Weather climatestehrmay2011


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  • Visible image of Venus
  • Radar composite of the surface of Venus
  • This illustrates one of the biggest problems with GEOENGINEERING, or altering/managing the climate using artificial means. Most geoengineering approaches affect the incoming visible light. The excess greenhouse effect reflects problems with the outgoing infrared. So every geoengineering approach addresses the problem in a way that is fundamentally different from the way it originated. There are also moral and societal issues with how much you want to geoengineer—if you can pick your climate, which one do you choose? Is it different if you’re poor or wealthy? If you live in the Arctic or the tropics?
  • So the greenhouse effect is actually a *good* thing. It’s just an excessive greenhouse that’s a problem.
  • Those in red are GCOS, Global Climate Observing System, sites
  • Every 10 years, we improve by about a day’s warning for large scale features: storm system tracks, hurricane tracks, but not features dictated by small-scale processes (e.g. hurricane strength).
  • S. Arrhenius from Philosophical Magazine and Journal of Science, “On the Influence of Carbonic Acid in the Air upon the Temperature of the Ground”, Phil. Mag. S 4. Vol. 41. No. 251. April 1896.
  • FAQ 2.1, Figure 1. Atmospheric concentrations of important long-lived greenhouse gases over the last 2,000 years. Increases since about 1750 are attributed to human activities in the industrial era. Concentration units are parts per million (ppm) or parts per billion (ppb), indicating the number of molecules of the greenhouse gas per million or billion air molecules, respectively, in an atmospheric sample. (Data combined and simplified from Chapters 6 and 2 of this report.)
  • Figure 3.1. Annual anomalies of global land-surface air temperature (°C), 1850 to 2005, relative to the 1961 to 1990 mean for CRUTEM3 updated from Brohan et al. (2006). The smooth curves show decadal variations (see Appendix 3.A). The black curve from CRUTEM3 is compared with those from NCDC (Smithand Reynolds, 2005; blue), GISS (Hansen et al., 2001; red) and Lugina et al. (2005; green).
  • Figure 5.14. Variations in global mean sea level (difference to the mean 1993 to mid-2001) computed from satellite altimetry from January 1993 to October 2005, averaged over 65°S to 65°N. Dots are 10-day estimates (from the TOPEX/Poseidon satellite in red and from the Jason satellite in green). The blue solid curve corresponds to 60-day smoothing. Updated from Cazenave and Nerem (2004) and Leuliette et al. (2004).
  • Source: IPCC, 2007
  • Dark shaded: range of average response Light shaded: range of response not allowing for feedbacks associated with land ice changes Black lines: range of response considering land ice changes (note: West Antarctic ice sheet highly uncertain) March 24, 2006 Science Overpeck and Otto-Bliesner Conditions in 2100 may be as warm as those 130,000 years ago, when sea level rose ~6 m. Melting of Greenland ice sheet raised sea level by ~2.2 to 3.4 m.
  • Greenland glaciers—moulins are meltwater plunging to the core.
  • 1m sea level rise
  • 6 m sea level rise: Losing roughly west Antarctic ice sheet + Greenland
  • FAQ 1.2, Figure 1. Schematic view of the components of the climate system, their processes and interactions.
  • Figure 10.4. Multi-model means of surface warming (relative to 1980–1999) for the scenarios A2, A1B and B1, shown as continuations of the 20th-century simulation. Values beyond 2100 are for the stabilisation scenarios (see Section 10.7). Linear trends from the corresponding control runs have been removed from these time series. Lines show the multi-model means, shading denotes the ±1 standard deviation range of individual model annual means. Discontinuities between different periods have no physical meaning and are caused by the fact that the number of models that have run a given scenario is different for each period and scenario, as indicated by the coloured numbers given for each period and scenario at the bottom of the panel. For the same reason, uncertainty across scenarios should not be interpreted from this figure (see Section for uncertainty estimates).
