Higher eccentricity amplifies the 23,000-y precessional cycles on 100,000 and 400,000 y cycles. The truncation of the summer monsoon response pattern at a critical threshold is called a clipped response. Monsoons on Pangaea are recoded in the sedimentary rocks deposited in the Newark Basin. Paleomagnetic data indicate that the site was about 10º north of the equator at 200 Ma. Thus its precipitation would have been dominated by precessional insolation changes Fossil dinosaur footprints indicate that the lakes occasionally dried out and the cycle was about 20,000 y. Fluctuations in sediment thickness in the Newark Basin lakes reflect three orbital cycles: 20,000, 100,000, and 400,000 y. The Newark Basin was in the interior of Pangaea at 200 Ma and at about 10° N. It contains 7,000 meters of sedimentary rocks. Newark Basin lake sediments varied in depth from very shallow to over 100 m deep at three tempos. The deep sediments are black and organic rich with thin varves. The shallow sediments are oxidized and show evidence of drying out at times. Combined tectonic and orbital-scale insolation signals show that a monsoon threshold may have been important in Earth’s past climate. And maybe today. Southeast Asia experienced slow uplift during the past 30 to 40 Ma. Increases in uplift and elevation create stronger monsoons. The monsoon threshold has a complex relationship with uplift and insolation. Insolation-Driven Monsoon Responses: A Chronometer for orbital tuning The tuning method is based on the relationship between the insolation signal (forcing) and the summer monsoon changes (response). The timing of orbital insolation forcing is known with great accuracy from astronomical calculations. Monsoon responses are well-recorded in the sediments in the oceans. The Present is the Key to the Past • Variability of Insolation • Orbital scale eccentricity and precession Changes summer insolation at high latitudes • 100,000 y, 41,000 y, 23,000 y • Solar variability • Faint young sun increasing by 25 to 30 percent over 4.5 Gy Natural Causes of Climate Change • Composition of the atmosphere • Greenhouse gasses • The carbon reservoirs: atmosphere, hydrosphere, lithosphere, biosphere exchange carbon and the rates vary only on tectonic scales 10s of ky to 100s of My • Tectonics – BLAG • Sea Floor spreading rate 100s of My • Uplift and erosion 100s of My • Atmosphere circulation • General circulation of the atmosphere – Hadley cell flow • Monsoon circulation seasonal to orbital scale – y to 10s of ky • Ocean circulation • Thermohaline flow • Catastrophes • Bolide impacts years to 10s of years • Super-volcano eruptions years to 10s of years Insolation Control of Ice Sheets Milankovic cycles account for glaciation 2019 Weather and climate 2020 2021 Insolation Control of Ice Sheets • Glaciations occurred at • 435 Ma • 310-230 Ma • 25 Ma to present • The recent glacial epoch beginning about 3 Ma can be explained by slowing of plate tectonics resulting in global cooling. Slowing of CO2 emissions from volcanoes and hydrolysis of CO2 in silicate rocks removed CO2 from the atmosphere. • More rapid changes of 100,000 years are the explained by orbital control of insolation. • When the recent glaciation ended about 11,000 years ago, the ice locked up on the continents melted and flowed into the oceans raising sea level by about 120 m (394 feet). Northern hemisphere ice sheet 20,000 years ago GRACE The Gravity Recovery and Climate Experiment, a joint mission of NASA and the German Aerospace Center, made detailed measurements of Earth’s gravity field anomalies for 15 years since its launch in March 2002. Period: 1.5 hours Launch mass: 487 kg (1,074 lb) each Start date: March 17, 2002 Names: GRACE; Tom and Jerry Reference system: Geocentric Inclination: 89.0° Rocket: Rokot GRACE Follow On (GRACE-FO) •Tom and Jerry have fallen to Earth and burned up in the atmosphere after lasting 15 years on what was a 5 -year mission. •GRACE Follow On satellites were launched into orbit by SpaceX on a Falcon 9 rocket May 22, 2018. The satellites are in the same orbit, 220 km apart, and use microwaves to measure distance between them with an accuracy of a few microns. (1 μ = 0.000001 meters) If our planet were a perfectly round, uniform sphere, every location would have the same mass. A satellite orbiting this unlikely Earth would undergo the same gravitational pull everywhere. The real Earth has mountains and flatlands, ocean trenches and seamounts, iron ore deposits and thick sand deposits. The mass below an orbiting satellite varies with crustal density causing variation in gravitational pull. Most of Earth’s mass hardly moves at all from one month to the next, barring natural disasters or human intervention. But water is always moving — flowing, evaporating, falling as rain or snow, and so on. Rapid changes in a region’s gravitational pull are mostly the result of moving water. file:///C:/Users/gtgos/Downloads/GRACE_FO_Fact_Sheet_REV4-6-18_508.pdf In the decades before GRACE, observations of melting ice in Greenland and Antarctica from the ground or by remote sensing were few— not enough even to be sure whether Antarctica’s ice sheet was growing or shrinking overall. GRACE measurements unequivocally show that Greenland has been losing about 280 gigatons of ice per year on average — a