Climate Change & Anthropocene Extinction 43: Expert explains biodiversity graph Royal Society report

It’s the end of the year so a good time to evaluate where we’re standing. Our climate-biodiversity series has progressed to episode 43. If you’ve missed it, we highly recommend you start at part 1, preferably under a Christmas tree (in a biodiverse spruce forest) with family. Sadly we cannot guarantee snow.

It’s also a very good time to try to summarise what we’ve been able to learn from the global community of climate researchers, our somewhat classical friends at the Royal Society thought. It’s been a while since the last IPCC report (AR5, 2013/2014) and apart from interesting upcoming IPCC publications about sea level rise and the important 1.5 degrees target it’ll be another long while until the next full-blown climate report will be released (IPCC AR6, 2022).

Let’s bridge that gap, the Royal Society thought – and compile our own climate updates report!
Biodiversity extinction risk under climate change - Mark Urban metastudy in Royal Society report
Direct extinction risk caused by climate change for all of Earth’s species, for different scenarios of atmospheric warming. It’s a very sad graph if you learn what ‘extinction risk’ actually means – and if you realise that climate change is just one of several extinction drivers. The graph represents the findings of a 2015 metastudy in Science by climate ecologist Mark Urban of the University of Connecticut. You may have seen it before in another form, perhaps while reading part 10 of this series, explaining biodiversity loss accelerates under warming – based on that same study. As the graph reappeared in the above updated form in the new Royal Society climate updates report we thought we should take another good look at it, and this time asked Mark Urban if during the busy end of the academic semester he could spare some time to personally help us better understand what it says and what it means. And he could! The interview questions are below.

Well, it’s actually a really good report – comprehensible, digestible and answering key questions with key figures – while the full PDF (‘Climate updates: What have we learnt since the IPCC 5th Assessment Report?’) is only 34 pages long, with a neat overview of all your preferred climate subtopics.

So before we get to our preferred subtopic (biodiversity effects of climate change) let’s first try to summarise ‘where climate change currently stands’ and how climate science progresses:

Quick summary of climate updates: global heat, climate sensitivity range & Antarctic sea level rise contributions…

First of all probably the biggest climate news of 2017 comes not from the scientific literature (is therefore not featured in the Royal Society report), but from direct global observations: based on preliminary data the current year is likely to rank among the three hottest years on the global record – including 2015 (2nd) and 2016 (1st), with 2014 ranking 4th. What is most noteworthy is that 2017 is now the hottest non-El Niño year on record (weak La Niña since September, neutral before) – offering a very solid indication of where the actual temperature trend is going: up.

La Niña 2017-2018 - NOAA
Just a quick reminder that steadily rising global temperatures are still subjected to natural fluctuations from year to year. One of the patterns behind this is the ‘El Niño/La Niña Southern Oscillation’ (ENSO). If you have a global heat record in an ENSO neutral year (like 2017), that record is likely close to the actual underlying temperature trend (just like 2014) – while the heat record of 2016 (El Niño dominated) is a peak superimposed on that trend. ENSO observation and forecast by NOAA, showing 2018 will likely be a (weak) La Niña-dominated year. (The average temperature for the years 2016-2020 is forecast to be above the 2015 value, the 2nd hottest year on record – another indication of where to draw the temperature trend.)

Now due to thermal climate inertia we are at current CO2 levels already committed to substantial extra warming. But of course the pace of the temperature trend also depends on the global future emissions outlook and on remaining uncertainties surrounding climate sensitivity – or the politically most relevant metric ‘Equilibrium Climate Sensitivity’ (ECS), the amount of warming expected on a decades timescale after doubling of the atmospheric CO2 concentration.

