The Earth is protected by a layer of greenhouse gases that keeps it at an average of 15 Celsius (59 Fahrenheit), a level that supports life in the modern climate age. The main gases keeping the Earth’s temperature at this level are water vapour, carbon dioxide, methane and nitrous oxide, which absorb heat and heat the Earth’s surfaces.
Climate change in the modern age is caused when the atmosphere traps heat radiating out from Earth into space, to create the so-called “greenhouse effect”. This is caused by gases that remain in the atmosphere for a long period of time and are unaffected by temperature changes.
While water vapour changes with a rise in temperature, it is important as a feedback gas with regards to the climate. When the Earth’s atmosphere warms, water vapour increases as does the possibility of clouds and rainfall.
Carbon dioxide (CO2)
CO2 is a greenhouse gas that is the most prominent cause of human-oriented climate change. Released mostly through respiration, volcanic eruptions and fossil fuel combustion, a 30 per cent increase in atmospheric CO2 concentration has been recorded since the 1750s. This is the greenhouse gas that is having the most impact on climate change due to its volume rather than its molecular strength.
A more active gas than CO2, methane is a formed from decomposed waste in landfills and from intensive cattle farming as well as human activities. The quantity of methane is much lower than carbon dioxide, but its effect is stronger. The United Nations Food and Agriculture Organisation (FAO) has found that agriculture is responsible for the release of more greenhouse gases than transportation, linking methane levels with increased levels of CO2 through the clearing of forests to create new grazing pastures for cattle and other livestock.
Nitrous oxide is a powerful greenhouse gas produced by the use of human-made fertilisers in soils, which then leaches into the water and the manufacture of acids and nylon. Nitrous oxide is also released from power stations and road vehicles, particularly those with catalytic converters. Although in comparison to CO2 and methane, the amounts of nitrous oxide released are small, each molecule is just over 300 times more powerful than a molecule of carbon dioxide when compared to climate change impact.
Now regulated through international agreements, CFCs are industrial compounds that destroy the ozone layer, making them greenhouse gases.
Changes in the Earth’s rotation and orbit around the sun create a form of slow feedback which causes natural climate change. The distribution of solar radiation received by the Earth changes over time and the resulting change in temperature affects ice age cycles that have taken place over the past million years. Yet the global surface temperature changes when compared with the Sun's energy received by the Earth since 1880, has shown no net increase in solar irradiance since the 1950s, despite global temperatures rising rapidly.
Natural causes of climate change
None of the causes of climate change work in isolation. A fall in solar activity, matched with an increase in volcanic eruptions, saw glaciers advance in the Alps between 400 and 200 years ago.
Half the measured natural greenhouse effect is caused by water vapour, the levels of which are mostly controlled by atmospheric temperatures and winds. While water vapour is not directly influenced by human behaviour, there is an indirect since water vapour almost doubles the sensitivity of the climate to human activities.
The natural carbon cycle
The natural carbon cycle circulates CO2 between the atmosphere, land and oceans to keep the Earth in balance. However, human actions have altered the CO2 cycle through human-made emissions through how the land is used for agriculture, fossil fuel combustion and other means. Scientists have been measuring the amount of CO2 that enters the atmosphere from fossil fuel emissions and net land use change and then how CO2 leaves the atmosphere absorbed into the oceans, to calculate the net annual CO2 accumulation in the atmosphere.
Human causes of climate change
The CO2 concentration has increased to 400+ppm today from 280 parts per million (ppm) before the year 1800. Prior to this increase, the ‘carbon cycle’ was roughly in balance and steady. For the last 200 years, the burning of fossil fuel, deforestation and other human activities have added more CO2 to the atmosphere than can be processed by the land biosphere and the oceans.
The volume of CO2 emissions from fossil fuels and other industrial processes (mostly cement manufacture) are following the highest-emission pathway set by the Global Carbon Project. The concentration of CO2 in the atmosphere varies each year, due to the effects of the weather on vegetation as well as the volcanic activity which causes a temporary drawdown of CO2 through the promotion of plant growth by the light-scattering and cooling effects of volcanic haze.
It has been possible for scientists to distinguish the effects of different human and natural influences on climate by studying particular characteristics of their effects. About a century ago, it was predicted that increases in CO2 would trap more heat near the surface and also make the stratosphere colder, and science has now produced the evidence to prove this hypothesis. It supports the interpretation that the observed surface warming on Earth is due to an enhanced greenhouse effect rather than an increase in the Sun’s radiation.
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