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Measuring Earth's energy imbalance

Posted on 20 September 2009 by John Cook

When the Earth is in energy imbalance, with more energy coming in than radiating back out into space, we experience global warming. How do we know if there's an energy imbalance? This can be determined empirically in two ways. Firstly, by using satellites to directly measure the difference between incoming energy from the sun and outgoing radiation from the earth. Secondly, by adding up the energy content of the atmosphere and ocean over time. The newly published paper An observationally based energy balance for the Earth since 1950 (Murphy 2009) does both.

Adding up the Earth's energy content

To calculate the Earth's total heat content, the authors took data of ocean heat content from the upper 700 metres. They included heat content from deeper waters down to 3000 metres depth. This is not insignificant  - the heat increase in waters deeper than 700 metres is around 40% of the heat increase in waters from 0 to 700 metres. They computed atmospheric heat content using the surface temperature record and the heat capacity of the troposphere. Land and ice heat content were also thrown into the mix.

A time history of the energy going to heat the Earth requires differentiating the heat content. As the heat content data was too noisy for single year differences, successive linear fits were performed to running 8-year segments of data. Eight years was chosen as it's the longest period that still separates the dips due to the El Chichon and Mt. Pinatubo volcanic eruptions. The resultant energy imbalance time series is seen in Figure 1:

Figure 1: Time history of energy flow into Earth's climate, calculated from the derivative of the Earth's heat content using Domingues 2008 for upper ocean heat (Murphy et al. 2009).

We see that since the mid 1970's, the planet has been in positive energy imbalance. This is consistent with satellite measurements which also find more energy coming in than radiating back into space. This energy imbalance is what is causing global warming.

Empirically calculating climate sensitivity

Climate sensitivity is an expression of how much global temperature changes for a given radiative forcing. The general consensus of peer reviewed estimates of climate sensitivity (both modelled and empirically determined) tend to cluster around a global warming of 3 ± 1°C for doubled CO2.

As the ERBE and CERES satellites measure the net energy imbalance, this data can be combined with temperature records to place constraints on climate sensitivity. Because the ERBE satellite record covers only 15 years, it doesn't encompass slower feedback processes such as receding Arctic sea ice. Hence the data provides only a weak upper bound of climate sensitivity (a maximum of around 10°C warming for doubled CO2). However, the analysis rules out climate sensitivities lower than 2°C. This finding is consistent with the general consensus estimate of climate sensitivity (in addition, the author Dan Murphy informs me he's currently doing follow-up work to calculate a more precise lower bound).

Cumulative energy budget

Possibly the most interesting section of the paper (at least to me) is analysing the various contributors to the energy imbalance and where the energy is going. Figure 2 shows the sum of positive, long-term climate forcings.

Figure 2: Cumulative energy budget for the Earth since 1950, showing mostly positive and mostly long-lived forcing agents from 1950 through 2004.

To close the energy budget, the authors also calculated how the positive forcings have been balanced by various negative forcings, as seen in Figure 3. Note that outgoing radiation is on the increase - another indication that the planet is indeed warming and consequently radiating more energy back into space.

Figure 3: Cumulative negative forcings such as stratospheric aerosols, direct and indirect aerosol forcing, increased outgoing radiation from a warming Earth, and the amount remaining to heat the Earth.

Of particular note is comparing the heat capacities of the land and atmosphere to the heat capacity of the ocean. The land and atmosphere contribution is not even visible in Figure 3 so here is a magnified look at the bottom corner of the graph:

Figure 4: cumulative energy storage of the ocean compared to land + atmosphere. 

It's worth remembering that global warming occurs over the entire globe - land, atmosphere and oceans. Consequently, to focus on one very small piece of the puzzle (eg - surface temperatures) while ignoring the larger picture can lead to misguided conclusions.

Internal variability is apparent in the surface temperature record - a measure of atmospheric heat content. This is no surprise from an energy budget point of view. Small changes in heat transfer into or from the ocean into the atmosphere can create significant changes in surface temperature. But when you look at the entire planet's heat content, a clearer picture emerges. The planet is steadily accumulating heat due to its energy imbalance.

