Claim: 5-6C Global Warming Could Even Kill Tardigrades

Guest essay by Eric Worrall

According to computer models created by Dr Giovanni Strona of the European Commission’s Joint Research Centre, working with Corey Bradshaw of Flinders University Australia, 5-6C global warming would wipe out most life on Earth by collapsing food chains, even though individual species might on their own be able to survive such a shift.

Losing species to climate change causes global ‘extinction domino effect’

New research reveals the extinction of plant or animal species from extreme environmental change increases the risk of an “extinction domino effect’”that could annihilate all life on Earth.

This would be the worst-case scenario of what scientists call ‘co-extinctions’, where an organism dies out because it depends on another doomed species, with the findings published today in the journal Scientific Reports.

Think of a plant’s flower pollinated by only one species of bee — if the bee becomes extinct, so too will the plant eventually.

“Even the most resilient species will inevitably fall victim to the synergies among extinction drivers as extreme stresses drive ecosystems to collapse,” says lead author Dr Giovanni Strona of the European Commission’s Joint Research Centre, based in Ispra in northern Italy.

Researchers from Italy and Australia simulated 2000 virtual earths, linking animal and plant species. Using sophisticated modelling, they subjected the virtual earths to catastrophic environmental changes that ultimately annihilated all life.

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What we were trying to test is whether the variable tolerances to extreme global heating or cooling by different species are enough to explain overall extinction rates,” says co-author Professor Corey Bradshaw of Flinders University.

“But because all species are connected in the web of life, our paper demonstrates that even the most tolerant species ultimately succumb to extinction when the less-tolerant species on which they depend disappear.

“Failing to take into account these co-extinctions therefore underestimates the rate and magnitude of the loss of entire species from events like climate change by up to 10 times.”

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“Another really important discovery was that in the case of global warming in particular, the combination of intolerance to heat combined with co-extinctions mean that 5-6 degrees of average warming globally is enough to wipe out most life on the planet,” says Dr Strona.

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Read more: https://news.flinders.edu.au/blog/2018/11/14/loss-animal-plant-species-climate-change-causes-global-extinction-domino-effect/

The abstract of the study;

Co-extinctions annihilate planetary life during extreme environmental change

Giovanni Strona & Corey J. A. Bradshaw

Climate change and human activity are dooming species at an unprecedented rate via a plethora of direct and indirect, often synergic, mechanisms. Among these, primary extinctions driven by environmental change could be just the tip of an enormous extinction iceberg. As our understanding of the importance of ecological interactions in shaping ecosystem identity advances, it is becoming clearer how the disappearance of consumers following the depletion of their resources — a process known as ‘co-extinction’ — is more likely the major driver of biodiversity loss. Although the general relevance of co-extinctions is supported by a sound and robust theoretical background, the challenges in obtaining empirical information about ongoing (and past) co-extinction events complicate the assessment of their relative contributions to the rapid decline of species diversity even in well-known systems, let alone at the global scale. By subjecting a large set of virtual Earths to different trajectories of extreme environmental change (global heating and cooling), and by tracking species loss up to the complete annihilation of all life either accounting or not for co-extinction processes, we show how ecological dependencies amplify the direct effects of environmental change on the collapse of planetary diversity by up to ten times.

Read more: https://www.nature.com/articles/s41598-018-35068-1

The strongest argument against the validity of this modelling exercise is that climate shifts of the magnitude described as catastrophic have already occurred in the recent past.

The Younger Dryas for example was an abrupt drop in temperature of between 2-6C in at least the Northern Hemisphere, which lasted for 1,200 years. The Younger Dryas occurred 12,900 years ago.

I’m not kidding when I say abrupt, some scientists suggest the Younger Dryas may have struck with full force in as little as a few months. In some locales such as Greenland the temperature shift was more extreme than 2-6C.

The recovery from the Younger Dryas was also very rapid in at least some locales, as you can see from the graph below.

The Younger Dryas did not cause a mass extinction event, not even close. If observations contradict your model, time to have a closer look at the model.

Where did the researchers go wrong? The following quote from the study suggests one possible problem with their model;

… At each step of the environmental-change trajectory, we removed from each locality all species with temperature-tolerance limits no longer compatible with the changed conditions (see Measuring environmental compatibility). This single mechanism defined species loss in the environmental-tolerance scenario. In the co-extinction scenario, in addition to the primary extinctions caused by climate change at each step, we also accounted for the loss of consumers driven to extinction by the depletion of their resources. In so doing, we explored various assumptions regarding the minimum amount of resources ensuring the survival of a consumer, and the ability of the food web to rearrange interactions following species loss (see Modelling co-extinctions). …

Read more: (Same link as above)

In the real world, species subject to heat or cold stress don’t just sit there and die, they migrate. Obviously there are isolated cases where migration is impossible, but the loss of say a handful of thick furred critters clinging to a mountain top in the middle of a tropical desert does not constitute a mass extinction event.

I’ll give the researchers credit that they provided the code associated with their research. If anyone has the time to analyse the code in depth, please post the result of your analysis in comments.

Update (EW): Forgot to include the money quote, the claim by the study authors that 5-6C could wipe out most life in the planet

Update 2 (EW): Percy Jackson points out that the study authors did attempt to emulate colonisation and migration due to environmental changes, so I was wrong about what is wrong with the model. Nevertheless there is something badly wrong with the model, because model predictions contradict evidence of resilience in the face of abrupt climate events which actually occurred, such as the Younger Dryas