The Earth's energy inventory
A recent publication by members of the PAGES 2k Network "Missing link in the Past – Downscaling paleoclimatic Earth System Models" (PALEOLINK) project provides an update on the land heat content of continental areas around the world.
The fundamental quantity for understanding how our climate will change in the future and how sensitive the Earth's climate is to increases in atmospheric concentrations of carbon dioxide is the change in Earth's energy content and its temporal evolution; that is, the way the Earth responds to additional atmospheric concentrations of greenhouse gases.
Authors, led by Francisco José Cuesta-Valero, discuss why the study of land heat change is important in their Climate of the Past paper "Long-term global ground heat flux and continental heat storage from geothermal data".
Teams of scientists around the world have been working on measuring the heat content of the ocean, atmosphere, ice masses and land. A recent publication (1) assesses the overall energy change in the recent past for all these heat reservoirs.
Land heat change is important for several reasons. The most obvious one is that we live on the land surface. Land heat affects the temperature above the ground and also acts as a trigger mechanism for extreme temperature events (2), some of which influence the intensity of natural forest fires.
In addition, the upper few meters of soil around the world contain about twice as much carbon as the whole atmosphere. Carbon is stored in the soils and in permafrost at higher latitudes. Increases in continental heat raises the potential of destabilizing soil and permafrost bounded carbon, leading to additional input of greenhouse gases into the atmosphere.
The estimate of the continental energy was done using a world-wide collection of underground temperature data measured in abandoned mineral exploration drillings to a depth of at least 300 m. Authors found that if the ground surface temperature and internal heat flow do not change over time, the temperature at any depth can be easily determined. However, if the Earth’s surface warms (or cools), a quantity of heat will be gained (or lost) by the ground. Such changes at the surface propagate into the underground and remain recorded as the temperature of the rocks. Analyses of these underground temperature changes allowed them to estimate the energy change of the continental areas for the first 300 m of the subsurface which retains information on ground surface temperature change over the last 700 years.
From a new world-wide database, the authors analyzed 1024 datasets of underground temperature and their variation with depth measured from the surface to a depth of 300 m. The retrieved changes in ground surface temperature indicate that the Earth's continental surface has warmed by 1.1K since pre-industrial times, which agrees with meteorological records. The new continental energy estimates also reveal that the continental subsurface has stored more energy during the last part of the 20th and first decade of the 21st century than previously reported, reaching around 12 ZJ (12 x1021J).
As current CMIP5 simulations are not able to account for the observed continental heat (3), the authors called on climate modelers to perform future climate simulations with deeper land surface model components in order to attempt to reproduce the land component of the Earth's energy budget, and enhance GCMs' representation of potentially powerful carbon feedbacks related to energy-dependent processes of the continental subsurface, such as the stability of the soil carbon pool and permafrost evolution (4).
Text: Hugo Beltrami
References
(1) von Schuckmann K et al. (2020) Heat stored in the Earth system: where does the energy go?, Earth Syst. Sci. Data, 12, 2013–2041
(2) Hirschi M et al. (2011) Observational evidence for soil-moisture impact on hot extremes in southeastern Europe. Nature Geosci 4, 17–21
(3) Cuesta-Valero FJ et al. (2016) First assessment of continental energy storage in CMIP5 simulations, Geophys. Res. Lett., 43, 5326–5335
(4) Hermoso de Mendoza I et al. (2020) Lower boundary conditions in land surface models – effects on the permafrost and the carbon pools: a case study with CLM4.5, Geosci. Model Dev., 13, 1663–1683