Appendix E Glossary Terms doi: 10.7930/J0TM789P Abrupt climate change Change in the climate system on a timescale The total effective radiative forcing due to both shorter than the timescale of the responsible aerosol–cloud and aerosol–radiation interactions forcing. In the case of anthropogenic forcing is denoted aerosol effective radiative forcing over the past century, abrupt change occurs (ERFari+aci). See also aerosol–radiation inter- over decades or less. Abrupt change need not be action. (condensed from IPCC AR5 WGI Annex externally forced. (CSSR, Ch. 15) III: Glossary) Aerosol–cloud interaction Aerosol–radiation interaction (RFari) A process by which a perturbation to aerosol The radiative forcing (or radiative effect, if affects the microphysical properties and evolu- the perturbation is internally generated) of an tion of clouds through the aerosol role as cloud aerosol perturbation due directly to aerosol– condensation nuclei or ice nuclei, particularly in radiation interactions, with all environmental ways that affect radiation or precipitation; such variables remaining unaffected. It is tradition- processes can also include the effect of clouds ally known in the literature as the direct aerosol and precipitation on aerosol. The aerosol pertur- forcing (or effect). bation can be anthropogenic or come from some natural source. The radiative forcing from such The total effective radiative forcing due to both interactions has traditionally been attributed aerosol–cloud and aerosol–radiation interactions to numerous indirect aerosol effects, but in this is denoted aerosol effective radiative forcing report, only two levels of radiative forcing (or (ERFari+aci). See also aerosol-cloud interaction. effect) are distinguished: (condensed from IPCC AR5 WGI Annex III: Glossary) The radiative forcing (or effect) due to aero- sol–cloud interactions (RFaci) is the radiative Agricultural drought forcing (or radiative effect, if the perturbation See drought. is internally generated) due to the change in number or size distribution of cloud droplets Albedo or ice crystals that is the proximate result of an The fraction of solar radiation reflected by a aerosol perturbation, with other variables (in surface or object, often expressed as a percent- particular total cloud water content) remaining age. Snow-covered surfaces have a high albedo, equal. In liquid clouds, an increase in cloud the albedo of soils ranges from high to low, and droplet concentration and surface area would vegetation-covered surfaces and oceans have a increase the cloud albedo. This effect is also low albedo. The Earth’s planetary albedo varies known as the cloud albedo effect, first indirect mainly through varying cloudiness, snow, ice, effect, or Twomey effect. It is a largely theoret- leaf area, and land-cover changes. (IPCC AR5 ical concept that cannot readily be isolated in WGI Annex III: Glossary) observations or comprehensive process models due to the rapidity and ubiquity of rapid ad- Altimetry justments. This is contrasted with the effective A technique for measuring the height of the radiative forcing (or effect) due to aerosol–cloud Earth’s surface with respect to the geocenter of interactions (ERFaci) the Earth within a defined terrestrial reference 460 U.S. Global Change Research Program Climate Science Special Report Appendix E | Gossary Terms frame (geocentric sea level). (IPCC AR5 WGI Biological pump Annex III: Glossary) The suite of biologically mediated processes responsible for transporting carbon against a Anticyclonic circulation concentration gradient from the upper ocean to Fluid motion having a sense of rotation about the deep ocean. (Passow and Carlson, 2012) the local vertical opposite to that of the earth’s rotation; that is, clockwise in the Northern Blocking Hemisphere, counterclockwise in the Southern Associated with persistent, slow-moving high Hemisphere, and undefined at the equator. It pressure systems that obstruct the prevailing is the opposite of cyclonic circulation. (AMS westerly winds in the middle and high latitudes glossary). and the normal eastward progress of extratrop- ical transient storm systems. It is an important Atlantic meridional overturning circulation component of the intraseasonal climate variabil- (AMOC) ity in the extratropics and can cause long-lived See Meridional overturning circulation weather conditions such as cold spells in winter (MOC). and heat waves in summer. (IPCC AR5 WGI Annex III: Glossary) Atmospheric blocking See Blocking. Carbon dioxide fertilization The enhancement of the growth of plants as a Atmospheric river result of increased atmospheric CO2 concentra- A long, narrow, and transient corridor of strong tion. (IPCC AR5 WGI Annex III: Glossary) horizontal water vapor