Climate change is a direct disturbing factor to ecosystem health. It also leads to geographical biome shifts and therefore forced species migrations. Invasive species and food chain disturbances can lead to plagues, creating further ecosystem damage. All these factors work synergistically with other current and future ecology stressing trends, like deforestation and ocean acidification, which makes it difficult to model and project biodiversity decline – yet all the more valuable to take notice of scientific attempts to come up with actual figures.

Ecological effects (species extinction risk and wider ecosystem damage), both on land and in the oceans, of climate change according to IPCC AR5 (2014)

One of the leading sources for global climate impact figures is of course the IPCC, with its series of subsequent reports, summarising the scientific consensus, evolving and building on an ever-larger foundation of scientific research.

The IPCC’s Fourth Assessment Report (IPCC AR4, published in 2007) was remarkably concrete about the effects of global climate change on global biodiversity decline, stating (WGII, chapter 3.3.1 – ‘Impacts on systems and sectors’) ’20 to 30 percent of species are at increased risk of extinction if global average warming exceeds 1.5 to 2.5 degrees’ [above a climate average baseline of 1980-1999 – so let’s say a warming a bit above the 2 degrees pre-industrial climate target (that for the sake of global temperature data quality actually works from the well-established 1880-1909 climate average, so not really ‘preindustrial’)]. In the executive summary of chapter 4 (Impacts, Adaptation and Vulnerability) it is stated as follows: “Approximately 20 to 30% of plant and animal species assessed so far (in an unbiased sample) are likely to be at increasingly high risk of extinction as global mean temperatures exceed a warming of 2 to 3°C above pre-industrial levels (medium confidence).”

According to IPCC AR4 the global uncertainty range lies at 10 to 40 percent, but with a large geographical spread, with the species extinction risk ‘varying among regional biota from as low as 1% to as high as 80%’ for a warming that exceeds 2 to 3 degrees above pre-industrial.

Of its series of reports summarising the global scientific consensus on climate change, IPCC AR4 was remarkably concrete – listing percentages for possible biodiversity decline under different warming scenarios. The subsequent report, IPCC AR5, offered a more qualitative assessment of climate change-induced extinction risks.

Under higher global average temperature rise, extinction risk would increase exponentially: “As global average temperature increase exceeds about 3.5°C” [relative to 1980-1999 – so more than 4 degrees warming compared to preindustrial climate], “model projections suggest significant extinctions (40 to 70% of species assessed) around the globe” – adding such biodiversity loss should be interpreted as irreversible damage of climate change.

Less concrete, more elaborate: IPCC AR5

It’s a bit more difficult to find such concrete biodiversity and extinction risk figures in the IPCC’s subsequent Fifth Assessment Report (IPCC AR5, published in 2013/2014) – that in contrast offers a more qualitative assessment.

In the summary for policy makers on page 17 the biodiversity effects of climate change are translated as follows:

“A large fraction of species faces increased extinction risk due to climate change during and beyond the 21st century, especially as climate change interacts with other stressors (high confidence). Most plant species cannot naturally shift their geographical ranges sufficiently fast to keep up with current and high projected rates of climate change in most landscapes; most small mammals and freshwater molluscs will not be able to keep up at the rates projected under RCP4.5” [one of four ‘representative concentration pathways’ assessed in IPCC AR5 – roughly in line with a greenhouse gas concentration between 580-720 ppm CO2 equivalents] “and above in flat landscapes” [where uphill (cooling) climate zone migration is impossible] “in this century (high confidence).”

“Future risk is indicated to be high by the observation that natural global climate change at rates lower than current anthropogenic climate change caused significant ecosystem shifts and species extinctions during the past millions of years. Marine organisms will face progressively lower oxygen levels and high rates and magnitudes of ocean acidification (high confidence), with associated risks exacerbated by rising ocean temperature extremes (medium confidence). Coral reefs and polar ecosystems are highly vulnerable. Coastal systems and low-lying areas are at risk from sea level rise, which will continue for centuries even if the global mean temperature is stabilized (high confidence).”

Currently observed ecological effects of climate change

In the main synthesis report IPCC AR5 states that extinction risk from climate change can be deduced from paleoclimatology, while ecological effects of anthropogenic climate change can (also) already be observed:

