How will climate change affect life in the oceans? In research to be published in Nature Climate Change* we, among several other authors, show that the answer is likely good and bad.
Our study models how species might move in response to different future climate scenarios. The good news is that overall, thanks to species migrations, most places will end up with greater numbers of species. According to our models, climate change is unlikely to directly cause extinction through warming waters for most species, except for those that can’t move or have very narrow thermal tolerances.
The bad news is that there are a few very special places that will lose species – particularly the spectacular ocean ecosystems of what’s known as the Coral Triangle, the epicentre of global marine biodiversity.
First, the good news
As ocean temperatures increase, marine life will likely move towards the poles - animals and plants will expand their ranges. We can already see this happening. In Australia, tropical species of fish are turning up in northern New South Wales.
We wanted to know how this would affect the overall numbers of animals and plants in the oceans – marine biodiversity – and the distinctive communities they comprise. While many things affect where marine life lives – habitat, competition, salinity – most species are affected fundamentally by temperature.
Using temperature to find out where species might move allowed us to look at an unprecedented number of species - nearly 13,000. These included animals and plants as diverse as fish, corals, jellies, snails, clams, crabs, shrimps and seaweeds.
We looked at two different climate scenarios, business as usual (known as RCP8.5) leading to warming of around 2.5ºC by 2100, and a scenario with medium mitigation (RCP4.5) leading to warming of around 1ºC over the same period.
Our model shows how fast different temperature zones will move and to where, using a measure known as “climate velocity”. This is a good way of predicting where species could move because it traces pathways connected by climate.
We should emphasise that our study shows where species could move. Our projections don’t necessarily mean that they will move, nor that they will successfully establish themselves at the locations where they arrive. That depends on a variety of factors, including their specific habitat requirements and how species interact with each other. But studies of invasive species suggest that species that can move will tend to do so.
Overall we found that biodiversity of the oceans will likely increase at local scales. As a result, we anticipate that marine ecosystems will become more similar. For instance, today on the east Australian coast, the types of species found along the central Queensland coast are quite different from those found in central New South Wales. As sea temperatures warm, we expect those boundaries to gradually break down, leading to what we call a “smearing” of biodiversity.
Bad news for the tropics
There are several theories as to why there are so many species in the tropics, and especially the Coral Triangle. Irrespective, we know that this area supports over 500 species of reef-forming corals, together with a massive diversity of fish, including whale sharks, and six of the seven extant species of sea turtles; it is also visited by many species of whales and dolphins. This concentration of marine biodiversity contributes significantly to livelihoods of the region’s 120 million or so human inhabitants.
Species living in tropical seas already live close to their thermal optimum. As temperatures increase, they will exceed the upper thermal limits of some species. When this happens, some species will adapt, for instance by seeking out micro-refuges, such as small patches of cool water caused by upwelling, or they might resort to living in deeper waters, if the water is clear enough.
But in the long term, most species will need to move. The reason we expect marine biodiversity to decrease in the tropics with warming is that there is no place warmer to act as a source of new species to replace those species moving out.
More than 5,000 of the 13,000 species we looked at in our study are found in the coral triangle. According to our projections, approximately 500 to 1,000 of these species will leave the region thanks to warming waters under RCP4.5 and RCP8.5, respectively.
What can we do?
Our modelling shows that the loss of marine life is strongly related to how much we mitigate climate change.
Even if we take only intermediate levels of action (under scenario RCP4.5), we can minimise the damage. But we can’t eliminate it entirely: under the emission-stabilisation RCP4.5 scenario we anticipate that the Coral Triangle will lose roughly half as many species as under the business-as-usual RCP8.5 scenario.
We can also look at how we manage the world’s oceans. Some regions, such as the northeast Atlantic and eastern Mediterranean, have seen greater impacts from people than others, and some of these overlap with regions likely to be affected by climate change.
Where there is overlap, we can look at alleviating the damage caused by people, such as pollution of coastal waters, or minimising the pressure on key species, for example by reducing fishing pressure on them.
In other areas, such as the poles, there is low human impact, but we project substantial changes in biodiversity. From a conservation perspective, we want representative sections of these areas to remain free from additional human pressure, for instance by using regulation to control future development.
And because climate change doesn’t respect national boundaries, all of these efforts will require international cooperation.
Only in that way will we ensure the seas remain rich and healthy in the future.
We acknowledge the contributions of all co-authors: Jorge Garcia Molinos, Benjamin S. Halpern, David S. Schoeman, Christopher J. Brown, Wolfgang Kiessling, Pippa J. Moore, John M. Pandolfi, Elvira S. Poloczanska, Anthony J. Richardson and Michael T. Burrows
*Update August 25: the paper on which this article is based has not yet been published. The article will be updated when the link is available.