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Comments 51 to 58 out of 58:

  1. There seems to be some confusion over the issue of heat transfer outward versus better insulation though GH gases. 1) Better insulation will reduce the IR emissions at the CURRENT TEMPERATURE. 2) Therefore the temperature will rise INSIDE of the insulating layer until through rising heat transfer though the atmosphere once again a radiative balance is reached where the outgoing energy is equal to the incoming energy, BUT AT A NEW HIGHER TEMPERATURE. 3) The the meanwhile, while the insulation value of our atmosphere so to speak is getting better and better due to rising GH gas concentrations, an imbalance between incoming and outgoing radiation is maintained. 4) This imbalance, or difference between incoming and outgoing radiation is the energy flow into the Earths long term heat stores such as the Oceans and the melt energy of ice etc. 5) In the end we will live in a warmer world which has also stored vast amounts of heat in raised ocean temperatures and molten ice. Thus even if we subsequently open the windows so to speak and lower GH gas concentrations, it will take a long time for all this excess heat to leave us again. The Earth climate is like a gigant pendulum. We have set in motion now at our peril.
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  2. I'm not sure where the right place to ask this is, so please redirect me if I'm in the wrong place. Does anyone know where I can track down the Top-Of-Atmosphere net energy/radiation data from CERES and prior to that ERBE? I've been looking and looking for an easy referencable observed energy balance graph covering the satellite era and can't seem to find them. I can find summaries of the overall trend for CERES, and pieces of ERBE from sometime in the 80's, but nothing by way of a simple graph. Surely with one of the express purposes of these satellites being to measure incoming and outgoing radiation somebody has put together a net radiation graphic of some form already somewhere? If I could be pointed in the right direction it'd be greatly appreciated.

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  3. Chpter 2 of IPCC AR5 discusses the measurement in 2.3.1. You might like to start with the references from there. I understand there are some difficulties with accuracy in the raw data though making it hard to get a precise measure of magnitude. ARGO data may be a more accurate way to get TOA imbalance.

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  4. bcglorfindel @52.

    I think you'll find the problem with creating a global data series using both ERBE & CERES mainly boils down to calibration issues. There is one graph that I can recall that does stitch the two together, fig 2e in Allan et al (2014). The green trace WFOV uses ERBE data.

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  5. @scaddenp, thanks I had started there actually and was only able to find stuff on Ceres from 2000 onwards.

    @MA Rodger, thanks for that, pretty much exactly what I'd been looking for. The articles I had found so far already had me suspecting your observation about troubles combining the series for instrumentation/calibration reasons. Even just having the two separate trends as you provided though is just what I was trying to learn. I'm still reasing through, but is it generally true then that the satellite observed energy imbalance has been largely without a trend at the inter-annual/decade level? Sure doesn't appear to be trending much in in the ERBE set, and the Ceres post 2000 data is declared unlikely to have a trend by the IPCC. I find that result counter intuitive though as CO2 concentration over that same time has verg steadily been increasing...

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  6. The very first sentence of the post is wrong: "When the Earth is in energy imbalance, with more energy coming in than radiating back out into space, we experience global warming."

    Earth is not the moon. On Earth, life transforms incoming solar energy into biochemical energy. So there has to be a radiative imbalance in order for life to develop and sustain itself.

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    Moderator Response:

    [DB] Thank you for taking the time to share with us.  Skeptical Science is a user forum wherein the science of climate change can be discussed from the standpoint of the science itself.  Ideology and politics get checked at the keyboard.  When making assertions running counter to accepted science, it is incumbent upon the user (you) to furnish citations to credible sources that support your contentions.

    Please take the time to review the Comments Policy and ensure future comments are in full compliance with it.  Thanks for your understanding and compliance in this matter.

  7. John Cook

    Mesuring Earth's energy imbalance, 20 September 2009

    This paper on Earth's energy balance is very interesting from a physicist's perspective.  The paper and summary are from 2009, and the most recent comment is from 2015.  Is there an up-to-date version of the Murphy et al paper?

    Thanks in advance

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  8. Richieb - you could try this one. Trenberth and Fusillo 2014.

    And same authors 2016

    The Argo system is helping constrain the energy imbalance.

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