In one year, in September 2018, the Intergovernmental Panel on Climate Change (IPCC) is going to publish a special report on the impacts of global warming of 1.5°C. Back in 2015, when the international community adopted the Paris Agreement, it requested that the IPCC would draft such a report. This is certainly linked to the targets contained in the Paris Agreement: Global average temperature rise shall not only be kept well below 2°C but efforts shall also be pursued to stay below the 1.5°C threshold. Many commentators back then doubted that this second target will indeed be abided by the major emitters. It was assessed that the recognition of 1.5°C would only be an appeasement to vulnerable nations and a political move by more powerful nations. Moreover, there are concerns on whether the 1.5°C target is even still attainable.
The special report of the IPCC shall thus bring light into the darkness. It will both offer an overview of the compatible mitigation pathways to reach the 1.5°C as well as an insight into the impacts of 1.5°C global warming on natural and human system, especially into the avoided impacts compared to the recognised 2°C target. Thereby, the state as well as the future development of coral reefs could play an integral part. The United Nations Educational, Scientific and Cultural Organization (UNESCO) has recently published the first global scientific assessment of climate change impacts on World Heritage coral reefs. It already sketches out to a certain extent what can be expected from the IPCC Special Report next year.
Coral reef ecosystems host more than 25 percent of all marine fish species, despite the fact that they cover only about 0.1 percent of the bottom of the ocean. This translates into more than one million species, making coral reef ecosystems the most biodiverse areas in the ocean. It is thus no overstatement to denote coral reefs as the “tropical rainforests of the ocean”. Besides, the coral reefs also provide viable social, economic and cultural services to human communities. For example, they represent a barrier against storm surges as well as important fishing grounds. In addition, they serve for recreational needs and attract a significant number of tourists. Thus, they provide benefits to over half a billion people and supply ecosystem services estimated at more than USD one trillion globally.
However, the coral reef ecosystems are increasingly under pressure, as the underlying symbiotic relationship between corals and certain types of algae becomes toxic under deteriorating environmental conditions. This leads to coral bleaching, which may eventually lead to the die-off of the corals. Reasons for coral bleaching are inter alia insufficient light levels and water quality due to water pollution. Climate change is another factor rapidly worsening the situation in these ecosystems due to higher sea temperatures, ocean acidification, sea-level rise, intensifying storms and disease spreading. Already one or two degrees above-average seawater temperatures can result in bleaching, although resilience depends on the specific location.
According to the assessment of UNESCO, nearly half (13) of the 29 World Heritage sites that contain coral reef systems were faced by heat stress more than twice per decade on average during the period 1985-2013. Only three areas were not exposed to any heat stress during this time and thus did not show any bleaching. However, in correspondence with the El Niño event 2015-2016, the worst global bleaching event occurred and lasted from 2014 until recently in 2017. During this event, 21 of the 29 of the places with “outstanding universal value” have experienced severe and/or repeated heat stress. Only four sites were not affected. This is quite alarming as it typically takes around 15 to 25 years for the corals to recover from such mass bleaching events.
With respect to the future prospects, the peer-reviewed scientific literature has already concluded that only limiting global average temperature rise to 1.5°C above pre-industrial levels provides a chance of preserving a high share of existing coral reef ecosystems. The UNESCO assessment predicts that under a business-as-usual scenario, most World Heritage reefs (25 of the 29) will be exposed to an intensity and frequency of heat stress that the reefs could not recover from already by 2040. The remaining four sites would be faced with such conditions as well before the end of this century. Under a more optimistic temperature scenario with 2.4°C warming until 2100, less than half of the World Heritage sites (14) will experience serious bleaching twice per decade before 2040, with only two reef systems not exposed to such conditions even in 2100. Nevertheless, also this scenario will not prevent a significant decline in coral reefs. Only in case of limiting warming to 1.5°C the frequency of severe bleaching and death of corals is significantly reduced.
Corals have some capacity to adapt to new environmental conditions, but it is unknown to which extent this is valid. The scientific literature indicates that coral reefs have never experienced conditions similar to the current combination of stressors. Especially the fast time scale of changing temperature conditions is alarming, since adaptation of coral reefs takes time given their long lifetimes of up to 100 years. In any case more ambitious emissions reduction pathways give more time for the corals to adapt and offer the opportunity to develop new approaches for management and rehabilitation as well as to reduce the other stressors. However, UNESCO warns that local management are not sufficient to protect the World Heritage coral reef properties.
Nonetheless, new management approaches provide for improvements at least in the short-term. For instance, in Belize, corals grown in sea nurseries have increased coral cover by 35 percent and the corals have survived natural and anthropogenic stressors for around the last ten years. Mexico just started a pilot programme to preserve coral reefs with an insurance system. Hotels and the government pay premiums, which enables close monitoring of the reef and restoration in case of storm surges. The approach of a payment for an ecosystem service is expected to be profitable, as the reef protects the hotels and the coastline as a natural barrier against the surges.
The IPCC Special Report on 1.5°C will not only focus on the impact on coral reefs. A 2016 study already indicated several other fields, where there are significant differences in impacts between 1.5°C and 2°C. This includes the frequency and intensity of extreme weather events, water availability in the Mediterranean, agricultural yields in Africa, Asia and America, and sea-level rise. Thus, there are large regional differences to be taken into account.
Still, the coral reef problem certainly makes the case for a shift to a more ambitious goal of 1.5°C. To lose the “rainforests of the oceans” with their staggering biodiversity would certainly have substantial global impacts, particularly for poorer nations dependent on the ecosystem services provided by the coral reefs. Therefore, the recent peer-reviewed scientific literature and the UNESCO assessment already indicate that the discussion on the 1.5°C target could gain further traction following the IPCC Special Report next year. It will be interesting how this discourse will influence the international climate talks and especially how the major emitters will position themselves. It may show how serious countries are about the efforts to reach 1.5°C and if they are willing to adjust their contributions to mitigate climate change respectively.
(Image: Coral reef, Pacific, Photo by Tom Nuget, 2008. Source: WorldFish, flickr)