CHAPTER 11 CLIMATIC CHANGE AND VARIABILITY

Introduction to Climatic Change and Variability

  • Random variability
  • Periodic variability
  • Variability changes
  • Temporal trends
  • Step changes

Climatic Changes in Geologic History

  • Temperature
  • Faint Young Sun Paradox
  • Global demise of methanogens may have caused first global ice age,nearly 2.5 billion years ago, in the beginning of the Proterozoic Eon
  • Snowball Earth hypothesis from evidence dating to 800-600 million years ago
  • Gondwanan Ice Age – named for the large southern hemisphere continent that resulted from the break-up of Pangaea320-250 million years ago
  • K-T extinction at 65 million years ago
  • Because “permanent” ice exists on earth today (ice sheets of Greenland and Antarctica, along with numerous mountain glaciers), we are currently in the final, major, known ice age in geologic history
  • Ice ages are subdivided into intervals when “permanent” ice is displaced equatorward (glacial phases)and intervals when ice retreats poleward (interglacial phases)
  • Currently, in interglacial phase
  • Wisconsin Glacial Phase in the Pleistocene Epoch(approximately 18,000 years ago), “permanent” ice covered most of North America north of the Ohio River and extended across the pole to cover most of Europe and Asia
  • Ice Ages and Sea Level
  • During the Wisconsin Glacial Phase, eustatic sea level may have been as much as 170 m (550 ft) below today’s level, exposing large sections of the continental shelves
  • A common misconception is that the extra sea ice during cold periods would displace the same amount of water as its liquid equivalent, so its formation does not contribute to sea level decreases
  • It is a misconception that melting sea ice during warm periods does not contribute to increased sea levels
  • If all of the world’s current cryosphere were to melt, then sea level would rise by about 67 m (220 ft), and substantial areas of the coastal plains would be flooded worldwide
  • Some places in Scandinavia and Alaska that were covered by substantial depths of ice during the Wisconsin Glacial Phase are now rising (Isostatic rebound) because of the release of the tremendous pressure/weight of the ice
  • Recent Trends
  • Younger Dryas, (in Europe known as the Loch Lomond Stadial) between approximately 8,000 and 5,000 years before present, a time known as the Hypsithermal, in which global average temperatures were perhaps 2 C° (4 F°) above today’s levels
  • Approximately 900 A.D., a warm interval known as the Little Climatic Optimum (LCO – also known as the Medieval Warm Period)began
  • During the Little Ice Age (LIA) from about 1450 to 1850, streams in Europe that do not freeze today were observed to freeze regularly
  • The ten warmest years of the instrumental record all occurred since 1990
  • An estimated global sea level rise of approximately 15 cm (6 in) has accompanied the warming of the 20th century

How Do We Know What We Know about Past Climatic Changes?

