© 2008 Nature Publishing Group nature geoscience | VOL 1 | JANUARY 2008 | www.nature.com/naturegeoscience 25 REVIEW ARTICLE THORNE LAY 1 *, JOHN HERNLUND 2 AND BRUCE A. BUFFETT 3 1 Earth and Planetary Sciences Department, University of California, Santa Cruz, California 91125, USA; 2 Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z4; 3 Department of Geophysical Sciences, University of Chicago, Chicago, Illinois 60637, USA *e-mail: thorne@pmc.ucsc.edu Energy is one of the most fundamental parameters of the Earth’s physical system, but it is difficult to determine robustly. e most accessible integrative energy measure for the planet is the total amount presently released at the surface, mainly comprising heat conducted through the surface rocks and from volcanic activity and hydrothermal circulation. Direct global measurements of heat conduction based on thermal gradients in shallow boreholes with calibrated rock- sample thermal conductivities indicate a total of 30–32 terawatts (TW), which increases to 43–49 TW when corrected according to an ocean lithosphere thermal model that accounts for observational underestimation due to hydrothermal circulation 1,2 . Although there has been some debate about the half-space cooling models used to correct the observations, the arguments for an upper value of 46 ± 3 TW appear sound. Crustal concentrations of radioactivity are estimated to account for 6–8 TW (out of ~20 TW of radiogenic heating in the chondritic model for the bulk silicate Earth) 2 , with some estimates of depleted upper-mantle radiogenic heat generation (~2 TW) and cooling (~3 TW), leaving as much as 33–35 TW that should be passing from the lower mantle to the upper mantle 3,4 . is large lower-mantle heat flow includes contributions from lower-mantle radiogenic heat generation (~10–12 TW), lower- mantle cooling (5–25 TW) and transfer of heat from the core into the base of the mantle (Fig. 1). Constraining any one source or sink bounds the residual balance. Diverse approaches have been pursued to estimate the heat flux at the core–mantle boundary (CMB), or at least upwards from the lowermost mantle. Early estimates 5–7 gave values in the range 3–4 TW, indicating that there is only minor heating of the mantle from below and that thermal plumes play a secondary role in mantle circulation. Recent estimates, however, have yielded values in the range 5–15 TW from independent considerations of core temperature, geodynamo energetics and buoyancy flux of lower-mantle thermal plumes. Further constraints have recently been provided by direct determinations of the temperature in the lowermost mantle, which were made by relating seismic velocity discontinuities to a laboratory-calibrated phase change in magnesium silicate perovskite (MgSiO 3 ). ese findings have important implications for the evolution of the deep Earth. LOWER-MANTLE TEMPERATURES AND PROPERTIES Determination of the CMB heat flow requires models for temperature, composition, material properties and/or dynamics of the deep interior, all of which are subject to large uncertainties. Absolute temperatures in the deep mantle are particularly difficult to constrain, and until recently have primarily been approximated by vast extrapolations from calibrated phase boundaries in the transition zone and at the inner-core boundary 8 . Upper bounds on CMB temperatures have been estimated by determinations of the lower-mantle solidus 9–11 and from outer-core melting temperature estimates 10,12 , whereas lower bounds are obtained by extrapolating transition-zone temperatures downwards along mantle adiabats (or subadiabats), giving values of 2,500–2,800 K. Allowing for the presence of a thermal boundary layer (TBL), these approaches lead to loosely constrained CMB temperatures (T CMB ) of 3,300–4,300 K (Fig. 2), which is a huge range for such an important Earth parameter. Increases of 500–1,800 K in temperature across a 200 km thick superadiabatic TBL with a thermal conductivity of ~10 W m –1 K –1 predict a net CMB heat flow of 5–13 TW (refs 10,13–15). us, relatively hot outermost core temperatures (>3,600 K) consistent with inner-core boundary Core–mantle boundary heat flow The Earth can be viewed as a massive heat engine, with various energy sources and sinks. Insights into its evolution can be obtained by quantifying the various energy contributions in the context of the overall energy budget. Over the past decade, estimates of the heat flow across the core–mantle boundary, or across a chemical boundary layer above it, have generally increased by a factor of 2 to 3. The current total heat flow at the Earth’s surface — 46 ± 3 terawatts (10 12 J s –1 ) — involves contributions from heat entering the mantle from the core, as well as mantle cooling, radiogenic heating of the mantle from the decay of radioactive elements, and various minor processes such as tidal deformation, chemical segregation and thermal contraction gravitational heating. The increased estimates of deep-mantle heat flow indicate a more prominent role for thermal plumes in mantle dynamics, more extensive partial melting of the lowermost mantle in the past, and a more rapidly growing and younger inner core and/or presence of significant radiogenic material in the outer core or lowermost mantle as compared with previous estimates.