  • Figure TS.5. (a) Global mean radiative forcings (RF) and their 90% confidence intervals in 2005 for various agents and mechanisms. Columns on the right-hand side specify best estimates and confidence intervals (RF values); typical geographical extent of the forcing (Spatial scale); and level of scientific understanding (LOSU) indicating the scientific confidence level as explained in Section 2.9. Errors for CH 4 , N 2 O and halocarbons have been combined. The net anthropogenic radiative forcing and its range are also shown. Best estimates and uncertainty ranges can not be obtained by direct addition of individual terms due to the asymmetric uncertainty ranges for some factors; the values given here were obtained from a Monte Carlo technique as discussed in Section 2.9. Additional forcing factors not included here are considered to have a very low LOSU. Volcanic aerosols contribute an additional form of natural forcing but are not included due to their episodic nature. The range for linear contrails does not include other possible effects of aviation on cloudiness. (b) Probability distribution of the global mean combined radiative forcing from all anthropogenic agents shown in (a). The distribution is calculated by combining the best estimates and uncertainties of each component. The spread in the distribution is increased significantly by the negative forcing terms, which have larger uncertainties than the positive terms. {2.9.1, 2.9.2; Figure 2.20}
  • SPM.6. Projected surface temperature changes for the late 21st century (2090-2099). The map shows the multi-AOGCM average projection for the A1B SRES scenario. Temperatures are relative to the period 1980-1999. {Figure 3.2}
  • Box 11.1, Figure 2. Robust findings on regional climate change for mean and extreme precipitation, drought, and snow. This regional assessment is based upon AOGCM based studies, Regional Climate Models, statistical downscaling and process understanding. More detail on these findings may be found in the notes below, and their full description, including sources is given in the text. The background map indicates the degree of consistency between AR4 AOGCM simulations (21 simulations used) in the direction of simulated precipitation change. (1) Very likely annual mean increase in most of northern Europe and the Arctic (largest in cold season), Canada, and the North-East USA; and winter (DJF) mean increase in Northern Asia and the Tibetan Plateau. (2) Very likely annual mean decrease in most of the Mediterranean area, and winter (JJA) decrease in southwestern Australia. (3) Likely annual mean increase in tropical and East Africa, Northern Pacific, the northern Indian Ocean, the South Pacific (slight, mainly equatorial regions), the west of the South Island of New Zealand, Antarctica and winter (JJA) increase in Tierra del Fuego. (4) Likely annual mean decrease in and along the southern Andes, summer (DJF) decrease in eastern French Polynesia, winter (JJA) decrease for Southern Africa and in the vicinity of Mauritius, and winter and spring decrease in southern Australia. (5) Likely annual mean decrease in North Africa, northern Sahara, Central America (and in the vicinity of the Greater Antilles in JJA) and in South-West USA. (6) Likely summer (JJA) mean increase in Northern Asia, East Asia, South Asia and most of Southeast Asia, and likely winter (DJF) increase in East Asia. (7) Likely summer (DJF) mean increase in southern Southeast Asia and southeastern South America (8) Likely summer (JJA) mean decrease in Central Asia, Central Europe and Southern Canada. (9) Likely winter (DJF) mean increase in central Europe, and southern Canada (10) Likely increase in extremes of daily precipitation in northern Europe, South Asia, East Asia, Australia and New Zealand. (11) Likely increase in risk of drought in Australia and eastern New Zealand; the Mediterranean, central Europe (summer drought); in Central America (boreal spring and dry periods of the annual cycle). (12) Very likely decrease in snow season length and likely to very likely decrease in snow depth in most of Europe and North America.
  • Snow: (nobody has them up to date yet)
  • Figure 11.12. Temperature and precipitation changes over North America from the MMD-A1B simulations. Top row: Annual mean, DJF and JJA temperature change between 1980 to 1999 and 2080 to 2099, averaged over 21 models. Middle row: same as top, but for fractional change in precipitation. Bottom row: number of models out of 21 that project increases in precipitation.