transport that is typical- ly associated with a low-level jet stream ahead Carbon dioxide removal of the cold front of an extratropical cyclone. A set of techniques that aim to remove CO2 di- The water vapor in atmospheric rivers is sup- rectly from the atmosphere by either (1) increas- plied by tropical and/or extratropical moisture ing natural sinks for carbon or (2) using chem- sources. Atmospheric rivers frequently lead ical engineering to remove the CO2, with the to heavy precipitation where they are forced intent of reducing the atmospheric CO2 concen- upward—for example, by mountains or by tration. CDR methods involve the ocean, land ascent in the warm conveyor belt. Horizontal and technical systems, including such methods water vapor transport in the midlatitudes occurs as iron fertilization, large-scale afforestation and primarily in atmospheric rivers and is focused direct capture of CO2 from the atmosphere using in the lower troposphere. (AMS glossary). engineered chemical means. (truncated version from IPCC AR5 WGI Annex III: Glossary) Baroclinicity The state of stratification in a fluid in which Climate engineering surfaces of constant pressure (isobaric) intersect See geoengineering. surfaces of constant density (isosteric). (AMS glossary). Climate intervention See geoengineering. Bias correction method One of two main statistical approaches used to Climate sensitivity alleviate the limitations of global and regional In Intergovernmental Panel on Climate Change climate models, in which the statistics of the (IPCC) reports, equilibrium climate sensitivity simulated model outputs are adjusted to those (units: °C) refers to the equilibrium (steady state) of the observation data. (The other approach is change in the annual global mean surface tem- empirical/stochastic downscaling, described perature following a doubling of the atmospheric under downscaling). The rescaled variables can equivalent carbon dioxide concentration. The remove the effects of systematic errors in climate effective climate sensitivity (units: °C) is an esti- model outputs. (derived from Kim et al., 2015) mate of the global mean surface temperature re- 461 U.S. Global Change Research Program Climate Science Special Report Appendix E | Gossary Terms sponse to doubled carbon dioxide concentration tative Concentration Pathways. (edited from that is evaluated from model output or observa- IPCC AR5 WGII Annex II: Glossary). tions for evolving non-equilibrium conditions. It is a measure of the strengths of the climate Compound event feedbacks at a particular time and may vary with An event that consists of 1) two or more extreme forcing history and climate state, and therefore events occurring simultaneously or successively, may differ from equilibrium climate sensitivity. 2) combinations of extreme events with under- The transient climate response (units: °C) is the lying conditions that amplify the impact of the change in the global mean surface temperature, events, or 3) combinations of events that are averaged over a 20-year period centered at the not themselves extremes but lead to an extreme time of atmospheric carbon dioxide doubling, in event or impact when combined. The contrib- a climate model simulation in which CO2 increas- uting events can be of similar or different types. es at 1% per year. It is a measure of the strength (CSSR, Ch. 15, drawing upon SREX 3.1.3) and rapidity of the surface temperature response to greenhouse gas forcing. (IPCC AR5 WGI Critical threshold Annex III: Glossary) A threshold that arises within a system as a result of the amplifying effects of positive Cloud radiative effect feedbacks. The crossing of a critical threshold The radiative effect of clouds relative to the commits the system to a change in state. (CSSR, identical situation without clouds (previously Ch. 15) called cloud radiative forcing). (drawn from IPCC AR5 WGI Annex III: Glossary) Cryosphere All regions on and beneath the surface of the Clouds can act as a greenhouse ingredient to Earth and ocean where water is in solid form, warm the Earth by trapping outgoing long- including sea ice, lake ice, river ice, snow cover, wave infrared radiative flux at the top of the glaciers and ice sheets, and frozen ground atmosphere (the longwave cloud radiative (which includes permafrost). (IPCC AR5 WGI effect [LWCRE]). Clouds can also enhance the Annex III: Glossary) planetary albedo by reflecting shortwave solar radiative flux back to space to cool the Earth (the Cyclonic circulation shortwave cloud radiative effect [SWCRE]). Fluid motion in the same sense as that of the The net effect of the two competing processes earth, that is, counterclockwise in the Northern depends on the height, type, and the optical Hemisphere, clockwise in the Southern