An atmospheric gas that absorbs and emits thermal radiation in the infrared range is known as a greenhouse gas.  Without such gases, the surface temperature of the earth would have been a frigid -18°C instead of the current 15°C.  Many climate scientists claim that human activities since the beginning of the Industrial Revolution have led to a steady increase in the atmospheric concentration of many greenhouse gases, with a consequent increase in the planet's surface temperature by about 0.85°C in the last 130 years.  However, in order to correctly estimate the anthropogenic changes in earth's surface temperature, it is important to quantify the effect of external natural factors like large volcanic eruptions on the planet's surface temperature.
 Though volcanic eruptions emit carbon dioxide, a greenhouse gas, the atmospheric concentration of this gas is about 16000 times the amount released by volcanic eruptions; therefore, no matter how large a volcanic eruption is, it cannot produce a significant change in the atmospheric carbon dioxide levels.  Further, the total amount of carbon dioxide emitted per year by volcanic eruptions – between 130 and 230 million tons – is roughly equivalent to that emitted in only three to five days of human activity.
 It is mainly through the emission of sulfur dioxide gas and ash particles into the atmosphere that large volcanic eruptions affect earth's surface temperature.  Sulfur dioxide reacts with the water vapor present in air to form fine particles called sulfate aerosols.  Winds spread the cloud of aerosols and ash particles around the globe in weeks.  These particles absorb incoming solar radiation and scatter it back into space, thereby producing a cooling effect on the earth.  They take several years to settle out of the atmosphere and thus impact the global surface temperature for many years.  Robock and Mao have shown that for two years after a great volcanic eruption, the surface temperature decreases by 0.1 – 0.2°C.  The 1991 Pinatubo eruption in Indonesia was one of the largest volcanic eruptions in the twentieth century and resulted in a global surface cooling of 0.5°C for about two to four years after the eruption.
 The El Chichón eruption in 1982 was the first major eruption whose climatic impact was studied in detail by modern instruments.  Though the emission volume of this eruption was similar to the Mount St. Helens eruption in 1980, El Chichón released seven times the amount of sulfate aerosols released by Mount St. Helens and lowered the earth's temperature by about 0.4°C as against a decrease of only 0.1°C for Mount St. Helens.