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Earth's Energy Imbalance: a fundamental measure of global climate change

January 2016 - International climate research scientists are calling for better monitoring of Earth's energy imbalance, with improved ocean observations to play a key role

How do we measure the rate of global warming? Traditionally, we tend to think of Earth's global surface temperature as the iconic indicator of climate change. However, in a new perspective piece in Nature Climate Change, scientists argue that there is a more fundamental measure of climate variability and the rate of global change: Earth's energy imbalance. 

Figure 1. Schematic representation of Earth's energy imbalance.

As the article explains, all the energy that enters or leaves the climate system does so radiatively at the top of Earth's atmosphere (Figure 1). Under an equilibrium climate, the solar radiation absorbed by the Earth is balanced by emitted longwave radiation. Increased greenhouse gases reduce the emitted longwave radiation and give rise to Earth's energy imbalance, leading to excess solar energy accumulating in the climate system. This is the most fundamental driver of observed climate change, and the various climate impacts that we are familiar with - such as warmer surface temperatures, sea level rise and loss of land-based ice -  are all symptoms of the energy imbalance. 

Observations and climate model simulations show that the vast majority of the excess energy accumulating in the climate system goes into warming the oceans (Figure 2). So measuring the rate of ocean heating is essential to monitor the rate of global climate change. The ocean also acts as the "memory" in the climate system, with the increased heat content resulting in warmer and moister atmospheric conditions over the oceans and the expectation of more intense rainfall events. Climate models show only a weak relationship between the energy imbalance and global surface temperature change over a decade or so, which implies that the so-called 'slowdown' in warming (also referred to as the 'hiatus' or 'pause') may tell us nothing about the recent rate of global climate change. 
Figure 2. Symptoms of Earth's energy imbalance.

To improve our ability to monitor the energy imbalance we would need to improve various Earth observations and how we process them.  In particular, we would need to build upon the recent Argo array of profiling floats and target the remaining poorly-observed areas of the ocean, including deep ocean (> 2000m), the ice-covered regions and the shallow and coastal seas. This information can be combined with satellite data and other measurements to improve coverage and help us attribute changes in Earth's energy imbalance to anthropogenic and other causes. The international community has come together under the CLIVAR CONCEPT-HEAT project to make progress in this most fundamental area of climate science. 

Lead author of the paper and co-chair of CONCEPT-HEAT, Karina von Schuckmann, summarised the challenges ahead:
"Advancing our capability to monitor the Earth's Energy Imbalance means increasing our knowledge on the status of global climate change - and the global ocean plays a crucial role. A concerted multi-disciplinary and international effort is needed to improve our ability to monitor this fundamental metric defining global warming."

Kevin Trenberth, co-author and co-chair of CONCEPT-HEAT adds:
"Earth's energy imbalance is a measure of how much climate change is underway and it is vital information for future projections of climate.   Yet our ability to track the imbalance and determine its absolute value and changes over time is fraught with difficulties, and different approaches are not yet fully reconciled.  We need a better observing and processing system, and the synthesis of the multiple techniques to obtain a much better climate information system."

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