A4.1.1 The Relationship between Biodiversity, Climate Change, and People

Human-induced climate change is leading to changes in precipitation, seasonality, storm intensity, and more.Footnote ¹⁵⁵ All ecosystems, marine, terrestrial and freshwater, are affected; the ways in which people relate to Nature will be significantly altered. An integrated response to climate change and biodiversity loss is needed, which is made difficult by the fact that different institutions are responsible for each (for example, the UN Framework Convention on Climate Change (UNFCCC) and the Convention on Biological Diversity (CBD)). Ecosystems, and the biodiversity they contain, are being altered by climate change, but can also help us adapt to and mitigate its effects (see Chapter 19 for discussion on Nature-based solutions).

Climate change will alter the way we relate to Nature. People living in rural areas will find that the food they can grow – and where they can grow it – changes, people in cities will find that there are higher costs of importing fuel, and supply chains for consumer goods will have vastly different environmental impacts. We need to understand the relationships between people and Nature, and how they are affected by climate change, in order to help the best elements of that relationship to be maintained – or even improved, where new relationships with Nature will be forged because of climate change

How can positive outcomes be achieved for people and ecosystems in different contexts as the climate changes? For example, communities living with coral bleaching face lowered fishery output and reduced income from tourism. Life with climate change looks very different for this community: a planned response may maintain some fishery output through creating climate-adapted habitats for reef fish. Without a response that considers climate change, the community could face loss of all benefits from the reef.

Different aspects of an ecosystem are affected differently by climate change and have roles in its recovery. For example, after reef bleaching, recovery is a race between recolonising corals and the algae that grow on the dead reef. Parrotfish are among the most effective consumers of algae, and they are also negatively affected by climate change. Helping protect parrotfish will help the reef recover, and maintain reef fishery output and communities’ livelihoods. So, a short-term reduction in fishing is needed in order to maintain the reef fishery’s long-term productivity.

A4.1.2 Climate-Related Changes to Biodiversity are Already Being Observed

Climate change is already harming biodiversity in many ways. Here we discuss a selection: globally coherent patterns of species distribution shifts; impacts on marine ecosystems, particularly coral reefs; and genetic signatures of climate change.

Species ranges are shifting towards the poles. This has been identified as a climate change ‘signal’. Meta-analyses have shown that about half of species on which there were data had changed their distributions significantly over the past 20 to 140 years (Reference Lovejoy and HannahLovejoy and Hannah, 2019). Boreal species diversity in the Yukon, Canada, increased over a 42-year period, during which temperatures increased by 2°C (Research Northwest and Morrison Hershfield, 2017; Reference Lovejoy and HannahLovejoy and Hannah, 2019). The poleward shift trend is consistent across taxa. Marine species are moving towards poles more quickly than those on land. Meta-analyses show that the speed of the leading edge of marine species distribution change is many times greater than the terrestrial, at ~72 km/decade. Poloczanska (2013) compared dynamics in marine species range and found that the leading edges of poleward range shift were expanding nearly five times faster than the trailing edges were contracting, which matches the trend seen in European butterflies. 63% non-migratory European butterfly species expanded their northern range boundary, but only 3% contracted their southern range boundary (Reference Parmesan, Ryrholm, Stefanescu, Hill, Thomas, Descimon, Huntley, Kaila, Kullberg, Tammaru, Tennent, Thomas and WarrenParmesan et al. 1999).

Climate change is already contributing to rapid, broad-scale ecosystem changes, with significant consequences for biodiversity. Several changes have already occurred at spatial scales sufficiently broad to represent biome changes. For example, inland water systems have already been significantly altered, and the spatial scale of changes in fire and precipitation frequency cover large proportions of tropical and boreal biomes respectively (Reference Gonzalez, Neilson, Lenihan and DrapekGonzalez et al. 2010; Reference Pachauri and MeyerIPCC, 2014). Rapid broad-scale changes differ from other patterns in vegetation dynamics in that they represent a ‘crash’ in one or more populations over large areas. Climate triggers these through megafires (as seen in Australia in late 2019 to early 2020), drought-triggered die-off, floods and hurricanes. Pest and pathogen outbreaks are frequently associated with these events, though the relationship between pests and pathogens and climate change is complex (Reference Rosenzweig, Iglesias, Yang, Epstein and ChivianRosenzweig et al. 2001; Reference Jactel, Koricheva and CastagneyrolJactel, Koricheva, and Castagneyrol, 2019).

Studies show impacts on individual species and communities, as well as changes at the biome level. Reference ParmesanParmesan (2006) reviewed studies that had focused on single species. For example, populations of the well-studied butterfly Euphydryas editha are observed to be declining due to warming causing their host plants to senesce before the insects diapause (a period of suspended development in their lifecycle), leading larvae to starve (Reference HellmannHellmann, 2002; Reference Singer and McBrideSinger and McBride, 2012).

