GEOL 108Lg 1st Edition Lecture 19Outline of Last Lecture-Environmental geology issues -Air pollution -Waste managementOutline of Current Lecture-Climate forcings and paleoclimate-Man made, global climate changeCurrent LectureGlobal ChangeClimate ProxiesPaleoclimatology is the study of past climates. Being that it is not possible to travel backwards in time to see what climates were like, scientists use imprints created during past climates to interpret paleoclimate. These are called proxies. Microbial life, such as diatoms, forams, and coral serve as useful climate proxies. Other proxies include ice cores, tree rings, and sediment cores (which include diatoms, foraminifera, microbiota, pollen, and charcoal within the sedimentand the sediment itself).Past climate can be reconstructed using a combination of different types of proxy records. These records can then be integrated with observations of Earth's modern climate and placed into a computer model to infer past as well as predict future climate.● Climate poxies: A short instrumental Era● almost no quantitative measurements before invention of thermometers and barometers (17th century) ● most reliable information from post-1850 AD● how can we tell what climate did before instruments’ time?● Climate proxies are:●geological objects sensitive to climate conditions●measurable quantities that substitute for something no longer measurable (e.g. global temperature 5 My ago)●5 major classes: Ice cores, sediment cores, corals, speleothems, tree ringsCombinations of proxy data are generally used to reconstruct records for past climates. - Ice core records- deep ice cores can be analyzed for trapped gas, stable isotope ratios, and pollen trapped within the layers to figure things out about past climates.- Tree rings can be counted to determine age. The thickness of each ring can then be used to infer fluctuations in temperature and rain fall/precipitation, since optimal conditions for the particular species will result in more growth (thicker rings for a given year). Burn marks and scars in the rings can indicate past natural events such as fire.- Sediment cores can be analyzed in many ways. Sediment layers can indicate sedimentation rate through time. Charcoal trapped within sediments can indicate past fire events. Remains of micro-organisms such as diatoms, foraminifera, microbiota and pollen within sediments can indicate changes in past climate (since each species has a limited rangeof habitable conditions). When these organisms and pollen sink to the bottom of a lake or ocean, they can become buried within the sediment. Thus, climate change can be inferred by species composition within the sediment.What are oxygen isotopes telling us?Processes that discriminate against the heavy isotope are often tied to a climate variable of interest. For instance:Evaporation–16O evaporates preferentially–evaporated water is said to be “isotopically lighter”–18O is left behind in seawater –seawater is said to be “isotopically heavier”When climate is cold, more water is tied up in glaciers Glacial ice becomes a place to store 16O. -18O/16O ratio in the ocean goes UPWhen climate is warm, more water is in the ocean : -18O/16O ratio goes DOWNIce Core Record of CO2 and Temperature: Based on cores from AntarcticaCO2 obtained from bubbles trapped in the ice, temperatures at certain times derived from δD (hydrogen isotopes)Summary: paleoclimatology● Time can be determined in 2 manners:● Relative (stratigraphy) vs absolute (radiometric dating)● Climate proxies:● geological objects sensitive to climate● allow to reconstruct temperature before thermometers● often tied to stable water isotopes (18O, 2H)● Proxies used to reconstruct past temperature:● δ18O in marine sediments (foraminifera)● δD in ice cores● δ18O in coral skeletons● tree-ring width + many, many more...The causes of climate change are various and complex. There are several major factors that can affect the climate system, including:- Changes in solar output- Changes in Earth's orbit- Changes in the distribution of continents- Changes in atmospheric content of greenhouse gases.The Milankovich Theory states that variations in Earth's orbit causes climate to change through time. According to this theory, changes in the shape of Earth's orbit around the sun (eccentricity), variations in Earth's axial tilt (obliquity), and the tendency for Earth to 'wobble' with respect to the direction of its rotational axis (precession) affect climate. This wobble can lead to fluctuations in the amount and distribution of incoming solar radiation, resulting in dramatic changes in climate over long time scales. Wobble may cause ice ages.It is important to consider scale when interpreting climate change through time. Four major timescales are generally considered, which include:- Long term- Hundreds of millions of years;- Medium term- One million years;- Short term- ~160,000 years;- Modern period- Hundreds of years.Time scale affects interpretations of climate change. Climate has both long term trends and shortterm variability. In looking at longer time scales, major shifts in climate such as the ice ages are easily recognizable, and viewing a long-term data set can provide the observer with a sense of the "big picture" of the climatic trends. Short term variations, like a colder than average month, can exist within longer term patterns such as the warming trend over the past thousand years. The coexistence of short and long term trends occurring simultaneously through time complicates our ability to unravel climate change.Past 500 Million YearsIce ages have occurred throughout Earth history, including at least 5 episodes in the last billion years. The Permian event was recognized by Wegner in his reconstruction of Gondwanaland.There have been four ice ages in the past 500 million yearsPast 65 Million Years (Initially warm followed by cooling!)-The Earth was much warmer during the Age of Dinosaurs (65250 m.y.) during the Mesozoic Era, but began to cool 50 m.y ago. -Warmth was due to high rates of sea floor spreading, volcanism, volcanic CO2. -Cooling – meteorite impact (short duration), continental collision and increased precipitation. Slowing plate motion and volcanism.