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Climate Change in the Past Palaeoclimate

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Climate Change in the Past Palaeoclimate Data collection and presentation by Carl Denef January 2014 1 Climate change in the Past Palaeoclimate Past climate is the key to preview future climate and helps to explain present climate change. Understanding present climate change and projecting climate change and impacts into the future can be greatly helped by knowledge of climate changes in the past. The next slides will show that most of the Earth’s geological history was characterized by a warm climate, with average global surface temperatures 9-12 °C warmer than now, and with atmospheric CO2 levels 3-5 times higher than in the pre-industrial Era. The warm climate sometimes turned into major glaciation periods (Ice Ages) that lasted 30-300 million years. At present we live again in a glaciation period interupted by cycles of warming every 100,000 years. During glacial periods CO2 levels dropped significantly, as did sea level. The major glaciations (blue areas) during the Earth's entire existence.[Ref] 2 How can we assess climate of the past? Past climate can not directly be assessed but can be reconstructed on the basis of what is called « proxies ». These are present physical parameters, that have signatures of certain climate parameters in the past. Temperature reconstruction proxies 1) Oxygen and Hydrogen isotope ratios in ice cores and in sediments in sea, land and lake floors : By drilling in polar ice sheets of Greenland and Antarctica and in mountain glaciers, cylindric specimens can be sampled and the relative quantity of the stable oxygen18 (18O) and deuterium (D) isotopes be determined. Water molecules containing the heavier 18O or D evaporate at a higher temperature than water molecules containing the normal 16O and hydrogen, due to the higher atomic weight of the former. Similarly, when water vapor condenses, heavier water molecules holding 18O atoms tend to precipitate first. Changes in 18O /16O ratio over time (δ18O) is therefore indicative of temperature change and can be followed in air trapped within fallen snow that compacts to ice or in the ice itself. δ18O in ice layers is indicative for the temperature at the time the ice was formed. The deeper the drilling the older the cores. The deepest drills are >4 km and contain proxies of > 800,000 years old. Sediments are often analyzed for δ18O in foraminifera (or forams) and diatoms. These are shelled microorganisms found in aquatic and marine environments. Forams are either planktonic (floating in the water column) or benthic (bottom dwelling). Foram shells are made up of calcium 18 carbonate (CaCO3) while diatom shells are composed of SiO2. Relatively more δ O in shells is found in carbonate when ocean waters are cold and ice. 3 18 covers the Earth, because at lower temperature the proportion of H2 O that evaporates becomes 18 lower, leaving more H2 O in the residual water for shell formation. 2) Alkenones and Mg/Ca ratio of calcite in foraminifera and diatoms in sediments may yield information about their temperature at formation. 3) Remnants of vegetation, animals, plankton, corals or pollen in land, lakes and ocean floor sediments may be characteristic of certain climatic zones 4) Direct temperature measurements in rocks: Rock has a very low thermal conductivity. It can take centuries for rocks underground to become aware of changes in surface temperatures. By taking very careful measurements of the temperature of rock in boreholes tens and hundreds of meters underground, it is possible to detect shifts in the long-term mean surface temperature at that location. As thermal diffusion is a very slow process, short term changes are averaged out. This technique only provides information about changes in the average temperature at the century resolution. The map on the left shows the locations of the 951 boreholes in the University of Michigan global database of boreholes. 5) The type of living species in fossils can be typical for a temperature range. For example, plankton live in narrow temperature ranges. 4 6) Tree rings are indicative for warmth, although also for humidity and nutrient conditions Reconstruction of CO2 levels: 1) Direct analysis in air trapped in ice core layers up to 800,000 years ago. 2) Reconstruction from carbon isotopic ratio (δ13C) in carbonate of fossilized soils (paleosols) or of phytoplankton (foramenifera) shells, that remained intact over millions of years. δ13C is the ratio of the stable carbon isotopes 13C/12C.[1] Carbon in inorganic carbonates not derived from living organisms shows the natural isotope ratio signature of 1/99 without preferential choice for 13C or 12C, while carbon in materials originated from photosynthesis is depleted of 13C, because plants prefer 12C over 13C in photosynthesis. Carbon in fossil shells is therefore also depleted in 13 13 C. δ C depends on the levels of CO2 in the atmosphere and on the amounts of CO2 being respired by organic matter in the soil itself. 