The bulk of recent research still suggests climate sensitivity values close to the most established numbers, numbers that have been around for many years. The lowest range (ECS values below 2 degrees) has become scientifically discredited (flawed methodology) – suggesting we’ll see the AR4 range reappear in AR6: climate sensitivity most likely between 2-4.5 degrees. Judging by our experts assessment of climate sensitivity the real world value could be comfortably in the middle of that range: ‘close to or just above 3 degrees’. (Some of the ‘fat tail’ assessments of climate sensitivity, like Tan et al, 2016 in Science, suggesting the possibility of climate sensitivity above 5 degrees was also included in the Royal Society climate updates report. Such higher values may represent a very long-term climate response to elevated CO2 – in line with paleo-derived evidence for ‘Earth System Sensitivity’.)

Now everyone who follows climate science a bit, would probably agree that next to climate sensitivity the other important field where research suggests we may need to think in slightly different numbers than we used to is sea level rise. Again the bulk of recent evidence shows ‘most likely’ projections for the year 2100 are still close to the old IPCC range, but improved understanding of possible instability of the Antarctic ice sheets does substantially raise high end projections (that are a combination of a high emissions scenario and high sensitivity) to well above that range.

The Royal Society report includes references to Clark et al, 2016 in Nature Climate Change, suggesting the final sea level rise on millennia timescale caused by anthropogenic climate change (partly depending on future emissions) lies in a range between 29 to 55 metres and to DeConto & Pollard, 2016 in Nature, a study suggesting hydro-fracturing and ice cliff collapse around Antarctic ice sheets increases high end projection for sea level rise by 2100 to ±2 metres. (The Royal Society report does not include reference to Hansen et al, 2016 in Atmospheric Chemistry and Physics – a study suggesting the possibility of even more rapid sea level rise within the current century.

Fortunately for this topic we won’t have to wait until 2022 as the IPCC will release a special report on sea level rise in 2019, probably confirming higher upper range values.

Royal Society climate updates report chapter 12: What is the influence of climate change on species extinction?

Now the impacts of climate change on global biodiversity are addressed on page 28 in a chapter under the above-mentioned title.

Of course we have ourselves also taken a good look at biodiversity projections in IPCC AR5 and tried to compare these to AR4 – the older IPCC report. What we concluded was that the IPCC paid more attention to the subject, but became less concrete in their projections.

The Royal Society describes this as follows:

“[In IPCC AR5] there was high confidence that species extinction rates would increase with the magnitude and rate of climate change, but low confidence in the fraction of species at increased risk, the regional and taxonomic focus, and the time frame over which extinctions could occur. Different mechanisms by which species might adapt to climate change, including dispersal, behavioural, genetic and evolutionary plasticity were all noted as significant but poorly understood.”

The reason seems quite obvious. Climatology is very complex – ecology is very complex. Combining knowledge of the two fields of science is therefore very, very complex. Everyone agrees we need more dedicated research, and fortunately many scientists across the world are indeed dedicated to just that.

One of them is the specialised climate ecologist Mark Urban of the University of Connecticut, whose 2015 metastudy in Science offered a solid foundation for the Royal Society’s overview of recent publications.

Those who follow our series will probably recall his name – thus far we’ve already discussed 6 of Urban’s publications about climate and extinction risk. These are for instance studies about how to factor in complexities like species’ different dispersal rates, competition, ecosystem interdependence and evolutionary responses to climate change. If you have no time to read them all, the one we would most strongly recommend is our recent article about the effects of climate change on temperate zone biodiversity, where possible advantages and disadvantages of coming from an environment with natural climate variability are discussed, based on ‘7 testable predictions’ in a recent publication in Trends in Ecology & Evolution.

Climate change's exponential decline of biodiversity
Now if you need to you can take another quick look at the extinction risk graph on top of this article, or the original version of the same graph that’s shown above. We have a couple of questions about it – and Mark Urban is going to help us better understand:

1. Extinction risk percentage = the number of ‘walking dead’

Q – Rolf: “The graph shows the extinction risk for species for different emissions/warming scenarios. What do those percentages mean?”

A – Mark: “These percentages represent the average expected extinction risk based on a given value of global warming. I calculated them from 131 separate studies that spanned most regions of the globe and types of species.

