Transcript

Glen Paul: G'day and welcome to CSIROpod, I'm Glen Paul.

You're probably familiar with the East Australian current, if from nothing more than the 2003 animated film Finding Nemo, where the current is portrayed as a super-highway for marine life to travel down the East Coast of Australia. In a recent paper published in the journal Nature Climate Change scientists have found that riding the current could now take Nemo further south than he might like to travel. Oceanographers have identified a series of ocean hotspots around the world generated by strengthening wind systems that have driven Oceanic currents, including the East Australian current, pole wards beyond their known boundaries.

The study, which involved an international scientist team, found that the hotspots are formed alongside ocean currents that wash the East Coast of the major continents and their warming is far exceeding the rate of Average Ocean surface warming. Co-authors of the paper, CSIRO's Dr Wen Ju Cai and visiting scientist Dr Mike McPhadden from the United States National Oceanic and Atmospheric Administration, worked together on the project and join me on the phone.

Firstly, Wen Ju, what actually prompted an international effort?

Cai: The initiation of this project was a study that we had done years ago, looking at the Tasman Sea warming. We have shown that over the past 50 years the Tasman Sea warming rate is actually faster than the global oceanic average, two to three times faster and it could link to the climate change ocean factors such as ozone depletion and perhaps also greenhouse warming but we were not sure. We'd like to see, if this is caused by climate change then we should have other evidence in other oceanic current systems.

Glen Paul: Right. So with evidence in mind how do you measure this accurately? Do you, for example, have reliable records to fall back on to make comparisons?

McPhadden: In fact, there are only a few of these ocean currents, Wen Ju mentioned the East Australian current, and we've seen this warming signal now in other currents around the world, for example, the Gulf Stream off the East Coast of the US, the Kuroshio current off the East Coast of Asia. These currents are very strong so it's difficult to make direct measurements of the current flows in these regions for sustained periods of time.

What we relied on for this study was a long history of occasional measurements plus a computer model that represented the dynamics of these currents and we basically blended the two, the sparse data and the computer model code for how these currents should respond to wind forcing. From that analysis we derived indications that the currents have undergone a systematic change over the last 50 to 100 years.

Glen Paul: Right, and these are as a result of these strengthening wind systems. Where does the cause of that lay?

McPhadden: The fingerprint of climate change is on this because we know from other studies, independent studies that the tropical zone is expanding, the trade wind system is moving northward and that links into all the other wind and current systems of the oceans. The fact that the global tropics are expanding, the fact that we see this warming in all the mid latitude currents like the East Australian current, Gulf Stream and Kuroshio current off of East Asia tell us that the likely culprit here is climate change.

Glen Paul: I see. Wen Ju, how sure are you that this is the result of climate change and not just natural changes that would otherwise occur in oceans over time?

Cai: Natural changes do occur but we have a lot of other evidence and, as Mike just pointed out, over Southern Australia as an example, this study pointed out that the current is moving forward but we also have the weather system that has been moving forward. Southern Australia, as you know, has just been through a long drought and that is associated with weather systems moving poleward.

In Perth, for example, that kind of drought condition has been persisting for more than 35 years. When you put all this evidence together we could say that there is at least a part that is driven by climate change.

Glen Paul: OK. Aside from these local ecological implications then what other impacts does a change in these currents bring?

Cai: There are some ecological impacts. We had a bit of observation showing that the boundaries of marine bio-diversity have been changing, the biotas has been changing and a lot of species have been moving further poleward, for example, the New South Wales Sea Urchin has been seen in Tasmania and while they are in Tasmania they are eating those giant kelp. Now kelp is a shelter for a lot of other species so it has quite a bit of knock on effect.

They also have implications for aquaculture as we know that Tasmania is a farm ground for salmon and farming salmon need a good temperature range and so the food that is produced for salmon also need to have a good temperature range. We could also quite a lot of other introduced species, in Victoria waters, for example, is the shore crab. We are now seeing them further and further south in Tasmanian waters. There is quite a bit of impact on ecosystems.

Glen Paul: Right. So, bringing all this together, what's the upshot?

Will this situation worsen and, if so, how quickly do you think?

Mcphadden: There is a trend for these currents, they appear to be moving forward over the past hundred years by a couple of hundred kilometres and in some cases they appear to be strengthening. These changes are related to changes in the wind field that we suspect strongly are related to climate change. The likelihood is, if we're to take these pieces of evidence and project into the future that these kinds of trends will continue. What the impacts are … are a little bit uncertain because there are so many feedbacks in the climate system that may either amplify or dampen some of these trends.

There are issues we should be concerned about because, for example, if you take the Gulf Stream which flows off the East Coast of the US then it carries a hundred times more water from the Equatorial Region to the pole, a hundred times more water in this current than all the Worlds Rivers combined. It's a huge, massive flow and that's just one of these current systems. They are transporting a tremendous amount of heat and water pole ward and the reason, for example, that Western Europe has a milder winter than the Eastern US is in part because of this transport of warm water from the tropics to the pole region and so if these currents are shifting northward it implies changes in the climates of Europe, North America, Australia and other regions of the globe.

It's a trend that we've noted, we're very interested in following up on this study about what some of the more detailed climate implications may be and already, as Wen Ju has mentioned, we can see the implications for marine ecosystems in the East Australia current region.

Glen Paul: So you as a representative of the Northern Hemisphere, what's the most important message then for policy makers to come from this research?

McPhadden: Well, this research is pointing to another manifestation of human impact on climate, one of many manifestations. It's yet another reason why we should be concerned about how our activities are affecting the world in which we live. From that point of view, as with all climate changes, there will be winners and losers depending on what the impacts are but it's real, it's happening and it's something that we should be concerned about.

Glen Paul: Wen Ju, your thoughts?

Cai: Yes, I would echo Mike's concern. A lot of implications we don't quite understand of yet so, for example, you have solid warming and that warming covers a great depth from surface to a thousand metres. That is going to do to our sea level rise locally faster than the global average and that would have its own set of impacts as well. I think we need to take this kind of change seriously.

Glen Paul: Absolutely. Well, if they ever decide to do a second Finding Nemo movie there are some serious plot lines there, that's for sure. Thank you both for talking to me about it today.

McPhadden: Thank you, Glen.

Cai: Thank you, Glen.

Glen Paul: Drs Wen Ju Cai and Mike McPhadden. For more information you can find us online at www.csiro.au, you can like us on Facebook or follow us o