Geological records stretching back millions of years indicate some large
variations in climate. These have been caused by many natural factors,
including the changes in the sun, volcanoes, earth’s orbit and CO2
levels. However, there is good evidence to show that the current rapid
warming is not primarily a natural change. Rather this is likely to be
due to human CO2 emissions.
Natural climate cycle
Over the last 800,000 years Earth’s climate has been cooler than
today on average, with a natural cycle between ice ages and warmer
interglacial periods. The transitions out of ice ages leading to an
eventual global temperature change of around 4-7°C, took about 5,000
years.
Atmospheric CO2 levels have been very tightly
linked to temperature throughout this cycle. In fact, the size of the
global temperature changes can only be explained by including the
varying greenhouse effect from CO2, without which temperature change would be have been much smaller.
Over the last 10,000 years (since the end of the last ice age) we have lived in a relatively warm period with stable CO2
concentration. This is the period during which human civilisation has
flourished. Some regional changes have occurred – long-term droughts
have taken place in Africa and North America, and the Asian monsoon has
changed frequency and intensity – but these have not been part of a
coherent global change.
The rate of CO2 accumulation
due to our emissions in the last 200 years looks very unusual in this
context (see chart below). Atmospheric concentrations are now well
outside the 800,000 year natural cycle and temperatures would be
expected to rise as a result.
Solar influences
The primary source of Earth’s heat is the Sun, so relatively small
changes in solar output can affect our climate. Low solar activity may
well have caused a series of very cold European winters during the
1700s, known as the ‘Little Ice Age’.
Satellite observations since the late 1970s have shown a slight decrease in the Sun’s total energy output. Instead of cooling however, Earth has warmed over this period.
Furthermore,
warming from the Sun would heat all parts of the atmosphere, including
the lowest few kilometres (the troposphere) and the layer above (the
stratosphere). Observations show that the stratosphere is in fact
cooling while the troposphere warms, consistent with greenhouse gas heating.
Greenhouse gas emissions
The link between CO2 emissions and rising temperature is very
robust. Scientists have known since the early 1800s that gases in the
atmosphere trap heat. The first calculations to show the link between
changes in CO2 levels and changes in the earth’s temperature were made over a hundred years ago. Global CO2 emissions resulting from human activity have increased by over 600% since 1950. As a result, the concentration of CO2 in the air is around 400 parts per million by volume (ppm) and rising, compared to about 280ppm in pre-industrial times.
Agriculture, industry and waste disposal all lead to further greenhouse
gas emissions (such as methane and nitrous oxide). While other forms of
air pollution (such as sulphates and nitrates) have partially offset
this greenhouse warming to date, human activity is having a strong
overall warming effect. After looking at a variety of factors, natural
and human, the Intergovernmental Panel on Climate Change concluded
there is a 90% probability that human-related greenhouse gas increases
have caused most of the observed increase in global average temperature
since the mid-20th century.
