Physical causes of global warming and cooling
include:
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variations in solar energy - sunspot activity
raises global temperature
volcanic eruptions - large quantities of
volcanic dust in the atmosphere shield the
earth from incoming insolation, lowering
global temperature. For example, the
eruption of Mount Pinatubo in 1991 caused a
dip in global temperatures in the early 1990s
Milankovitch cycles or variations in the tilt
and/or orbit of the earth around the sun
changing oceanic circulation such as the
periodic warming (El Nino) and cooling (La
Nina) of areas of the tropical Pacific
Oceans.
These physical causes of global temperature
change have always existed and have been
responsible for alternate heating and cooling
cycles of the earth's temperature.
The human causes of global warming have been in
the news a lot in recent years - you can probably
think of a few examples. Human factors are the
result of growing population and economic
developments.
They include:
 the burning of fossil fuels for transport,
industry and power, producing carbon
dioxide
 world-wide deforestation, sometimes
involving rainforest burning, which also
produces carbon dioxide
 car exhausts and nitrogen fertilisers,
producing nitrous oxide
 CFCs found in fridges, air conditioning and
aerosols and as a biproduct of the
production of polystyrene packaging, like
pizza and burger boxes
 methane, produced from rice fields, landfill
sites and from both ends of cattle
These different greenhouse gases - carbon
dioxide, methane, nitrous oxide - have caused an
enhanced greenhouse effect, trapping some
outgoing infra-red radiation and keeping the
earth warmer than it might otherwise be.
Burning fossil fuels releases carbon dioxide
stored millions of years ago as oil, coal or natural
gas. In the last 200 years we have burned a large
part of these stores, resulting in an increase in
CO2 in our atmosphere. Deforestation also
releases CO2 stored in trees and in the soil.
The increase of CO2 in the atmosphere thickens
the 'greenhouse blanket', with the result that
too much heat is trapped into the Earth's
atmosphere. This causes global warming: global
temperatures rise and cause climate change.