abruptly warms the climate • The Younger Dryas event involved dammed-up water from glacial Lake Agassiz entering the St. Lawrence

  33. What if …? • Extended global warming will • Increase precipitation over the North Atlantic • Melt sea ice and ice caps • The conveyor will decrease over the 21st century • The Achilles’ heel of our climate system: weakening of the conveyor and a changed climate • Especially in the northern latitudes

  34. Ocean-atmosphere oscillations • These processes produce globally erratic climates • The North Atlantic Oscillation (NAO): atmospheric pressure centers switch back and forth across the Atlantic • Switching wind and storms • El Niño/La Niña Southern Oscillation (ENSO): shifts in atmospheric pressure over central equatorial Pacific Ocean • Dominates global climate for over a year at a time • 1997–2000 ENSO cost $36 billion and killed thousands • Interdecadal Pacific Oscillation (IPO): a warm-cool cycle that swings over the Pacific over several decades

  35. More ocean-atmosphere oscillations • Understanding these processes helps clarify some puzzling developments • A global warming trend was stopped in the mid-1940s • But resumed 30 years later • This mid-century cooling resulted from shifting ocean circulation linked to the NAO • Ocean-atmosphere oscillations can offset global warming • But greenhouse warming will overcome them and intensify by the end of this decade

  36. The Earth as a greenhouse • Factors that influence climate • Internal components: oceans, atmosphere, snow, ice • External factors: solar radiation, Earth’s rotation and orbit, gaseous makeup of the atmosphere • Radiative forcing: the influence of any factor on the energy balance of the atmosphere-ocean-land system • Positive (negative) forcing: leads to warming (cooling) • Forcing is measured in Watts/m2 • Solar radiation entering the atmosphere = 340 W/m2 • Radiation is acted on by forcing factors

  37. Warming processes • Greenhouse gases (GHGs): water vapor, CO2, other gases • Light energy goes through the atmosphere to Earth • Earth absorbs and converts energy to heat • Infrared heat energy radiates back to space • GHGs (but not N2 and O2) in the troposphere absorb some infrared radiation • Direct it back to Earth’s surface • The greenhouse effect was first recognized in 1827 • It is now firmly established

  38. GHGs insulate Earth • GHGs delay the loss of infrared heat (energy) • Without insulation, Earth would be -19°C instead of +14°C • Life would be impossible • Earth’s global climate depends on the concentration of GHGs • Changing amounts of GHGs change positive forcing agents, which would change the climate • Tropospheric ozone has a positive forcing effect • Varying with time and location

  39. The greenhouse effect

  40. Cooling processes • Planetary albedo: sunlight reflected by clouds • Contributes to overall cooling by preventing warming • Low-flying clouds have a negative forcing effect • High-flying, wispy clouds have a positive forcing effect • Absorb solar radiation and emit infrared radiation • Snow and ice contribute to albedo by reflecting sunlight • Black carbon soot darkens snow and ice • Dark snow/ice absorbs radiant energy instead of reflecting it • Reduces albedo

  41. Volcanoes and aerosols • Volcanic activity can lead to planetary cooling • Reflects radiation from particles and aerosols • Aerosols: microscopic liquid or solid particles from land or water • Industrial aerosols (pollution) cancel some GHG warming • Sulfates, nitrates, dust, soot from industry and forest fires • Sooty aerosols (from fires): warming effect • Sulfate aerosols create cooling through more clouds • Reduced pollution in the U.S. and Europe decreased aerosols • China’s and India’s pollution has increased aerosols

  42. Global warming and cooling

  43. Solar variability • Variation in the Sun’s radiation influences the climate • Changes in solar radiation occur on 11-year cycles • Solar radiation increases during high sunspot activity • Sunspots block cosmic ray intensity • Reduce cloud cover and increase solar radiation • Solar output declined in 1985 and continued for 20 years • But global temperatures rose rapidly • The IPCC AR4 concluded that GHGs were 13 times more responsible for warming temperatures than solar changes

  44. Thus … • Global atmospheric temperatures are a balance between positive and negative forcing from natural causes (volcanoes, clouds, natural GHGs, solar irradiance) and forcing from anthropogenic causes (sulfate aerosols, soot, ozone, increased GHGs) • Forcing agents result in climate fluctuations • It is hard to say any one event or extreme season is due to humans • But climate has shifted enough to generate international attention

  45. Evidence of climate change • Weather varies naturally year to year • Local temperatures may not follow global averages • But the 10 warmest years on record were 1997–2008 • 2005 set a record high—the warmest since the late 1800s • The average global temperature has risen 0.6°C since the mid-1970s (0.2°C/decade) • Warming is happening everywhere • Most rapidly at high latitudes of the Northern Hemisphere • The warming is a consequence of an “enhanced greenhouse effect”

  46. Satellites • Measurements of tropospheric temperatures by 13 satellites over 20 years did not show temperature increases • Skeptics made much of this discrepancy • The Climate Change Science Program (CCSP) • Created in 2002 to address unresolved questions about climate change • Its 2006 report stated that errors caused the discrepancy • Corrected and new data agree that surface and tropospheric temperatures are rising

  47. Ocean warming • Recently, the upper 3,000 meters of the ocean have warmed • Dwarfing warming of the atmosphere • 90% of the heat increase of Earth’s systems • Over the last decade, oceans have absorbed most of the non-atmospheric heat • A long-term consequence: the impact of this stored heat as it comes into equilibrium with the atmosphere • It will increase atmospheric and land heat even more • A short-term consequence: unprecedented rising sea levels • Thermal expansion and melting glaciers and ice caps

  48. Heat capture by the oceans

  49. The rise in global mean sea level

  50. Other observed changes by the IPCC AR4 • Changes are consistent with GHG-caused climate change • Increased warm temperature extremes • Decreased cold temperature extremes • Spring comes earlier, fall later, in the Northern Hemisphere • Ecosystems are out of sync • Tree deaths and insect damage • Heat waves are increasing in intensity and frequency • Droughts are increasing in intensity and frequency • 60% of the U.S. is in a drought that started in the 1990s