  • Milankovitch, M. 1920. Theorie Mathematique des Phenomenes Thermiques produits par la Radiation Solaire. Gauthier-Villars Paris.Milankovitch, M. 1930. Mathematische Klimalehre und Astronomische Theorie der Klimaschwankungen, Handbuch der Klimalogie Band 1 Teil A Borntrager Berlin.Milankovitch, M. 1941 Kanon der Erdbestrahlungen und seine Anwendung auf das Eiszeitenproblem Belgrade. 
(New English Translation, 1998, Canon of Insolation and the Ice Age Problem. With introduction and biographical essay by Nikola Pantic. 636 pp. $79.00 Hardbound. Alven Global. ISBN 86-17-06619-9.)
  • Weather climatestehrmay2011

    1. 1. -- of a small rock in space Weather and Climate Jeffrey W. Stehr, Ph.D. University of Maryland Atmospheric & Oceanic Science With contributions from John S. Perry, Ph.D. May 24, 2011
    2. 2. Shameless Self-Promotion <ul><li>University of Maryland now has a professional masters program! </li></ul><ul><li>And an undergraduate program is on its way! </li></ul><ul><li>Your son/daughter/self will be able to go from a high school diploma to a professional masters in 5 years. </li></ul><ul><li>All our graduates get jobs in their chosen fields . </li></ul><ul><li>You’re looking at the new associate director of both. </li></ul>
    3. 3. General Plan for our Hour <ul><li>Why our rock has a nicer climate than most rocks </li></ul><ul><li>What determines the climates of our rock </li></ul><ul><li>The distribution of climates, and where Virginia fits </li></ul><ul><li>How weather arises in all its diversity </li></ul><ul><li>How weather predictions come about </li></ul><ul><li>How all this is changing, and what we humans have done </li></ul>Overall Goal: To equip YOU with the basic understanding you will need as a Virginia Naturalist.
    4. 4. 2010: Global Temperatures Global Top 10 Warmest Years (Jan-Dec) Anomaly °C Anomaly °F 2010 0.62 1.12 2005 0.62 1.12 1998 0.60 1.08 2003 0.58 1.04 2002 0.58 1.04 2009 0.56 1.01 2006 0.56 1.01 2007 0.55 0.99 2004 0.54 0.97 2001 0.52 0.94 The 1901-2000 average combined land and ocean annual temperature is 13.9°C (56.9°F), the annually averaged land temperature for the same period is 8.5°C (47.3°F), and the long-term annually averaged sea surface temperature is 16.1°C (60.9°F).
    5. 5. Surf for yourself! <ul><li>NOAA’s climate Web site </li></ul><ul><li> </li></ul><ul><li>IPCC Web site: </li></ul><ul><li> </li></ul>
    6. 6. Climategate: Proof! (of what?) <ul><li>Himalayan glaciers will not melt by 2035 (~90% receding) </li></ul><ul><ul><li>IPCC has printed a retraction </li></ul></ul><ul><ul><li>IPCC didn’t follow their own rules in allowing a non-peer-reviewed source to come into its writing </li></ul></ul><ul><li>Phil Jones has a tough job, is human, spends a fair bit of time responding to skeptics </li></ul><ul><li>No evidence of a trick, but some trouble… </li></ul><ul><ul><li>In scientific parlance, “trick” means “a clever method”, not an attempt to deceive </li></ul></ul><ul><ul><li>Problems with the tree-ring data, yes. </li></ul></ul><ul><li>Allegations that a peer-reviewed article was kept out of the IPCC report were false. </li></ul>
    7. 7. Climategate: good stuff <ul><li>Climate researchers will be more open about their data and methods </li></ul><ul><ul><li>This is very, very good for science </li></ul></ul><ul><li>Note: two much more thorough investigations have still found nothing more </li></ul>
    8. 8. A Rock in Space (Moon) Max: 123°C Min: - 233°C (*or -243°C?) Not a nice place!
