IPCC Assessment in depth, Part 3: What impact is global warming having on marine life?

The threat that climate change poses to human well-being and the health of the planet is “unequivocal”, says the latest report from the United Nations’ Intergovernmental Panel on Climate Change (IPCC). In this in-depth Q&A series, Carbon Brief unpacks the key findings of the report and the developments since the IPCC’s last assessment. (Read previous section here)

By Carbon Brief

3. What impact is global warming having on marine life?

Previous IPCC reports have found, with “growing confidence”, that humanity’s emissions are impacting the oceans and their ecosystems. In the time since the last assessment report, more research – including observational work, modelling studies, palaeoclimate proxies and Indigenous knowledge – has “provide[d] increasing evidence” of climate change’s impacts on marine ecosystems, the new report says.

The report states with very high confidence that human-driven chemical and physical changes to the ocean are altering the distribution and abundance of marine organisms “from microbes to mammals and from individuals to ecosystems, in every region” of the world. It also says with high confidence that “climate change has exposed ocean and coastal ecosystems to conditions that are unprecedented over millennia”.

As the oceans warm, marine species are being forced out of their natural habitats, “generally follow[ing] the pace and direction of climate warming”. Since the 1950s, these poleward shifts are happening at a rate of almost 60km per decade – although the report notes that there is “substantial variation” in this rate, depending on the species and region.

While there is high confidence that fish and marine mammals are being pushed poleward by warming waters, there is low-to-medium confidence that species’ distributions are shifting deeper in the ocean in response.

Warming is also affecting the timing of key biological events, such as phytoplankton blooms, commercial fish spawning and marine reptile breeding, the authors say. Nearly three-quarters of published studies are “consistent” with these changes, although the report notes there is a significant northern-hemisphere bias, with more than 95% of observations recorded there.

In many cases, warming exacerbates the effects of other stressors that organisms may be experiencing. The report divides these into “climate-induced drivers” and “non-climate drivers”. Other climate drivers include ocean acidification and deoxygenation, while non-climate drivers include pollution, overfishing, invasive species and habitat degradation.

The chart below shows observed changes in the ocean between 1925-2016: warming rate (top), climate velocity – the speed and direction that a given point on a map would need to move to maintain its current climate state – (middle) and the change in total number of marine heatwave days, calculated as the difference between the time periods 1925-54 and 1987-2016 (bottom). Darker colours show stronger positive (red) and negative (blue) effects.

The report notes that organisms “frequently experience” climate and non-climate drivers at the same time. As a result, disentangling the effects of climate change and other non-climate drivers is difficult.

The observational records of many marine ecosystems are short, or have a geographic bias, which also contributes to the challenge of attribution. However, a “growing body” of work “increasingly provides multiple lines of evidence” for attributing these impacts to climate change, the authors say.

The report notes a “wide range of responses” of commercially exploited fish and invertebrate species to warming, the majority of which have been “detrimental”. However, overexploitation of fisheries also has a significant impact on fishery yields, and these “combined effects…make it difficult to assess” what portion of this comes from climate change.

There are some cases where attribution is more clear than in others. For example, climate change is exacerbating the magnitude, extent and frequency of marine heatwaves – as it is with many other types of extreme weather.

There is high confidence that marine heatwaves can lead to “mass mortality” events among “key foundational species”. In “most regions”, seaweeds such as kelp are already experiencing such mass mortality events, along with range shifts. This is partially due to the direct effects of warming and partially due to indirect effects, such as changing ranges of plant-eating species.

The report warns of “irreversible phase shifts” in these ecosystems if 1.5C warming is surpassed and that they are at “high risk”, even if warming is kept to below 1.5C with periods of “temperature overshoot” above that threshold.

The report projects with medium confidence that even under SSP1-1.9, mangroves and saltmarshes are likely at “high risk” from future sea-level rise, “with impacts manifesting in the mid-term”.

Coral reefs, which are home to one-quarter of marine biodiversity, face a range of threats from both climate and non-climate drivers. Increasingly frequent and severe heat stress have increased both mass bleaching events and disease outbreaks. There is high confidence that  with warming above 1.5C, reefs are “under threat” of reaching erosion rates that are larger than the rate at which new corals can grow.

