Earth’s Energy Imbalance and Energy Flows through the Climate System

Description:

Governments all over the world have recognized anthropogenic climate change as a major threat to our livelihoods, economy, and ecological treasures. Measuring Earth’s energy imbalance associated with greenhouse gas forcing of the Earth system is a fundamental aspect of tracking the rate of climate change. Understanding the changes and variability in flows of energy into and through the climate system represents a key challenge in advancing our knowledge of future climate change. The contributions in this special collection consolidate and advance the current knowledge on Earth’s energy imbalance and energy flows through the climate system through observational and modelling studies, including: intercomparisons of upper-ocean heat uptake in a multitude of data products; the introduction of Deep SOLO autonomous floats to measure the ocean’s temperature below 2000 meters depth; characterizing heat flows in the atmosphere and in the ocean; assessing regional energy budgets, such as for the Arctic; establishing linkages to the global water cycle; examining the role of cloud types on radiative effects; and utilizing spectral signatures to better understand the spatio-temporal variability in radiation fluxes at Earth’s surface and top of atmosphere. Research into Earth’s energy imbalance is an interdisciplinary research field, connecting the different components of our climate system with phenomena that act on numerous timescales and have direct consequences for global and regional sea level, surface temperatures, weather patterns, and the potential for dramatic future impacts on the habitability of our planet. This special collection is one of the outcomes of the World Climate Research Programme (WCRP) workshop The Earth’s Energy Imbalance and its implications held in Toulouse, France, in November 2018. The WCRP has identified the improved quantification and understanding of Earth’s energy imbalance and its spatio-temporal variability to be major challenges in contemporary climate research across disciplines and provides a strong motivation for this special collection.

Collection organizers:
Maria Hakuba, Jet Propulsion Laboratory/Caltech
Matt Palmer, Met Office Hadley Centre and University of Bristol
Seiji Kato, NASA Langley Research Center

Earth’s Energy Imbalance and Energy Flows through the Climate System

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Lijing Cheng
,
Grant Foster
,
Zeke Hausfather
,
Kevin E. Trenberth
, and
John Abraham

Abstract

The increased concentrations of greenhouse gases in the atmosphere create an increase in Earth’s thermal energy, which is mainly stored in the ocean. Quantification of the rate of increase in ocean heat content (OHC) is vital for understanding the current and future climate of Earth. Linear trend lines have been frequently used to quantify long-term rates of change, but are inappropriate because they cannot capture nonlinearity in trends, have large start- and end-point sensitivity, and the assumption of linearity is nonphysical. Here observed and model-based linear regressions with higher-order polynomial (quadratic), piecewise linear, and locally weighted scatterplot smoothing (LOWESS) are compared. Piecewise linear and LOWESS perform best in depicting multidecadal trends. It is shown that linear rates are valid for up to about 15-yr segments (i.e., it is valid to compute linear rates within a 15-yr time window). Using the recommended methods, ocean warming for the upper 2000 m increases from about 0 to 0.06 ± 0.08 W m−2 for 1958–73 to 0.58 ± 0.08 W m−2 for 2003–18, indicating an acceleration of ocean warming that happens in all four ocean basins and from near the sea surface to 2000 m. There is consistency between multimodel-mean historically forced climate models and observations, which implies that the contribution of internal variability is small for global 0–2000 m OHC. Notable increases of OHC in the upper ocean (i.e., 0–300 m) after about 1980 and the deeper ocean (300–2000 m) after the late 1980s are also evident. This study suggests alternative methods to those currently used to estimate ocean warming rates to provide a more accurate quantification of long-term Earth’s energy changes.

Significance Statement

Quantifying long-term rates of change is needed to understand the time evolution of ocean warming and to assess the changing ocean and Earth’s energy budgets. Linear trend lines have been frequently used but cannot capture nonlinearity in trends, and have large start- and end-point sensitivity. Based on an analysis of the statistical features of ocean heat content time series, this study proposes two alternative methods to quantify the rates of change, including piecewise linear fit and LOWESS. Robust increases in warming for the upper 2000 m detected through observational records and climate models from 1958 to 2020, indicate a robust acceleration of ocean warming. Slow penetration of heat from the upper ocean into the deeper ocean is also evident.