    9. 9. Another bad rock: Mars <ul><li>Thin atmosphere 0.7% Earth’s (95% CO 2 ) </li></ul><ul><li>Little water </li></ul><ul><li>CO 2 frozen in ice cap </li></ul><ul><li>Very cold </li></ul><ul><li>Little to no greenhouse effect </li></ul><ul><li>Min: -140°C </li></ul><ul><li>Max: 20°C </li></ul><ul><li>Average: -63°C </li></ul>
    10. 10. Another Bad Rock: Venus <ul><li>THICK atmosphere, 92x Earth’s (96% CO 2 , 3% N 2 ) </li></ul><ul><li>Mean surface Temperature: 482°C </li></ul><ul><li>Sulfuric acid clouds </li></ul><ul><li>Runaway greenhouse effect </li></ul><ul><li>No oceans </li></ul><ul><li>Little water </li></ul>
    11. 11. Radar composite of Venus
    12. 12. Our Rock in Space <ul><li>Much nicer! </li></ul><ul><li>What’s different? </li></ul><ul><ul><li>Water </li></ul></ul><ul><ul><li>Green stuff </li></ul></ul><ul><ul><li>Air! – Our Atmosphere </li></ul></ul>
    13. 13. Our Atmosphere <ul><li>Not much of it – half below 4 miles </li></ul><ul><li>Many gases </li></ul><ul><ul><li>Mostly N 2 , O 2 , Ar </li></ul></ul><ul><ul><li>Variable: </li></ul></ul><ul><ul><ul><li>Water vapor </li></ul></ul></ul><ul><ul><ul><li>Carbon Dioxide </li></ul></ul></ul><ul><ul><ul><li>Methane </li></ul></ul></ul><ul><ul><ul><li>Ozone </li></ul></ul></ul><ul><li>Made by life (the dynamic gases) and vital for life </li></ul>
    14. 14. Our Atmosphere and our Sun <ul><li>Sun’s heat mostly in the visible, where the atmosphere is transparent. </li></ul><ul><li>Earth’s heat radiation is in the infrared , where trace gases absorb much of it. </li></ul><ul><li>The most important absorbing gases are water vapor and carbon dioxide. </li></ul><ul><li>These gases absorb about 90% of the heat radiated by the Earth. </li></ul><ul><li>This was clearly understood by 1847. </li></ul>
    15. 16. The “Greenhouse Effect” <ul><li>Average temperature without greenhouse gases: 2 °F </li></ul><ul><li>Average temperature with greenhouse gases: 59 °F </li></ul>
    16. 17. More GHG = More Heating
    17. 18. Natural Climate Changes: Ice Ages <ul><li>Winter vs. summer: Earth tilts toward the sun in summer, away from it in winter </li></ul><ul><li>Earth’s orbit is an ellipse, not a circle. Earth is closer to the sun in NH winter, farther away in NH summer…this changes. </li></ul><ul><li>What changes: </li></ul><ul><ul><li>Timing of the closeness vs. tilt: precession </li></ul></ul><ul><ul><li>The tilt itself: obliquity </li></ul></ul><ul><ul><li>Oval vs. circular shape of the orbit: eccentricity </li></ul></ul><ul><ul><li>Solar output </li></ul></ul>
    18. 20. Where axis points when Tilt of axis “ Ovalness” of orbit
    19. 21. What about right now? <ul><li>We’re actually in a very quiet period </li></ul><ul><li>Little eccentricity </li></ul><ul><li>Closer in NH winter, farther away in summer (most of the land is here) </li></ul><ul><li>Should be gentle warming for next 25,000 years </li></ul><ul><li>No cooling sufficient to cause an ice age in next 50,000-100,000 years </li></ul>
    20. 22. Our Rock under the Greenhouse
    21. 23. General Circulation and Climate <ul><li>Circulation driven by heating in tropics and cooling near poles </li></ul><ul><li>Rotation of the Earth forces breakup into three major cells. </li></ul><ul><li>These cells determine the general distribution of climates over the planet. </li></ul><ul><li>Mountains, configuration of continents, ocean currents influence the details. </li></ul>