  • Paleoclimatologists can make educated guesses using proxy evidence to ascertain climatic conditions of the distant past and assess the timing and magnitude of climatic change or variability
  • Basic principles
  • Preponderance of evidence is used to reach conclusions about the climate of a particular place at a particular time in the past
  • Principle of Uniformitarianism- evidence that would accompany a certain climatic type today would be assumed to suggest the same kind of climate at the time in the past when similar evidence was produced
  • Catastrophism - Catastrophic changes do not occur uniformly, but from one equilibrium to a drastically different equilibrium in only a short time geologically
  • Radiometric dating
  • Based on the principle of radioactive decay – rate at which spontaneous nuclear disintegration of certain isotopes of certain elements occurs
  • Compare the amount of parent isotope remaining to the amount of resulting daughter isotope using mass spectrometry techniques
  • Radioactive isotopes that have relatively short half-lives are more useful for relatively young samples
  • Advantages - the only absolute dating techniques that go back farther in geologic history than a few hundred years; scientifically solid method
  • Lithospheric and Cryospheric Evidence
  • Principle of Superposition– Older evidence will be found successively lower in sedimentary rock layers
  • Cross-cutting relationships – Many regions exhibit igneous rock formations that cross-cut pre-existing sedimentary rock layers
  • Relic landforms can also suggest features of past climates
  • Glaciers leave behind erosional and depositional landforms or features that provide geological hints of their presence in the past including glacial moraines, rock striations, deposits of loess,and specific erosional and depositional landforms related to past glacier formation, motion, and ablation
  • Glacial erratics suggest that a glacier must have moved rock from a location near other rocks similar to its composition to present location
  • The discrepancy between local soils and vegetation characteristic of colder climates and a warmer regional climate present in their locations today can give hints of a past climate to which the local environment has not adjusted
  • Ice coresfrom deep holed drilled into ice sheets can be sampled in a laboratory, and clues about the paleochemistry of the atmosphere can be found from air preserved within bubbles trapped in ice when the ice formed
  • Varvethickness can be related to climatic fluctuations on an annual or even seasonal scale
  • Biological Evidence
  • Fossilized rock allows paleoscientists to infer the climatic conditions that must have existed at the time and place that the fossil was formed
  • Foraminifera can provide clues about the salinity of the oceans during their existence, and salinity is related to the energy and water budgets of the atmosphere
  • Composition of the exoskeletons of other fossilized microscopic marine organisms also provides clues about ancient atmospheric temperature
  • Palynology– the scientific study of pollen and spores – coring and radiocarbon dating can identify a temporal sequence of pollen/spore types; a useful tool that can provide important information about the paleoclimatic conditions, including circulation, especially when used in conjunction with other types of paleoclimate evidence
  • Dendroclimatology can provide climatic evidence dating back hundreds or even thousands of years before present and offers the advantage of an annual and even a sub-annual record; especially useful in precipitation and streamflow reconstructions
  • Other sources of biological evidence include: analysis of the proportion of calcium carbonate (CaCO3) generated from exoskeletons in marine sediments, coral reefs, and treeline fluctuations
  • Historical Data refers to written information about climate but does not include formal meteorological records
  • Advantage: abundance of evidence for recent times
  • Disadvantage: results are not usually quantified, so it can be difficult to ascertain meaning
  • Converging evidence from several proxy data sources must be used before reaching a firm conclusion

Natural Causes of Climatic Change and Variability

  • Continental Drift and Landforms
  • Positions of continents can affect ocean circulation, and therefore temperature, to a significant extent
  • The positions of mountainous regions can be important producers of climatic change
  • Mountain uplift can affect the synoptic-scale atmospheric circulation
  • Milankovitch Cycles
  • Eccentricity of earth's orbit around the sun – gradually changes from a more circular to a more elliptical orbit, with a periodicity of approximately 95,000 years.
  • Tilt – the angle of tilt of earth’s axis does not change through the course of the year on human time scales, but on geologic time scales the tilt does slowly change from 21.8° from the vertical to 24.4° from the vertical
  • Precession of the equinoxes - Earth's axis wobbles like a spinning top because the earth is not a perfect sphere
  • Climatic changes can result from changes in eccentricity, tilt, and/or precession
  • Volcanic Activity
  • The uneven frequency of volcanic activity may be explained by differences in the number and strength of subduction zones through geologic time
  • Periods of active volcanism may enhance the greenhouse effect by contributing CO2 to the atmosphere, leading the earth into an ice-free period, and the slow removal of that CO2 may allow for the onset of an ice age
  • On shorter time scales, high concentrations of dust ejected during periods of intense and frequent volcanism may diminish insolation at the surface, cooling the climate
  • The addition of matter to the atmosphere from volcanism can disrupt the global atmospheric circulation, resulting in unusual and/or extreme weather events
  • Variations in Solar Output
  • Although t is assumed that increased solar output would be associated with a warmer climate, an inverse relationship between solar output and temperature could possibly exist
  • Sunspots – huge magnetic storms that appear as darker regions on the sun’s surface, varying in number and intensity over time
  • ENSO Events
  • ENSO affects the mid-latitude atmospheric circulation systems, because the shearing motion associated with circulation at the poleward edges of the Hadley cells affects mid-latitude circulation
  • Monsoon circulations tend to weaken during El Niño, and Atlantic tropical cyclone activity tends to be suppressed during El Niño because of the abnormally strong westerly winds, which shear off the tops of developing tropical cyclones.
  • El Niño or La Niña events produce significant climatic variability
  • Quasi-biennial Oscillation (QBO) is a periodic reversal of the equatorial stratospheric winds between westerly and easterly, with a cycle of approximately 26 months