According to a new report published in Nature magazine by a team of international scientists, the global ocean’s upper layer is definitely warming up. Sophie Perri talks to CSIRO Climate modeller Tony Hirst about climate change, and the state of our oceans.

An example of coral bleaching. Image: Matt Kieffer.

Tony: Over the periods of good instrumentation we are getting warming of about point four degrees near the surface, this is over about the past 18 years or so and about point one degree warming at about 500 metres or so, roughly speaking. This may not sound like a huge amount but it’s expected to be ongoing and is contributing as I say about one millimetre a year to see level rise.

Sophie: Why are the oceans so critical to our understanding of climate change?

Tony: They provide quite a bit of buffering, where they absorb a lot of the heat which is being trapped by the increased greenhouse gasses in the atmosphere and then they go and mix it down to a considerable depth away from the surface and that leaves the surface rather cooler than it otherwise would be. Unfortunately though the heat of the water at depths causes it to expand and as it expands that causes sea level rise to happen, and so sea level rise is occurring we measure at about two to three millimetres a year and about half of that is due to the warming of the sub-surface waters.

Sophie: What does ocean warming suggest about the pattern of climate change? What is this telling us exactly?

Tony: It’s providing us with more evidence that the pattern of climate change is progressing as expected. Which means warming currently, over the surface over-all, of about point two degrees or so per decade is increasing as greenhouse gas levels in the atmosphere increase.

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Sophie: So Tony, what does this mean for the coral reef and fish stock?

Tony: For coral reefs, if you get temperatures above a certain level then there’s increased risk of bleaching of the coral reefs, in which case the coral may become damaged which may well result in damage to the reef. At exactly what temperatures that will occur is still a matter of scientific investigation. But we have had two severe bleaching events in the past decade and we expect more as the upper ocean waters warm up.

Sophie: What will be the impacts of the seas warming on the El Niño la Niña weather pattern?

Tony: That’s an area of very active research and currently the research suggests that we would be more inclined to have more El Niño like conditions going forward… So as to what’s likely to happen next, well that all depends on the future amount of greenhouse gasses in the atmosphere. If the greenhouse gasses stay in the atmosphere for a long time at about similar levels as they are today then we would expect the oceans to keep warming at similar rate and that warming will continue for centuries because the heat will gradually mix down into deeper and deeper ocean, and taker centuries to get down to the deepest oceans. If however, the level of greenhouse emissions continues to increase, that warming of the ocean will intensify and that will cause increased rates of sea level rise which will then be ongoing for centuries after that. Sea level rise is a very long term thing, and once you get it started its hard to stop it. Unless, the only thing that would stop it would be a major reduction in greenhouse gasses in the atmosphere and that’s going to be probably quite hard to achieve.