Ocean acidification is also decreasing both coral cover and diversity; these effects can be “exacerbate[d]” by warming, although some

evidence suggests that these responses differ between species. Other calcifying organisms, such as mussels and oysters, are also at “high risk of decline” due to a combination of climate stressors, the report warns.

There is medium confidence that continued warming, acidification and sea level rise will increase the risk of both regional and global extinctions of certain species – even if warming is limited to less than 2C by the end of the century.

Another major climate driver is hypoxia, or low oceanic oxygen levels. Hypoxia is caused by the decreased solubility of oxygen in warmer waters as well as increased respiration rates of marine microbes and changes in the circulation of the ocean.

The report states that low-oxygen zones are increasing in both size and number around the world “with growing impacts on fish species diversity and ecosystem functioning”. Declining oceanic oxygen levels “have already decreased suitable habitat” for important open-water fish species such as tuna by 15%. There is medium confidence that the subsurface ocean will see “historically unprecedented” low levels of oxygen over the remainder of this century.

Organisms whose habitats are more “thermally stable”, such as the polar regions or the deep sea, “are often more sensitive to warming” than those from environments with more temperature variation, the authors say. The report states with high confidence that warming has contributed to “major community shifts, both gradual and abrupt” in fish, marine mammals and seabirds in the polar regions.

The report also notes that organisms in certain life stages – such as eggs and embryos – are less tolerant to heat than adult organisms.

Many organisms’ responses to stressors are affected by the availability of food. Thus, the report states, “studies conducted under an excess of food risk underestimating the ecological effects of climate-induced drivers”.

The chart below shows a summary of the impacts of different climate drivers on a range of oceanic ecosystems, as well as their capacity to

adapt to change. The colours show the strength of the impact, with darker colours indicating stronger impacts; the dots in each box show the confidence level from low confidence (one dot) to high (three dots).

Evolution can provide one avenue for relief from the changing climate. Experimental evidence has shown that microbial populations can adapt to conditions mimicking those of climate change on rapid timescales.

However, research into the ability of “complex and long-lived” organisms to evolve and adapt to climate change remains a “major gap”, the report says. And the longevity of marine birds and mammals “suggests limited evolutionary resilience to rapid climate change”.

Looking more broadly, ecosystem models are a useful way of understanding potential future outcomes of a changing climate. But, the report states, they “cannot be expected to accurately project the trajectories of complex marine ecosystems”.

Both CMIP5 and CMIP6 ensembles project declines in global net primary production – the net carbon gained by phytoplankton and other plants in the ocean – by 2080-99 as compared to 1996-2015 levels, but CMIP6 shows much smaller decreases, and there is low confidence in this projection due to regional variations and uncertainties in the underlying drivers.

Global models also project a loss in marine biomass (the total weight of all animal and plant life in the ocean) of around -6% (±4%) under SSP1-2.6 by 2080-99, relative to 1995-2014. Under SSP5-8.5, this rises to a -16% (±9%) decline. In both cases, there is “significant regional variation” in both the magnitude of the change and the associated uncertainties, the report says.

Next: How is climate change affecting the world’s water?

All sections:

1. What is the IPCC report?
2. How is climate change affecting land ecosystems?
3. What impact is global warming having on marine life?
4. How is climate change affecting the world’s water?
5. What does the IPCC report say about impacts on food and agriculture?
6. What risks does climate change pose to the world’s cities?
7. What does the report say about public health, conflict and migration?
8. How does climate change affect poverty and progress towards the sustainable development goals?
9. How is the world adapting to climate change?
10. What the latest IPCC report says about carbon fee and dividend

Authors:

Aruna Chandrasekhar holds an MSc in environmental change and management from the University of Oxford. She previously worked as a freelance journalist for a variety of publications, including the New York Times, Guardian, Scroll and Caravan.

Daisy Dunne holds a BSc in biology from the University of Bristol and a science journalism MA from City, University of London. She was The Independent’s climate correspondent from November 2020 to 2021. Prior to this, she was Carbon Brief’s science writer from 2017 to 2020.

Josh Gabbatiss covers climate and energy policy. Josh holds a BSc in zoology from the University of Bristol. He previously worked as science correspondent at The Independent and prior to that was a freelance journalist for a variety of scientific publications including New Scientist and BBC Earth.