Guest essay by Eric Worrall

Scientists claim that a 5C warming will kill off 60% of the world’s fish. But such claims ignore major abrupt temperature excursions which occurred in the not too distant past.

Climate change will make world too hot for 60 per cent of fish species

ENVIRONMENT 2 July 2020

By  Adam Vaughan

Fish are at a far greater risk from climate change than previously thought, as researchers have shown that embryos and spawning adults are more susceptible to warming oceans.

In a worst-case scenario of 5°C of  global warming, up to 60 per cent of fish species around the world would be unable to cope with temperatures in their geographical range by 2100, when different stages of their lives are taken into consideration. Even if humanity meets the Paris deal’s tough goal of holding warming to 1.5°C, it would be too hot for 10 per cent of fish.

Previously, we thought that just 5 per cent of fish species would struggle to cope with 5°C of global warming, but that was based on analysis of adult fish alone.

“We can say 1.5°C is not paradise, there will be changes. But we can limit those changes if we manage to stop climate change. Fish are so important for human nutrition, so this study makes a strong case for protecting our ecosystems and natural environments,” says Hans-Otto Pörtner at the Alfred Wegener Institute in Bremerhaven, Germany, part of the team behind the research.

…

Read more: https://www.newscientist.com/article/2247602-climate-change-will-make-world-too-hot-for-60-per-cent-of-fish-species/#ixzz6REJS463Z

The abstract of the study;

Thermal bottlenecks in the life cycle define climate vulnerability of fish

Flemming T. Dahlke¹,*, Sylke Wohlrab¹,², Martin Butzin¹, Hans-Otto Pörtner¹,³,*

 See all authors and affiliationsScience  03 Jul 2020:
Vol. 369, Issue 6499, pp. 65-70
DOI: 10.1126/science.aaz3658 

Species’ vulnerability to climate change depends on the most temperature-sensitive life stages, but for major animal groups such as fish, life cycle bottlenecks are often not clearly defined. We used observational, experimental, and phylogenetic data to assess stage-specific thermal tolerance metrics for 694 marine and freshwater fish species from all climate zones. Our analysis shows that spawning adults and embryos consistently have narrower tolerance ranges than larvae and nonreproductive adults and are most vulnerable to climate warming. The sequence of stage-specific thermal tolerance corresponds with the oxygen-limitation hypothesis, suggesting a mechanistic link between ontogenetic changes in cardiorespiratory (aerobic) capacity and tolerance to temperature extremes. A logarithmic inverse correlation between the temperature dependence of physiological rates (development and oxygen consumption) and thermal tolerance range is proposed to reflect a fundamental, energetic trade-off in thermal adaptation. Scenario-based climate projections considering the most critical life stages (spawners and embryos) clearly identify the temperature requirements for reproduction as a critical bottleneck in the life cycle of fish. By 2100, depending on the Shared Socioeconomic Pathway (SSP) scenario followed, the percentages of species potentially affected by water temperatures exceeding their tolerance limit for reproduction range from ~10% (SSP 1–1.9) to ~60% (SSP 5–8.5). Efforts to meet ambitious climate targets (SSP 1–1.9) could therefore benefit many fish species and people who depend on healthy fish stocks.

Read more (paywalled): https://science.sciencemag.org/content/369/6499/65

The temperature excursion I was thinking of is the Younger Dryas, an abrupt return to ice age conditions just under 13,000 years ago, which lasted around 1200 years. The temperature shift was extreme (4-10C in Greenland), and occurred very rapidly, much faster than today’s mild global warming. The recovery was slower – but there were rapid periods of both warming and cooling.

Big freeze plunged Europe into ice age in months

… Carbon isotopes in each slice reveal how productive the lake was, while oxygen isotopes give a picture of temperature and rainfall. At the start of the ‘Big Freeze’ their new record shows that temperatures plummeted and lake productivity stopped over the course of just a few years. “It would be like taking Ireland today and moving it up to Svalbard, creating icy conditions in a very short period of time,” says Patterson, who presented the findings at the European Science Foundation BOREAS conference on humans in the Arctic, in Rovaniemi, Finland. …

Read more: https://phys.org/news/2009-11-big-plunged-europe-ice-age.html

One thing which did not happen during this period of wild temperature swings was the extinction of a large number of fish species.

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