3) Determination of stomata in fossil plants: Stomata are pores to breathe in the CO2 that plant leaves need for photosynthesis. When CO2 is abundant, plants down regulate the number of stomata in their leaves. Stomata density in fossil plants is therefore an important proxy for atmospheric CO2 concentrations. Reconstruction of ocean currents. Ocean sediment made up of microfossils and mineral grains delivered to the sea from continent erosion can tell about ocean currents in the past. Diatoms particularly take advantage of upwelling ocean water that is richer in nutrients. Reconstruction of ocean pH: Ocean pH can be reconstructed up to >20 million years ago from the ratio of stable boron isotopes 11B/10B in ocean sediments of foraminifera. 5 Reconstruction of Wind directions. Volcanic dust, sea salt, black carbon and desert dust in the air are deposited on glaciers and ice sheets and accumulated with snow in the ice. When the dust shows up in ocean cores, its chemistry can be used to determine where it came from. By mapping the distribution of the dust, wind direction and strength can be inferred. The dust also may reveal how dry and dusty the climate may have been at a particular time. Read more Climate models. It has become possible to represent the different physical processes associated with the climate system as differential equations that can be numerically resolved by computers. These physical processes include atmospheric and ocean circulation, the reciprocal relationships of the latter, ice formation and melting dynamics, the distribution of δ18O in the oceans, ecological parameters of carbonate forming organisms and others. If the various forcing parameters are known over time they can be entered in the model that then computes the likely climate evolution over time. The calculated climate is then compared to the climate evolution constructed from proxies. The closer the modeled changes match those observed in the sediments or ice cores, the greater the confidence in the realism of the models. Multiple regression analysis can dissect out the different forcing factors involved in the past climate change. 6 Ice Cores (source) Ocean Foranimifera samples (Wikipedia) sediment cores 7 (source) Palaeoclimate drivers At time scales of 1-10 million years tectonic activity is the major driver of climate change. Increased tectonic activity causes continental drift and increases CO2 release by volcanism and sea-floor spreading, promoting global warming by the increased greenhouse effect. Continental drift, in turn , determines the position of the continents and affects the thermohaline circulation. The latter transfers heat between the equatorial regions and the poles and in this way affects climate. When landmasses are concentrated near the poles, there is an increased chance for Source snow and ice to accumulate as there is more cooling over polar regions. Small changes in the Earth’s obliquity, eccentricity and precession change the amount of solar radiation reaching the Earth (Milankovitch cycles) which can tip the balance between summers in which the packed winter snow completely melts and summers in which the winter snow persists until the following winter. If snow remains accumulating it strongly increases albedo, resulting in cooling. Moreover, accumulation of snow and ice on land decreases sea level. 8 Geological time scales (source) Period Epoch Date Holocene 0–0.0117 0.0117–0.126 Quaternary 0.126–0.781 Pleistocene 0.781–1.806 1.806–2.588 Look here or here how the Earth land and sea surface evolved over those times ! Source Numbers are million 9 years Continents during geological history Ma = million years ago Source Mid-Ordovician (470 Ma) Mid-Jurassic (170 Ma) Gondwana Ice Cretaceous-Paleogene boundary (65 Ma) Eocene (50 Ma) Present Pleistocene10 (0.5 Ma) Climate changes in the Phanerozoic - from 500 million years ago Temperature: There have been periods when global average temperature was 9-12 °C higher than present (15 °C), as derived from δ18O in fossils. [1] The Earth was a hothouse without any ice caps on the poles. Tropical waters are assumed to have been around 45°C. Sporadically there were long periods of glaciation (indicated by the blue bars in the Figure) over the Antarctic area. During warm periods, masses of water evaporated from the oceans that, upon cooling over the huge supercontinent known as ‘Gondwana’, that was located over the Southern hemisphere and Antarctic region at that era, precipitated as snow. Packed snow became ice, giving cooling of the Earth (increased ice-albedo). 11 Source (adapted) CO2: Overall, temperature changes correlated with atmospheric CO2 levels, lowest levels being reached during periods of glaciation. However, absolute values were very divergent according to the proxy used (See Figure). Boron, phytoplankton and fossil plant stomata proxies generally 13 give lower CO2 levels. A recent paper in PNAS showed previous δ C determinationsin in soil carbonates overestimated atmospheric CO2 levels; the paper calculated with a new method that CO2 levels during the Phanerozoic was maximum 1500 ppm and that the fluctuations were best in line with those reconstructed from the stomata proxies.
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