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  1. Recent Climate Changesand Climate Archives Martin Visbeck DEES, Lamont-Doherty Earth Observatory visbeck@ldeo.columbia.edu

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

  3. General Atmosphere Ocean CirculationThe surface energy balance Top of atmosphere seafloor Imbalance of energy flux at the top can be balanced by: Atmospheric Heat Transport Oceanic Heat Transport Air-sea interface

  4. Sea Surface Temperature • The link between ocean and climate depends on exchange of energy (mainly heat and radiation) and materials (water, gases) across the sea surface. T atmosphere 'sees', influences and responds to the sea surface temperature (SST), by way of sea-air heat flux. SST generally cools with increasing latitude, but important deviations from a pure latitudinal dependence occurs. These are generally due to the movement of sea water in both the horizontal and vertical directions. Temperature and density of ocean water are related inversely: warm water means low density, cold water means denser sea water.

  5. Ocean Salinity • As the range of salt concentration in the ocean varies from about 3.2 to 3.8%, oceanographers, who refer to salt content as 'salinity', express salt concentration as parts per thousand; 34.9 ppt is the average salinity.


  6. Effect of Atmospheric Forcing on Ocean

  7. Wind Driven Ocean Circulation

  8. Cool water Ocean simulation in tank light

  9. Buoyancy Driven Ocean Circulation • The buoyancy forces are capable of inducing overturning that reach from the sea surface to the sea floor. • Buoyancy fluxes are those fluxes between air and water that alter the density of the sea water. • Cooling of the ocean and evaporation makes the ocean (colder, saltier) denser, removing buoyancy. • Heating and excess precipitation has the opposite effect, they add buoyancy to the ocean.

  10. Overturning • North Atlantic Deep Water • Antarctic Bottom Water • Intermediate Water

  11. Oceanic Heat Transport

  12. The Oceans Role in Climate The sum of the wind driven and buoyancy driven ocean current transport large amounts of heat and fresh water over large distances. Can you rationalise the signs?

  13. Recent Climate Changesand predicting the future 1.What factors influence climate change? 2.On what time scales do these factors operate and what controls the time dependence.

  14. Review of planetary energy balance A. Remember that in equilibrium, absorbed solar energy equals emitted heat. B. Absorbed solar energy depends on solar constant (intensity of Sun at Earth's distance) and planetary albedo (fraction of incident sunlight reflected) C. Emitted heat depends on temperature at which Earth radiates to space; difference between this temperature and surface temperature is an indicator of the greenhouse effect, which depends on the concentration of greenhouse gases. D. Climate change occurs when either side of energy balance is perturbed.

  15. Climate change occurs when either side of energy balance is perturbed. Example 1: • Increase greenhouse gases -> • decrease IR radiation to space -> • absorbed solar exceeds emitted thermal -> • temperature must increase to restore balance.

  16. Climate change occurs when either side of energy balance is perturbed. Example 2: • Increase planetary albedo -> • decrease absorbed solar -> • emitted thermal exceeds absorbed solar -> • temperature must decrease to restore balance. • In general real climate changes involve changes of both sides of the energy balance.

  17. 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.

  18. 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

  19. 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

  20. Recent historical climate change 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

  21. Recent historical climate change 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. Notice the regional difference between tropics and northern and southern hemisphere.

  22. Recent historical climate change 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?

  23. 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). II. Natural variability.

  24. 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).

  25. 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.

  26. Solar luminosity variations Sunspots are dark, decrease luminosity, but are surrounded by bright faculae which cover a larger area; thus, Sun is brightest at peak of sunspot cycle Satellite observations since 1980 indicate that solar luminosity oscillates slightly with the 11-year sunspot cycle.

  27. Solar luminosity variations Larger long-term variations may explain Little Ice Age (Maunder minimum), but mechanism is not understood (related to sunspot number, cycle length); unknown potential contributor to future climate change.