    22. 24. Climates of the Planet
    23. 25. Recipe for Climate <ul><li>A nice, hot Sun (but not too hot) </li></ul><ul><li>A comfortable orbit (right distance, nearly circular) </li></ul><ul><li>Reasonable rotation at a moderate inclination (day not too long, inclination not 90°) </li></ul><ul><li>An atmosphere with a good assortment of heat-absorbing gases (some greenhouse) </li></ul><ul><li>An assortment of oceans and land masses with varied topography. (land not all in one place) </li></ul>
    24. 26. Weather
    25. 27. Species of turbulence – “Weather”
    26. 28. How do we predict weather? What’s happening now? Global network of surface observing stations
    27. 30. Upper Air Observations (weather balloons)
    28. 31. Weather satellites Geostationary satellites Polar-orbiting satellites
    29. 32. Assimilate Data and Analyze
    30. 33. Numerical Forecast Model <ul><li>Basic equations </li></ul><ul><li>Many approximations </li></ul><ul><li>Clever numerical methods </li></ul><ul><ul><li>Supercomputers </li></ul></ul>
    31. 34. Numerical Forecast
    32. 35. Numerical Forecast
    33. 36. Numerical Forecast
    34. 37. Numerical Forecast
    35. 38. Numerical Forecast
    36. 39. The #@$% weather-guessers never get it right! Forecasts are better than ever!
    37. 40. How can we predict climate if we can’t predict the weather? The same way we can predict the tide but not the individual waves
    38. 41. Climate prediction through the ages
    39. 42. 2950-1600 BCE(?)
    40. 44. Olmec Long Count <ul><li>32 BCE </li></ul>
    41. 47. Arrhenius, 1896 (equilibrium) Present day best estimate: 2.5-4.0°C with a best estimate of 3.0°C for 2100 from doubling CO 2 from the IPCC 4 th assessment, 2007
    42. 48. Svante Arrhenius (1859-1927)
    43. 49. What about future climate? <ul><li>Earth’s climate has changed frequently and radically in the past </li></ul>
    44. 50. Could we be changing climate?
    45. 51. We are changing the atmosphere
    46. 52. The world has been warming
    47. 53. Sea level has been rising
    48. 55. Chesapeake Bay and Climate Change 6 m Sea Level Rise <ul><ul><li>more coastline than California! </li></ul></ul><ul><ul><li>more susceptible to sea level rise than all but 2 other states </li></ul></ul>
    49. 59. Climate Modeling
    50. 60. Future climate depends on us.
    51. 61. Emissions
    52. 62. Figure TS.5
    53. 64. Projections of Future Climate 2090-2099
    54. 65. Global Precipitation Projections
    55. 66. Virginia’s Temperate Climate Virginia Long-Term Average Temperature and Precipitation (1895-1998) Month Maximum °F Minimum °F Average °F Precipitation (Inches) Jan 45.8 26.0 35.9 3.13 Feb 47.7 26.7 37.2 3.08 Mar 56.9 34.1 45.5 3.86 Apr 67.1 42.7 54.9 3.29 May 75.8 52.2 64.0 3.99 Jun 82.9 60.2 71.5 3.69 Jul 86.1 64.3 75.2 4.31 Aug 84.6 63.2 73.9 4.14 Sep 79.2 57.0 68.1 3.50 Oct 69.2 45.0 57.1 3.36 Nov 57.8 35.4 46.6 3.21 Dec 47.8 28.0 37.9 3.18 Annual 66.7 44.6 55.7 42.70
    56. 67. Virginia Climate <ul><li>BIG factors in VA Climate: </li></ul><ul><ul><li>Atlantic Ocean and its warm Gulf Stream </li></ul></ul><ul><ul><li>the Blue Ridge and </li></ul></ul><ul><ul><li>Appalachian mountain systems </li></ul></ul><ul><li>Cold air from North, Warm from the south clash over Virginia </li></ul><ul><li>In the southern warmth in summer, northern cold in winter </li></ul><ul><li>A shift in that pattern means a shift in climate </li></ul>
