Pollution's Impact: Global Warming's Unseen Cause

Air pollution and global warming are closely linked. Air pollution is defined as the presence of harmful amounts of certain substances in the Earth's atmosphere, including gases, dust, smoke from fires, and fumes. These substances, or pollutants, can have a range of effects on the climate, with some causing warming and others having a cooling effect. Greenhouse gases, such as carbon dioxide, are significant contributors to global warming by trapping heat from the sun in the Earth's atmosphere. Other pollutants, such as black carbon particles from burning fossil fuels, also contribute to warming by absorbing sunlight. On the other hand, some atmospheric particles, such as sea salt particles, can reflect sunlight back into space, temporarily cooling the climate.

Greenhouse gases

The main greenhouse gases are carbon dioxide, methane, nitrous oxide, ozone, chlorofluorocarbons, and water vapour. Each gas has a different effect on climate change, depending on its concentration in the atmosphere, how long it remains in the atmosphere, and how strongly it affects the Earth's atmospheric blanket.

Carbon dioxide (CO2) is the primary greenhouse gas emitted through human activities, accounting for 80% of all US greenhouse gas emissions in 2022. It is released into the atmosphere through the burning of fossil fuels, solid waste, trees and other biological materials, and certain chemical reactions. Methane (CH4) is emitted during the production and transport of coal, natural gas, and oil, as well as from livestock and other agricultural practices. Nitrous oxide (N2O) is emitted during agricultural, industrial, and combustion activities. Fluorinated gases, such as hydrofluorocarbons, perfluorocarbons, and nitrogen trifluoride, are synthetic and emitted from household, commercial, and industrial applications. They are particularly potent, with global warming potentials ranging from thousands to tens of thousands. Water vapour is the most abundant greenhouse gas, and its atmospheric concentration is linked to the warming caused by other greenhouse gases.

The increase in greenhouse gas emissions is causing global warming, which is altering the planet's climate system. The burning of fossil fuels for energy is artificially amplifying the natural greenhouse effect, resulting in an increase in global temperatures. This, in turn, is causing more frequent and intense extreme weather events, precipitation extremes, rising sea levels, and alterations to ecosystems and natural habitats.

Black carbon

The largest sources of black carbon emissions are Asia, Latin America, and Africa, with China and India contributing 25-35% of global emissions. Household energy, including the burning of solid fuels and kerosene for cooking, lighting, and heating, accounts for nearly half of global black carbon emissions. The transport sector, particularly diesel engines, is another major source, contributing about 70% of emissions in Europe and North America.

Reducing black carbon emissions is considered one of the easiest and fastest ways to slow down short-term global warming. Improved fuel and vehicle standards, cleaner household energy sources, and the adoption of clean technologies can significantly reduce black carbon emissions. The CCAC (Climate and Clean Air Coalition) estimates that globally implemented control measures could reduce global black carbon emissions by up to 80%.

Ozone

Human activities have damaged the ozone layer, particularly through the use of gases like chlorofluorocarbons (CFCs) in spray cans and refrigerants, which break down ozone molecules in the upper atmosphere. CFCs are also potent greenhouse gases, although they are present in small concentrations in the atmosphere. CFCs account for about 13% of the total energy absorbed by human-produced greenhouse gases.

The ozone hole itself has a minor cooling effect (about 2% of the warming effect of greenhouse gases) because ozone in the stratosphere absorbs heat radiated to space by gases in a lower layer of Earth's atmosphere (the upper troposphere). The loss of ozone means slightly more heat can escape into space from that region.

While the ozone hole cannot be said to have caused global warming, it has left a mark on the Southern Hemisphere summer climate. When the ozone hole forms each spring, total ozone concentrations in the lower stratosphere dip, causing a strong springtime cooling trend in the lower polar stratosphere over the past few decades. This cooling trend has, in turn, amplified the temperature contrast between the poles and the equator, which often intensifies atmospheric circulation. Models indicate that springtime cooling due to ozone loss is largely responsible for changes in atmospheric circulation across the Southern Hemisphere.

These circulation changes have affected the climate at the surface, primarily in the summer. The prevailing mid-latitude westerly winds have strengthened and shifted closer to the pole, drawing mid-latitude storm tracks and associated rainfall southward. More intense surface winds appear to have intensified the subtropical ocean gyres and increased the overturning circulation in the Southern Ocean. In subtropical latitudes, summertime precipitation has increased.

Aerosols

Since the Industrial Revolution, humans have pumped more and more aerosols into the air, and this has actually counteracted global warming to a significant degree. Using climate models, researchers estimate that aerosols have masked about 50% of the warming that would otherwise have been caused by greenhouse gases trapping heat near the Earth's surface. Without the presence of these aerosols in the air, the planet would be about 1°C (1.8°F) hotter.

Climate forcers

Long-Lived Climate Forcers (LLCFs)

Long-lived climate forcers are greenhouse gases that remain in the atmosphere for an extended period, ranging from several decades to centuries. The most well-known LLCFs are:

• Carbon Dioxide (CO2): The primary greenhouse gas emitted through human activities, particularly the burning of fossil fuels and deforestation. CO2 can remain in the atmosphere for hundreds to thousands of years.
• Methane (CH4): Methane is considered a short-lived climate forcer due to its shorter atmospheric lifetime of about 12 years. However, it has a long-term impact on the climate as it is much more effective at trapping heat compared to CO2.
• Nitrous Oxide (N2O): Emitted from agricultural activities and industrial processes, nitrous oxide has an atmospheric lifetime of about 114 years.
• Fluorinated Gases: These include hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6), which are synthetic gases used in various industrial applications. They can remain in the atmosphere for thousands of years.

Short-Lived Climate Forcers (SLCFs)

Short-lived climate forcers are substances with a much shorter atmospheric lifetime, typically ranging from a few days to a few decades. Despite their short lifespan, they can have a significant impact on the climate and air quality. Key SLCFs include:

• Black Carbon: A component of particulate matter (PM) produced from the incomplete combustion of fossil fuels, biofuel, and biomass. Black carbon absorbs sunlight and warms the atmosphere but remains in the atmosphere for only days to weeks.
• Tropospheric Ozone (O3): Formed by the reaction of sunlight with pollutants like nitrogen oxides (NOx) and volatile organic compounds (VOCs). It has a lifetime of a few weeks and is considered a pollutant.
• Hydrofluorocarbons (HFCs): Some HFCs have shorter lifetimes and are used as refrigerants and in other industrial applications.

Other Climate Forcers

In addition to the main short- and long-lived climate forcers, there are other substances that impact the climate:

• Water Vapour: Water vapour is the most abundant greenhouse gas. It amplifies the effects of other greenhouse gases. As the atmosphere warms, more water evaporates, trapping more heat and creating a feedback loop that increases global warming.
• Aerosols (Reflective Particles): Aerosols reflect sunlight back into space, cooling the Earth. They are often produced by industrial pollution, the burning of fossil fuels, and volcanic eruptions. While they have a cooling effect, it does not counterbalance the warming from greenhouse gases.
• Clouds: Clouds can act as both climate forcers and climate dampeners. Low clouds reflect sunlight, having a cooling effect, while high clouds trap heat and contribute to warming. Their overall effect depends on various factors such as altitude and type.

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