Climate changes: present, past, and future

Take away ideas and understandings

I. Review of planetary energy balance.

  1. Remember that in equilibrium, absorbed solar energy equals emitted heat.
  2. Absorbed solar energy depends on solar constant (intensity of Sun at Earth's distance) and planetary albedo (fraction of incident sunlight reflected) (Fig 1).
  3. 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.
  4. Climate change occurs when either side of energy balance is perturbed.
  5. Example 1: Increase greenhouse gases -> decrease IR radiation to space -> absorbed solar exceeds emitted thermal -> temperature must increase to restore balance.

    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.

II. Past climate change.

  1. Historical (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 (Fig 2. This record contains information from a large number of land stations as well as measurements of sea surface temperature. The latter include satellite measurements over the last two decades.

    1. Land measurements (Fig 3).
      1. Global coverage between 1880 to 2000.
      2. Urban heat island effect.
        1. Two data sets one rural and the other containing data from near or within cities. The two data sets show similar trends and the differences between them are not statistically significant. The warming in both far exceeds the uncertainties.
      3. Types of data and how measurements are weighted.
        1. More land in Northern hemisphere than southern.
        2. Oceans warm more slowly than land.
        3. Temperature measurements now cover 54% of land.
      4. Diurnal temperature range.
        1. Warming in recent decades has involved faster rise in daily minimum than daily maximum temperatures (Fig 4).
        2. Also a correlation between this trend and an increase in cloudiness.
    2. Sea Surface Temperature.
      1. Temperature within the upper few meters of the ocean.
      2. In the early days buckets of water were brought on deck and the temperature measured.
      3. Since 1941 most measurements from ships have been engine intake water temperature (Fig 5).
    3. Land and sea surface temperatures show similar trends over the last 100yrs. Additional data does little to change the pattern (Fig 6).
    4. Glacial advances and retreats in the Swiss Alps correlate with North Atlantic sea surface temperatures.
  2. 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 (See Fig 2 again).
    1. Proxy climatic indicators.

      Proxy indicators of past climate are data derived from natural recorders not man-made instruments. Such natural recorders include tree rings recording annual tree growth, various properties of cores, including some that are dependent on temperature, taken from the worlds great ice caps, various measurements made on deep sea cores or cores removed from tropical corals.

    2. During the last 1000 years tree rings have been used extensively to estimate past climatic conditions prior to the instrumental record. They are in fact the major contributor to the temperature curve for the last 1000 years (Fig 7).
    3. How are tree rings used as indicators of past climate?

      Tree ring width's vary because of a number of local and regional environmental factors. In general when conditions are "good" trees grow more than when conditions are "adverse". What is good or adverse depends on regional and local conditions. In arid regions rainfall may be the most important control of tree ring width while in high latitudes or high altitudes temperature may be most important. A trees local position is also important to its growth response so trees in the same region may show different growth histories depending on their local setting. Replication of data from different local trees is essential.

    4. How are climate chronologies developed from tree rings?

      Chronologies are developed by counting rings in individual trees and matching ring patterns between trees. For a particular region, old wood preserved in swamps or buildings can be used to extend the chronology backward to before the age of living trees. In general continuous chronologies can be developed for a region and they can extend back hundreds of years. If a chronology is not continuous it can be dated by carbon 14 dating.

    5. How is a climatic information extracted from tree rings?

      Since tree rings respond to a variety of climatic factors the rings record more than one climatic variable. The information about a single variable, say temperature, is extracted in the following way. A tree ring data set for a region is first subdivided in to subsets of ring chronologies. One set is used to match against a known climatic record such as the instrumental record of temperature for the last 80 years. This is done through what is called a regression analysis in which the tree ring widths for the set of years chosen are correlated with measured temperature for that same set of years. The result of this correlation develops an equation or model of the relationship between ring width and temperature. To verify or test the ability of this model to predict temperature from tree ring width, the model is applied to the tree ring chronologies that were not part of the original calibration set, but grew when there are temperature measurements with which to compare them.

  3. Climate change of the past several hundred thousand years.
    1. The record of ice cores.
      1. How does ice record climate change? (Fig 8)
      2. How does ice record changes in atmospheric composition? (Fig 9).
    2. Sedimentary archives

      Sediments are laid down consecutively from oldest to youngest (this is called the principle of superposition), and they contain material that provides information on the time of their deposition and the environment in which they were deposited (climate). Extracting the information from sediments requires the application of proxies. Proxies are measurements or observations that can be related to a climate parameter such as temperature, salinity, aridity, wind speed, current speed, etc.

  4. Climate change over the history of the Earth.
    1. Important factors in long term climate change
      1. Changes in the astronomical configuration (Milankovitch)
      2. Changes in the carbon cycle
      3. Changes in the hydrological cycle
      4. Changes in the configuration of the continents (plate tectonics)
    2. Examples of past climate change.
      1. "Snowball Earth"
      2. Permo-Carboniferous glaciation
      3. Hothouse conditions in the Cretaceous
      4. Cenozoic build-up of glaciation
      5. Pleistocene glacial cycles
      6. Millenial scale variability of climate

III. External climate forcings (other than greenhouse gases).

  1. Solar luminosity variations.
  2. Volcanic eruptions.
  3. Anthropogenic (tropospheric) aerosols.
  4. Biogenic regulation of climate (Gaia).

IV. Natural variability.

  1. Short time scales (1-2 years): Random weather-related variations of turbulent, chaotic atmosphere.
  2. Interannual (2-8 years): Primarily ENSO; longer time scale due to interaction of atmosphere with more massive ocean mixed layer and thermocline.
  3. 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.
  4. Modeling results of all important radiative forcings during the last 100+ years. (Figures in this section are from: Hansen, James E. Dangerous Anthropogenic Interference: A Discussion of Humanity's Faustian Climate Bargain and the Payments Coming Due (pdf - scroll to end of file). Presentation given at the Distinguished Public Lecture Series at the Department of Physics and Astronomy, University of Iowa, on Oct. 26, 2004.)

    The question now is how much of the temperature change during the last century (Fig. 21) can be explained by the known forcings discussed above.

    1. Summation of temperature forcings and their direction (1850 – 2000) (Fig. 22).
    2. Time series of individual radiative forcings during the last 150 years (Fig. 23).
    3. Comparison of the model simulated global mean surface temperature change and the instrumental record of that change since 1880 (Fig. 24).

    The answer to the above question is now clearer. Most of the recorded global mean temperature change of the last century can be explained by known radiative forcings. Greenhouse gasses are not the whole story but they are an important and in fact dominant driving force.

Lecture text by Martin Stute, Julian Sachs, Stephanie Pfirman. Updated, 2004 by Jim Hays. Updated 2005, Sidney Hemming.