So many climate models, so little time … A new way of measuring oceantemperatures helps scientists sort the likely from unlikely scenarios ofglobal warming.
We’ve heard that rising temperatures will lead to rising sea levels, but whatmany may not realise is that most of the increase in energy in the climatesystem is occurring in the ocean.
Now a study from UNSW Sydney and CSIRO researchers has shown that a relativelynew ocean temperature measuring program – the Argo system of profiling floats– can help tell us which climate modelling for the 21st century we should bepaying attention to the most.
Professor John Church from UNSW’s Climate Change Research Centre in the Schoolof Biological, Earth and Environmental Sciences says the study published todayin Nature Climate Change is an attempt to narrow the projected range offuture ocean temperature rises to the end of the 21st century using modelsimulations that are most consistent with the Argo’s findings in the years2005 to 2019.
“The models that projected very high absorption of heat by the ocean by 2100also have unrealistically high ocean absorption over the Argo period ofmeasurement,” Prof. Church says.
“Likewise, there are models with lower heat absorption in the future that alsodon’t correspond to the Argo data. So we have effectively used the Argoobservations to say, ‘which of these models best agree with the observationsand therefore constrain projections for the future?’”
Named after the boat which Greek mythological hero Jason travelled on insearch of the golden fleece, the Argo floats are loaded with high-techequipment that measures ocean temperatures to depths of up to 2000 metres.
Each Argo float sends measurements to satellites which then beams the resultsto analysis centres around the world. There are more than 3500 floatsdispersed around the globe, with the bulk of them managed by the US (more than2000) while Australia accounts for the next highest number of floats,numbering 317.
Prof. Church says the Argo floats offer a new level of accuracy in temperaturemeasurement. Not only are the high-tech instruments more reliable than in thepast, but the coverage of the planet’s oceans is so much more thorough.
“Previously we depended on research ships making very high accuracymeasurements, but only in very restricted areas. Or we would get merchantships to drop expendable instruments into the water which gave bettercoverage, but with much less accurate instruments.
“Using these approaches, there were much larger gaps in the Southern Oceanbecause these waters were less trafficked.”
The ubiquity of the Argo floats offers unprecedented real-time monitoring ofocean temperatures that will help oceanographers and climate scientistsconstrain their climate projections based on this higher resolution andaccuracy of data.
The full story
One of the takeaways from the team’s analysis of the Argo data is that landand air temperatures only tell part of the story about the planet’s overallheat absorption. Prof. Church says the apparent stability of temperatures inthe early 21st century did not correspond with the Argo’s recording of seatemperatures in the same period.
“More than 90 per cent of the energy trapped by greenhouse gases is stored inthe ocean and only about 1 per cent in the warming atmosphere,” he says.
“In the first decade of this century, average surface temperature didn’tincrease that much. And it created a lot of room for climate sceptics to say‘what climate change?’. But throughout that period, oceans continued to takeup heat – and this is where the 90 per cent of the energy in the ocean versusthe 1 per cent in the atmosphere becomes critically important in the totalclimate system.
“The climate system was still increasing its overall heat content over thisperiod.”
It turns out that the projections of rising air and sea temperatures by somemodels in a recent set of modelling in the Coupled Model IntercomparisonProject are warming too rapidly.
The models in CMIP5, which was completed from 2010 to 2014, fits in with theArgo observations more closely than CMIP6, which was only recently completed.
“Some of the 28 CMIP6 models we used were more sensitive to greenhouse gasesfor their projections,” Prof. Church says.
“Projections of climate change to the end of the century all take into accountthe effects of greenhouse gas emissions which are already responsible for muchof the increased temperatures we’ve seen in the 20th and early 21st centuries.
“Even if we take strong steps now to limit emissions to the upper bound of theParis Agreement target of 2oC global surface warming, ocean temperatures arestill projected to rise five to nine times the observed warming by 2081-2100,with 8 to 14cm rise in sea levels from the expansion of warmed ocean watersalone.”
With no concerted efforts to rein in emissions, oceans are set to warm by 11to 15 times the warming observed by Argo in 2005-2019, with sea levelsprojected to rise 17 to 26 cm from the expansion of warmed ocean waters alone,and further rises from the addition of water to the ocean from glaciers andice sheets.
Prof. Church says the Argo data has given scientists much more solid figuresto work with when making projections about warming. In fact, the likely rangeof the constrained projections using the Argo data is 17 per cent narrowerthan the IPCC’s Sixth Assessment Report.
“While a longer Argo observational period will allow us to further tighten therange of projected ocean warming and sea level rise, the Argo observations andthe climate model projections already highlight the need to urgently and verysignificantly mitigate our greenhouse gas emissions if we are to avoiddangerous impacts of ocean warming and sea level rise.”
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