  28. Solar luminosity variations Larger long-term variations may explain Little Ice Age (Maunder minimum), but mechanism is not understood (related to sunspot number, cycle length); unknown potential contributor to future climate change.

  29. Volcanic Eruptions Large ash and dust particles fall out of atmosphere quickly, do not affect climate. Climate impact is favored by (a) material reaching stratosphere, above altitude of scavenging by rain, (b) small particles, which fall out slowly; under these conditions, volcanic aerosols can reflect sunlight (increase albedo) for several years and cool climate. Small particle formation: Injection of sulfur-bearing gases (e.g., SO2) into stratosphere, photochemical reactions form small sulfuric acid (H2SO4) droplets.

  30. Volcanic Eruptions Not all volcanoes affect climate: Mt. St. Helens (1980) exploded sideways sulfur-poor, thus no climate impact; Mt. Pinatubo (1991) exploded vertically, sulfur-rich, thus biggest climate impact of 20th Century. The eruption of Mount Pinatubo presented modern climatologists with an opportunity to test the reliability of numerical climate models to predict cooling from a volcanic eruption. They could estimate the amount of aerosols propelled into the stratosphere from this eruption, their spread around the world, and eventual removal from the stratosphere. Using these estimates as input to the model they asked the model to predict the effect of these aerosols on our planets mean global temperature. These estimates are compared with instrumental measurements of Earth's temperature over a period of several years after the eruption.

  31. Volcanic Eruptions It is clear from this figure that the model did a very good job. This does not mean that the model would do as good a job with other factors forcing climate change such as increasing CO2 input. About 10 other volcanoes have probably affected climate in past century.

  32. Antropogenic (tropospheric) Aerosols Anthropogenic (tropospheric) aerosols include atmospheric particles and droplets: sulphate, soot, dust, sea salt. Some aerosols increase light reflection/scattering Sulphur dioxide transforms to sulfuric acid which attracts water vapor and condenses to form sulphate particles that reflect/scatter light. Sources include combustion, volcanoes, biology. SO2 emissions have probably more than doubled sulfate aerosol concentration in 20th Century; systematic upward trend in albedo (direct effect) may have offset part of greenhouse warming; climate models agree better with observed temperature trend when aerosols are included.

  33. Antropogenic (tropospheric) Aerosols Anthropogenic (tropospheric) aerosols include atmospheric particles and droplets: sulphate, soot, dust, sea salt. SO2 emissions have probably more than doubled sulfate aerosol concentration in 20th Century; systematic upward trend in albedo (direct effect) may have offset part of greenhouse warming; climate models agree better with observed temperature trend when aerosols are included.

  34. Antropogenic (tropospheric) Aerosols Other types of aerosols, for example soot from combustion and biomass burning, also are important, but are darker, so reduce albedo, absorb sunlight, reduce cloud cover due to atmospheric heating, mayb cause warming. Additional indirect effect of sulphate aerosols is to modify cloud properties Increase cloud brightness because aerosols lead to larger number and smaller size of cloud droplets, causing higher cloud albedo Increase cloud cover because smaller droplets inhibit rainfall and increase cloud liftetime.

  35. Antropogenic (tropospheric) Aerosols Aerosol forcing regional in nature (mostly Northern Hemisphere, near and downwind of industrialized areas); some regions may cool while others warm.

  36. Biogenic regulation of Climate (GAIA) Hypothetical example: Daisyworld (planet populated by white daisies that like warm temperatures, black daisies that like cool temperatures); if climate warms, white daisies thrive, albedo increases, limiting warming; if climate cools, black daisies thrive, albedo decreases, limiting cooling. Real world possibility: dimethylsulfide (DMS) emissions by plankton, leading to sulfate aerosol formation; primary nucleation source for oceanic clouds; possible impact on climate change, but DMS dependence on temperature not established.

  37. Natural Variability A. Short time scales (1-2 years): Random weather-related variations of turbulent, chaotic atmosphere (NAO) 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.