    57. 68. Virginia Climate <ul><li>5 Regions: </li></ul><ul><ul><li>Tidewater, Piedmont, Northern Virginia, Western Mountain and Southwestern Mountain </li></ul></ul><ul><li>Highest temperature: 110° F. in Balcony Falls in Rockbridge County on July 15, 1954; </li></ul><ul><li>Lowest is -30° F on January 21, 1985, at the Mountain Lake Biological Station near Blacksburg </li></ul><ul><li>Greatest snowfall during a single storm: Big Meadows, SNP, 48” fell January 6-7, 1996. </li></ul><ul><li>Most snow to fall in a month: Warrenton in February 1899, 54”. </li></ul>
    58. 69. Mid-Latitude Storms <ul><li>Fueled by temperature contrasts </li></ul><ul><li>Track west to east across N. America </li></ul><ul><li>Near the Atlantic coast, they move Northeast </li></ul><ul><ul><li>Due to contrasting land (cold) Gulf Stream (warm) temperatures </li></ul></ul><ul><li>Pulls in cold air from NW, warm moist air from E </li></ul><ul><li>Produces impressive storms! </li></ul><ul><li>VA typically a little too far south for the truly mean stuff (e.g. Nova Scotia) </li></ul>
    59. 70. Tropical storms <ul><li>Fueled by warm, moist tropical airmasses, which come from warm ocean waters </li></ul><ul><li>Move from the Gulf or the Atlantic up the Eastern Seaboard </li></ul><ul><li>Get their knees chopped out from under them by the land </li></ul><ul><li>Interact in nasty ways with mountains!!! </li></ul><ul><ul><li>Lots of wind, rain, landslides, flooding, etc. </li></ul></ul><ul><li>September VA can see 10-40% of its rainfall from tropical systems </li></ul>
    60. 71. Rain shadows <ul><li>When winds are from the west: </li></ul><ul><ul><li>New River and Shenandoah River are in the Rain shadow of the Appalachians </li></ul></ul><ul><li>When winds are from the east: </li></ul><ul><ul><li>New River and Shenandoah River are in the Rain shadow of the Appalachians </li></ul></ul><ul><li>These valleys are drier than the rest of the state </li></ul>
    61. 72. Thunderstorms <ul><li>Can occur in any month of the year (think “Tidewater”) </li></ul><ul><li>Are most frequent in the late afternoon (~4:30 PM) </li></ul><ul><li>Most frequent in southern Virginia </li></ul><ul><ul><li>Especially in the SW corner of the state </li></ul></ul><ul><ul><li>Least frequent in N. Virginia </li></ul></ul>
    62. 75. “ Take-home” Ideas <ul><li>Our rock has a nice climate because of a happy combination of sun, orbit, rotation, water, land and above all our atmosphere. </li></ul><ul><li>Virginia happily lies in the temperate zone </li></ul><ul><li>Weather derives from heating-driven turbulence on many scales </li></ul><ul><li>Weather forecasts are good and getting better </li></ul><ul><li>Our atmosphere is changing; our climate is changing with it; and it’s because of us. There’s really no serious doubt about this. </li></ul><ul><li>Virginia faces a warmer – maybe 6 °F – and somewhat wetter (0-10%?) future </li></ul>
    63. 76. What can I do? <ul><li>“ Citizen Science” </li></ul><ul><li>Bud Burst </li></ul><ul><ul><li>Track plant phenophase in your area </li></ul></ul><ul><ul><li> </li></ul></ul><ul><li>Global Invasions Network (invasives are first in!) </li></ul><ul><ul><li>Garlic Mustard Survey </li></ul></ul><ul><ul><li> </li></ul></ul>
    64. 77. No, really, what can I do? <ul><li>Can choose to get 100% of energy from wind </li></ul><ul><li>Can buy an electric vehicle </li></ul><ul><li>Can use less </li></ul>
    65. 78. Thank You!