The changing climate is also altering marine biomes, including unique megadiverse systems such as coral reefs. The ocean plays a crucial role in stabilising the Earth System, mainly due to its huge capacity to absorb CO₂ and heat while experiencing minimal change in temperature. The ocean has absorbed 93% of the extra energy existing due to the greenhouse effect, and approximately 30% of human-generated CO₂. These absorptions have had an impact: sea levels have risen, sea ice extent has decreased and ocean pH has dropped rapidly, which is associated with concentrations of key ions such as carbonate and bicarbonate. Oxygen levels are being driven down in deeper areas of the ocean; oxygen-dependent organisms are beginning to disappear from these so-called ‘dead zones’. Elsewhere, levels of productivity are rapidly increasing or decreasing due to retreating ice, changing winds, and altering nutrient compositions (Reference Pörtner, Karl, Boyd, Cheung, Lluch-Cota, Schmidt and ZavialovPörtner et al. 2014).

Coral reefs only occupy 0.1% of Earth’s surface, but they provide habitat for 25% of known marine organisms (Hoegh-Guldberg, 2019). Coral reefs have experienced small shifts in temperature and ocean chemistry over the past 420,000 years at least. This is due to the fact that even large environmental changes, such as shifts between ice-ages and interglacial periods, are experienced as small changes over relatively long periods of time compared to the current pace of change. The great barrier reef has waxed and waned in the past as sea levels have risen and fallen, but today’s pace of change makes current fluctuations more significant. As much as 75% coral reefs are threatened, and as much as 95% is in danger of being lost by 2050 (Reference Hoegh-GuldbergHoegh- Guldberg, 1999; Reference Hoegh-Guldberg, Mumby, Hooten, Steneck, Greenfield, Gomez, Harvell, Sale, Edwards, Caldeira, Knowlton, Eakin, Iglesias-Prieto, Muthiga, Bradbury, Dubi and HatziolosHoegh-Guldberg et al. 2007). Reef-building corals have contracted over the past 30–50 years, which is associated with loss of reef 3D structure (Reference Bruno and SeligBruno and Selig, 2007). Reef-building corals rely on the symbiosis between coral and small photosynthetic organisms (Symbiodinium). Rapid temperature and pH changes cause this symbiosis to break down, leading to coral bleaching (Reference GlynnGlynn, 1993; Reference Hoegh-GuldbergHoegh-Guldberg, 1999). Around 1980, large-scale bleaching began in tropical regions, with no precedent in scientific literature; these bleaching events were associated with short periods where maximum sea temperatures rose by 1–2°C (Reference Hoegh-GuldbergHoegh-Guldberg, 1999). Reefs occasionally recover from bleaching, but most do not. Protecting the remaining 10% of coral reefs will deliver huge benefits for biodiversity, ecosystem services and human well-being (Reference Pachauri and MeyerIPCC, 2014). Climate change mitigation will form an important part of this response, but should take place alongside conservation and restoration efforts (such as the expansion of the marine protected area networks) (Reference Pachauri and MeyerIPCC, 2014).

In addition to its impact on coral reefs, ocean acidification has been responsible for changes in animal behaviour: reductions in gastropod shell thickness (due to reduced calcification as a result of reduced pH) lead to enhanced escape activity when a predator is present, demonstrated in a study on Littorina littorea (Reference Bibby, Cleall-Harding, Rundle, Widdicombe and SpicerBibby et al. 2007). Another example is foraging in deep-sea urchins, which increased under lowered pH conditions, possibly to compensate for reduced ability to detect food (Reference Barry, Lovera, Buck, Peltzer, Taylor, Walz, Whaling and BrewerBarry et al. 2014).

As well as changing species distribution and altering ecosystems, climate change has already left signatures in organisms’ genomes. Selection for genes enabling organisms to survive in warmer temperatures has been identified: an example is changes to the mitochondrial DNA (mtDNA) NADH gene in American pika (Ochotona princeps, a small relative of rabbits and hares), suggesting local adaptation to different thermal and respiratory conditions (Reference Lemay, Henry, Lamb, Robson and RusselloLemay et al. 2013). Spatially divergent selection of climate-associated genes in oak trees (Quercus lobata) has also been observed, including genes involved in bud burst and flowering, growth, and osmotic and temperature stress (Reference Sork, Fitz-Gibbon, Puiu, Crepeau, Gugger, Sherman, Stevens, Langley, Pellegrini and SalzbergSork et al. 2016). Evolution is a stochastic process, dependent on the genetic options available and their underlying architecture, so we can never be entirely certain about how change will happen. As well as genetic evolution, environmentally determined plasticity is an important aspect of adaptation to changing temperatures: examples include increases in body size in marmots, and changes to clutch size in birds (Reference Hoffmann and SgróHoffmann and Sgró, 2011).

