Climate change is the changes in average weather conditions that persist over multiple decades or longer. Climate change does not necessarily mean an increase in global and regional temperatures. Climate change can be both increases and decreases in temperature, changes in precipitation patterns, changing frequency of severe weather events, and changes to other features of the climate system.
The Earth's climate can be affected by natural factors that are external to the climate system, such as changes in volcanic activity, solar output, El Niño-Southern Oscillation (ENSO), and the Earth's orbit around the Sun. Of these, the three factors relevant on timescales of contemporary climate change are changes in volcanic activity, changes in solar radiation and ENSO. ENSO is comprised of El Niño and La Niña, the warm and cool phases of a recurring climate pattern across the tropical Pacific. The pattern can shift back and forth irregularly every two to seven years, and each phase triggers predictable disruptions of temperature, precipitation, and winds. In terms of the Earth's energy balance, volcanic activity, changes in solar radiation are factors influence the amount of incoming energy. Volcanic eruptions are episodic and have relatively short-term effects on climate. Changes in solar irradiance have contributed to climate trends over the past century but since the Industrial Revolution.
Climate change can also be caused by human activities, such as the burning of fossil fuels and the conversion of land for forestry and agriculture. Since the beginning of the Industrial Revolution, these human influences on the climate system have increased substantially. In addition to other environmental impacts, these activities change the land surface and emit various substances to the atmosphere. These in turn can influence both the amount of incoming energy and the amount of outgoing energy and can have both warming and cooling effects on the climate. The dominant product of fossil fuel combustion is carbon dioxide (CO2), a greenhouse gas. The overall effect of human activities since the Industrial Revolution has been a warming effect, driven primarily by emissions of CO2 and enhanced by emissions of other greenhouse gases.
The build-up of anthropogenic greenhouse gases in the atmosphere has led to an enhancement of the natural greenhouse effect. It is this human-induced enhancement of the greenhouse effect that is of concern because ongoing emissions of greenhouse gases have the potential to warm the planet to levels that have never been experienced in the history of human civilization.
Carbon dioxide is the main cause of human-induced climate change. It has been emitted in vast quantities from the burning of fossil fuels and it is a very long-lived gas, which means it continues to affect the climate system during its long residence time in the atmosphere. CO2 emissions cause increases in atmospheric concentrations of CO2 that will last thousands of years. However, fossil fuel combustion, industrial processes, agriculture, and forestry-related activities emit other substances that also act as climate forcers. Some, such as nitrous oxide (N2O), are long-lived greenhouse gases like CO2, and so contribute to long-term climate change. Nitrous Oxide has a global warming potential that is 265–298 times that of CO2 for a 100-year timescale. Nitrous oxide emitted today remains in the atmosphere for more than 100 years, on average. Other substances have shorter atmospheric lifetimes because they are removed fairly quickly from the atmosphere. Therefore, their effect on the climate system is similarly short-lived. Together, these short-lived climate forcers are responsible for a significant amount of current climate forcing from anthropogenic substances. Some short-lived climate forcers have a climate warming effect (‘positive climate forcers') while others have a cooling effect (‘negative climate forcers').
If atmospheric levels of short-lived climate forcers are continually replenished by ongoing emissions, these continue to exert a climate forcing. However, reducing emissions will quickly lead to reduced atmospheric levels of such substances. A number of short-lived climate forcers have climate warming effects and together are the most important contributors to the human enhancement of the greenhouse effect after carbon dioxide. This includes methane (CH4), tropospheric ozone (O3), and black carbon, a small solid particle formed from the incomplete combustion of carbon-based fuels (coal, oil and wood for example).
Other short-lived climate forcers have climate cooling effects, most notably sulphate aerosols. Fossil fuel combustion emits sulfur dioxide (SO2) into the atmosphere (in addition to CO2) which then combines with water vapor to form tiny droplets (aerosols) which reflect sunlight. Sulphate aerosols remain in the atmosphere for only a few days (washing out in what is referred to as acid rain), and so do not have the same long-term effect as greenhouse gases. The cooling from sulphate aerosols in the atmosphere has, however, offset some of the warming from other substances. That is, the warming we have experienced to date would have been even larger had it not been for elevated levels of sulphate aerosols in the atmosphere.