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Two trillion light bulbs burning in our oceans

By Virginia Tressider

February 24, 2015

A network of floating data monitors across the world’s oceans has revealed a noticeable rise in temperature, particularly around Northern Australia, in as little as eight years – something that usually takes a lot longer to be recorded.

The global Argo array is a network of 3750 floats, jointly funded by the US National Ocean and Atmospheric Administration, Australia’s Integrated Marine Observing System, the National Institute of Water and Atmospheric Research in New Zealand, and other international agencies. It enables scientists to observe the basic physical state of all world oceans simultaneously.

CAST OFF: BWR2015 deployment. Image by J. JCOMMOPS
CAST OFF: BWR2015 deployment. Image by J. JCOMMOPS

So what have they found? A report in Nature Climate Change, ‘Unabated planetary warming and its ocean structure since 2006’ reveals the results.

The top 2000m of the world’s oceans warmed at a rate of 0.4 to 0.6 watts per square metre (W/m 2) between 2006 and 2013. The translates to a warming of roughly 0.005°C a year in the top 500m metres of ocean and 0.002°C a year at depths between 500 and 2000m. This doesn’t sound like much, but expressed in everyday terms, it’s equivalent of adding the heat of two trillion continuously burning 100-watt light bulbs to the world’s oceans.

Lead study author, Dean Roemmich, said the rate of ocean heat gain is not unusual.

“What is new is that the rate and patterns of ocean heat gain are revealed over a period as short as eight years, thanks to the Argo array, that the warming signal is shown to extend to 2000m and deeper, and that it is occurring predominantly in the Southern Hemisphere ocean south of 20°S’. Northern Australia is at this latitude.”

He says the study puts a widely reported ‘hiatus’ in global surface air temperatures since 1998 into context. It illustrates that the pause in warming of the sea surface and the lower atmosphere is not representative of the steady, continuing heat gain by the climate system. Scientists measure that heat gain in terms of increasing temperature averaged over the water column. What appears to be happening is a separation into upper layers with high variability over the years, and large uncertainty in the trend, and deeper steadier warming with lower uncertainty.

OH BUOY: Floats ready for deployment at CSIRO Hobart. Image by J. JCOMMOPS
OH BUOY: Floats ready for deployment at CSIRO Hobart. Image by J. JCOMMOPS

Co-author Susan Wijffels from CSIRO explains, “When we measure globally and deep enough, we see a steady rise in the earth’s heat content, consistent with the expected greenhouse gas-driven imbalance in our planet’s radiation budget”.

The data come from battery-powered autonomous floats that spend most of their life drifting at depth. In a nifty bit of engineering, they remain stable because they are neutrally buoyant at the ‘parking depth’ pressure. Most floats descend to a target depth of 1000m to drift, then drop to 2000m to start the temperature and salinity profile. At regular intervals – usually 10 days – the floats rise to the surface, over about six hours, taking about 200 pressure, temperature, and salinity measurements as they go. Satellites or GPS determine the floats’ positions when they surface, and the floats transmit their data to the satellites. The floats then sink to drift again until the cycle is repeated.

UP PERISCOPE: Argo float cycle

Previously, scientists had to rely on ships for periodic readings to inform opinion about temperatures and conditions in our seas. The equipment available meant readings could only be taken at 700m or less. Now the organisations behind the Argo technologies are looking to develop a new generation of deep ocean floats that can record fundamental data at depths of as much as 6000m.

“The ability to consistently detect a global ocean heat gain of 0.4 to 0.6 W/m² over this short period is historically unprecedented”, the researchers note in their report.

“Homogeneous global coverage, high data quality and temporal resolution of seasonal and interannual fluctuations are key attributes for enabling the Argo-only analyses that underpin this result.”

All Argo data is available online and free to use.

4 comments on “Two trillion light bulbs burning in our oceans

  1. Robert White says:

    The increase in ocean heat load of 0.4-0.6 watts/square metre between 2006 and 2013 is very similar to the increase in the Sun’s irradiance between 2006 and 2013 (see Climate Change Evidence and Causes. The Royal Society and National Academy of Sciences 2014). Is this comparable to the burning of 2 trillion 100 watt light bulbs? What puzzles me is why there should be disequilibrium between the oceans (which are absorbing heat and showing a temperature rise) and the lower atmosphere, given that, as your article says, the hiatus (approximate constancy in mean global surface temperature, allowing for inter-annual variation) that has been observed since 1998. Any advice on this?

    1. Chris McKay says:

      Thanks for your comment Robert. From our scientists…

      The observed reduction in the rate of surface warming over the period 1998 to 2012, as compared to the period 1951 to 2012, is due in roughly equal measure to a reduced trend in net external forcing (associated with volcanic eruptions and a downward phase of the 11-year solar cycle) and a cooling contribution from internal variability (associated with a redistribution of heat within the ocean). Remember that this reduction refers to the lower rate of air temperature increase; global air temperatures themselves are not decreasing, and there has been no slow-down in the rate of global land and ocean temperature increase.

      Regarding the contribution of solar irradiance to warming, this is small compared to the contribution of greenhouse gases, as can be seen in this diagram from the latest IPCC report: http://www.ipcc.ch/report/graphics/images/Assessment%20Reports/AR5%20-%20WG1/SPM/FigSPM-05.jpg

      1. Robert White says:

        I am sceptical of the IPCC reports because that body is too politically motivated. However, the recent Royal Society-National Academy of Sciences document, to which I referred, strikes me as being more balanced in its explanations and projections and less politically influenced. Surely you don’t mean ‘the contribution of solar irradiance to warming …. is small compared to the contribution of greenhouse gases’. Solar irradiance is the primary source of energy coming into the earth and its atmosphere. Perhaps you mean changes in solar irradiance are small etc. However, you do acknowledge a downward trend in solar irradiance between 1998 and 2012. In fact, if one looks at the RS-NAS document (figure 2) one sees that the decrease in solar irradiance, amounting to approximately 1.1 watts per metre square, occurred between about 2002 and 2008-9. This would, to use your analogy, correspond to switching off several trillion 100 watt light bulbs! However, from 2009 to the present there is an increase in solar irradiance of approximately 0.5 watts per metre squared, which is what I referred to previously in referring to ocean warming. You refer to the forcing effect of greenhouse gases. Certainly the main culprit, CO2, is going up inexorably and its effect is amplified by the link with water vapour, which is a potent GHG. However, what is not made clear in these discussions is what is the quantitative linkage between CO2 and water vapour in the models that gives such a big amplification of the change in CO2 concentration and its effect on warming? Can this be explained simply?

        1. Chris McKay says:

          Many of our scientists contributed a lot of their time to the research assessment efforts of the IPCC, and we would suggest it as a good source of summary and assessment of peer-reviewed research.

          A lot has been written about greenhouse gases and feedbacks in the water cycle, you might try:
          http://www.skepticalscience.com/water-vapor-greenhouse-gas.htm
          http://www.skepticalscience.com/humidity-global-warming.htm
          http://www.skepticalscience.com/positive-feedback-runaway-warming.htm

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