Greenhouse Effect

Global Climate: the Earth's thermostat

   For at least the past 1/2 billion years, the temperature of Earth's atmosphere has
       remained close to 25 C. Why?

   The "greenhouse effect"
       Certain constituents of Earth's atmosphere are opaque to infrared radiation.
          Most important:  H₂O and CO₂

       The atmosphere is mostly transparent to solar radiation, which heats the ground.
       The ground re-radiates this heat as infrared, which is partially blocked by the opacity
           of greenhouse gasses.
       The role of H₂O is complex - the most important greenhouse gas, but clouds, etc.
       More CO₂ -> more trapping of heat -> warmer temperatures

    What is the thermostat?
       The carbon cycle:


          On timescales longer than 100,000 years or so,

       - CO₂ combines with  H₂O in air to form carbonic acid H₂CO₃
       - carbonic acid reacts with silicate minerals in rock (chemical weathering)
       - the products of weathering are transported to the oceans
       - where they are used by plankton to produce CaCO₃ (calcium carbonate)
             and SiO₂ (silica) shells
       - These organisms die, sink to the bottom of the ocean, and become sediments
       - Sediments are subducted by plate techtonic motions and melted
       - Volcanism at mid-ocean ridges and volcanoes release CO₂ back into the atmosphere

      On shorter timescales, photosynthesis removes CO₂ from the atmosphere and delivers
          it to sediments as well.
      Seawater can "buffer" changes in atmospheric CO₂.

      The rate of chemical weathering doubles for each 10° C increase in temperature
      Warmer temperatures lead to more evaporation -> more rain, increasing weathering
      Warmer temperatures lead to more vegitation -> more CO₂ removal

   The thermostat:
   The Earth's climate is kept, more or less, at a constant temperature.
       warming -> faster removal of CO₂ from the atmosphere -> less CO₂ -> cooling
       cooling -> slower removal of CO₂ from the atmosphere -> more CO₂ -> warming

This process occurs on geologic timescales (100,000 years or more) and is responsible
for returning the Earth to its natural equilibrium temperature.

What can cause a deviation?

    Change in solar radiation striking Earth:
       The sun is now 50% or so brighter than it was just after the formation of the Earth.
       Clouds or dust can block solar radiation from striking Earth, reducing solar heating
       Changing reflectivity of the ground
          ocean vs. continent, snow/ice vs. rock

    Change in atmospheric composition:
        Increased/decreased photosynthesis
        Volcanism
        Fires
        release/uptake of gasses dissolved in the oceans