Climate Change & Anthropocene Extinction 48: large biodiversity benefits of 1.5 degrees limit (IPCC)

Then of course we also have our climate-ecology series that wanted an update based on IPCC SR15. Judging by IPCC’s special report on 1.5 degrees the ecological benefits of strengthening the global climate target from 2 to 1.5 degrees are relatively large. For instance insect extinction risk (expressed in this case as species losing over half of their geographic range) declines threefold, while range-deduced survival changes for climate-threatened plant and vertebrate species double.

That’s a staggering* difference, for ‘just’ 0.5 degrees of (additional) limited warming, illustrating how biodiversity loss from climate change is exponential to temperature rise – escalating as global average temperatures steadily increase.

[*) To illustrate this is highly non-linear: from 1.5 to 2 degrees warming increases 33%, while insect habitat decline increases threefold. Or: from 2 to 1.5 degrees warming is reduced by 25 percent, while insect habitat loss decreases by 67 percent.]

Climate change biome shift over Amazon basin
Schematic representation of biome shift across South America (Amazon basin) under a high 21st century warming scenario. Many species will lose suited range, an important mechanism through which climate change leads to species’ extinctions. Image from University of Sussex report from 2014 in BioScience (PDF).

Let’s start with a powerful quote from the executive summary of IPCC SR15’s third chapter about climate change impacts:

Risks of local species losses and, consequently, risks of extinction are much less in a 1.5°C versus a 2°C warmer world (high confidence). The number of species projected to lose over half of their climatically determined geographic range at 2°C global warming (18% of insects, 16% of plants, 8% of vertebrates) is projected to be reduced to 6% of insects, 8% of plants and 4% of vertebrates at 1.5°C warming (medium confidence). Risks associated with other biodiversity-related factors, such as forest fires, extreme weather events, and the spread of invasive species, pests and diseases, would also be lower at 1.5°C than at 2°C of warming (high confidence), supporting a greater persistence of ecosystem services.”

Now an interesting paragraph is 3.4.3.3, about ‘changes in species range, abundance and extinction’.

One of the mechanisms through which climate change can lead to biodiversity decline is that suited habitats for species are linked to specific climatic conditions that are often found in latitude- or altitude-dependent zones. As the climate warms up, these zones move poleward or uphill, leading to large-scale biome shifts and forcing species to disperse in the proper direction to keep within these optimal habitat boundaries. The average pace over the last couple of decades of this forced migration is 17 kilometers poleward and 11 metres in increased altitude per decade. For some species this is easy to keep up with (think of birds), whereas slow-moving (plants) and slow-growing (trees) species will have a much harder time. These differences in dispersal ability leads to temporal and spatial mismatches between inter-dependent species, further stressing biodiversity.

“Recent trends confirm this finding; for example, the spatial and interspecific variance in bird populations in Europe and North America since 1980 were found to be well predicted by trends in climate suitability (Stephens et al., 2016).”

Over the course of the 20th century, during which the global climate only warmed by some 0.8 degrees, these stresses already had a profound detrimental effect on biodiversity:

A recent meta-analysis of 27 studies concerning a total of 976 species (Wiens, 2016) found that 47% of local extinctions (extirpations) reported across the globe during the 20th century could be attributed to climate change, with significantly more extinctions occurring in tropical regions, in freshwater habitats and for animals.”

As warming progresses mismatches between species responses may worsen the situation:

“Owing to lags in the responses of some species to climate change, shifts in insect pollinator ranges may result in novel assemblages with unknown implications for biodiversity and ecosystem function (Rafferty, 2017).”

Now an updated assessment of over 100,000 species’ climate-induced extinction risk finds that even in a strongly warming-limited scenario in which global average temperature rise is limited to no more than 2 degrees, this poses significant extinction risks for various realms:

“Warren et al. (2013) simulated climatically determined geographic range loss under 2°C and 4°C of global warming for 50,000 plant and animal species, accounting for uncertainty in climate projections and for the potential ability of species to disperse naturally in an attempt to track their geographically shifting climate envelope. This earlier study has now been updated and expanded to incorporate 105,501 species, including 19,848 insects, and new findings indicate that warming of 2°C by 2100 would lead to projected bioclimatic range losses of >50% in 18% (6–35%) of the 19,848 insects species, 8% (4–16%) of the 12,429 vertebrate species, and 16% (9–28%) of the 73,224 plant species studied (Warren et al., 2018a).”

Now if warming is limited only 0.5 degrees further and global temperatures don’t rise to a max of 2 degrees but to 1.5 degrees instead, the extinction risk associated with species’ declining geographical ranges drops by two-thirds for insects and fifty percent for plants and invertebrates:

“At 1.5°C of warming, these values fall to 6% (1¬18%) of the insects, 4% (2–9%) of the vertebrates and 8% (4–15%) of the plants studied. Hence, the number of insect species projected to lose over half of their geographic range is reduced by two-thirds when warming is limited to 1.5°C compared with 2°C, while the number of vertebrate and plant species projected to lose over half of their geographic range is halved (Warren et al., 2018a) (medium confidence). These findings are consistent with estimates made from an earlier study suggesting that range losses at 1.5°C were significantly lower for plants than those at 2°C of warming (Smith et al., 2018).”

So is 1.5 degrees ‘good for biodiversity’? No, biodiversity thrives on stability, so good is no climate change at all. Even if we manage to limit warming to ‘just’ 1.5 degrees global average temperature rise species on average still lose close to about a quarter of their climatically suited ranges(!)