bit less than twice the weight of Mt. Everest — and Antarctica has lost slightly under 120 gigatons a year. In 2019 Greenland lost 600 gigatons of ice. Water stored within soil or farther below ground in aquifers is very sparsely measured from the ground. By measuring changing mass on what appears to be dry land, GRACE has found that in a third of Earth’s largest aquifers, humans are pumping out groundwater faster than it is being replenished. file:///C:/Users/gtgos/Downloads/GRACE_FO_Fact_Sheet_REV4-6-18_508.pdf Rising sea levels have been monitored very precisely by satellite altimetry missions for about 25 years. These measurements, however, give only the grand total. To understand the pace of future sea level rise, scientists need to know how much of this total comes from each of two main causes: melting ice flowing into the ocean from land, and the expansion of seawater as it warms. By combining GRACE and altimetry data, scientists are able to partition these two causes. file:///C:/Users/gtgos/Downloads/GRACE_FO_Fact_Sheet_REV4-6-18_508.pdf The viscous mantle under Earth’s crust moves ever so slightly in response to changes in water mass near the surface. GRACE has a community of users who calculate these shifts for their research. Scientists at NASA’s Jet Propulsion Laboratory in Pasadena, California, used GRACE data to calculate how ice sheet loss and groundwater depletion have literally changed the rotation of Earth as the system adjusts to these movements of mass. file:///C:/Users/gtgos/Downloads/GRACE_FO_Fact_Sheet_REV4-6-18_508.pdf GRAVITY RECOVERY AND CLIMATE EXPERIMENT GRACE Gravity anomaly map after 15 years of data from GRACE https://www.nasa.gov/missions/grace-fo Greenland Ice Sheet melt-water channel In 2019 Greenland lost 600 gigatons of ice. Melt water streams on Greenland ice sheet During the exceptionally warm Arctic summer of 2019, Greenland lost 600 billion tons of ice — enough to raise global sea levels by nearly a tenth of an inch (2.2 millimeters) in just two months, a new study shows.”We knew this past summer had been particularly warm in Greenland, melting every corner of the ice sheet,” said lead author Isabella Velicogna, senior project scientist at JPL and a professor at UCI. “But the numbers really are enormous.“ For context, last summer’s losses are more than double Greenland’s 2002-2019 yearly average. https://www.nasa.gov/feature/jpl/nasa-led-study-reveals-thecauses-of-sea-level-rise-since-1900 NASA-led Study Reveals the Causes of Sea Level Rise Since 1900 Ian J. O’Neill / Jane J. Lee Jet Propulsion Laboratory, Pasadena, Calif. 818-354-2649 / 818-354-0307 firstname.lastname@example.org / email@example.com The researchers found that estimates of global sea level variations based on tide-gauge observations had slightly overestimated global sea levels before the 1970s. (Located at coastal stations scattered around the globe, tide gauges are used to measure sea level height.) They also found that mountain glacier meltwater was adding more water to the oceans than previously realized but that the relative contribution of glaciers to sea level rise is slowly decreasing. And they discovered that glacier and Greenland ice sheet mass loss explain the increased rate of sea level rise before 1940. In addition, the new study found that during the 1970s, when dam construction was at its peak, sea level rise slowed to a crawl. Dams create reservoirs that can impound freshwater that would normally flow straight into the sea. “That was one of the biggest surprises for me,” said lead researcher Thomas Frederikse, a postdoctoral fellow at JPL, referring to the peak in global dam projects at that time. “We impounded so much freshwater, humanity nearly brought sea level rise to a halt.” NASA-led Study Reveals the Causes of Sea Level Rise Since 1900 Ian J. O’Neill / Jane J. Lee Jet Propulsion Laboratory, Pasadena, Calif. 818-354-2649 / 818-354-0307 firstname.lastname@example.org / email@example.com Since the 1990s, however, Greenland and Antarctic ice sheet mass loss and thermal expansion have accelerated sea level rise, while freshwater impoundment has decreased. As our climate continues to warm, the majority of this thermal energy is absorbed by the oceans, causing the volume of the water to expand. In fact, ice sheet melt and thermal expansion now account for about two-thirds of observed global mean sea level rise. Mountain glacier meltwater currently contributes another 20%, while declining freshwater water storage on land adds the remaining 10%. All told, sea levels have risen on average 1.6 millimeters (0.063 inches) per year between 1900 and 2018. In fact, sea levels are rising at a faster rate than at any time in the 20th century. But previous estimates of the mass of melting ice and thermal expansion of the ocean fell short of explaining this rate, particularly before the era of precise satellite observations of the world’s oceans, creating a deficit in the historic sea level budget. Antarctic ice loss and global sea level rise Back to insolation control of ice sheets Ice sheets form where the rate of snow and ice accumulation exceeds the rate of ice loss (ablation). The blue zone represents accumulation rate. The red zone represents ablation rate. Accumulation rate increases with temperature because the water content of