Glossary

When most people say "aerosol," they often refer to a specific type of product that produces an aerosol spray, like hairspray. When you press the nozzle, liquid particles suspended in the gas come out. When scientists talk about aerosols, they refer to a broad category of tiny solid or liquid particles or droplets- a typical size between 0.01 and 10 mm - suspended in a gas, which can occur naturally or be a result of human activities. For example, when you see dust floating in a beam of sunlight you see a natural example of aerosols suspended in the air, while soot from gas powered cars is an example of aerosols from human activities. (See IPCC 2018)

The fraction of solar radiation reflected by a surface or object, often expressed as a percentage. Snow-covered surfaces have a high albedo; the albedo of soils ranges from high to low; vegetation-covered surfaces and oceans have a low albedo. The Earth’s albedo varies mainly through varying cloudiness, snow, ice, leaf area, and land-cover changes. (IPCC 2018)

Resulting from or produced by human beings. (IPCC 2018)

A naturally occurring gas, also a by-product of burning fossil fuels and biomass, as well as from land-use changes and other industrial processes. It is a greenhouse gas which has the ability to trap heat.

A carbon sink absorbs carbon dioxide from the atmosphere.

Climate in a narrow sense is usually defined as the “average weather,” or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands of years. The classical period is 3 decades, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state including a statistical description, of the climate system. (IPCC 2018)

Climate change refers to any change in climate over time, whether due to natural variability or as a result of human activity (IPCC 2018).  This usage differs from that in the United Nations Framework Convention on Climate Change (UNFCCC), which defines “climate change” as: “a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods.” (IPCC 2018)

The climate system is the highly complex system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface, and the biosphere, and the interactions between them. The climate system evolves in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations and human-induced forcings such as the changing composition of the atmosphere and land use. (IPCC 2018)

Energy balance is "the difference between the total incoming and total outgoing energy. If this balance is positive, warming occurs; if it is negative, cooling occurs. Averaged over the globe and over long time periods, this balance must be zero. Because the climate system derives virtually all its energy from the Sun, zero balance implies that, globally, the absorbed solar radiation, that is, incoming solar radiation minus reflected solar radiation at the top of the atmosphere and outgoing longwave radiation emitted by the climate system are equal." (IPCC AR6 Glossary)

Evaporation is the process that changes liquid water to gaseous water (water vapor). Water moves from the Earth’s surface to the atmosphere via evaporation. (USGS 2019).

A process that triggers changes in a second process that in turn influences the original one; a positive feedback intensifies the original process, and a negative feedback reduces it. (IPCC 2018)

Gigatonne or metric gigaton (unit of mass) is equal to 1,000,000,000 metric tons.

Global warming is the long-term heating of Earth’s surface observed since the pre-industrial period (between 1850 and 1900) due to human activities, primarily fossil fuel burning, which increases heat-trapping greenhouse gas levels in Earth’s atmosphere. (NASA)

Greenhouse gases effectively absorb infrared radiation e mitted by the Earth’s surface, by the atmosphere itself due to the same gases, and by clouds. Atmospheric radiation is emitted to all sides, including downward to the Earth’s surface. Thus greenhouse gases trap heat within the surface troposphere system. This is called the “natural greenhouse effect.” Atmospheric radiation is strongly coupled to the temperature of the level at which it is emitted. In the troposphere, the temperature generally decreases with height. Effectively, infrared radiation emitted to space originates from an altitude with a temperature of on average -19°C, in balance with the net incoming solar radiation, whereas the Earth’s surface is kept at a much higher temperature of on average 14°C. An increase in the concentration of greenhouse gases leads to an increased infrared opacity of the atmosphere, and therefore to an effective radiation into space from a higher altitude at a lower temperature. This causes a radiative forcing, an imbalance that can only be compensated for by an increase of the temperature of the surface-troposphere system. This is called the “enhanced greenhouse effect.” (IPCC 2018)

Greenhouse gases are those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of infrared radiation emitted by the Earth’s surface, the atmosphere, and clouds. This property causes the greenhouse effect. Water vapor (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4), and ozone (O3) are the primary greenhouse gases in the Earth’s atmosphere. Moreover, there are a number of entirely human-made greenhouse gases in the atmosphere, such as the halocarbons and other chlorine and bromine-containing substances which are dealt with under the Montreal Protocol. Beside CO2 , N2O, and CH4, the Kyoto Protocol deals with the greenhouse gases sulfur hexaflouride (SF6 ), hydrofluorocarbons (HFCs) and perfluorocarbons (PFCs). (IPCC 2018).

The Intergovernmental Panel on Climate Change (IPCC) is the United Nations body for assessing the science related to climate change. The IPCC issues periodic reports assessing the scientific basis of climate change, its impacts and future risks, and options for adaptation and mitigation. The reports are created by hundreds of scientists from around the world and are considered a leading authority on climate change science. (See IPCC webpage)

Ozone, the triatomic form of oxygen (O3 ), is a gaseous atmospheric constituent. In the troposphere, it is created both naturally and by photochemical reactions involving gases resulting from human activities (photochemical smog). In high concentrations, tropospheric ozone can be harmful to a wide-range of living organisms. Tropospheric ozone acts as a greenhouse gas. In the stratosphere, ozone
is created by the interaction between solar ultraviolet radiation and molecular oxygen (O2 ). Stratospheric ozone plays a decisive role in the stratospheric radiative balance. Depletion of stratospheric ozone, due to chemical reactions that may be enhanced by climate change, results in an increased ground-level flux of ultraviolet (UV-) B radiation. (IPCC 2018)

The IPCC definition of pre-industrial (period) is "The multi-century period prior to the onset of large-scale industrial activity around 1750. The reference period 1850–1900 is used to approximate pre-industrial global mean surface temperature (GMST).  (IPCC ARG Glossary)

Radiative forcing is the change in the net vertical irradiance [expressed in Watts per square meter (Wm-2)] at the tropopause due to an internal change or a change in the external forcing of the climate system, such as a change in the concentration of CO2 or the output of the Sun. Usually radiative forcing is computed after allowing for stratospheric temperatures to readjust to radiative equilibrium, but with all tropospheric properties held fixed at their unperturbed values. (IPCC 2018)

It is the second layer of the atmosphere as you go upward, above the troposphere. The bottom of the stratosphere starts around 6.2 miles (10km) while the top is around 31 miles (50km)  above the surface of the Earth at middle latitudes. Where the stratosphere starts varies with latitude and with the seasons. (UCAR Center for Science Education).

The Earth Radiation Budget (at the top of the atmosphere) describes the overall balance between the incoming energy from the sun and the outgoing thermal (longwave) and reflected (shortwave) energy from the earth. It can only be measured from space. (Global Climate Observing System)

According to NASA, "Technically, there is no absolute dividing line between the Earth’s atmosphere and space, but for scientists studying the balance of incoming and outgoing energy on the Earth, it is conceptually useful to think of the altitude at about 100 kilometers above the Earth as the “top of the atmosphere.” … It is the place where solar energy (mostly visible light) enters the Earth system and where both reflected light and invisible, thermal radiation from the Sun-warmed Earth exit. (NASA)

The lowest part of the atmosphere, from the surface to about 10 km (roughly 6.2 miles) in altitude at mid-latitudes (ranging from 9 km (5.59 miles) at high latitudes to 16 km (9.94 miles) in the tropics on average), where clouds and weather phenomena occur. In the troposphere, temperatures generally decrease with height. (IPCC 2018)

Short-term changes to local atmospheric conditions such as air temperature, humidity, wind, and cloud cover. Short-term changes typically occur within minutes, hours, or days.