Climate commitment describes the fact that Earth's

climate Climate is the long-term weather pattern in a region, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteoro ...

reacts with a delay to influencing factors ("

climate forcing Radiative forcing (or climate forcing) is a concept used to quantify a change to the Earth's energy budget, balance of energy flowing through a planetary atmosphere. Various factors contribute to this change in energy balance, such as concentration ...

s") such as the growth and the greater presence of

greenhouse gas Greenhouse gases (GHGs) are the gases in the atmosphere that raise the surface temperature of planets such as the Earth. Unlike other gases, greenhouse gases absorb the radiations that a planet emits, resulting in the greenhouse effect. T ...

es. Climate commitment studies attempt to assess the amount of future

global warming Present-day climate change includes both global warming—the ongoing increase in global average temperature—and its wider effects on Earth's climate system. Climate change in a broader sense also includes previous long-term changes ...

that is "committed" under the assumption of some constant or some evolving level of forcing. The constant level often used for illustrative purposes is that due to doubling or quadrupling relative to the pre-industrial level; or the present level of forcing.

Definition

Climate commitment is the "unavoidable future climate change resulting from inertia in the geophysical and socio-economic systems".IPCC, 2021
Annex VII: Glossary
atthews, J.B.R., V. Möller, R. van Diemen, J.S. Fuglestvedt, V. Masson-Delmotte, C. Méndez, S. Semenov, A. Reisinger (eds.) I
Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change
[Masson-Delmotte, V., P. Zhai, A. Pirani, S.L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M.I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T.K. Maycock, T. Waterfield, O. Yelekçi, R. Yu, and B. Zhou (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 2215–2256, doi:10.1017/9781009157896.022. Different types of climate change commitment are discussed in the literature. These include the "constant composition commitment"; the "constant emissions commitment" and the "zero emissions commitment".

Basic idea

If a perturbation — such as an increase in greenhouse gases or

solar activity Solar phenomena are natural phenomena which occur within the Stellar atmosphere, atmosphere of the Sun. They take many forms, including solar wind, Solar radio emission, radio wave flux, solar flares, coronal mass ejections, Stellar corona#Coron ...

— is applied to Earth's

climate system Earth's climate system is a complex system with five interacting components: the Atmosphere of Earth, atmosphere (air), the hydrosphere (water), the cryosphere (ice and permafrost), the lithosphere (earth's upper rocky layer) and the biosphere ( ...

the response will not be immediate, principally because of the large

heat capacity Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K). Heat capacity is a ...

and

thermal inertia Thermal inertia is a term commonly used to describe the observed delays in a body's temperature response during heat transfers. The phenomenon exists because of a body's ability to both store and transport heat relative to its environment. Sinc ...

of the

ocean The ocean is the body of salt water that covers approximately 70.8% of Earth. The ocean is conventionally divided into large bodies of water, which are also referred to as ''oceans'' (the Pacific, Atlantic, Indian Ocean, Indian, Southern Ocean ...

s. :''As an analogue, consider the heating of a thin metal plate (by the sun or by a flame): the plate will warm relatively quickly. If a thick metal block is heated instead, it will take much longer for the entire block to reach equilibrium with the imposed heating because of its higher heat capacity.'' Land only stores

heat In thermodynamics, heat is energy in transfer between a thermodynamic system and its surroundings by such mechanisms as thermal conduction, electromagnetic radiation, and friction, which are microscopic in nature, involving sub-atomic, ato ...

in the top few meters. Ocean water, by contrast, can move vertically and store heat within the ocean's depth (

convection Convection is single or Multiphase flow, multiphase fluid flow that occurs Spontaneous process, spontaneously through the combined effects of material property heterogeneity and body forces on a fluid, most commonly density and gravity (see buoy ...

). This is why the land surface is observed to warm more than the oceans. It also explains the large difference in global surface temperature response between * " transient" climate simulations in which the planet's incoming/outgoing energy flows are substantially out-of-balance and only a shallow ocean model might be utilized, and * " equilibrium" climate simulations in which the energy flows approach a new balance and a full ocean model is needed. The "commitment" can apply to variables other than temperature: because of the long mixing time for heat into the deep ocean, a given surface warming commits to centuries of

sea level rise The sea level has been rising from the end of the last ice age, which was around 20,000 years ago. Between 1901 and 2018, the average sea level rose by , with an increase of per year since the 1970s. This was faster than the sea level had e ...

from thermal expansion of the ocean. Also once a certain threshold is crossed, it is likely that a slow melting of the Greenland ice sheet will commit us to a sea level rise of 5m over millennia.

Models

Recent

model A model is an informative representation of an object, person, or system. The term originally denoted the plans of a building in late 16th-century English, and derived via French and Italian ultimately from Latin , . Models can be divided in ...

s forecast that even in the unlikely event of greenhouse gases stabilizing at present levels, the Earth would warm by an additional 0.5°C by 2100, a similar rise in temperature to that seen during the 20th century. In 2050, as much as 64% of that commitment would be due to ''past'' natural forcings. Over time, their contribution compared to the human influence will diminish. Overall, the warming commitment at 2005 greenhouse gas levels could exceed 1°C. As ocean waters expand in response to this warming, global

sea level Mean sea level (MSL, often shortened to sea level) is an mean, average surface level of one or more among Earth's coastal Body of water, bodies of water from which heights such as elevation may be measured. The global MSL is a type of vertical ...

s would mount by about 10 centimeters during that time. These models do not take into account

ice cap In glaciology, an ice cap is a mass of ice that covers less than of land area (usually covering a highland area). Larger ice masses covering more than are termed ice sheets. Description By definition, ice caps are not constrained by topogra ...

and

glacier A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...

melting; including those climate feedback effects would give a 1–1.5°C estimated temperature increase.

History

The concept has been discussed as far back as 1995 in the

IPCC The Intergovernmental Panel on Climate Change (IPCC) is an intergovernmental body of the United Nations. Its job is to "provide governments at all levels with scientific information that they can use to develop climate policies". The World M ...

TARbr>
and in the IPCC_Second_Assessment_Report, SAR.

Misuse

Climate commitment studies span a range of emissions scenarios which are intimately tied to past, present and future human choices. The "commitment" concept is misused when worst cases are asserted to be inevitable regardless of social agency. Models rather indicate that additional surface warming can be halted almost simultaneous with rapid emissions reductions.

References

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External links

{{Global warming Climatology