Download Greenhouse Effect

The Greenhouse Effect
The term 'Greenhouse Effect' is commonly used to describe the
increase in the Earth's average temperature that has been
recorded over the past 100 years. However, without the 'natural
greenhouse effect', life on Earth would be very different to that
seen today. The greenhouse effect is very important when we talk
about climate change as it relates to the gases which keep the
Earth warm. It is the extra greenhouse gases which humans have
released which are thought to pose the strongest threat.
The 'natural greenhouse effect'
The Earth receives its life sustaining warmth from the Sun. On its
way to the Earth's surface most of the heat energy passes through
the Earth's atmosphere, while a smaller proportion is reflected
back into space.
The energy warms the Earth's surface, and as the temperature
increases, the Earth radiates heat energy (infrared energy) back
into the atmosphere. As this energy has a different wavelength to
that coming from the sun, some is absorbed by gases in the
atmosphere.
As a result, the Earth's average surface temperature is kept at
about 15C by the blanket of atmosphere that surrounds it. Without
this, the temperature would be about minus 18C - too cold for life.
The gases keep the Earth warm in a similar (but not identical) way
as a greenhouse keeps plants warm.
The 'enhanced greenhouse effect'
Although most of the greenhouse gases occur naturally in the
atmosphere, some are man-made and the most well-known of
these are the fluorocarbons. Since the industrial revolution, human
activities have also resulted in an increase in natural greenhouse
gases, especially carbon dioxide. An increase in these gases in
the atmosphere enhances the atmosphere's ability to trap heat,
which leads to an increase in the average surface temperature of
the Earth.
Key Points
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Global temperatures have risen by over 0.7C in the last 300
years - climate change is therefore already taking place.
0.5C of this warming occurred during the 20th century. Most
of the warming was from 1910 to 1940 and from 1976
onwards
Four out of five of the warmest years ever recorded were in
the 1990's
The 1990's was the warmest decade of the last millennium
with 1998 being the warmest year globally since records
began in 1861
1999 was the warmest year on record in the UK
January-June 2002 was the warmest start to a year in the
northern hemisphere
The total number of cold days (where the average
temperature was under 0C) has fallen from between 15 and
20 per year prior to the twentieth century, to around 10 per
year in recent years
1995 saw the most hot days in 225 years of daily
measurements - 26 days above 20C
Sea levels are rising globally, arctic sea ice is thinning and
rainfall is becoming heavier in some parts of the World.
Average global sea levels have increased by between 0.1
and 0.2 metres over the last 100 years
Snow cover in the Northern Hemisphere has declined since
the late 1960's by around 10%
Mountain glaciers in non polar regions have retreated
significantly during the 20th century
There is evidence of more precipitation in large parts of the
World - an increase of 0.5-1% per decade in many mid and
high level areas of the northern hemisphere
In the same area of the World there has been a 2-4%
increase in the frequency of heavy rainfall events
In Asia and Africa there has been an increased frequency
and intensity of droughts in the last few decades