CAUSES OF CLIMATE CHANGE: Natural

Solar irradiance

• • Changes in the sun’s energy output would cause the climate to change, since the sun is the fundamental source of energy that drives our climate system.
  • Studies show that solar variability has played a role in past climate changes, a decrease in solar activity is thought to have triggered the Little Ice Age between approximately 1650 and 1850, when Greenland was largely cut off by ice from 1410 to the 1720s and glaciers advanced in the Alps.
  • But several evidences show that current global warming cannot be explained by changes in energy from the sun:
    • Since 1750, the average amount of energy coming from the sun either remained constant or increased slightly.
    • If the warming were caused by a more active sun, then scientists would expect to see warmer temperatures in all layers of the atmosphere. Instead, they have observed a cooling in the upper atmosphere, and a warming at the surface and in the lower parts of the atmosphere. That’s because greenhouse gases are trapping heat in the lower atmosphere.
    • Climate models that include solar irradiance changes can’t reproduce the observed temperature trend over the past century or more without including a rise in greenhouse gases.
  • Sunspot Cycles:
    • The increased sunspot activity (increase in the number of sunspots) causes warming of the earth’s surface and its atmosphere and vice versa.
    • Sunspots are darker and cooler areas in the photosphere of the sun.

Astronomical Theories

• The Milankovitch Theory or Milankovitch Oscillations have tried to explain the causes of climate change through change in the eccentricity of earth’s elliptical orbit, obliquity including of the earth’s rotational axis and precession of the equinoxes
• On the basis of above theory, he tried to explain the advancement and retreat of ice sheets during Pleistocene Ice Age.
• Atmospheric Dust Hypothesis (mainly volcanic eruptions and dusts)
  • The atmospheric solid particulate matters include dust particles, salt particles, pollen, smoke and soot, volcanic dusts and ashes, etc.
  • Black carbon (BC) is a solid particle or aerosol, not a gas, but it also contributes to warming of the atmosphere.
    • Unlike GHGs, BC can directly absorb incoming and reflected sunlight in addition to absorbing infrared radiation.
    • BC can also be deposited on snow and ice, darkening the surface and thereby increasing the snow’s absorption of sunlight and accelerating melt.
  • Sulfates, organic carbon, and other aerosols can cause cooling by reflecting sunlight.