Lecture 15 Defining climate, climate controls Climate classification Past climates Historical climate paleoclimate Data and mechanism for change

Climate, the collective state of the atmosphere for a given location and over a specified time interval Location, regional or global scale climate Time Averages and extremes of variables Temperature Precipitation winds

Climate controls Latitude Elevation Topography Proximity to large bodies of water Prevailing atmospheric circulation

Classifying the climate zones (ancient Greek)

Classifying climate zones --- Koppen A. Humid tropical B. Dry C. Humid middle-latitude, mild winters D. Humid middle-latitude, severe winters E. Polar H. Highland Note that boundaries fluctuate from year to year

Fluctuating boundaries of dry/humid

Tropical humid climates (A)

Dry climates (B)

Dry climates (B), not tropical

Moist subtropical to Mid-Latitude (C) Marine west coast climates

Humid subtropical (C)

Mediterranean climates (C)

Severe midlatitude climates (D) (humid, continental)

Subarctic (D)

Polar climates (E)

Past climates Historical climate (past several K years) Instrumental record (measurements) Historical data (for example diaries, ship logs) Paleoclimate, study of climate of the distant past Use environmental records, such as Tree rings Pollen records Air bubbles and dust in ice Marine sediments Fossil record

Concentration of atm CO2 and CH4 from ice bubbles in the Vostok ice core 2083 m long ice core. Dated by counting the number of ice layers

Ice cores Dust in ice sheets can be caused by volcanoes Or by dry windy conditions that lead to soil erosion Colder periods in Earth’s history are usually much dustier Did the dust block the sun or did the colder temperatures cause drier conditions?

Past climates, the change mechanisms Volcanic eruptions Asteroid impacts Solar variability Variations in Earth’s orbit: Milankovitch cycles Precession Obliquity Eccentricity Plate tectonics

Volcanic activity and climate change Explosive eruptions emit huge quantities of gases and fine grained debris into the atmosphere The greatest eruptions at low latitudes are powerful enough to inject the material into stratosphere where it will filter out a portion of the incoming solar radiation Mount St Helens El Chichon Mount Pinatubo

Solar variability Variations in the amount of energy from the sun Variations in number of sunspots follow an 11 year cycle Maunder minimum– some believe that a reduction in output of the sun during this time cooled Earth Little Ice Age, 1400--1850

Yearly averaged sunspot numbers, 1610-2000

Precession: Earth’s axis wobbles (like that of a spinning top) every 27K years

Obliquity: the angle Earth’s axis makes with the plane of Earth’s orbit (41K years)

Eccentricity: change in the shape of Earth’s orbit around the sun (100K years)

Plate tectonics and climate change 300million years ago

Ice age is a period global cooling that leads to the creation of vast ice sheets across land