The term, extinction risk, requires a bit more explanation. I use extinction risk to mean that the process of extinction is occurring and will reach its unfortunate end state unless conservation actions are put into place or global warming is reversed. Note that I don’t suggest a time frame for these extinctions. These models do not make any predictions about when the species will become officially extinct.

That’s because it can take a long time for a species to go extinct after the process started. Species might be around for a long time, but are the “walking dead,” meaning they are declining toward extinction. Others might become extinct quickly. Unfortunately, we seldom know which course a species will take.

Think of the total number of individuals of a species as being the water in a large urn. Global warming cracks the urn and water starts leaking out. This crack might be big or small, but eventually the water will run out unless the crack is repaired.”

2. Why biodiversity declines exponentially with warming

Q – Rolf: “Your graph, based on many individual studies, shows biodiversity loss tends to accelerate under continued warming. Could you explain (to a layman) the main mechanism behind this non-linear response?”

A – Mark: “One of the big surprises for me was that extinction risk accelerates with climate change. The models can’t really tell us why, but I can offer a few suggestions:

Most models evaluate the remaining habitat for a species under climate change. Basically, you take the existing range, and then find what part of that range will exist under future climates. If climate is changing only from top to bottom, then the range size declines proportionally with climate change, and we don’t expect an acceleration of extinction loss. However, if the climate is changing both from top to bottom and right to left, as might be the case for multiple climate variables, then the range gets smaller faster and extinction risk accelerates.

Think of it as the difference between folding a paper from top to bottom versus from both top to bottom and from right to left. In the second case, the size of the paper diminishes rapidly.

A similar argument can be made for mountain species that are forced to smaller and smaller regions near peaks. Another reason could be a peak in smallish range sizes that is reached by these temperature rises.”

3. Including more realistic assumptions in climate-biodiversity models

Q – Rolf: “You find there’s a range in scientific literature for biodiversity loss per climate scenario. You write that studies that show a higher extinction risk tend to include more complexities, like ‘realistic assumptions about extinction debt and dispersal capacity’. Should we therefore pay more attention to the upper range?”

A – Mark: “I think we need to be careful here because of the many uncertainties. Indeed there are many factors not included in most models that would increase extinction risks. However, other missing factors like adaptive evolution will decrease those risks.

I think the way forward is to build better models with these complexities included. My hope is that in five years, scientists will be using more sophisticated models that can help us to refine overall extinction risk estimates and pinpoint which particular species are most at risk.”

4. Are biodiversity projections changing?

Q – Rolf: “If we compare IPCC reports AR4 (2007) and AR5 (2014) this summary of the climate science seems to have become a bit less concrete about global climate-induced biodiversity loss figures. As you evaluate the available literature, do you see a chronological trend in climate-biodiversity projections? (+ Do you expect IPCC AR6 will have new global biodiversity figures?)”

A – Mark: “I think we’re steadily improving our understanding of biological risks of climate change, but we still have a long way to go. I think we’ll see more certainty about these projections as we build toward IPCC AR6.”

Thank you Mark! And thank you for reading this!

Now as it may be time to think of your New Year’s resolutions, let’s get a bit more practical. In science uncertainties will always remain, and will always require more research. Due to that endeavour we now have a light shining on the road ahead of us – a very practical thing when your full speed en route to the abyss of a mass extinction.

But you can’t expect to see everything beyond the normal horizon of your actual eyes. This will not be different in the year 2022, when the new IPCC report is released, or at any other point thereafter. Let’s act on all that we do know.

2017: third hottest year on record - NASA GISS data
Monthly and 12-month average global temperature development, showing (with only December data still missing) 2017 will likely rank as the third hottest year on record, despite a developing La Niña. Graph by Stephan Okhuijsen, data NASA GISS. Please note the shown baseline is 1951-1980 – so the actual global temperature trend is already well above 1 degree Celsius compared to the preindustrial climate and due to hemispherically skewed warming the northern hemisphere already breached 2 degrees last year. That was an El Niño-triggered spike on top of the trend, but it definitely shows where we’re heading.

© Rolf Schuttenhelm |

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