change refers to a statistically significant variation in either the mean state of the
climate or in its variability, persisting for an extended period (typically decades or
longer). Climate change may be due to natural internal processes or external forcings, or
to persistent anthropogenic changes in the composition of the atmosphere or in land use.
The Earth is the only planet in our solar
system that supports life. The complex process of evolution occurred on Earth only because
of some unique environmental conditions that were present: water, an oxygen-rich
atmosphere, and a suitable surface temperature.
Mercury and Venus, the two planets that lie between Earth
and the sun, do not support life. This is because Mercury has no atmosphere and therefore
becomes very hot during the day, while temperatures at night may reach -140 ºC. Venus, has a thick
atmosphere which traps more heat than it allows to escape, making it too hot (between 150
and 450 ºC) to
Only the Earth has an atmosphere of the proper depth and
chemical composition. About 30% of incoming energy from the sun is reflected back to space
while the rest reaches the earth, warming the air, oceans, and land, and maintaining an
average surface temperature of about 15 ºC.
The chemical composition of the atmosphere is also
responsible for nurturing life on our planet. Most of it is nitrogen (78%); about 21% is
oxygen, which all animals need to survive; and only a small percentage (0.036%) is made up
of carbon dioxide which plants require for photosynthesis.
The atmosphere carries out the critical function of
maintaining life-sustaining conditions on Earth, in the following way: each day, energy
from the sun (largely in the visible part of the spectrum, but also some in the
ultraviolet, and infra red portions) is absorbed by the land, seas, mountains, etc. If all
this energy were to be absorbed completely, the earth would gradually become hotter and
hotter. But actually, the earth both absorbs and, simultaneously releases it in the form
of infra red waves (which cannot be seen by our eyes but can be felt as heat, for example
the heat that you can feel with your hands over a heated car engine). All this rising heat
is not lost to space, but is partly absorbed by some gases present in very small (or
trace) quantities in the atmosphere, called GHGs (greenhouse gases).
Greenhouse gases (for example, carbon dioxide, methane,
nitrous oxide, water vapour, ozone), re-emit some of this heat to the earth's surface. If
they did not perform this useful function, most of the heat energy would escape, leaving
the earth cold (about -18 ºC) and unfit to support life.
However, ever since the Industrial Revolution began about
150 years ago, man-made activities have added significant quantities of GHGs to the
atmosphere. The atmospheric concentrations of carbon dioxide, methane, and nitrous oxide
have grown by about 31%, 151% and 17%, respectively, between 1750 and 2000 (IPCC 2001).
|Variations of the Earth's surface temperature for
the past 140 years
The Earths surface temperature is shown
year by year (red bars) and approximately decade by decade (black line, a filtered annual
curve suppressing fluctuations below near decadal time-scales). There are
uncertainties in the annual data (thin black whisker bars represent the 95% confidence
range) due to data gaps, random instrumental errors and uncertainties, uncertainties in
bias corrections in the ocean surface temperature data and also in adjustments for
urbanisation over the land. Over both the last 140 years and 100 years, the best estimate
is that the global average surface temperature has increased by 0.6 ± 0.2 °C.
Source IPCC Third
Assessment Report: Climate Change 2001 (The Scientific Basis, Summary for
From year 1000 to year 1860 variations in average surface
temperature of the Northern Hemisphere are shown (corresponding data from the Southern
Hemisphere not available) reconstructed from proxy data (tree rings, corals, ice cores,
and historical records). The line shows the 50-year average, the grey region the 95%
confidence limit in the annual data. From years 1860 to 2000 are shown variations in
observations of globally and annually averaged surface temperature from the instrumental
record; the line shows the decadal average. From years 2000 to 2100 projections of
globally averaged surface temperature are shown for the six illustrative SRES scenarios
and IS92a using a model with average climate sensitivity. The grey region marked
"several models all SRES envelope" shows the range of results from the full
range of 35 SRES scenarios in addition to those from a range of models with different
climate sensitivities. The temperature scale is departure from the 1990 value.
Source IPCC Third
Assessment Report: Climate Change 2001 (Synthesis Report)
An increase in the levels of GHGs could lead to greater
warming, which, in turn, could have an impact on the world's climate, leading to the
phenomenon known as climate change. Indeed, scientists have observed that over the 20th
century, the mean global surface temperature increased by 0.6 °C (IPCC 2001). They
also observed that since 1860 (the year temperature began to be recorded systematically
using a thermometer), the 1990's have been the warmest decade.
However, variations in temperature have also occurred in
the past - the best known is the Little Ice Age that struck Europe in the early Middle
Ages, bringing about famines, etc. It is therefore difficult to determine whether current
observations of increasing temperature are due to natural variabilities or whether they
have been forced by anthropogenic (man-made) activities.
Scientific studies and projections are further complicated
by the fact that the changes in temperature that they have been observing do not occur
uniformly over different layers of the lower atmosphere or even different parts of the
The Earth's climate system constantly adjusts so as to
maintain a balance between the energy that reaches it from the sun and the energy that
goes from Earth back to space. This means that even a small rise in temperature could mean
accompanying changes in cloud cover and wind patterns. Some of these changes may enhance
the warming (positive feedback), while others may counteract it (negative feedback).
Negative feedback (causing a cooling effect) may result from an increase in the levels of
aerosols (small particles of matter or liquid that can be produced by natural or man-made
activities). Positive feedback may result from an increase in water vapour (because of
greater evaporation with temp rise), which itself is a GHG and can further add to the
All the factors described above complicate the work of
scientists who try to predict the fallout of climate change. Despite these uncertainties,
the Third Assessment Report published by the IPCC states, 'there is new and stronger
evidence that most of the warming observed over the last 50 years is attributable to human
activities' (IPCC 2001).
Climate Change 2001
Cambridge: Cambridge University Press
The global climate
system: a schematic