What is the cause of glacial-interglacial cycles?GEOL 108Lg 1st Edition Lecture 17Outline of Last Lecture- Weather vs. Climate- Greenhouse Effect-Earth’s (usually, though maybe not this week) nice & pleasant, habitable climate as a consequence of solar radiation balance and the Greenhouse Effect Variability in climate (tropics at low latitudes, bands of deserts at mid-latitudes) as a consequence of uneven solar energy and convective transportOutline of Current Lecture- PaleoclimatologyCurrent LectureEarth’s Climate IIEl Nino Southern Oscillation is a quasi-periodic climate pattern—not a storm— that occurs acrossthe tropical Pacific Ocean every five years or so. The Southern Oscillation refers to variations in the temperature of the surface of the tropical eastern Pacific Ocean (warming and cooling knownas El Niño and La Niña respectively) and in air surface pressure in the tropical western Pacific. The two variations are coupled: the warm oceanic phase, El Niño, accompanies high air surface pressure in the western Pacific, while the cold phase, La Niña, accompanies low air surface pressure in the western PacificPrecipitation Anomaly: difference during a given year from the long term averageExamples: La Nina, El NinoGlacial Interglacial Cycles Caused by VARIATIONS IN LEVELS OF Co2Large, continental ice-sheets in the Northern Hemisphere have grown and retreated many times in the past. Times with large ice-sheets are known as glacial periods (or ice ages) and times without large ice-sheets are interglacial periods. The most recent glacial period occurred between about 120,000 and 11,500 years ago. Since then, the Earth has been in an interglacial period called the Holocene (Figure 2). Glacial periods are colder, dustier and generally drier than interglacial periods. These glacial-interglacial cycles are apparent in many marine and terrestrial paleo-climate records from around the world.Historical Record of the Millennial Climate: What is the cause of glacial-interglacial cycles?Variations in the Earth's orbit through time have changed the amount of solar radiation received by the Earth in each season (Figure 3). Interglacial periods, shown as the periods of higher temperature (shaded in yellow) in the Dome Fuji ice core from Antarctica, tend to happen during times of more intense summer solar radiation in the Northern Hemisphere. These glacial-interglacial cycles have waxed and waned throughout the Quaternary Period (the past 1.8 millionyears). Since the middle Quaternary, glacial-interglacial cycles have had a frequency of about 100,000 years. In the solar radiation time-series, cycles of this length (known as "eccentricity") are present but are weaker than cycles lasting about 23,000 years, (which are called "precession of the equinoxes.") Warming at the end of glacial periods tends to happen more abruptly than the increase in solar insolation. There are several positive feedbacks that are responsible for this. One is the ice-albedo feedback. A second feedback involves atmospheric CO2. Direct measurement of past CO2trapped in ice core bubbles show that the amount of atmospheric CO2 decreased during glacial periods (Figure 3), in part because more CO2 was stored in the deep ocean due to changes in either ocean mixing or biological activity. Lower CO2 levels weakened the atmosphere's greenhouse effect and helped to maintain low temperatures. Warming at the end of the glacial periods liberated CO2 from the ocean, which strengthened the atmosphere's greenhouse effectand contributed to further warming.The Puzzle of the Ice Ages/Global Glaciationchanges in climate are not symmetricUnsolved Mystery of the Ice Ages: Asymmetric changes in climate may be associated with amplification by Greenhouse forcing, from changes in CO2 – but why does CO2 change in the first place?Why did climate change over the past 1,000 years?Variability in solar energy: There was a medieval warm period (1000-1300)Corresponding to a time of increased solar radiationThe Little Ice Age (14th Century) corresponding the minimum solar activityVariability in atmospheric conditions: volcanic eruptions – contributed to cooling of the Little Ice AgeAstronomical Theory of Glacial-Interglacial CyclesJames Croll (Scottish natural historian, 1820-1890) hypothesis that changes in the Earth’s orbit caused glaciationMilutin Milankovitch (Serbian mathematician) formalized theory relating changes in the Earth’s orbital parameters to climate change“Milankovitch Cycles”Orbital changes => changes solar energy reaching Earth => changes in global climateExplains the repeating, cyclical changes from glacial to interglacial•Solar – changes in the intensity of solar radiation reaching the Earth•Volcanic – eruptions add aerosols to the atmosphere that increase reflection of radiationover short periods of time, but also add CO2 to the atmosphere that increases greenhouse warming over long periods of time•Greenhouse forcing – addition of CO2 to atmosphere contributes to climate warmingIt is the job of paleoclimatologists to tease these apart…Ice Ages of the QuaternaryRecent glaciations were cold and icy, but not frozen at the low, equatorial latitudes – unlike the crazy Snowball EarthRecent glaciations were cold and icy but not frozen at the low equatorial latitudeHow do we know there were low latitude glaciationscharacteristically glacial sediment formed in deposits1. We know that we had glaciation reaching down to equatorSnowball earth? How about Slushball Earth! The big debateGlaciers covering all the way down across the equatorHow do we know Snowball Earth happened?Snowball Earth EvidenceHow do we know there were low-latitude glaciations?Characteristically glacial sediment formed in depositswhere:(1) We know the ages from fossils and isotopes (~700 Ma)(2) We know the latitude from magnetic mineralsThe CO2 vs. Solar Radiation Argument, repeated…In summary:Climate has been remarkably stable over geologic time, but has also changed.Both of these features may well be due to changing greenhouse conditions (i.e. changing CO2).All of this is worth bearing in mind when thinking about