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Outline. Review of Ocean Stratification and Circulation Recent historical Climate Change External Climate Forcings Natural Climate Variability Paleoclimatology Ice Ages. Recent historical climate change.

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  1. Outline • Review of Ocean Stratification and Circulation • Recent historical Climate Change • External Climate Forcings • Natural Climate Variability • Paleoclimatology • Ice Ages

  2. Recent historical climate change A. Past 1000 years: evidence from winter severity information, tree rings, etc. suggests that there was a medieval warm period about 1000 years ago, then a "Little Ice Age" from about 1400 to the late 19th Century B. Past 100+ years: Direct surface weather station measurements of temperature indicate slowly rising global temperatures from late 19th Century until about 1940, then weak cooling until 1965, then sharply rising temperatures up to the present C. Greenhouse gas concentrations have increased steadily since the beginning of the Industrial Revolution; why has global temperature not increased monotonically? Can we predict with confidence that the globe will continue to warm in the future? D. Must consider other external climate forcings and natural (unforced, internal, random) variability of climate system.

  3. Recent historical climate change Must consider other external climate forcings and natural (unforced, internal, random) variability of climate system. I. External climate forcings (other than greenhouse gases). A.Solar luminosity variations B.Volcanic eruptions. C.Anthropogenic (tropospheric) aerosols include atmospheric particles and droplets: sulphate, soot, dust, sea salt. D.Biogenic regulation of climate (Gaia).

  4. Recent historical climate change II. Natural variability. A.Short time scales (1-2 years): Random weather-related variations of turbulent, chaotic atmosphere. Global temperature animation:1971-1999. B.Interannual (2-8 years): Primarily ENSO; longer time scale due to interaction of atmosphere with more massive ocean mixed layer and thermocline. C.Decadal-to-century scale: Due to changes of intermediate/ deep ocean circulation and interaction with atmosphere; unknown magnitude and triggering mechanisms leave open question of whether climate change is predictable.

  5. Recent historical climate change II. Natural variability. A.Short time scales (1-2 years): Random weather-related variations of turbulent, chaotic atmosphere. Global temperature animation:1971-1999. B.Interannual (2-8 years): Primarily ENSO; longer time scale due to interaction of atmosphere with more massive ocean mixed layer and thermocline. C.Decadal-to-century scale: Due to changes of intermediate/ deep ocean circulation and interaction with atmosphere; unknown magnitude and triggering mechanisms leave open question of whether climate change is predictable.

  6. Recent historical climate change A. Past 1000 years: evidence from winter severity information, tree rings, etc. suggests that there was a medieval warm period about 1000 years ago, then a "Little Ice Age" from about 1400 to the late 19th Century

  7. Why we study past climates (paleoclimatology) A. We can measure many aspects of the climate system today and, from these measurements, understand fundamental physical and chemical processes. B. These measurements can be used as input to climate models and the complex processes and their feedbacks can be simulated.

  8. Why we study past climates (paleoclimatology) C. We cannot instrument the past; the complex atmospheric processes can not be measured. The climate system must remain a black box D. However, the mysterious climate system of the past produces a record and from this record we learn about past climate systems.

  9. The kinds of paleoclimatic archives Proxy Records: naturally record and store climate information. 1.Sensitivity Best to have present-day analogue for calibration. Ability to isolate parameter of concern. Understand reasons for complications. 2.Storage, accumulation. Sedimentary records. Growth rings. 3.Dating. Figure out when things happened. Are there gaps in the record? Cross compare with other records, events. Determine rate of change.

  10. The kinds of paleoclimatic archives Marine sediments: accumulate slowly (typically 2-6 cm per 1000 years) but relatively continuously. Lake sediments: Tree rings: annual growth layers in trees and shrubs. Peat deposits: Glacial Ice Cores:

  11. The Milankovitch MechanismPacemaker of the ice ages A. The Milankovitch or astronomical theory of climate change is an explanation for the changes in the seasons which result from changes in the earth's orbit around the sun. The theory is named for Serbian astronomer Milutin Milankovitch, who calculated the slow changes in the earth's orbit by careful measurements of the position of the stars, and through equations using the gravitational pull of other planets and stars.

  12. The Milankovitch Mechanism There are three main components to Earth orbital variabilityEccentricity (100,000 year period), Tilt (41,000 year period), and precession (23,000 and 19,000 year periods). Only variations in orbital tilt and precession significantly affect the amount of radiation received during a given season.

  13. The Milankovitch Mechanism These orbital changes cause large changes (up to ±15%) in the amount of sunlight received during a given season. The key variable is the amount of summer radiation at high northern latitudes.

  14. The climate record of ice cores 1.Result of snow accumulation 2.Snow contains air 3.Some air gets trapped in bubbles Bubbles then contain "fossil air." 4.Ice contains water and water contains isotopes of both hydrogen and oxygen 5.Factors controlling the behavior of hydrogen and oxygen isotopes 6. Relation of dO18 values to temperature.

  15. The climate record of ice cores 6. Relation of dO18 values to temperature. Then you can reconstruct time series of past climate change...

  16. The climate record of ice cores

  17. Antropogenic CO2 Increase A. The anthropogenic rise in CO2 is ~1.5% each year due to fossil fuel combustion and deforestation. B. The preindustrial CO2 level was ~280 ppm, it is now (1998) at 375 ppm By the time most of you are 30 years old it will be close to 460-470 ppm.Doubling of atmospheric CO2 is expected by the year 2050, perhaps sooner depending on the emission scenario which plays out.

  18. Antropogenic CO2 Increase C. Here is a comparison of the CO2 variations over the full glacial to interglacial cycle of the last 140,000 years compared to the anthropogenic rise in CO2 just over the past 300 years (Fig. 34).

  19. The climate record of ice cores C. Here is a comparison of the CO2 variations over the full glacial to interglacial cycle of the last 140,000 years compared to the anthropogenic rise in CO2 just over the past 300 years

  20. Recent historical climate change

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