Adaptation The process of adjustment to actual or expected climate and its effects. In human systems, adaptation seeks to moderate harm or exploit beneficial opportunities. In natural systems, human intervention may facilitate adjustment to expected climate and its effects.
Aerosols A suspension of airborne solid or liquid particles, with a typical size between a few nanometres and 10 μm that reside in the atmosphere for at least several hours.
Albedo 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, and vegetation-covered surfaces and oceans have a low albedo. The Earth’s planetary albedo varies mainly through varying cloudiness, snow, ice, leaf area and and cover changes.
Anthropogenic Resulting from or produced by human activities.
AR4 The 4th Assessment Report of the IPCC, published in 2007. The 4AR was superseded by the 5th Assessment report, published in 2013/ 2014, on which this update booklet is based.
Attribution The process of evaluating the relative contributions of multiple causal factors to a change or event with an assignment of statistical confidence
Biological pump The process of transporting carbon from the ocean’s surface layers to the deep ocean by the primary production of marine phytoplankton, which converts dissolved inorganic carbon (DIC), mainly CO2,  and nutrients into organic matter through photosynthesis.
Climate The average weather, or more rigorously, the statistical description in terms of the mean and variability of relevant quantities over a period of time rang­ing from months to thousands or millions of years. The classical period for averaging these variables is 30 years, as defined by the World Meteorologi­cal Organization. The relevant quantities are most often surface variables such as temperature, precipitation and wind.
Climate Change A change in the state of the climate that can be identified (e.g., by using statistical tests) by changes in the mean and/or the variability of its properties, and that persists for an extended period, typically decades or longer. Climate change may be due to natural internal processes or external forcings such as modulations of the solar cycles, volcanic eruptions, and persistent anthropogenic changes in the composition of the atmosphere or in land use.
Climate Model A numerical representation of the climate system based on the physical, chemical and biological properties of its components, their interactions and feedback processes, and accounting for some of its known properties.  Climate models are applied as a research tool to study and simulate the climate, and for operational purposes, including monthly, seasonal and interannual climate predictions.      
Cryosphere All regions on and beneath the surface of the Earth and ocean where water is in solid form, including sea ice, lake ice, river ice, snow cover, glaciers and ice sheets, and frozen ground (which includes permafrost).
Drought A period of abnormally dry weather long enough to cause a serious hydrological imbalance. Drought is a relative term; therefore any discussion in terms of precipitation deficit must refer to the particular precipitation-related activity that is under discussion.
Energy Budget The Earth is a physical system with an energy budget that includes all gains of incoming energy and all losses of outgoing energy. The Earth’s energy budget is determined by measur­ing how much energy comes into the Earth system from the Sun, how much energy is lost to space, and accounting for the remainder on Earth and energy flows between the atmosphere and the ocean or land surface.
Emissions Scenario A plausible representation of the future develop­ment of emissions of substances that potentially influence the earth’s energy budget (e.g., greenhouse gases, aerosols) and are based on a coherent and internally con­sistent set of assumptions about driving forces (such as demographic and socioeconomic development, technological change) and their key relationships.
Feedback An interaction in which a perturbation (change) in one climate quantity causes a change in a second, and the change in the second quantity ultimately leads to an additional change in the first. A negative feedback is one in which the initial perturbation is weakened by the changes it causes; a positive feedback is one in which the initial perturbation is increased.  For example, melting ice can expose dark-coloured land. The dark-coloured land absorbs more heat than the white ice, leading to further warming and melting.  This is positive feedback.
Forcings Forcing represents any external factor that influences global climate by heating or cooling the planet.  They may be either natural or anthropogenic.  Natural forcings include volcanic eruptions, solar variations and orbital forcing; the amount of solar energy reaching Earth changes with orbital parameters eccentricity, tilt and precession of the equinox. Anthropogenic forcings include changes in the composition of the atmosphere and land use change.