A4.1.3 What Does the Future Hold?

Biodiversity changes have been projected under different scenarios, combining climate change and mean species abundance, showing the negative correlation between temperature rise and species abundance for plants and warm-blooded mammals (Figure A4.1, Reference Schipper, Hilbers, Meijer, Antão, Benítez-López, De Jonge, Leemans, Scheper, Alkemade, Doelman, Mylius, Stehfest, Van Vuuren, Van Zeist and HuijbregtsSchipper et al. 2019).

Figure A4.1 Mean Species Abundance (MSA) Plotted Against Global Mean Temperature Increase (GMTI)

Source: Reference Schipper, Hilbers, Meijer, Antão, Benítez-López, De Jonge, Leemans, Scheper, Alkemade, Doelman, Mylius, Stehfest, Van Vuuren, Van Zeist and HuijbregtsSchipper et al. (2019). Note: Red: warm-blooded mammals; green: plants.

Figure A4.2 Change in Mean Species Abundance (MSA) under Different Scenarios

Source: Reference Schipper, Hilbers, Meijer, Antão, Benítez-López, De Jonge, Leemans, Scheper, Alkemade, Doelman, Mylius, Stehfest, Van Vuuren, Van Zeist and HuijbregtsSchipper et al. (2019). Note: Scenarios represent combinations of shared socio-economic pathways (SSPs) and representative concentration pathways (RCPs).

Reference Schipper, Hilbers, Meijer, Antão, Benítez-López, De Jonge, Leemans, Scheper, Alkemade, Doelman, Mylius, Stehfest, Van Vuuren, Van Zeist and HuijbregtsSchipper et al. (2019) used three shared socio-economic pathways (SSPs) to project changes in mean species abundance (MSA) to 2100, each associated with different pressures placed on the environment by human activity. Even in the most sustainable scenarioFootnote ¹⁵⁶, global changes in MSA are projected to be negative. The regional rivalry scenario projects high population growth and resource-intensive consumption, while the fossil-fuelled development scenario is also characterised by a consumption-oriented society, which is even more energy-intensive. Both lead to high levels of climate change, and widespread negative changes to MSA. Future impacts of climate change on marine biodiversity are discussed in Chapter 16.

Climate change is affecting, and will continue to affect, tropical forests. These forests play a huge part in regulating global climate, accounting for a third of land surface productivity and evapotranspiration (Reference MalhiMalhi, 2012). Forest biodiversity will be affected, as tropical forests fall outside their historic climate variability ranges up to a decade faster than any other major terrestrial ecosystem (Reference Mora, Frazier, Longman, Dacks, Walton, Tong, Sanchez, Kaiser, Stender, Anderson, Ambrosino, Fernandez-Silva, Giuseffi and GiambellucaMora et al. 2013). Mean land surface temperatures for tropical forest regions are expected to increase by at least 3°C this century (Reference Zelazowski, Malhi, Huntingford, Sitch and FisherZelazowski et al. 2011) which is similar to the warming extent of the Paleo-Eocene Thermal Maximum (around 55.5 million years ago), but occurring at an unprecedented speed. This will cause more intense dry seasons, stronger and more frequent droughts, and stronger, longer-lasting heat waves (Reference Malhi, Gardner, Goldsmith, Silman and ZelazowskiMalhi et al. 2014). Eastern Amazonia in particular will experience significant declines in total rainfall (Reference Pachauri and MeyerIPCC, 2014). There is evidence that tropical trees have narrower thermal niches than temperate trees (Reference Araújo, Ferri-Yáñez, Bozinovic, Marquet, Valladares and ChownAraújo et al. 2013). The effect of increased productivity from increased CO₂ concentration may have a compensatory effect, however, the narrowness of the thermal niche will probably cause a marked decline in fitness for most tropical forest systems.

The decline in fitness and rainfall in the Amazon in particular will have serious consequences for the entire Earth System. At the moment, the Amazon acts as a planetary cooling system, influencing global circulation of air and vapour by evaporating vast amounts of water into the Earth’s atmosphere. Even in long dry seasons, the deep-rooted Amazon trees absorb water many metres below the soil surface. Deforestation has reduced the amount of vapour in the atmosphere, leading to reduced rainfall in neighbouring areas, and climate-induced decline in forest fitness would have similar consequences. Where light penetrates to the forest floor, fires become more likely, releasing large amounts of carbon into the atmosphere, and contributing to the greenhouse effect. An estimated amount of carbon equivalent to a decade’s worth of human emissions is stored within the wood of Amazon trees. Releasing it through burning would have disastrous consequences for global warming (Reference Lovejoy and HannahLovejoy and Hannah, 2019).