Why exactly is Global Warming happening now?

There are numerous factors that are linked to changes in temperature (Figure 3.1); such as the Sun, the oceans, volcanic activity, changes in topography and changes in land usage. This complex array of interacting elements makes it very difficult to pinpoint a specific factor that can be singularly held responsible for a change in temperature. There is, however, one very important aspect that has been altered greatly in the past century; the greenhouse effect.

Figure 3.1
Global Climate System; components and feedbacks.

The greenhouse effect is the Earth’s natural method of regulating its temperatures. The Sun’s rays reach the Earth in the form of light wave radiation. The absorbed rays warm the Earth
and because the Earth is much cooler than the Sun, some of
it is some is re-radiated back towards space in longer wavelengths, infrared radiation (Albedo Effect). However there are certain gases, called greenhouse gases, in the atmosphere that interfere with this process. These greenhouse gases absorb some
of the infrared radiation and also re-
radiates it in different directions. When the infrared is emitted downwards
towards the Earth, it is known as the "greenhouse effect". The more greenhouse gases
there are, the more escaping infrared radiation they will absorb. Eventually, the infrared radiation will espace the atmosphere, but is greatly delayed by the greenhouse gases. Without the greenhouse gas effect, the average global temperature may be as low as -18^oC^[4]. The problem arises when there are too much greenhouse gases in the atmosphere, and actually cause the temperatures to rise instead of being moderated. This is what is believed to be happening currently and is referred to as the enhanced greenhouse effect.

[Flash 3.1] Below is a Flash Animation by Clay Frost of MSNBC demonstrating how the greenhouse effect works.

Greenhouse gases that contribute to the effect are carbon dioxide (CO₂), methane (NH₄), nitrous oxide (N₂O), chlorofluorocarbons (CFC’s) and tropical ozone (O₃).^[5] Below is a table comparing the levels of greenhouse gases from 1750 and 2006:

Greenhouse Gas	1750 concentration	2006 concentration	Percentage Change
Carbon Dioxode	280 ppm	377 ppm	34.6%
Methane	730 ppb	1847 ppb	153.0%
Nitrous Oxide	270 ppb	319 ppb	40.4%
Chlorofluorocarbons (CFC’s)	0	0.880 pp	---
Ozone (Troposphere)	25 ppb	34 ppb	46.0%

Data retrieved from PhysicalGeography.net^[6] and Carbon Dioxide Information Analysis Center^[7]
ppm = parts per million
ppb = parts per billion

The chart indicates that all of these greenhouse gases have become more abundant in the atmosphere. The greenhouse gas that is most present in our atmosphere, other than water vapour, is carbon dioxide. CO₂ also has the strongest correlation with temperature. It is shown that as CO₂ levels rise, temperature also rises (Figure 3.3). Currently, CO₂ levels are at an all time high are and continue to dramatically rise and at the same time, temperature records are also being set. In the past 420 000 years, CO₂ levels have never gone below 180ppm and above 300ppm, yet we currently have over 377ppm. This exceeds
the previous levels by over 20%^[8].

Figure 3.3
Trends in Arctic temperature, CO2 and methane in the last 420 000 years.

Carbon dioxide is constantly removed from the atmosphere by absorption into water and photosynthesis. It is also constantly released, as mentioned, by plant and animal respiration, organic decay, combustion and volcanic eruptions. Active carbon storage areas, or carbon sinks, are the living terrestrial biosphere, the atmosphere, the oceans, the soil, rocks and fossil fuels^[9]. Carbon sinks exchange carbon with the atmosphere very slowly, over long periods of time. The past natural exchange has been very balanced, maintaining the carbon levels in a constant range. This balance has been disturbed in the recent centuries; carbon levels in the atmosphere have risen to levels never seen before.

Water vapour is actually the most significant contributor to the greenhouse effect but it is only involved in natural reactions. Higher temperatures would cause more evaporations and warmer air can hold more moisture, so it would seem that water vapour would add to the changes but it would also increase cloud formations which block out more sun, offsetting the warming effects^[10]. It acts more as a reactor to the currently occurring changes. Methane, like CO₂, is exchanged between the surface and atmosphere. It is released from wetlands, certain insects and animals, industrial processes, fossil
fuel extraction and garbage decomposition. Methane is removed from the atmosphere by processes involving OH, and also absorbed by soils. Methane levels have skyrocketed lately. Nitrous acid concentrations are increasing, but not as significantly as CO₂ and methane. It is emitted through production of nylon and nitric acids, biomass burning and combustion of fossil fuels equipped with catalytic converters. CFC’s are emitted from refrigerators, aerosol spray propellants and cleaning solvents. They contribute to the greenhouse effect but they
also thin the ozone in the stratosphere. This decrease in ozone however is offset by the increase in the troposphere. Much of the ozone found in the troposphere comes from the stratosphere, where it is produced from a reaction between oxygen and solar radiation. Lately, the surface has increasingly produced ozone through chemical processes involving nitrogen oxides, carbon monoxide and other air pollutants^[11].

The current climate represents the Earth’s response to all these changes.

Who or what is responsible?

There is no sole factor that can be singled out and be blamed for the current warming but a very significant factor has been the actions of humans. CO₂ is released naturally but human activity, particularly in the last 100 years (Figure 3.4), is believed to have had a direct impact on the emission of greenhouse gases. Human activities since the Industrial Revolution have treated the world in a completely different way.

Figure 3.4
Carbon content of CO2 emission from fossil fuel combustion and cement production, 1900-2000.

Humans have completely changed the way the land is used. Forested areas and other areas with natural plant life have been cleared for agricultural fields, asphalt, concrete and urban uses. This eliminates carbon sinks and releases much of the carbon previously stored in these areas. There is also an increase in the abundance of rice paddies, which is a form of wetland. Domestic cattle population has quadrupled in the past century. These changes to the land also alter the way the Earth absorbs and reflects light; wetlands absorb more than forests and asphalt more than grass^[12]. Cities tend to be much warmer than the countryside surrounding them.

A major activity that humans have become dependant on is the burning of fossil fuels for energy. We have a continually increasing demand for more energy to support our
everyday activities and we burn fossil fuels to meet our needs. Fossil fuels are carbon and methane reserves, thus burning them releases those greenhouses gases that were once stored in them. Fuel extracted and burned to power cars also emits CO₂ into the atmosphere during combustion, as well as nitrous oxide if the car is equipped with a catalytic converter^[13]. Aerosols are also added to the atmosphere which affects the radiation balance in the atmosphere. If carbon emission are not reduced,
concentrations will continute to rise to even higher levels.