Warming Oceans are Changing Marine Habitats. Study Explores Impact on Thousands of Species

Andreas Schwarz Meyer, University of Cape Town and Christopher Trisos, University of Cape Town

Every year, human activities release billions of tons of greenhouse gases into the atmosphere. These gases trap heat from the Sun, making the Earth warmer than it would be without them. Over 90% of the extra heat from greenhouse gases gets absorbed by the ocean, driving marine temperatures to alarmingly high levels. Some estimates suggest that oceans have warmed over 1.5°C since the Industrial Revolution started, more than 250 years ago.

Marine organisms are particularly sensitive to warming, partly because they have narrow “thermal safety margins”. This is the difference between the environmental temperature and the maximum temperature an organism can tolerate without experiencing harm.

A narrow thermal safety margin means there is little room for temperatures to rise before they start causing damage to marine life. To make matters worse, marine organisms have limited options for avoiding high temperatures. One option is for them to migrate to greater depths, which is not always possible.

As the planet becomes hotter, thousands of marine species will be exposed to potentially unsafe conditions. This will affect their ability to survive and reproduce, causing their numbers to decline, even to the point of local extinction.

At the same time, the warming ocean could make it possible for some species to colonise regions that were previously too cold for them. This is known as a thermal opportunity.

When marine species start shifting to places of new thermal opportunity, it could lead to mismatches between where fish stocks are located and where fishing communities are based. This could affect local economies and availability of food for people.

We are scientists focused on understanding how climate change can affect species, ecosystems and people. Together with a team of specialists in ecological forecasts and marine ecology, we set out to examine the effects of warmer seas on 21,000 marine species. We built a model that integrates species distribution data and future climate projections to look at two aspects: the emergence of thermal opportunities and the dangers of exposure to warmer water.

We wanted to find out when these new thermal opportunities and threats will emerge. Is the emergence a gradual process? How long will they last?

We found that thermal opportunities are expected to emerge earlier and more gradually, whereas threats from exposure to higher temperatures will arrive later but much more abruptly. Understanding when and where opportunities could emerge can help guide conservation efforts to protect marine ecosystems and the critical services they provide for humans.

Where 21000 marine species are headed

To find out what’s likely to happen, we estimated the thermal preferences for over 21000 marine species by analysing the historical temperatures they have experienced across their ranges. These species include fish, crustaceans, shellfish, jellyfish, and sea urchins and other invertebrates.

We also analysed yearly projections of sea surface temperature until 2100, using models from the same data sets used in latest assessment report from the Intergovernmental Panel on Climate Change (IPCC). We looked at three climate scenarios for the oceans based on low, intermediate and high levels of greenhouse gas emissions.

This showed us where and when thermal opportunities are likely to take place and how long they will remain open. In other words, if a species moves because their natural habitat has become too hot, the cooler part of the ocean they move to will also only remain at the correct temperature for a certain number of years.

We also evaluated which species will be exposed to potentially harmful temperatures and when this will happen.

Our study shows that shifts in ocean temperatures due to climate change are already creating thermal opportunities for hundreds of marine species to colonise new habitats, mainly in temperate and polar regions. These opportunities will continue to increase for thousands of species until 2050, even if greenhouse gas emissions are reduced rapidly.

If exposure to unsafe temperatures accelerates after 2050, especially if we fail to keep global warming levels below 2°C as established in the Paris Agreement, our projections show that tropical ecosystems will be most affected.

We initially expected that thermal opportunities would not last long, especially where temperatures are projected to increase quickly. However, our findings show that, regardless of the rate of warming, opportunities tend to remain open (at least until the end of the century). This suggests that species might still be able to move into these new, suitable habitats even many years after those habitats become available.

For example, imagine a fish species struggling in a warming ocean. If a cooler area nearby becomes suitable for it, the fish might be able to migrate there, even decades later, as long as the conditions stay favourable. This increases the chances that species will be able to shift where they live as the climate changes.

For some species, thermal opportunities could help prevent population declines or even extinction by providing them with places outside their current habitats where they can thrive.

If these species are also important for local economies or food security, these changes could also benefit humans.

The negatives

The arrival of new species can harm local ecosystems by introducing new predators, competitors or diseases. These can threaten native species and disturb the balance of ecosystems.

Even in areas with low exposure to unsafe temperatures, the arrival of new species could disrupt ecosystems and create challenges. In parts of the ocean that are not heating rapidly, the arrival of new species could disturb those already living there.

Solutions

Our findings suggest that climate change will reshape marine ecological communities. However, we can still reduce the number of species at risk from increasing ocean temperatures.

Our model shows that limiting global warming to well below 2°C could halve the number of species that might have to change habitats by 2100.

It would also reduce the number of species exposed to unsafe temperatures 100-fold.

On the other hand, uncontrolled climate change would greatly increase the risk of ecological disruption from thermal exposure and opportunities, potentially affecting three in five ecological communities across the world’s oceans.

Every fraction of a degree of warming that we can avoid matters.

Andreas Schwarz Meyer, Postdoctoral Research Fellow in Ecology & Evolutionary Biology, University of Cape Town and Christopher Trisos, Senior Research Fellow in Climate Change Risks, University of Cape Town

This article is republished from The Conversation under a Creative Commons license. Read the original article.