Internal factors are those associates to the complexity derived from the fact of the climatic systems to be chaotic systems not linear. External factors can be natural or antropognicos. The main natural external factor is the variability of the solar radiation, that depends on the solar cycles and the fact of that the internal temperature of the sun comes increasing. Antropognicos factors are those of the influence human being leading to the effect greenhouse, main of which it is the emission of sulfatos that go up until the stratosphere causing depletion of the ozone layer (source: IPCC) Scientists agree that natural internal and external factors can cause significant climatic changes. In the last millenium two important periods of temperature variation had occurred: a known hot period as Hot Medieval Period and a known cold as Small Age of the Ice. The variation of temperature of these periods has similar magnitude to the one of the current heating and is given credit to only have been caused for internal and external factors.
The Small Age of the Ice is attributed to the reduction of the solar activity and some scientists agree that the observed terrestrial heating since 1860 Source is a natural reversion of the Small Age of the Ice (: The Skeptical Environmentalist). However great amounts of gases have been emitted for the atmosphere since that the industrial revolution started, from 1750 the emissions of carbon dioxide had increased 31%, methane 151%, nitrogen oxide 17% and tropospheric ozone 36% (Source IPCC). Most of these gases is produced by the fsseis fuel burning. The scientists think that the reduction of the areas of tropical forests has contributed, as well as the old forests, for the increase of carbon. However new forests in the United States and Russia contribute to absorb carbon dioxide and since 1990 the amount of absorbed carbon is greater that the set free amount in the deforestation.