Guest post: How meeting the 2C goal cuts the climate risks for 25,000 marine species

Climate change is rewiring marine ecosystems at an alarming rate. Warming waters are causing the distribution of marine life to shift, with species such as bluefin tuna showing up in the Arctic while others are moving out of the tropics. 

These – and many other – changes will only continue to increase, reshuffling food webs and impacting virtually all life in the ocean from bacteria to whales. 

However, the complexity of these impacts – to which species have different tolerance levels – makes it incredibly difficult to understand the net risks to individual marine species and ecosystems.

Our new study, published in Nature Climate Change, outlines a new index – the Climate Risk Index for Biodiversity (CRIB) – that assesses the climate risk for nearly 25,000 marine species and their ecosystems.

The findings show that, under very high emissions, almost 90% of these species are put at high or critical risk, with species at risk across 85% of their native areas, on average. However, in an emissions mitigation scenario consistent with the 2C global warming limit in the Paris Agreement, the risk is reduced for virtually all marine species and ecosystems. 

Our climate risk assessment can help prioritise vulnerable species and ecosystems and lays the groundwork for supporting climate-smart approaches to conservation.

Climate report card

The “risk” in the CRIB framework is measured as the likelihood that a species will no longer be able to persist in a location where it is currently found. 

Using a statistical approach, we essentially created a climate report card for each species and ecosystem that tells us which will be winners or losers under climate change. 

Just as a report card grades students on subjects such as maths and science, we used a data-driven approach to score individual species on 12 specific climate risk factors in all parts of the ocean where they live. 

The framework is based on an analysis of how the innate characteristics of a species – such as body size and temperature tolerance – interact with past, present, and future climate conditions. You can see these characteristics in section b of the graphic below. 

We then use these indices to calculate the climate vulnerability and risk of each species according to the present-day sensitivity to climate change, projected future exposure and innate potential to adapt (see section c). 

From here, we evaluate climate vulnerability on a relative scale from zero to one (section d), where a score of one typically corresponds to a species and location where sensitivity and exposure are at their extreme highest and adaptivity is at its lowest. 

We then convert this score to an absolute risk scale ranging from negligible (lowest) to critical (highest), as shown in section e. This threshold approach is comparable to that used by the Intergovernmental Panel on Climate Change (IPCC) reasons for concern framework that assesses climate risk to humans, as well as the International Union for Conservation of Nature (IUCN) Red List index of risk for species.

This captures both the likelihood and the magnitude of the impact of climate change for each species across the locations in which they are found.

Contrasting climate futures

Our study focuses on two scenarios of how future society – and the greenhouse gas emissions it produces – could pan out. These scenarios are known as shared socioeconomic pathways (SSPs) and the results paint two contrasting pictures of climate futures for marine life. 

Under the very high emissions SSP5-8.5 scenario – in which the global average ocean temperature increases by 3-5C by 2100 – we found that almost 90% of the 25,000 species were at a high or critical climate risk. On average, species were at risk across 85% of their distributional range.

Under this scenario, top predator species – such as sharks, tuna and billfishes – had a significantly higher risk than species further down the food chain, such as forage fishes and invertebrates. As these apex species can exert a disproportionate influence on ecosystems, it further suggests that ecosystem structure and functioning will be affected. 

As the map below shows, climate risk is largest in coastal ecosystems that support the highest fishery catches and in many subtropical and tropical ecosystems that tend to be biodiversity hotspots. The map shows the projections under SSP5-8.5 in 2100, with red shading indicating the worst-affected areas.

Climate risk may also interact with other threats to marine life. Risks appear to be higher in locations with larger numbers of species assessed as having higher extinction risk and those with more endemic species, which are only found in one geographic area and are inherently more vulnerable.

The findings also suggest severe knock-on impacts for people who rely on the ocean most.

Under the high emissions scenario, climate risks for species fished by humans for food or revenue – such as for instance cod, anchovies and lobsters – were systematically greater within the territories of low-income nations. 

The map below shows the proportion of fished species projected to be at high or critical risk under high emissions in 2100. Dark red shading indicates areas where almost all fished species are at high risk.

Such countries typically rely more on fisheries to meet their population’s nutritional needs. This then represents yet another example of climate inequality. While low-income countries have made the smallest contribution to climate change, they are likely to bear the brunt of the impacts while being the least well positioned to adapt. 

However, our study also shows the benefits to ocean life of climate mitigation and a rapid switch to a more sustainable world. And these benefits would be seen in lower-income marine-dependent nations the most. 

The low emissions SSP1-2.6 scenario is consistent with meeting the 2C global warming limit set out in the Paris Agreement. This would see an approximate average ocean temperature increase of 1-2C by 2100. The map below shows the change in the percentage of high risk species when moving from high emissions to low. The purple shading indicates areas showing lower risks.

Our findings show a reduced climate risk for virtually all species and ecosystems under the low emissions scenario. Thus, sticking to the goals of the Paris Agreement would have substantial benefits for marine life, with the disproportionate climate risk for ecosystem structure, biodiversity hotspots, fisheries and low-income nations being greatly reduced or eliminated.

Reducing risks with mitigation and adaptation

While mitigating emissions is the most direct approach to reducing the climate risk for marine life, the reality is that climate change is already impacting the oceans and, even with effective climate mitigation, they will continue to change. 

Therefore, adapting to a warming climate is crucial to building resilience for both ocean species and people. Yet, as a society, our approaches to fisheries management, protected area planning and biodiversity conservation were primarily developed in a world where the climate was relatively stable. 

This means that our approaches to managing the ocean must change to ensure they are not undermined. This will rely on a combination of continued development of new methods and adaptation strategies, capacity development in under-resourced parts of the world and a clear and well-articulated communication of the benefits of adaptation. 

Our study provides a new framework that can help inform stakeholders and decision-makers when navigating these complex issues, and assist with developing strategies to manage and conserve species and ecosystems more effectively under climate change. 

We designed the framework specifically with climate adaptation in mind. It is transparent, reproducible, data-driven, flexible and delivers a measure of risk – a valuable metric for those weighing up costs and benefits. In addition, it can be monitored over time to help evaluate trends.

So, while some aspects of our study paint a potentially bleak future for many marine species, it also measures how much our oceans and the life within them stand to benefit from both climate change mitigation and adaptation.