EOSC 112 Lecture 6: Long Term Climate Change lecture notes

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Earth and Ocean Sciences
EOSC 112
Debeare Bart

LONG TERM CLIMATE CHANGE Geological evidence indicated that liquid water has been continuously present on the surface of the Earth since at least 3.8 billion years ago, which implies that temperature on Earth’s surface cannot fluctuate beyond the freezing or boiling temperature of water. Yet, there is evidence of extreme ice ages and extreme tropical ages with no ice. - The geological time is divided into intervals at different levels - At the broadest level, there are two primary eons (Precambrian (first ~4 billion years) and the Phanerozoic (last 554 million years). o Organisms alive during the Precambrian Eon were mostly single celled organism that left a sparse fossil record. The oldest are stromatolites (layered minerals deposited by ancient colonies of cyanobacteria) o During the Phanerozoic = complex organisms producing hard skeletons which leave fossils, this fossil record is what gives us information about this era. - Eons are subdivided into Eras o Phanerozoic = Paleozoic (“old life”), Mesozoic (“middle life”) and Cenozoic (“new life” - Eras are then divided into periods – Cenozoic (0-65 million years)  tertiary (65-1.8 million years) and Quaternary (last 1.8 million years) Climate: The climate is characterized by long periods of warm climate separated by shorter episodes of cold climate. Generally, it was warmer during the first 2 billion years, then somewhere between 2.5 and 2.3 billion years ago there was evidence of the first glaciation, the Huronian glaciation. Following this, there was another 1 billion years of warmer weather before the large glaciation that appeared in the Late Proterozoic (800 – 600 Million years ago) called the “snowball earth”. Then the weather warmed again until the Quaternary glaciation happened (300-270 million years ago). - During the Quaternary ice ages, the volume of continental ice in the northern hemisphere changed substantially, during the ice ages, huge ice sheets covered what is now Canada and Northern USA as well as Northern Europe – this cause a 130m sea level drop! Factors: One of the primary factors is the input of solar radiation. The “Faint Young Sun” paradox happened between 4.5 and 1.8 billion years ago when the Earth should have been totally frozen yet there were was still liquid water on the planet. The evidence of water comes from sedimentary rocks so… why was there liquid water? - the most likely explanation for this is that greenhouse gases were present at higher concentration in the early atmosphere that today so compensate for the lower Sun radiations flux and to maintain the early Earth temperature above freezing. - CO 2ay have been high during the earlier stages of Earth’s history for several reasons: o Continents were much smaller (cratons); throughout Earth’s history, sedimentary rocks are added to the periphery of the cratons at subduction zones, as marine sediments get scraped off the top of the plunging oceanic plates, transformed into sedimentary rocks by pressure and cementation and added to the edge of the continental plate. o Volcanism is associated with subduction also produces volcanic rocks that add material to the periphery of the growing continents – removal rate of CO fro2 atmosphere happens through chemical weathering – which must have been smaller since the land mass available for chemical weather was smaller. (long term Atmospheric CO is 2ontrolled by the balance between uptake during weathering and released by volcanism) If surface area covered by rocks that can react with carbonic acid in rain water is smaller = higher atmospheric CO (l2ss chemical weathering). If volcanism doesn’t change, CO starts2to increase because input > output. - Then negative feedback comes into the picture o CO in2reases  acidity of water increases  increases rate of reaction between rocks and carbonic acid  gradually increases rate of CO2 removal/surface area. It is believed that the addition rate of CO2by volcanism was likely higher during this time since the interior Earth was hotter at the beginning, so more volcanism  faster seafloor spreading. - Which results in higher rates of CO2 emissions to the atmosphere The Earth system contains a simple natural thermostat that regulates its temperature in the long term and maintains within the range of liquid water: - Rate of reaction between rocks and acid rain during chemical weathering not only increases with the acidity of the rain, but it also increases as the temperature increase. o Faster dissolving rate when temperature is higher ▪ Which produces a natural thermostat that stabilizes the temperature of the planet on long time scale and compensates for the increase in solar emission with time. - Assuming that the CO2 level are at a steady state and input by outgassing = uptake by weathering o Increase in solar intensity  increase in temperature o Higher temperature increases the rate of CO2 uptake by chemical weathering o If outgassing stays the same, CO2 drops, lowering the weathering rate until they match up o Ending up with a stable, but LOWER CO2 level and lower greenhouse warming compensates for the increase in solar radiation – keeping the temperature constant. SO, as the sun has gradually increases in size and intensity over time, the atmospheric CO2 levels have decreased to compensate. If CO2 were the only GHG, it would have to have been ~1000x higher than todays! - But there were other gases that affected the climate and atmosphere in the early atmosphere, such as CH4 (methane) which is a potent GHG that is also released from volcanoes. METHANOGENS: they were among the first organisms to evolve and they produce CH4 and the early atmosphere may have had 1000ppm instead of 1.7ppm like today. Today, methane is quickly oxidized by oxygen but this did not happen in the early atmosphere which did NOT contain O2. Due to this, the level of CO2 needed to keep the Earth above freezing temperature could have been much lower. - So when we had low solar intensity, it was compensated for by higher greenhouse warming from the presence of high atmospheric CO2 and CH4. - Then, 2.5 billion years ago we find evidence for the first glaciation – the HURONIAN HURONIAN: We know that there was major ice accumulate based on geological deposits dating from that time which could only have been produced by glaciers or ice sheets growing on the continents. - Around Lake Huron, rocks that date between 2.5-2.5 billion years ago consist of tillites – large, angular, detached rocks cemented in a matrix of fine rock flour. - They are formed when the glacier is moving slowly from a high elevation towards the toe of the glacier where all the ice melts, as a result
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