INTRODUCTION TO CAUSES
OF CLIMATE CHANGE
The main external influences on climate are:
• Solar activity
• Volcanic emissions
• Greenhouse gases.
Climatologists agree that the earth has warmed
recently. The detailed causes of this change remain an active field
of research, but the scientific consensus identifies greenhouse
gases as the primary cause of the recent warming. Outside of the
scientific community, however, this conclusion can be controversial.
greenhouse gases - the main cause of global warming
Adding carbon dioxide or methane to Earth’s atmosphere,
with no other changes, will make the planet’s surface warmer. These
gases create a natural greenhouse effect without which temperatures
on Earth would be an estimated 30 °C (54 °F) lower, and the Earth
uninhabitable. Scientists do not dispute that greenhouse gases help
to warm Earth. Debates occur over how much these greenhouse gases
will affect our climate in the future, and exactly how much human
activity is increasing global warming.
How do greenhouse gases warm Earth?
Greenhouse gases are transparent to shortwave
radiation from the sun, the main source of heat on the Earth. However,
they absorb some of the longer infrared radiation emitted by the
Earth, thereby reducing radiational cooling and hence raising the
temperature of the Earth. How much they warm the world by is shown
in their global warming potential.
How are humans increasing greenhouse gases?
About three-quarters of the man-made emissions
of carbon dioxide to the atmosphere during the past 20 years are
due to fossil fuel burning. The rest of the anthropogenic emissions
are predominantly due to land-use change, especially deforestation.
The atmospheric concentrations of carbon dioxide
and methane have increased by 31% and 149% respectively above pre-industrial
levels since 1750. This is considerably higher than at any time
during the last 650,000 years, the period for which reliable data
has been extracted from ice cores. From less direct geological evidence
it is believed that carbon dioxide values this high were last attained
40 million years ago.
The longest continuous instrumental measurement
of carbon dioxide mixing ratios began in 1958 at Mauna Loa. Since
then, the annually averaged value has increased monotonically by
approximately 21% from the initial reading of 315 ppmv, as shown
by the Keeling curve, to over 380 ppmv in 2006. The monthly CO2
measurements display small seasonal oscillations in an overall yearly
uptrend; each year’s maximum is reached during the northern hemisphere’s
late spring and declines during the northern hemisphere growing
season as plants remove some CO2 from the atmosphere.
Sources of methane
Methane, the primary constituent of natural gas,
enters the atmosphere both from biological production and leaks
from natural gas pipelines and other infrastructure. Some biological
sources are natural, such as termites or forests], but others have
been increased or created by agricultural activities such as the
cultivation of rice paddies. Recent evidence indicates that methane
concentrations have begun to stabilize, perhaps due to reductions
in leakage from fuel transmission and storage facilities.
Methane clathrate, also called methane hydrate,
is a form of water ice that contains a large amount of methane within
its crystal structure. Large amounts of this lie trapped in sea
beds and some scientists theorize that this could be released with
sufficient global warming, leading to a 5 degree increase in temperature.
Sources of carbon dioxide
Future carbon dioxide levels are expected to continue
rising due to ongoing fossil fuel usage, though the actual trajectory
will depend on uncertain economic, sociological, technological,
and natural developments. The IPCC Special Report on Emissions Scenarios
gives a wide range of future carbon dioxide scenario, ranging from
541 to 970 parts per million by the year 2100. Fossil fuel reserves
are sufficient to reach this level and continue emissions past 2100,
if coal and tar sands are extensively used.
Globally, the majority of greenhouse gas emissions
caused by humans arise from fuel combustion. The remainder is accounted
for largely by “fugitive fuel” (fuel consumed in the production
and transport of fuel), emissions from industrial processes (excluding
fuel combustion), and agriculture: these contributed 5.8%, 5.2%
and 3.3% respectively in 1990. Current figures are broadly comparable.
Around 17% of emissions are accounted for by
the combustion of fuel for the generation of electricity. A small
percentage of emissions come from natural and anthropogenic biological
sources, with approximately 6.3% derived from agriculturally produced
methane and nitrous oxide.
All life on Earth is carbon-based, so carbon dioxide
is also released into the atmosphere when organisms die and decay.
Large amounts of carbon are stored in trees, coal deposits and permafrost.
Positive feedback effects, such as the expected release of methane
from the melting of permafrost peat bogs in Siberia (possibly up
to 70,000 million tonnes), may lead to significant additional sources
of greenhouse gas emissions.
The man-made emissions of other pollutants (notably
sulfate aerosols) exert a cooling effect. This partially accounts
for the plateau/cooling seen in the temperature record in the middle
of the twentieth century, though this may also be due to intervening
natural cycles.
Due to the thermal inertia of the earth’s oceans
and slow responses of other indirect effects, the Earth’s current
climate is not in equilibrium with the forcing imposed by increased
greenhouse gases. Climate commitment studies indicate that, even
if greenhouse gases were stabilized at present day levels, a further
warming of perhaps 0.5 °C to 1.0 °C (0.9–1.8 °F) would still occur.
proposed Alternative causes of climate change
There is largely consensus in the scientific community
that greenhouse gases are the main cause of climate change, and
that human activity plays a key role in this. However, alternative
hypotheses have been proposed to explain all or part of the observed
increase in global temperatures. Some of these hypotheses include:
• The warming is within the range of natural variation.
• The warming is a consequence of coming out of a prior cool period,
namely the Little Ice Age
• The warming is primarily a result of variances in solar irradiance,
possibly via modulation of cloud cover
• Most readings are done in heavily populated areas which are expanding
with growing population.
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