Climate Change
"Climate change is a change in
global or regional climate patterns, in particular a change apparent from the
mid to late 20th century onwards and attributed largely to the increased levels
of atmospheric carbon dioxide produced by the use of fossil fuels"
Natural Causes
The
Earth’s climate can be affected by natural factors that are external to the
climate system, such as changes in volcanic activity, solar output, and the
Earth's orbit around the Sun. Of these, the two factors relevant on timescales
of contemporary climate change are changes in volcanic activity and changes in
solar radiation. In terms of the Earth’s energy balance, these factors
primarily influence the amount of incoming energy. Volcanic eruptions are
episodic and have relatively short-term effects on climate. Changes in solar radiance have contributed to climate trends over the past century but since
the Industrial Revolution, the effect of additions of greenhouse gases to the
atmosphere has been about ten times that of changes in the Sun’s output.
Human Causes
Climate
change can also be caused by human activities, such as the burning of fossil
fuels and the conversion of land for forestry and agriculture. Since the
beginning of the Industrial Revolution, these human influences on the climate
system have increased substantially. In addition to other environmental
impacts, these activities change the land surface and emit various substances
to the atmosphere. These in turn can influence both the amount of incoming
energy and the amount of outgoing energy and can have both warming and cooling
effects on the climate. The dominant product of fossil fuel combustion is
carbon dioxide, a greenhouse gas. The overall effect of human activities since
the Industrial Revolution has been a warming effect, driven primarily by
emissions of carbon dioxide and enhanced by emissions of other greenhouse
gases.
The
build-up of greenhouse gases in the atmosphere has led to an enhancement of the
natural greenhouse effect. It is this human-induced enhancement of the greenhouse
effect that is of concern because ongoing emissions of greenhouse gases have
the potential to warm the planet to levels that have never been experienced in
the history of human civilization. Such climate change could have far-reaching
and/or unpredictable environmental, social, and economic consequences.
Short-lived and long-lived climate forcers
Carbon
dioxide is the main cause of human-induced climate change. It has been emitted
in vast quantities from the burning of fossil fuels and it is a very long-lived
gas, which means it continues to affect the climate system during its long
residence time in the atmosphere.
However, fossil fuel combustion, industrial
processes, agriculture, and forestry-related activities emit other substances
that also act as climate forcers. Some, such as nitrous oxide, are long-lived
greenhouse gases like carbon dioxide, and so contribute to long-term climate
change.
Other substances have shorter atmospheric lifetimes because they are
removed fairly quickly from the atmosphere. Therefore, their effect on the
climate system is similarly short-lived. Together, these short-lived climate
forcers are responsible for a significant amount of current climate forcing
from anthropogenic substances.
A number of short-lived climate forcers have climate warming
effects and together are the most important contributors to the human
enhancement of the greenhouse effect after carbon dioxide. This includes
methane and tropospheric ozone – both greenhouse gases – and black carbon, a
small solid particle formed from the incomplete combustion of carbon-based
fuels (coal, oil and wood for example).
Other
short-lived climate forcers have climate cooling effects, most notably sulphate
aerosols. Fossil fuel combustion emits sulphur dioxide into the atmosphere (in
addition to carbon dioxide) which then combines with water vapour to form tiny
droplets (aerosols) which reflect sunlight. Sulphate aerosols remain in the
atmosphere for only a few days (washing out in what is referred to as acid
rain), and so do not have the same long-term effect as greenhouse gases.
