12.1.2. Natural and Human-Generated Greenhouse Gases

The greenhouse effect is a naturally occurring process that aids in heating the Earth's surface and atmosphere. It results from the fact that certain atmospheric gases, such as carbon dioxide, water vapor, and methane, are able to change the energy balance of the planet by absorbing long wave radiation emitted from the Earth's surface. Without the greenhouse effect life on this planet would probably not exist as the average temperature of the Earth would be a chilly -18° Celsius, rather than the present 15° Celsius.

The greenhouse effect is essential for life on Earth. This process involves certain gases in the atmosphere that trap heat. When the sun's energy reaches Earth, some of it is reflected back into space, while the rest warms the planet. The Earth's surface then emits this heat as longwave radiation. Greenhouse gases absorb this heat and reradiate it, keeping the earth warmer than it would be otherwise. Without this process, Earth would be too cold for most forms of life.

You can think of the greenhouse effect like a warm blanket at night. Just as a blanket keeps you warm by trapping your body heat, greenhouse gases trap warmth from the sun, keeping the Earth at a temperature that can support life.

Some greenhouse gases occur naturally in the atmosphere, while others result from human activities. Naturally occurring greenhouse gases include water vapor, carbon dioxide, methane, nitrous oxide, and ozone. Certain human activities, however, add to the levels of most of these naturally occurring gases.

Natural greenhouse gases are gases that are emitted into the atmosphere through natural processes. Water vapor is the most abundant greenhouse gas, followed by carbon dioxide and methane. These gases are part of Earth's natural cycle and help to maintain a stable climate. However, human activities such as burning fossil fuels and deforestation increase their concentrations, leading to enhanced greenhouse effects and climate change.

Imagine a well-balanced scale where one side represents natural gases and the other side represents human-generated gases. If people start adding weights (like carbon and methane from cars and factories) to one side, the scale tips, indicating a problem in balance. Similarly, our climate has been tipped due to increased human emissions.

Carbon dioxide is released to the atmosphere when solid waste, fossil fuels (oil, natural gas, and coal), and wood and wood products are burned. Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from the decomposition of organic wastes in municipal solid waste landfills, and the raising of livestock. Nitrous oxide is emitted during agricultural and industrial activities, as well as during combustion of solid waste and fossil fuels.

Human activities contribute significantly to the increase of greenhouse gases. For instance, carbon dioxide is mainly produced by burning fossil fuels, which we rely on for energy. Methane, often referred to as a more potent greenhouse gas, is released in ways such as livestock farming and landfills where organic matter decomposes. Nitrous oxide emissions primarily come from agricultural fertilizers and other industrial processes.

Think of a garden. If you keep adding fertilizer (representing emissions such as nitrous oxide) without considering its impact, it can harm the plants. In the same way, the continuous release of greenhouse gases without control is detrimental to our atmosphere.

Very powerful greenhouse gases that are not naturally occurring include hydro fluorocarbons (HFCs), per fluorocarbons (PFCs), and sulfur hexafluoride (SF6), which are generated in a variety of industrial processes.

Apart from natural greenhouse gases, there are artificial gases produced through industrial activities. HFCs, PFCs, and SF6 are particularly concerning due to their high global warming potential. While they exist in lower concentrations in the atmosphere compared to carbon dioxide, they are far more effective at trapping heat.

It’s like having a small but very powerful flashlight. Even though it's small (like HFCs), when you turn it on, it can shine much brighter than a larger, dimmer light. Similarly, these artificial gases may be present in small amounts, but their effects on warming are substantial.

Often, estimates of greenhouse gas emissions are presented in units of millions of metric tons of carbon equivalents (MMTCE), which weights each gas by its Global Warming Potential or GWP value.

To compare the impacts of different greenhouse gases, scientists convert their emissions into a common unit called 'carbon equivalents.' This involves using a factor known as Global Warming Potential (GWP), which indicates how much heat a greenhouse gas can trap in the atmosphere over a specified period compared to carbon dioxide.

Imagine you want to compare the strength of two different types of snacks. You might convert their weights into a standard size package. In the same way, using carbon equivalents helps us understand the overall impact of various greenhouse gases.

As energy from the Sun passes through the atmosphere a number of things take place. A portion of the energy (26% globally) is reflected or scattered back to space by clouds and other atmospheric particles. About 19% of the energy available is absorbed by clouds, gases (like ozone), and particles in the atmosphere. Of the remaining 55% of the solar energy passing through the Earth's atmosphere, 4% is reflected from the surface back to space. On average, about 51% of the Sun's radiation reaches the surface. This energy is then used in a number of processes, including the heating of the ground surface; the melting of ice and snow and the evaporation of water; and plant photosynthesis.

When sunlight reaches Earth, not all of it stays. Some energy is reflected back into space, while some is absorbed by atmospheric components. The remaining energy warms the surface of the Earth. This heat can cause physical processes, such as melting ice, evaporating water, and aiding in photosynthesis (the process plants use to make food).

It's like watering a plant. Not all the water you pour in is absorbed; some evaporates. The water that does get absorbed helps the plant grow. Similarly, when sunlight hits the Earth, most of it is used, but some escapes back into space, contributing to Earth's energy balance.

The heating of the ground by sunlight causes the Earth's surface to become a radiator of energy in the long wave band (sometimes called infrared radiation). This emission of energy is generally directed to space. However, only a small portion of this energy actually makes it back to space. The majority of the outgoing infrared radiation is absorbed by the greenhouse gases. Absorption of long wave radiation by the atmosphere causes additional heat energy to be added to the Earth's atmospheric system.

Once the Earth's surface absorbs solar energy, it releases it back as heat in the form of longwave radiation. This radiation usually escapes into space, but greenhouse gases capture and hold some of this heat, which contributes to warming the atmosphere over time. This cycle continues, with the atmosphere heating further as more radiation is absorbed.

Think of a pot on a stove. Once you heat the pot, it transfers heat back into the air around it. However, if you keep it covered, much of the heat stays trapped inside. Similarly, greenhouse gases trap some of the heat the Earth tries to release, causing the atmosphere to warm.

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