It should be noted that at 1.5°C of warming, and if species’ ability to disperse naturally to track their preferred climate geographically is inhibited by natural or anthropogenic obstacles, there would still remain 10% of the amphibians, 8% of the reptiles, 6% of the mammals, 5% of the birds, 10% of the insects and 8% of the plants which are projected to lose over half their range, while species on average lose 20–27% of their range (Warren et al., 2018a).”

And the fact that some species can more easily adapt or even profit from these changes does not change the overall picture, as also for birds, which have relatively high dispersal ability, under 1.5 degrees of warming a net range decline is projected:

“Given that bird and mammal species can disperse more easily than amphibians and reptiles, a small proportion can expand their range as climate changes, but even at 1.5°C of warming the total range loss integrated over all birds and mammals greatly exceeds the integrated range gain (Warren et al., 2018a).”

Underestimating complex factors means underestimating biodiversity loss

Now a big problem is these are all probably underestimation, as climate-ecology models still do not incorporate many complex feedback factors that are associated with the cascadic nature of extinction events. IPCC SR15 notes the following:

“A number of caveats are noted for studies projecting changes to climatic range. This approach, for example, does not incorporate the effects of extreme weather events and the role of interactions between species. As well, trophic interactions may locally counteract the range expansion of species towards higher altitudes (Bråthen et al., 2018). There is also the potential for highly invasive species to become established in new areas as the climate changes (Murphy and Romanuk, 2014), but there is no literature that quantifies this possibility for 1.5°C of global warming.”

Another problem that’s very hard to model and that scientists know from paleoclimatological extinction events (and possibly linked to invasive species) is that climate disturbance can lead to pests and plagues, further stressing (endemic) biodiversity:

“In some cases, when species (such as pest and disease species) move into areas which have become climatically suitable they may become invasive or harmful to human or natural systems (Settele et al., 2014).”

As a result the IPCC SR15 authors conclude the following:

There is no literature that directly estimates the proportion of species at increased risk of global (as opposed to local) commitment to extinction as a result of climate change, as this is inherently difficult to quantify. However, it is possible to compare the proportions of species at risk of very high range loss; for example, a discernibly smaller number of terrestrial species are projected to lose over 90% of their range at 1.5°C of global warming compared with 2°C (Figure 2 in Warren et al., 2018a). A link between very high levels of range loss and greatly increased extinction risk may be inferred (Urban, 2015). Hence, limiting global warming to 1.5°C compared with 2°C would be expected to reduce both range losses and associated extinction risks in terrestrial species (high confidence).”

Global loss of biodiversity as a ‘reason for concern’

Now in the special paragraph (3.5.2.4) about ‘Reason For Concern’ (RFC) 4, on ‘global aggregate impacts’ of climate change, another good way is presented to express extinction risk under 2 and 1.5 degrees warming scenarios: biome shifts. A range shift of 17 kilometres per decade (over last decades) may not seem much in itself, but it does add up substantially as time and warming progress. Under 2 degrees warming about 13 percent of on-land biomes are projected to undergo a state change. When warming is limited to 1.5 degrees these biome shifts are almost halved:

“Biome shifts, species range loss, increased risks of species extinction and risks of loss of ecosystem functioning and services: 13% (range 8–20%) of Earth’s land area is projected to undergo biome shifts at 2°C of warming compared to approximately 7% at 1.5°C (medium confidence) (Section 3.4.3; Warszawski et al., 2013), implying a halving of biome transformations.”

Biodiversity loss reason for concern climate change
‘Reason for Concern’ number 4 is not the only way to express extinction risk under climate change. Even more worrying is RFC1, the ‘unique and threatened systems’. This includes the Arctic marine ecosystem and coral reef ecosystems, that are at risk of very severe impacts even at 1.5 degrees global average warming…

This paragraph with other large-scale impacts on biodiversity, again emphasising the expected benefits of the 1.5 degrees target for insect life:

“Overall levels of species loss at 2°C of warming are similar to values found in previous studies for plants and vertebrates (Warren et al., 2013, 2018a), but insects have been found to be more sensitive to climate change, with 18% (6–35%) projected to lose over half their range at 2°C of warming compared to 6% (1–18%) under 1.5°C of warming, corresponding to a difference of 66% (Section 3.4.3). The critical role of insects in ecosystem functioning therefore suggests that there will be impacts on global ecosystem functioning already at 2°C of warming, whilst species that lose large proportions of their range are considered to be at increased risk of extinction (Section 3.4.3.3).”

Now the temperature threshold where climate change becomes a ‘high risk’ for biodiversity (under reason for concern 4) is now thought to be significantly lower than in IPCC’s previous assessment (AR5, 2013) – as IPCC SR15 concludes the following:

“In AR5, the transition from undetectable to moderate impacts was considered to occur between 1.6°C and 2.6°C of global warming reflecting impacts on the economy and on biodiversity globally, whereas high risks were associated with 3.6°C of warming to reflect the high risks to biodiversity and accelerated effects on the global economy. New evidence suggests moderate impacts on the global aggregate economy and global biodiversity by 1.5°C of warming, suggesting a lowering of the temperature level for the transition to moderate risk to 1.5°C (Figure 3.21). Further, recent literature points to higher risks than previously assessed for the global aggregate economy and global biodiversity by 2°C of global warming, suggesting that the transition to a high risk level is located between 1.5°C and 2.5°C of warming (Figure 3.21), as opposed to at 3.6°C as previously assessed (medium confidence).

Now meanwhile another important UN report has been released, by The Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES). We’ll quickly follow up and find out what exactly this important biodiversity report has to say about climate change, as just one of many major extinction drivers…

© Rolf Schuttenhelm | www.bitsofscience.org

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