DARGAN M. W. FRIERSON DEPARTMENT OF ATMOSPHERIC SCIENCES 11/27/2013 ATM S 587, Fundamentals of Climate ChangeNext Topic: Paleoclimate ! Climate of the past ¡ Snowball Earth ¡ Hot climates ¡ Ice ages ¡ Last few thousand yearsWhat Sets the Climate Over Earth’s History? ! The Sun ¡ It’s changed in magnitude over its lifespan ÷ Over last billion years, increased 10% ÷ Initially 75% as strong as it is nowWhat Sets the Climate Over Earth’s History? ! The orbit of the Earth around the Sun ¡ If the solar system was just the Earth & the Sun, the orbit would be a perfect ellipse that never changed ¡ However, there are other planets/moons in the solar system which causes orbits to change with time ÷ Ex: the tilt of the Earth changes over a 41,000 year cycle (pretty quickly!) ¡ What would a higher tilt mean for the climate? ÷ More seasonality (colder winters, warmer summers)What Sets the Climate Over Earth’s History? ! Location of the continents ¡ These have shifted with time ¡ Mountain ranges appear, sometimes high latitude ice sheets aren’t possible, etcWhat Sets the Climate Over Earth’s History? ! Location of the continents ¡ These have shifted with time ¡ Mountain ranges appear, sometimes high latitude ice sheets aren’t possible, etc Exact representation of continental drift… à àWhat Sets the Climate Over Earth’s History? ! Volcanoes ¡ On short timescales, cause cooling ¡ Over very long timescales, can add significant CO2 to the atmosphereHow Do We Know About Paleoclimate? ! Proxy data: tells us about temperature, precipitation, etc through other indicators ¡ Biological data ÷ Tree rings, pollen, corals, fossils ÷ Ex: Alligator skeletons at relatively high latitudes tell us the Eocene winter temperatures must have been very mild then!How Do We Know About Paleoclimate? ! Proxy data: tells us about temperature, precipitation, etc through other indicators ¡ Ice core data ÷ Ice at the bottom of Greenland/Antarctica is over 100,000 years old ÷ Ex: Ice cores have tiny bubbles of air trapped inside that reveal past atmospheric compositionHow Do We Know About Paleoclimate? ! Proxy data: tells us about temperature, precipitation, etc through other indicators ¡ Geological data: ÷ Rocks, sediments, shape of the land, etc ÷ Ex: land in the Seattle area is cut out by glacier flows from when ice sheets used to have a much larger extentHow Do We Know About Paleoclimate? ! Proxy data: tells us about temperature, precipitation, etc through other indicators ¡ Isotopic data: ÷ Many of the previously mentioned datasets can be dated using carbon dating or other radiometric dating techniques ÷ Also isotopes can tell us about precipitation and temperature as we’ll seeHistory of the World, Part I ! We’ll look at this timeline: ¡ Lifetime of Earth (4.5 billion years) ¡ Past 250 million years ¡ Past million years ¡ Past 20,000 years ! Equivalent timeline for 20 yr old student: ¡ Whole life ¡ 250 million yrs = last year ¡ 1 million yrs = last 36 hours ¡ 20,000 yrs = last 45 minutesTemperature through Time Kasting et al!“deep” (distant) past was mostly warmer than today!PRESENT DAY Note: time scale is very non-linear Millions of years ago EARTH BIRTH“Faint Young Sun” Paradox ! The Sun was initially around 75% as strong as it is now ! “Faint Young Sun” Paradox: raised by Carl Sagan in 1972 ¡ Earth was warm most of this time when the Sun was weak ÷ We know this from geologic evidence ¢ Rounded pebbles, mud cracks, ripple marks, microfossil algae ÷ High greenhouse gas concentrations are likely key to keeping it warmSnowball Earth– occurred several times in the last billion years when ice-albedo feedback spiraled out of controlTemperature through Time Kasting et al!Snowball Earth Events!PRESENT DAY Millions of years ago EARTH BIRTHAn ice-covered Earth would have a very high albedo and an extremely low temperature If the Earth ever became ice covered, how could the ice ever melt?Extremely high greenhouse gas concentrations would be required to deglaciate Let’s discuss controls on carbon dioxide over very long timescales (this is important not just for the Snowball Earth question)Controls on Carbon Dioxide Over Time ! Release by volcanoes is a relatively efficient way of getting CO2 into the atmosphere ¡ Remember this is small as compared to current human emissions ¡ Volcanoes are very important over hundred thousand year timescales ! When we’re considering long timescales, have to think about how CO2 is removed as wellChemical Weathering ! How does CO2 get removed from the atmosphere over long times? ¡ Land masses are key in a process called chemical weathering ¡ When rain/snow falls on silicate rocks, it reacts and takes CO2 out of the atmosphere ! Chemical weathering is a negative feedback ¡ When climate is hotter, it’s easier for weathering to take CO2 out of the atmosphere ¡ Likely key for stabilization of climate over millions of yearsSnowball Earth– ice-albedo feedback out of control During Snowball Earth, volcanic activity injected CO2 into the atmosphere With the land and oceans covered by ice, there was no chemical weathering to remove CO2 from the atmosphere, so it accumulated.Eventually the greenhouse effect became so strong that the ice began to melt, despite its high albedo.Once initiated, melting would proceed very rapidly as the albedo dropped.Post-Snowball “Hothouse” Climate ! Immediately after Snowball Earth thaws, CO2 concentrations would have been tremendously high ! Was likely the hottest period in Earth’s history right after the coldest! ¡ Temperatures jumped from -50o C to 50o C in only 1000 years! ! Massive weathering would gradually bring down CO2 and temperaturesHow Did Life Survive? ! We know life existed before Snowball events ! How would it have survived the ice-covered surface? Snowball Earth Events 3,500 …2,000 800 540 0 Time in millions of years 11 animal phyla emergeHydrothermal vents? Cracks in the ice? How did life survive Snowball Earth?Ice in Snowball Earth ! Steve Warren (UW Atmos Sci/ESS)