From Greenhouse to Icehouse The last 50 million years News Item February 3, 2021 • Scientists in the Arctic’s ‘Ice Factory’ Found a Worrying Sign of Climate Change • Sea Ice in the Laptev Sea is lower • Wave action stirring the water to 40-50 meters depth is warming the sea • Thawing of the subsea permafrost is releasing methane September 7, 2021 • SAT in 2020 was 2.1°C above the 1981-2010 average. • 7th consecutive year with SAT above the 1981-2010 average. • The SAT anomalies contributed to the early widespread fires and record retreat of the Laptev Sea ice. • Loss of sea ice created reduced albedo and enhanced warming of the ocean water. • Continental and mountain glaciers formed • Fossil vegetation shows cold-adapted forms replaced warm-adapted forms and their ranges moved toward the tropics • Fossil land animals show warmadapted types disappearing from high latitudes • Oxygen isotopes show cooling of the ocean • Mg/Ca ratios show cooling of the ocean Earth’s climate has cooled 15 °C during the past 50 my. What is the Evidence? What were the causes? • This evidence relies on the principles of natural science we covered in week one. • Superposition, original horizontality, faunal succession, uniformitarianism Fossil plants from Arctic Canada show that a warm climate existed there 60 million years ago. Now the ground cover is tundra. A fossil turtle from Arctic Canada is evidence of a warm ice-free climate 90 million years ago. 60 Ma Today Estimated mean annual Temperature for N. A. Tree leaves with smooth edges live in the tropics and trees with serrated edges live in colder climates. Antarctica today is mostly covered with ice although mountains along the northeast tip protrude and expose rocks containing fossil fauna and flora. Elsewhere the continent is covered by 4 km of ice. The fossils show that 30 million years ago nothofagus trees like those now living in Argentina lived in Antarctica. UND Geology PhD Student Marie Berglin in a dry valley in Antarctica January, 2019 UND campsite 2020 https://eos.org/articles/antarctic-ice-cores-might-be-older-than-dirt? utm_source=eos&utm_medium=email&utm_campaign=EosBuzz021420 Ice core depth Top Hunt for the Oldest Ice on Oxygen Cosmogenic nuclide Bottom # of Cosmogenic Nuclides Physics and Chemistry in Climate Research Oxygen isotope ratios in shells and ice Magnesium and calcium ratios in shells δ18O (delta O 18) • Ratios of • • • 18O/16O in foraminifera and ice are a proxy for water temperature. 18O is the heavier and lesser abundant isotope, 16O is the lighter and more abundant isotope. 18O has 8 protons and 10 neutrons, 16O has 8 protons and 8 neutrons ppt (0/00) SMOW = Standard Mean Ocean Water • Low δ18O values indicate high temperatures and high δ18O values indicate low temperatures. • δ18O: surface ocean = 0 to +2, deep ocean = +3 to +4, ice sheets = -30 to -55 Mass spectrometer Oxygen isotope evidence for oceanic cooling of 14 ºC since 70 Ma Benthic Planktonic Foraminifera make their CaCO3 shells from HCO3-1 in sea water. Benthic forams live on the sea floor and planktonic forams live in near-surface water. Over time, deep waters have cooled and have moved δ18O toward positive values. Evaporation favors 16O and enriches sea water in 18O. As ice sheets grow the ocean becomes enriched in 18O further moving δ18O toward positive values. Mg/Ca evidence for oceanic cooling of 14 ºC since 70 Ma Chemical reactions are temperature dependent, and the potential for substitution of Mg for Ca in the crystal lattice of calcite (CaCO3) increases with temperature. Over time the Mg/Ca ratio in the shells of benthic foraminifera has decreased indicating cooling of the deep ocean. Caveat: Neither oxygen isotope nor Mg/Ca ratios capture all aspects of climate change. Note that an overlay of the Mg/ Ca plot on the δ18O plot shows some • http://lorraine-lisiecki.com/stack.html Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records, Paleoceanography, 20, PA1003, doi:10.1029/2004PA001071 Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records, Paleoceanography, 20, PA1003, doi:10.1029/2004PA001071 The δ18O record of deep-sea temperatures is a proxy record of polar glaciations. The trend in δ18O from 3 to 4 during the past 5 My indicates cooling of the global ocean. Lisiecki, L. E., and M. E. Raymo (2005), A Pliocene-Pleistocene stack of 57 globally distributed benthic δ18O records, Paleoceanography, 20, PA1003, doi:10.1029/2004PA001071 During the past 600 My, the glacial cycles averaged about 100 ky. Glaciations built slowly over an average of 86 ky. Interglacials occurred rapidly and averaged less than 15 ky. We are presently in an interglacial that has lasted for 18 ky. For the past 1,260 ky the glacial cycle peaks average about 89 ky and the interglacials average about 75 ky. The values have become closer over time. This variability is a hint to the material in Chapter 8. 1260 ky 6 δ18O 5 4 3 2 0 315 630 ky 945 1260 From Greenhous e to Icehouse • Summary of the evidence for cooling •The changing climate brought mountain glaciers, continental-scale ice sheets, replacement of warmadapted vegetation and animals by cold-adapted forms. •Oxygen isotope and Mg/Ca temperature proxies •This section explores three possible explanations for the cooling trend: • Change in ocean heat transport by opening and closing of interoceanic gateways. • The BLAG spreading hypothesis • The uplift and weathering hypothesis Climate Models Overview from NOAA • This image shows the concept used in climate models. • Each of the thousands of 3-dimensional grid cells can be represented by mathematical equations that describe the materials in it and the way energy moves through it. • The advanced equations are based on the fundamental laws of physics, fluid motion, and chemistry. • To “run” a model, scientists specify the climate forcing (for instance, setting variables to represent the amount of greenhouse gases in the atmosphere) and have powerful computers solve the equations in each cell. • Results from each grid cell are passed to neighboring cells, and the equations are solved again. Repeating the process through many time steps represents the passage of time. Image source: NOAA. Geographic change as a factor to explain cooling of the deep ocean during the past 50 million years. The GATEWAY HYPOTHESIS Opening and closing of oceanic gateways can allow or impede exchange of waters between basins and affect heat transport from the equator to the poles. Two tests of the Gateway Hypothesis The Gateway hypothesis would have warm tropical waters flowing to Antarctica. But, separation of South America opened Drake’s passage and created a circumpolar cold current Drake’s Passage Simulations indicate that Antarctica would be cold regardless and opening of Drakes passage was not a factor in Antarctic glaciation. However, the geography models in Chapter 6 were not so great and I wonder how this factors into Ruddiman’s conclusion. Second Test The Central American Seaway The cut off of Pacific water flow would have increased precipitation in the North Atlantic aiding snow and ice build up. In fact the opposite would have happed due to lack of sea ice and to the warm summer ablation of the ice sheet. Conclusion: the Gateway hypotheses fails. Hypotheses that invoke changes in CO2 Antarctic glaciation could occur when CO2 content is below 750 ppm and Arctic glaciation could occur when CO2 content is below 280 ppm
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