    66. 79. Radiative Balance <ul><li>The atmosphere warms from the ground up </li></ul><ul><li>The sun’s warmth comes in at or near the visible part of the spectrum (aka “shortwave”) and heats the ground </li></ul><ul><ul><li>The atmosphere is transparent to visible light </li></ul></ul><ul><li>The ground radiates energy back into space </li></ul><ul><ul><li>The atmosphere is nowhere near as transparent to infrared light (aka “longwave”), so the atmosphere behaves like a see-thru blanket </li></ul></ul><ul><li>Weather systems and ocean currents move energy, but in the end, something has to get warmer in order to release this excess energy back into space </li></ul><ul><li>Note: clouds, particles, etc., make this more complex </li></ul>
    67. 80. Blackbody Radiation <ul><li>Everything emits infrared (aka “longwave”) light </li></ul><ul><ul><li>You, me, toasters, trees, squirrels, ice, everything </li></ul></ul><ul><li>This goes according to the blackbody radiation law: </li></ul><ul><ul><li>Power =  T 4 (watts/square meter) </li></ul></ul><ul><li>If we have: </li></ul><ul><ul><li>more energy being trapped and </li></ul></ul><ul><ul><li>the only way to get rid if it is through blackbody radiation, then </li></ul></ul><ul><ul><li>the temperature must increase </li></ul></ul>
    68. 81. Scenarios
    69. 82. Glacier melt? <ul><li>Certainly an issue </li></ul><ul><li>Probably will accelerate sea level rise </li></ul><ul><li>NOT in IPCC 2007 (missed cutoff) </li></ul><ul><li>Not as large as thought in 2005 (fast year for Greenland’s glaciers) </li></ul><ul><li>Stay tuned… </li></ul>
    70. 83. North American Projections
    71. 84. Structure of the Atmosphere <ul><li>Almost ¾ of the atmosphere is in the troposphere, in which all our weather occurs . </li></ul><ul><li>Temperature normally decreases with height by about 3.6°F per 1000 ft in the troposphere. </li></ul><ul><li>But dry air moved up or down changes temperature at about 5½°F per 1000 ft. </li></ul><ul><li>Jet aircraft fly near the tropopause , where temperatures are near -70°F. </li></ul>
    72. 85. How do we get ice ages? <ul><li>Early-mid 1800s: Idea of an ice age proposed, no explanation for how </li></ul><ul><li>1842, Frenchman Joseph Alphonse Adhémar suggested that the varying lengths of winter and summer, an effect of the precession , causes ice to accumulate in the hemisphere with the longer winter. </li></ul><ul><li>Scotsman James Croll combined the eccentricity of the orbit and the precession and in the 1860s and 1870s presented his ideas on the effects of the cycles and how they might influence climate, especially the colder winters when they correspond with the aphelion, when Earth is farthest from the Sun </li></ul>
    73. 86. How do we get ice ages? <ul><li>Milankovitch gets most of the credit for relating the cycles to ice ages because he incorporated all of the pertinent cycles, dealt with them in much greater mathematical precision and showed much more thoroughly how they affect climate. </li></ul><ul><li>Often, these cycles are called Milankovitch-Croll cycles </li></ul>
    74. 87. What about future climate change? <ul><li>Orbital changes occur over thousands of years, and the climate system may also take thousands of years to respond to orbital forcing. </li></ul><ul><li>Theory suggests that the primary driver of ice ages is the total summer radiation received in northern latitude zones where major ice sheets have formed in the past, near 65 degrees north. </li></ul><ul><li>Past ice ages correlate well to 65N summer insolation (Imbrie 1982). </li></ul>
    75. 88. <ul><li>Astronomical calculations show that 65N summer insolation should increase gradually over the next 25,000 years </li></ul><ul><li>No 65N summer insolation declines sufficient to cause an ice age are expected in the next 50,000 - 100,000 years ( Hollan 2000, Berger 2002). </li></ul>