Open access
Abhishek Savita
,
Catia M. Domingues
,
Tim Boyer
,
Viktor Gouretski
,
Masayoshi Ishii
,
Gregory C. Johnson
,
John M. Lyman
,
Josh K. Willis
,
Simon J. Marsland
,
William Hobbs
,
John A. Church
,
Didier P. Monselesan
,
Peter Dobrohotoff
,
Rebecca Cowley
, and
Susan E. Wijffels

Abstract

The Earth system is accumulating energy due to human-induced activities. More than 90% of this energy has been stored in the ocean as heat since 1970, with ∼60% of that in the upper 700 m. Differences in upper-ocean heat content anomaly (OHCA) estimates, however, exist. Here, we use a dataset protocol for 1970–2008—with six instrumental bias adjustments applied to expendable bathythermograph (XBT) data, and mapped by six research groups—to evaluate the spatiotemporal spread in upper OHCA estimates arising from two choices: 1) those arising from instrumental bias adjustments and 2) those arising from mathematical (i.e., mapping) techniques to interpolate and extrapolate data in space and time. We also examined the effect of a common ocean mask, which reveals that exclusion of shallow seas can reduce global OHCA estimates up to 13%. Spread due to mapping method is largest in the Indian Ocean and in the eddy-rich and frontal regions of all basins. Spread due to XBT bias adjustment is largest in the Pacific Ocean within 30°N–30°S. In both mapping and XBT cases, spread is higher for 1990–2004. Statistically different trends among mapping methods are found not only in the poorly observed Southern Ocean but also in the well-observed northwest Atlantic. Our results cannot determine the best mapping or bias adjustment schemes, but they identify where important sensitivities exist, and thus where further understanding will help to refine OHCA estimates. These results highlight the need for further coordinated OHCA studies to evaluate the performance of existing mapping methods along with comprehensive assessment of uncertainty estimates.

Open access
Wouter Dorigo
,
Stephan Dietrich
,
Filipe Aires
,
Luca Brocca
,
Sarah Carter
,
Jean-François Cretaux
,
David Dunkerley
,
Hiroyuki Enomoto
,
René Forsberg
,
Andreas Güntner
,
Michaela I. Hegglin
,
Rainer Hollmann
,
Dale F. Hurst
,
Johnny A. Johannessen
,
Christian Kummerow
,
Tong Lee
,
Kari Luojus
,
Ulrich Looser
,
Diego G. Miralles
,
Victor Pellet
,
Thomas Recknagel
,
Claudia Ruz Vargas
,
Udo Schneider
,
Philippe Schoeneich
,
Marc Schröder
,
Nigel Tapper
,
Valery Vuglinsky
,
Wolfgang Wagner
,
Lisan Yu
,
Luca Zappa
,
Michael Zemp
, and
Valentin Aich

ABSTRACT

Life on Earth vitally depends on the availability of water. Human pressure on freshwater resources is increasing, as is human exposure to weather-related extremes (droughts, storms, floods) caused by climate change. Understanding these changes is pivotal for developing mitigation and adaptation strategies. The Global Climate Observing System (GCOS) defines a suite of essential climate variables (ECVs), many related to the water cycle, required to systematically monitor Earth’s climate system. Since long-term observations of these ECVs are derived from different observation techniques, platforms, instruments, and retrieval algorithms, they often lack the accuracy, completeness, and resolution, to consistently characterize water cycle variability at multiple spatial and temporal scales. Here, we review the capability of ground-based and remotely sensed observations of water cycle ECVs to consistently observe the hydrological cycle. We evaluate the relevant land, atmosphere, and ocean water storages and the fluxes between them, including anthropogenic water use. Particularly, we assess how well they close on multiple temporal and spatial scales. On this basis, we discuss gaps in observation systems and formulate guidelines for future water cycle observation strategies. We conclude that, while long-term water cycle monitoring has greatly advanced in the past, many observational gaps still need to be overcome to close the water budget and enable a comprehensive and consistent assessment across scales. Trends in water cycle components can only be observed with great uncertainty, mainly due to insufficient length and homogeneity. An advanced closure of the water cycle requires improved model–data synthesis capabilities, particularly at regional to local scales.

