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♤ Climate “forcings” are factors in the climate system that either increase or decrease the effects to the climate system.
♤ Positive forcings such as excess greenhouse gases warm the earth while negative forcings, such as the effects of most aerosols and volcanic eruptions, actually cool the earth.
♤ Atmospheric aerosols include volcanic dust, soot from the combustion of fossil fuels, particles from burning forests and mineral dust.
♤ Dark carbon-rich particles such as soot from diesel engines absorb sunlight and warm the atmosphere.
♤ Conversely, exhaust from high-sulphur coal or oil produce light aerosols that reflect sunlight back to space, producing a cooling effect. Aerosols that form naturally during volcanic eruptions cool the atmosphere. Large volcanic eruptions can eject enough ash into the atmosphere to lower temperature for a year or more until the sulfate particles settle out of the atmosphere.
1 7.4.1. Forcing Altering the Energy Balance
♤ The power of a process to alter the climate is estimated by its “radiative forcing,” the change in the Earth’s energy balance due to that process.
♤ Some climate forcings are positive, causing globally averaged warming, and some are negative, causing cooling. Some, such as from increased CO2 concentration, are well known; others, such as from aerosols, are more uncertain.
Natural Forcings
♤ Natural forcings include changes in the amount of energy emitted by the Sun, very slow variations in Earth’s orbit, and volcanic eruptions.
♤ Since the start of the industrial revolution, the only natural forcing with any long-term significance has been a small increase in solar energy reaching Earth. However, this change is not nearly enough to account for the current warming.
Human-Induced Forcings
♤ Climate forcing can also be caused by human activities. These activities include greenhouse gas and aerosol emissions from burning fossil fuels and modifications of the land surface, such as deforestation.
Human-Generated Greenhouse Gases
♤ Greenhouse gases are a positive climate forcing; that is, they have a warming effect. Carbon dioxide emitted from the burning of fossil fuel is presently the largest single climate forcing agent, accounting for more than half of the total positive forcing since 1750.
Human-Generated Aerosols
♤ Burning fossil fuels adds aerosols to the atmosphere. Aerosols are tiny particles in the atmosphere composed of many things, including water, ice, ash, mineral dust, or acidic droplets.
♤ Aerosols can deflect the Sun’s energy and impact the formation and lifetime of clouds. Aerosols are a negative forcing; that is, they have a cooling effect.
Causes of Climate Change
♤ While natural forcings do exist, they are not significant enough to explain the recent global warming. Human activities are very likely responsible for most of the recent warming.