4.1 Statistical meaning of “normal variation”.
As pointed out above, there is an upward trend in the global temperature over a period of decades to centuries, statistically incompatible with variation around a stable mean. When the mean moves outside its prior zone of uncertainty, it is necessary to conclude that it is “changing”. Modelling and observation both definitively support the role of increasing atmospheric CO2 in that process.
Climate has varied before and there obviously continues underlying year-to-year fluctuations. But such past changes have occurred on much slower time-scales than occurring currently unless precipitated by a cataclysmic event such as a meteor. Also, because much of the warming can be attributed to the increase in heat trapping gases arising from human activity, it is not “normal” variation (see Section 3).
4.2 Evidence from impact on marine species:
Despite this, the “normal variation” claim is often raised. Another way to benchmark current climate heating against natural variation is to look at its impact on biodiversity. One of the core assumptions of evolution is that species adapt to survive in their ecological niche. On this measure, climate change is contributing to the extinction of both land and marine species. Rapid and widespread extinction events due to heat are not consistent with “normal variation.”
Perhaps the clearest example is the bleaching and collapse of the Great Barrier Reef (GBR), which is a diverse ecological phenomenon comprising thousands of coral and related species. Widespread bleaching events occurred on the GBR in 1998, 2002, 2016 and 2017. Although coral can recover from local bleaching events,
“The time between recurrent events is increasingly too short to allow a full recovery of mature coral assemblages, which generally takes from 10 to 15 years for the fastest growing species and far longer for the full complement of life histories and morphologies of older assemblages … we are already approaching a scenario in which every hot summer, with or without an El Niño event, has the potential to cause bleaching and mortality at a regional scale” [paper].
In other words, current rates and depths of coral bleaching exceed the bounds of the ability of coral to adapt and recover, incompatible with natural variation.
Opponents to this view often argue that there have been previous bleaching events. But, like global temperature, the current rate and depth of bleaching is not normal, a calculation recently made possible by the generation of a high-resolution global mass coral bleaching database [paper],
“Recurrent regional-scale (>1000 km) bleaching and mortality of corals is a modern phenomenon caused by anthropogenic global warming: Bleaching before the 1980s was recorded only at a local scale of a few tens of kilometres because of small-scale stressors such as freshwater inundation, sedimentation, or unusually cold or hot weather. The modern emergence of regional-scale bleaching is also evident from the growth bands of old Caribbean corals: synchronous distortions of skeletal deposition (stress bands) along a 400-km stretch of the Mesoamerican Reef have only been found after recent hot conditions, confirming that regional-scale heat stress is a modern phenomenon caused by anthropogenic global warming.”
“The 2015-2016 global bleaching event is a watershed for the Great Barrier Reef, and for many other severely affected reefs elsewhere in the Indo-Pacific. The most likely scenario, therefore, is that coral reefs throughout the tropics will continue to degrade. The 2016 marine heatwave has triggered the initial phase of that transition on the northern, most pristine region of the Great Barrier Reef, changing it forever as the intensity of global warming continues to escalate. The large-scale loss of functionally-diverse corals is a harbinger of further radical shifts in the condition and dynamics of all marine ecosystems.” [paper].
Not just the GBR, but also“Coral reefs across the world’s oceans are in the midst of the longest bleaching event on record … the period of summer-like temperatures has become longer through the record, with a corresponding shortening of the ‘winter’ reprieve from warm temperatures. The frequency of bleaching-level thermal stress increased three-fold between 1985–91 and 2006–12.” [paper]
As recently concluded in a Nature article, “Immediate global action to curb future warming is essential to secure a future for coral reefs.” [paper]
It is tempting to view a coral bleaching event as an isolated incident due to a combination of local ocean currents and cloudless days. But of fundamental concern is that coral bleaching is just one manifestation of marine heatwaves that are increasing across all marine environments. From 1925 to 2016, the number of marine heatwaves occurring globally increased by 54% [paper]. For example, an unprecedented marine heatwave in the Tasman Sea in 2015 was 300 times more likely to be due to anthropogenic climate change than to natural variation [paper].
Marine heatwaves represent a substantial threat to global marine species, from predator species [paper] to molluscs [paper], seaweed (kelp) [paper] and to species at the bottom of the food chain such as plankton [paper]. Marine heatwaves can also cause large-scale deaths and reproductive failures of birds [paper]. Because of the enormous biological reservoir of the ocean, and the role of marine species in atmospheric gas exchange, extinction events amongst marine species are of substantial concern. This is particularly true of plankton, which fixate ∼45 gigatons of organic carbon per annum [paper] and are a food source for many other marine species.
4.3 Evidence from impact on terrestrial species:
The broader lens to view the global collapse of coral reefs is through the widespread, accelerated loss of terrestrial and marine species: Current extinction rates are approximately 100 times above the background rate, far outstripping the appearance of new species [paper]. As with marine species, terrestrial species across the breadth of the food chain are being lost, with as many as one million species facing extinction [paper]. This includes the loss of as much as three quarters of total insect biomass (including bees, moths, butterflies) since 1990 [paper]; Over 40% of insect species are threatened with extinction [paper]. As with plankton, the loss of insects is particularly concerning as in addition to their role in pollination and nutrient cycling, they are a major food source for birds, mammals and amphibians. The extent of future global heating will impact substantially here: If warming is limited to 1.5°C as compared with 2°C, the numbers of insect species projected to halve in number is reduced by two thirds [paper].
The causes of this accelerated extinction rate are diverse, but clearly do not sit within the boundaries of “normal variation”. The extinction of native Australian species has many causes, including deforestation, ecological fragmentation, disease and domestic pets such as dogs and cats [paper]. In addition, climate heating – particularly heat waves – are playing an increasing role, with extreme heat waves causing “extinction events” (repeated and unsustainable mass deaths) for many species. A telling example is the wild flying-fox (Pteropus spp.) of which there are four species on the Australian mainland (Little Red, Black, Grey-headed and Spectacled). Flying foxes are keystone species, with a crucial ecological role of pollination and seed dispersal, which have declined in numbers by up to 95% in the past century and may become functionally extinct (decline to irreversible numbers) in the next few decades [site].
Flying-foxes show a sequence of thermoregulatory behaviours with rising temperatures (wing fanning, clustering, salivating, panting) [site]. However, these mechanisms become overwhelmed beyond 42º C and mass fatalities in bat colonies soon occur [paper]. Single heat-wave events can kill as many as 23-49% of young female bats [paper], killing over 100 000 in the last decade and occurring with increasing frequency. On January 4th, 2014, record heat temperatures were recorded across nine locations in south east Queensland, killing 45,500 flying foxes in one day and leaving 1000 orphaned. Reports suggested that this represented half of the black flying fox population in the region before the heatwave [media].
Other Australian native species are threatened by global heating, including Koalas, which are vulnerable to heat waves and other climate changes including longer dry spells: For example, Koala numbers declined by 80% between 1995 and 2009 in south-west Queensland [paper]. In late 2009, heatwaves killed an estimated 25% of a koala population in Gunnedah, NSW. Global heating can magnify other threats to Koala survival, such as land clearing which concentrates koala populations in sub-optimal habitat away from permanent water. Global heating also selectively threatens the viability of tall trees in old-growth forests [paper] which represent the natural habitat of koalas and many other Australian native species.
Next section: Climate change and bushfires