Full access
Seiji Kato
,
Norman G. Loeb
,
John T. Fasullo
,
Kevin E. Trenberth
,
Peter H. Lauritzen
,
Fred G. Rose
,
David A. Rutan
, and
Masaki Satoh

Abstract

Effects of water mass imbalance and hydrometeor transport on the enthalpy flux and water phase on diabatic heating rate in computing the regional energy and water budget of the atmosphere over ocean are investigated. Equations of energy and water budget of the atmospheric column that explicitly consider the velocity of liquid and ice cloud particles, and rain and snow are formulated by separating water variables from dry air. Differences of energy budget equations formulated in this study from those used in earlier studies are that 1) diabatic heating rate depends on water phase, 2) diabatic heating due to net condensation of nonprecipitating hydrometeors is included, and 3) hydrometeors can be advected with a different velocity from the dry-air velocity. Convergence of water vapor associated with phase change and horizontal transport of hydrometeors is to increase diabatic heating in the atmospheric column where hydrometeors are formed and exported and to reduce energy where hydrometeors are imported and evaporated. The process can improve the regional energy and water mass balance when energy data products are integrated. Effects of enthalpy transport associated with water mass transport through the surface are cooling to the atmosphere and warming to the ocean when the enthalpy is averaged over the global ocean. There is no net effect to the atmosphere and ocean columns combined. While precipitation phase changes the regional diabatic heating rate up to 15 W m−2, the dependence of the global mean value on the temperature threshold of melting snow to form rain is less than 1 W m−2.

Open access
Goodwin Gibbins
and
Joanna D. Haigh

Abstract

A recent paper by Kato and Rose reports a negative correlation between the annual mean entropy production rate of the climate and the absorption of solar radiation in the CERES SYN1deg dataset, using the simplifying assumption that the system is steady in time. It is shown here, however, that when the nonsteady interannual storage of entropy is accounted for, the dataset instead implies a positive correlation; that is, global entropy production rates increase with solar absorption. Furthermore, this increase is consistent with the response demonstrated by an energy balance model and a radiative–convective model. To motivate this updated analysis, a detailed discussion of the conceptual relationship between entropy production, entropy storage, and entropy flows is provided. The storage-corrected estimate for the mean global rate of entropy production in the CERES dataset from all irreversible transfer processes is 81.9 mW m−2 K−1 and from only nonradiative processes is 55.2 mW m−2 K−1 (observations from March 2000 to February 2018).

Open access
Jan D. Zika
,
Jonathan M. Gregory
,
Elaine L. McDonagh
,
Alice Marzocchi
, and
Louis Clément

Abstract

Over 90% of the buildup of additional heat in the Earth system over recent decades is contained in the ocean. Since 2006, new observational programs have revealed heterogeneous patterns of ocean heat content change. It is unclear how much of this heterogeneity is due to heat being added to and mixed within the ocean leading to material changes in water mass properties or is due to changes in circulation that redistribute existing water masses. Here we present a novel diagnosis of the “material” and “redistributed” contributions to regional heat content change between 2006 and 2017 that is based on a new “minimum transformation method” informed by both water mass transformation and optimal transportation theory. We show that material warming has large spatial coherence. The material change tends to be smaller than the redistributed change at any geographical location; however, it sums globally to the net warming of the ocean, whereas the redistributed component sums, by design, to zero. Material warming is robust over the time period of this analysis, whereas the redistributed signal only emerges from the variability in a few regions. In the North Atlantic Ocean, water mass changes indicate substantial material warming while redistribution cools the subpolar region as a result of a slowdown in the meridional overturning circulation. Warming in the Southern Ocean is explained by material warming and by anomalous southward heat transport of 118 ± 50 TW through redistribution. Our results suggest that near-term projections of ocean heat content change and therefore sea level change will hinge on understanding and predicting changes in ocean redistribution.

Open access
Seiji Kato
and
Fred G. Rose

Abstract

This reply addresses a comment on the study by Kato and Rose (herein referred to as KR2020). The comment raises four points of criticism. These are 1) on notations used, 2) on a steady-state assumption made, 3) on the result of entropy production change with Earth’s albedo, and 4) disputing the statement that a simple energy balance model cannot produce absorption temperature change with Earth’s albedo. We concur on points 2 and 3 raised by the comment and recognize the significance of entropy storage due to ocean heating in the analysis of how entropy production changes with the shortwave absorptivity of Earth. Once entropy storage is considered, the results of KR2020 indicate that the increase of entropy production rate by irreversible processes, including by radiative processes, is smaller than the increase of entropy storage when absorptivity is increased. This is a manifestation of the primary contribution of positive top-of-atmosphere net irradiances (i.e., energy input to Earth) to heating the ocean and is consistent with an energy budget perspective. Once entropy storage is separated, the entropy production by irreversible processes increases with the shortwave absorptivity.