Geoengineering A broad set of methods and technologies that aim to deliberately alter the climate system in order to alleviate the impacts of climate change. Most, but not all, methods seek to either (1) reduce the amount of absorbed solar energy in the climate system (Solar Radiation Management) or (2) increase net carbon sinks from the atmosphere at a scale sufficiently large to alter climate (Carbon Dioxide Removal).
Greenhouse Gas Those gaseous constituents of the atmosphere, both natural and anthropogenic, that absorb and emit radiation at specific wavelengths within the spectrum of terrestrial radiation emitted by the Earth’s surface, the atmosphere itself, and by clouds.
Hadley Cell A direct, thermally driven overturning cell in the atmosphere consisting of poleward flow in the upper troposphere, subsiding air into the subtropical anticyclones, return flow as part of the trade winds near the surface, and with rising air near the equator in the so-called Inter-Tropical Convergence Zone.
Internal variability Variations in the mean state and other statistics (such as the occurrence of extremes) of the climate on all spatial and temporal scales beyond that of individual weather events, due to natural unforced processes within the climate system because, in a system of components with very different response times and complex dependencies, the components are never in equilibrium and are constantly varying. An example of internal variability is El Niño, a warming cycle in the Pacific Ocean which has a big impact on the global climate, resulting from the interaction between atmosphere and ocean in the tropical Pacific.
Mitigation A human intervention to reduce the amount of climate change for example by reducing the sources or enhance the sinks of greenhouse gases.
Radiatively active Tiny objects in the air including liquid water, ice or dust and other aerosols, which interact with either the infrared or solar radiation fields through absorption, scattering and emission.
Reconstruction Approach to reconstructing the past temporal and spatial characteristics of a climate variable from predictors. The predictors can be instrumental data if the reconstruction is used to infill missing data or proxy data if it is an indirect measure used to develop paleoclimate reconstructions.
Solubility pump An important physic- chemical process that transports dissolved inorganic carbon from the ocean’s surface to its interior. Because carbon dioxide is more soluble in colder water, and the thermohaline circulation of the oceans is driven by cold, dense water sinking at high latitudes, deep water contains more dissolved inorganic carbon.
Stratosphere The highly stratified region of the atmosphere above the troposphere extending from about 10 km (ranging from 9 km at high latitudes to 16 km in the tropics on average) to about 50 km altitude.
Thermohaline Circulation Large-scale circulation in the ocean that transforms low-density upper ocean waters to higher-density intermediate and deep waters and returns those waters back to the upper ocean. The circulation is asymmetric, with conversion to dense waters in restricted regions at high latitudes and the return to the surface involving slow upwelling and diffusive processes over much larger geographic regions. The THC is driven by high densities at or near the surface, caused by cold temperatures and/or high salinities, but despite its suggestive though common name, is also driven by mechanical forces such as wind and tides. The Atlantic meridional overturning circulation, in which warm shallow water flows northwards, mainly along the western boundary of the ocean, and cold deep water flows southwards, is partly a thermohaline circulation, but mechanical forcing from winds, especially in the Southern Ocean, and tides are also partly responsible.
Tipping point A hypothesized critical threshold when global or regional climate changes rapidly from one stable state to another stable state. The tipping point event may be irreversible.
Troposphere The lowest part of the atmosphere, from the surface to about 10 km in altitude at mid-latitudes (ranging from 9 km at high latitudes to 16 km in the tropics on average), where clouds and weather phenomena occur. In the troposphere, temperatures generally decrease with height.
Turnover time A measure of how long a component stays in a reservoir.  It is the ratio of the mass M of a reservoir (e.g., a gaseous compound in the atmosphere) and the total rate of removal S from the reservoir: T = M/S. For each removal process, separate turnover times can be defined. In soil carbon biology, this is referred to as Mean Residence Time.
Uncertainty A state of incomplete knowledge that can result from a lack of information or from disagreement about what is known or even knowable. It may have many types of sources, from imprecision in the data to ambiguously defined concepts or terminology, or uncertain projections of human behaviour.

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