The DEEP-C project was funded by the Natural Environment Research Council (NERC) with the grant reference - NE/K004387/1 - led by Professor Richard Allen (University of Reading) and Dr Elaine McDonagh (National Oceanographic Centre).

A global warming trend since the 1970s has slowed over the most recent 10-15 years despite the continuing build up of carbon dioxide in the atmosphere (due primarily to the burning of fossil fuels). This research sought to understand the reasons for this "hiatus" in global warming and in particular the roles of the ocean and atmosphere in contributing to this hiatus through movement of energy around the climate system. This project has helped us monitor changes in climate and understand the processes that are important in enabling us to predict climate change more accurately over the coming decades.

Warming of the planet is caused by a small yet persistent imbalance between the amount of sunlight absorbed by the Earth and the outgoing flow of thermal (infra-red) radiative energy constantly emanating from our planet to space: if more energy is arriving than leaving then the climate heats up. To understand why the heating has apparently slowed required a detailed assessment of the flows of energy arriving from space, how this energy is transported by the atmosphere, taken up by the surface ocean and subducted deep below the sea surface. Previously, scientists had identified a discrepancy between these energy flows, or "missing energy" in the climate. A primary objective of this research program was to resolve the discrepancy between these energy flows and understand the root causes of the hiatus in the warming of the Earth's surface.

The project combined the latest, improved satellite measurements of Earth's radiative energy imbalance (reflected sunlight and emitted thermal radiation) with the best estimates of energy flows in the atmosphere (from reanalysis simulations) and detailed 3-dimensional ocean heating measurements made by thousands of automated floating buoys, to determine the observed flows of energy in the climate system. The project combined these measurements with state-of-the-art depictions of Earth's climate from sophisticated computer simulations to understand the mechanisms by which the build up of energy due to greenhouse gas increases are redistributed into the oceans.

It is plausible that increased amounts of reflective aerosols in the atmosphere (due to human activities or naturally through emissions by volcanic eruptions) may have diminished the heating of the planet. However, preliminary analyses lead to the hypothesis that in fact more heat has been entering the deep ocean rather than heating the planets surface. Getting to the bottom of this question was vital for understanding current climate variability and future change over the coming 10 years or more. The project considered that the research is also important for understanding regional sea level rise (since warmer water occupies a larger volume leading to rising sea level), fluctuations in clouds and whether they magnify or reduce warming tendencies (climate feedbacks) and simulating the ocean circulation and heat uptake, crucial for representing climate change over the coming decades.

This research was only possible by combining the expertise from three institutions (the University of Reading, the National Oceanography Centre Southampton and the Met Office) covering satellite data, reanalyses of the atmosphere and ocean, ocean measurements and numerical computer simulations of the climate system. The current planetary changes are unusual and present a timely opportunity for understanding how our climate system works: to discover the cause of the global warming hiatus and to understand and simulate the mechanisms important in representing climate variability and change over the coming decades.

The primary objective of the project was to quantify Earth's Energy Imbalance, its variability and implications for climate change over the coming decades. The driving goal was to observe, understand and explain the slow rates of surface warming since 2000 using satellite data, atmospheric reanalyses, in situ observations of ocean heat content and climate simulations and to elucidate physical mechanisms operating during "hiatus" decades of slow surface warming rates through the following specific objectives:

1) Combine CERES satellite radiation budget measurements with ERA Interim reanalysis and additional datasets, providing improved 2D estimates of surface heat fluxes entering the ocean surface since 2000
2) Develop and compute global 3D ocean heat and freshwater content and its changes since 2003 using ARGO observations, leading to improved understanding of energy propagation through the climate system
3) Conduct multi-model assessment of simulated variability in ocean heat content, freshwater content and steric sea-level and evaluate the processes fundamental for ocean heat uptake and redistribution
4) Provide accurate estimate of Earth's net radiative energy balance and its variability over the period 2000-2015 by combining results from (1-3) and compare with state-of-the-art CMIP5 climate simulations.
5) Quantify and understand lags between OHC and top of atmosphere radiation involving additional minor energy storage terms in the climate system
6) Characterise spatial signatures and mechanisms of ocean and atmospheric heat re-distribution during the period 2000-2015 using observations and multi-model simulations of variability in ocean heat content, freshwater content and steric sea-level

Meeting these objectives also resulted in:

7) Explain the previously documented discrepancy between energy flows (or"missing energy") in the climate system
8) Generate legacy software and methodologies for monitoring changes in Earth's radiative imbalance

Additional outputs that stemmed from the project were:

9) Computations of ocean-basin scale estimates of energy budget,
10) Identification of key regions of energy subduction into the ocean on interannual-decadal time-scales
11) Explore the potential for using deep hydrographic sections for quantifying changes deeper than 2000m and evaluate their significance.
12) New knowledge for regional sea level, climate feedback and reanalysis/climate modelling communities (see Academic Beneficiaries)