“Many terrestrial, freshwater and marine species have shifted their geographic ranges, seasonal activities, migration patterns, abundances and species interactions in response to ongoing climate change (high confidence). While only a few recent species extinctions have been attributed as yet to climate change (high confidence), natural global climate change at rates slower than current anthropogenic climate change caused significant ecosystem shifts and species extinctions during the past millions of years (high confidence). Increased tree mortality, observed in many places worldwide, has been attributed to climate change in some regions. Increases in the frequency or intensity of ecosystem disturbances such as droughts, windstorms, fires and pest outbreaks have been detected in many parts of the world and in some cases are attributed to climate change (medium confidence). Numerous observations over the last decades in all ocean basins show changes in abundance, distribution shifts poleward and/or to deeper, cooler waters for marine fishes, invertebrates and phytoplankton (very high confidence), and altered ecosystem composition (high confidence), tracking climate trends. Some warm-water corals and their reefs have responded to warming with species replacement, bleaching, and decreased coral cover causing habitat loss (high confidence). Some impacts of ocean acidification on marine organisms have been attributed to human influence, from the thinning of pteropod and foraminiferan shells (medium confidence) to the declining growth rates of corals (low confidence). Oxygen minimum zones are progressively expanding in the tropical Pacific, Atlantic and Indian Oceans, due to reduced ventilation and O2 solubility in warmer, more stratified oceans, and are constraining fish habitat (medium confidence).”

“Risks of harmful impacts on ecosystems and human systems increase with the rates and magnitudes of warming, ocean acidification, sea level rise and other dimensions of climate change (high confidence). Future risk is indicated to be high by the observation that natural global climate change at rates lower than current anthropogenic climate change caused significant ecosystem shifts and species extinctions during the past millions of years on land and in the oceans (high confidence). Many plant and animal species will be unable to adapt locally or move fast enough during the 21st century to track suitable climates under mid- and high-range rates of climate change (RCP4.5, RCP6.0 and RCP8.5) (medium confidence). Coral reefs and polar ecosystems are highly vulnerable.”

Climate change and other ecological stressors are synergistic drivers of extinction:

“A large fraction of terrestrial, freshwater and marine species faces increased extinction risk due to climate change during and beyond the 21st century, especially as climate change interacts with other stressors (high confidence). Extinction risk is increased relative to pre-industrial and present periods, under all RCP scenarios, as a result of both the magnitude and rate of climate change (high confidence). Extinctions will be driven by several climate-associated drivers (warming, sea-ice loss, variations in precipitation, reduced river flows, ocean acidification and lowered ocean oxygen levels) and the interactions among these drivers and their interaction with simultaneous habitat modification, over-exploitation of stocks, pollution, eutrophication and invasive species (high confidence).”

What happens on land, happens in the oceans too

IPCC AR5 states that climate change and ocean acidification, synergistically with overfishing, poses a large threat to global fish biodiversity and productivity:

“Global marine species redistribution and marine biodiversity reduction in sensitive regions, under climate change, will challenge the sustained provision of fisheries productivity and other ecosystem services, especially at low latitudes (high confidence).”

“The progressive expansion of Oxygen Minimum Zones and anoxic ‘dead zones’ in the oceans will further constrain fish habitats (medium confidence). Open-ocean net primary production is projected to redistribute and to decrease globally, by 2100, under all RCP scenarios (medium confidence). Climate change adds to the threats of over-fishing and other non-climatic stressors (high confidence). Marine ecosystems, especially coral reefs and polar ecosystems, are at risk from ocean acidification (medium to high confidence). Ocean acidification has impacts on the physiology, behaviour and population dynamics of organisms. The impacts on individual species and the number of species affected in species groups increase from RCP4.5 to RCP8.5. Highly calcified molluscs, echinoderms and reef-building corals are more sensitive than crustaceans (high confidence) and fishes (low confidence) (Figure 2.6b). Ocean acidification acts together with other global changes (e.g., warming, progressively lower oxygen levels) and with local changes (e.g., pollution, eutrophication) (high confidence), leading to interactive, complex and amplified impacts for species and ecosystems.”

Warming-induced biodiversity decline might in turn also act as a positive feedback on that climatic warming, through large-scale disturbances of the carbon cycle:

“Carbon stored in the terrestrial biosphere is susceptible to loss to the atmosphere as a result of climate change, deforestation and ecosystem degradation (high confidence). The aspects of climate change with direct effects on stored terrestrial carbon include high temperatures, drought and windstorms; indirect effects include increased risk of fires, pest and disease outbreaks . Increased tree mortality and associated forest dieback is projected to occur in many regions over the 21st century (medium confidence), posing risks for carbon storage, biodiversity, wood production, water quality, amenity and economic activity.”

Even climate change-induced sea level rise can create ecological damage and biodiversity loss:

“Coastal systems and low-lying areas will increasingly experience submergence, flooding and erosion throughout the 21st century and beyond, due to sea level rise (very high confidence). The population and assets projected to be exposed to coastal risks as well as human pressures on coastal ecosystems will increase significantly in the coming decades due to population growth, economic development and urbanization (high confidence). Climatic and non-climatic drivers affecting coral reefs will erode habitats, increase coastline exposure to waves and storms and degrade environmental features important to fisheries and tourism (high confidence).”

Both IPCC AR4 and IPCC AR5 state that 21^st century climate change – after a possible initial phase of ‘CO2 fertilisation’ – could turn the global biosphere into a net carbon source. (Above image from IPCC AR4.)

© Rolf Schuttenhelm | www.bitsofscience.org