Diagnosing Earth's Energy Pathways in the Climate system (DEEP-C)

Lead Research Organisation: NATIONAL OCEANOGRAPHY CENTRE
Department Name: Science and Technology

Abstract

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). Our proposed research seeks 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 will help 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 requires 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 to the proposed research program is to resolve the discrepancy between these energy flows and understand the root causes of the hiatus in the warming of the Earth's surface.

Our proposed project combines the latest, improved satellite measurements of Earth's radiative energy imbalance (reflected sunlight and emitted thermal radiation) with our 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. We will combine 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 is 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, our preliminary analyses lead us 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 is vital for understanding current climate variability and future change over the coming 10 years or more. We consider 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.

We consider that this research is 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.

Planned Impact

This project will investigate the reasons for the apparent slow down in surface warming over the globe since 2000. This apparent slow down is often quoted by groups sceptical about climate change as evidence that anthropogenic climate change is not happening and that the models used to make projections of climate change are flawed, thus casting doubt on the credibility of climate science. It is important that we engage the general public in this project and present an objective picture of the scientific understanding of climate change as well as educating the public in the way that scientific research is carried out. This research is particularly valuable since it deals with the fundamental driver of climate change, Earth's energy balance, and how this energy is redistributed in the oceans, in addition to utilising exciting new satellite instruments, automated ocean profiling floats and state-of-the-art climate simulations.

These developments will also be of substantial interest to the UK Department for Energy and Climate Change (DECC) who need briefings on the latest science to be more effective in international negotiations on climate change (through the United Nations Framework Convention on Climate Change).

This project will not provide projections of future climate directly, but the results will help the wider scientific community to improve predictions of climate and climatic impacts - such as regional temperature, precipitation and sea-level - for the coming decades (e.g., through Met Office PI on the project). Therefore, this project will deliver information that is highly relevant to the field of climate change adaptation. Results from the project will be particularly valuable to the UK Climate Change Risk Assessment UK, Department for Environment, Food and Rural Affairs (DEFRA) and the Committee on Climate Change (CCC).

Comparisons between the satellite and ocean datasets will be of interest to the NASA project partners in terms of assessing quality of data products, allowing efficient exchange of ideas and prioritising datasets for the climate community.

Publications

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King B (2017) Corrigendum in Journal of Climate

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Garry F (2019) Model-Derived Uncertainties in Deep Ocean Temperature Trends Between 1990 and 2010 in Journal of Geophysical Research: Oceans

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Firing Y (2017) Deep temperature variability in Drake Passage in Journal of Geophysical Research: Oceans

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Desbruyères D (2016) Deep and abyssal ocean warming from 35 years of repeat hydrography in Geophysical Research Letters

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Desbruyères D (2020) Importance of Boundary Processes for Heat Uptake in the Subpolar North Atlantic in Journal of Geophysical Research: Oceans

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Desbruyères D (2016) Observational Advances in Estimates of Oceanic Heating in Current Climate Change Reports

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Clément L (2020) Signature of Ocean Warming at the Mixed Layer Base in Geophysical Research Letters