Weather, Climate and Society ATMO 336 Seasons Orbital Variations and Ice Ages

Reasons for Seasons Eccentricity of Earth’s Orbit Elongation of Orbital Axis Tilt of Earth’s Axis - Obliquity Angle between the Equatorial Plane and the Orbital Plane

Earth is 5 million km closer to sun in January than in July. Eccentricity of Orbit Aphelion Perihelion Ahrens (2nd Ed.), akin to Fig Solar radiation is 7% more intense in January than in July. Why is July warmer than January in Northern Hemisphere?

147 million km152 million km Ahrens, Fig Animation

Solar Zenith Angle Depends on latitude, time of day & season Has two effects on an incoming solar beam Surface area covered or Spreading of beam Path length through atmosphere or Attenuation of beam Ahrens, Fig Large Area Small Area Short Path Long Path Equal Energy 23.5 o

Ahrens, Fig Large Zenith Angle Zero Zenith Angle Large Zenith Angle Small Zenith Angle Beam Spreading High Sun – Power Spread over Smaller Area Low Sun – Power Spread over Larger Area Animation

Quantifying Beam Spreading Schematic Ignores Earth’s Curvature

Atmospheric Path Length Schematic Ignores Earth’s Curvature Cloud

Length of Day Lutgens & Tarbuck, p33 Animation

Day Hours at Solstices - US Sites Summer-Winter Tucson (32 o 13’ N) 14: :03 Seattle (47 o 38’ N) 16:00 - 8:25 Anchorage (61 o 13’ N) 19:22 - 5:28 Fairbanks (64 o 49’ N) 21:47 - 3:42 Hilo (19 o 43’ N) 13: :46 Gedzelman, p67 Arctic Circle Earth Sun Applet

Path of Sun Hours of daylight increase from winter to summer pole Equator always has 12 hours of daylight Summer pole has 24 hours of daylight Winter pole has 24 hours of darkness Note different Zeniths Danielson et al., p75

Noon Zenith at Solstices Summer-Winter Tucson AZ (32 o 13’ N) 08 o 43’ - 55 o 43’ Seattle WA (47 o 38’ N) 24 o 08’ - 71 o 08’ Anchorage AK (61 o 13’ N) 37 o 43’ - 84 o 43’ Fairbanks AK (64 o 49’ N) 41 o 19’ - 88 o 19’ Hilo HI (19 o 43’ N) 3 o 47’ (north) - 43 o 13’ Aguado & Burt, p46

Incoming Daily Solar Radiation (Insolation) at the Top of the Atmosphere C C W W

Is Longest Day the Hottest Day? USA Today WWW Site Consider Average Daily Temperature for Chicago IL: equilibruim warming cooling

Annual Energy Balance Heat transfer done by winds and ocean currents NHSH Radiative Warming Radiative Cooling Ahrens, Fig Differential heating drives winds and currents We will examine later in course

Summary Tilt (23.5 o ) is primary reason for seasons Tilt changes two important factors Angle at which solar rays strike the earth Number of hours of daylight each day Warmest and Coldest Days of Year Occur after solstices, typically a month later Requirement for Heat Transport Done by Atmosphere-Ocean System

Most Recent Ice Age Extend of continental glaciers 18,000 years BP. Sea level was m lower than present. Bering land bridge between Siberia and Alaska. Aguado and Burt, Fig 16-4

SST 18,000 years BP Much cooler over the North Atlantic Ocean. Ocean currents were undoubtedly different. North Atlantic Drift was probably much weaker. 18,000 BPToday Ahrens, Fig 13.2

Temperatures Since Last Ice Age Rapid warming occurred at end of Younger-Dryas period. Ice cores indicate that Ice Age conditions ended in 3 years! Glacial retreat Rapid melt Glacial advance Apline advance Ahrens, Fig 13.3

Climate Changes Affect Humans Temperatures for eastern Europe during the last 1200 years. Viking settlements lost in Greenland Viking colonization in Greenland Ahrens, Fig 13.4

Milankovitch Theory of Ice Ages Attempts to explain ice ages by variations in orbital parameters Three cycles: Eccentricity (100,000 yrs) Tilt (41,000 yrs) Precession (23,000 yrs) Changes the latitudinal and seasonal distributions of solar radiation.

Eccentricity (orbital shape) E = now E = ,000 yrs E = 0.5 shownE = 0.0 shown

Obliquity (axial tilt) O = 23.5˚ 41,000 yrs

Precession (wobble) Gyroscopic like motion 26,000 yrs Varies time of perihelion aphelioin

Milankovitch Theory Change in daily solar radiation at top of atmosphere at June solstice Changes as large as ~15% occur

Milankovitch Theory of Ice Ages Ice ages occur if summers are cool and less snow melts. Partially agrees with observations, but many questions unanswered. What caused the onset of the first Ice Age?

Take Home Points Milankovitch Cycles Explains some observations Raises many questions