air increases. Cold air is dry. The accumulation rate does not exceed 0.5 meters per year regardless of temperature. Ablation rate increases rapidly with temperature and is driven by summer temperatures above 0º C. The net balance between accumulation and ablation is called the ice mass balance. What season of year does the temperature represent? SUMMER Milutin Milanković was a Serbian mathematician, astronomer, climatologist, geophysicist, civil engineer and popularizer of science. Milanković Theory on orbital control of insolation and glaciation Large tilt exposes the poles to greater summer insolation and small tilt reduces summer insolation. Snow and ice remain when the summer insolation is low allowing ice sheets to grow. A major point about today’s climate. The Greenland and Antarctic ice sheets were stable and behaving according to Milankovic’s Theory until recently. Now they are melting, but not because of increased insolation due to increasing tilt. In fact, summer insolation at the poles is decreasing because axial tilt is slowly decreasing. What is happening? Back radiation of the thermal infrared portion of the EMS is trapped greenhouse Model of ice balance along a north-south line in the northern hemisphere ice sheets. Although the Climate Point is shown touching at sea level, it can touch ground at high elevations in mountains… alpine glaciers. When the equilibrium line is driven north by high values of summer insolation, the continents lie in a regime of net ablation and no ice can accumulate. When it is driven south by summer insolation minima, the northern landmasses lie in a zone of net accumulation and ice sheets can grow. Large tilt increases summer insolation and increases ablation. Small tilt decreases summer insolation and allows ice accumulation Ice Elevation Feedback As ice accumulates vertically the surface reaches colder air thus diminishing the ablation rate. The Greenland ice sheet is currently 3,200 m thick and the average elevation is 1,932 m. The average elevation of Antarctica is 2,500 m (8,200 ft). The ice sheet varies in thickness from 3,200 m to 4,900 m. Ice sheet growth and melting lags behind summer insolation forcing by almost ¼ of a wavelength. Tilt cycle is 41,000 y and lag is 10,000 y. Precession cycle is 23,000 years and lag is just under 6,000 y. If ice accumulation rate on Greenland were steady at 0.3 m/y, it would take 10,667 years to reach 3,200 m thickness. Of course it is unrealistic to assume a steady accumulation rate. Why? Orbital control varies on 23 ky, 41 ky, and 100 ky It is not linear. Orbital Control of Summer Insolation It is Not Constant What would the phase lag look like if the 41 ky and 23 ky were superimposed? A modulated signal with varying amplitudes. Amplitudes 1:1:1 Amplitudes 2:1:1 Amplitudes 800,000 y. Flexure of the lithosphere due to ice loading Change in slope of the Glacial Lake Agassiz beaches due to post-glacial rebound Flexure of the crust due to post-glacial rebound in the River Valley changed the drainage pattern of streams during the past 10,000 years. Typical dendritic drainage pattern Streams experience headward erosion and it is up hill. Onset of glaciation driven by orbital control of insolation. The delayed response of flexure keeps the elevation high and in colder air. Onset of an interglacial period driven by orbital control of insolation The delayed response of flexure keeps the elevation low and in warmer air. Full Cycle of Ice Sheet Growth and Decay Where is Canada today? F Where was Canada 20,000 years ago? E Two interpretations of δ18O for the past 100s of ky Cesare Emiliani: a record of past temperatures Nick Shackleton: a record of past ice volume It is a combination of the two 1976 James Hays, John Imbrie, Nick Shackleton: Linked δ18O to orbital cycles during the past 300,000 years that showed that the δ18O changes lagged summer insolation by several thousand years. 1980: A continuous δ18O record for the past 2.5 My δ18O data for the past 150,000 years show three orbital cycles that could reveal past ice volumes, but the temperature overprint makes that difficult. Fossil coral reefs provide a method for determining past sea levels. The useful corals live in the photic zone, shallow water, and the reefs are essentially within 25 feet (8 meters) of sea level. Milankovitch died in 1985 and all of the δ18O research occurred since. Then the only available records of glaciation were deposits on land and it was widely accepted that there had been only 4 glaciations in North America. Now the δ18O evidence shows fifty glacial maxima. These diagrams show a preglaciation phase when no ice accumulates. These diagrams show ice accumulates during summer minima but melts entirely during summer maxima. Ice persists during summer weak summer maxima and melts only in large summer maxima. Spetcral Analysis and Ice Volume •Seasonal summer insolation changes at 65° N are strong at 23,000 years, weaker at 41,000 years, and negligible at 100,000 years. •In contrast, the δ18O signal of northern hemisphere ice volume (and deep-ocean temperature) between 2.75 and 0.9 million years ago was very strong at 41,000 years but much weaker at 23,000 years. •Since 0.9 million years ago, the δ18O has been strongest at 100,000 years.
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