Open access
Damien Irving
,
Will Hobbs
,
John Church
, and
Jan Zika

Abstract

Coupled climate models are prone to “drift” (long-term unforced trends in state variables) due to incomplete spinup and nonclosure of the global mass and energy budgets. Here we assess model drift and the associated conservation of energy, mass, and salt in CMIP6 and CMIP5 models. For most models, drift in globally integrated ocean mass and heat content represents a small but nonnegligible fraction of recent historical trends, while drift in atmospheric water vapor is negligible. Model drift tends to be much larger in time-integrated ocean heat and freshwater flux, net top-of-the-atmosphere radiation (netTOA) and moisture flux into the atmosphere (evaporation minus precipitation), indicating a substantial leakage of mass and energy in the simulated climate system. Most models are able to achieve approximate energy budget closure after drift is removed, but ocean mass budget closure eludes a number of models even after dedrifting and none achieve closure of the atmospheric moisture budget. The magnitude of the drift in the CMIP6 ensemble represents an improvement over CMIP5 in some cases (salinity and time-integrated netTOA) but is worse (time-integrated ocean freshwater and atmospheric moisture fluxes) or little changed (ocean heat content, ocean mass, and time-integrated ocean heat flux) for others, while closure of the ocean mass and energy budgets after drift removal has improved.

Open access

Observational Constraint on Greenhouse Gas and Aerosol Contributions to Global Ocean Heat Content Changes

Elodie Charles
,
Benoit Meyssignac
, and
Aurélien Ribes

Abstract

Observations and climate models are combined to identify an anthropogenic warming signature in the upper ocean heat content (OHC) changes since 1971. We apply a new detection and attribution analysis developed by Ribes et al. that uses a symmetric treatment of the magnitude and the pattern of the climate response to each radiative forcing. A first estimate of the OHC response to natural, anthropogenic, greenhouse gas, and other forcings is derived from a large ensemble of CMIP5 simulations. Observational datasets from historical reconstructions are then used to constrain this estimate. A spatiotemporal observational mask is applied to compare simulations with actual observations and to overcome reconstruction biases. Results on the 0–700-m layer from 1971 to 2005 show that the global OHC would have increased since 1971 by 2.12 ± 0.21 × 107 J m−2 yr−1 in response to GHG emissions alone. But this has been compensated for by other anthropogenic influences (mainly aerosol), which induced an OHC decrease of 0.84 ± 0.18 × 107 J m−2 yr−1. The natural forcing has induced a slight global OHC decrease since 1971 of 0.13 ± 0.09 × 107 J m−2 yr−1. Compared to previous studies we have separated the effect of the GHG forcing from the effect of the other anthropogenic forcing on OHC changes. This has been possible by using a new detection and attribution (D&A) method and by analyzing simultaneously the global OHC trends over 1957–80 and over 1971–2005. This bivariate method takes advantage of the different time variation of the GHG forcing and the aerosol forcing since 1957 to separate both effects and reduce the uncertainty in their estimates.

Open access
Yang Liu
,
Jisk Attema
, and
Wilco Hazeleger

Abstract

Interactions between the atmosphere and ocean play a crucial role in redistributing energy, thereby maintaining the energy balance of the climate system. Here, we examine the compensation between the atmosphere and ocean’s heat transport variations. Motivated by previous studies with mostly numerical climate models, this so-called Bjerknes compensation is studied using reanalysis datasets. We find that atmospheric energy transport (AMET) and oceanic energy transport (OMET) variability generally agree well among the reanalysis datasets. With multiple reanalysis products, we show that Bjerknes compensation is present at almost all latitudes from 40° to 70°N in the Northern Hemisphere from interannual to decadal time scales. The compensation rates peak at different latitudes across different time scales, but they are always located in the subtropical and subpolar regions. Unlike some experiments with numerical climate models, which attribute the compensation to the variation of transient eddy transports in response to the changes of OMET at multidecadal time scales, we find that the response of mean flow to the OMET variability leads to the Bjerknes compensation, and thus the shift of the Ferrel cell at midlatitudes at decadal time scales in winter. This cell itself is driven by the eddy momentum flux. The oceanic response to AMET variations is primarily wind driven. In summer, there is hardly any compensation and the proposed mechanism is not applicable. Given the short historical records, we cannot determine whether the ocean drives the atmospheric variations or the reverse.

Open access