Not even 1.5°C is good enough

The international community is still far from agreeing on a clearly defined maximum limit for global warming. Yet, parts of marine ecosystems face catastrophic risks already at current levels of warming.

The special report on “Impacts of 1.5°C of Global Warming on Natural and Human Systems” (SR 1.5)1, which was published by the International Panel on Climate Change (IPCC) in October last year, very clearly highlights the tremendous difference in effects at 1.5°C compared to those at 2°C. The Paris Agreement from 2016 states that the warming should be kept “well below 2 degrees Celsius”, whilst pursuing “efforts to limit the temperature increase even further to 1.5 degrees Celsius”. Evidently, the IPCC special report shows that the latter of these statements should be taken more seriously than ever, especially as 1.5°C already is predicted to cause severe damage to nature and human societies (and indeed damage is already observed at present warming of 1°C).

This is especially true for marine ecosystems, and whereas 1.5°C is already a politically highly ambitious aim, it is also evident for these systems that serious effects will be observed in oceans around the world even if this goal is achieved.

Ocean ecosystems were specifically analysed in chapter 3.4.4 of the SR 1.5, followed by further in depth analysis in the very recent Summary for Policy Makers of the IPCC Special Report on the Ocean and Cryosphere in a Changing Climate (SPM SROCC)². Based on these reports, ocean ecosystems are expected to experience (or are already experiencing) effects caused by, for instance, physical disturbance (e.g. storms, rising water levels), changes in ocean physics (e.g. currents, stratification), rising seawater temperatures, and acidification. Many of these factors interact, and effects are in many cases strengthened by other human induced disturbances, such as coastal infrastructure, habitat destruction, overfishing, etc.

It is particularly alarming that all groups of organisms that the IPCC used to exemplify and illustrate the risks at different levels of global warming (in this case as warming of sea surface temperature, SST) in the SR 1.5 are expected to experience at least moderate effects already at warming below 1.5°C SST (Figure)³.

The highest risks are for warm water corals, which are already heavily impacted at current levels of warming, as evidenced by large scale mortalities. As also other human impacts have heavily affected warm water coral reefs, they have declined substantially over the last three decades. As stated in Box 3.4 of the SR 1.5 (on Tropical coral reefs): “Even with warming until today […], a substantial portion of coral-reefs have experienced large-scale mortalities that have led to much reduced coral populations”. In recent years, climate effects have become more and more prominent factors. In, for instance, the Great Barrier Reef a loss of 50% has been observed following elevated temperatures and bleaching of corals in 2016 - 2017. Adding to this, a very recent study⁴ that has been published after the special report, shows that recovery is hampered by a severe crash in recruitment (production of new corals), as evidenced by an 89% loss of new corals. Even if the world succeeds in achieving the 1.5°C aim, the estimated prediction for warm water corals is an additional loss of 70-90%. With 2°C warming, the estimated effect approaches a total loss.

Mid latitude bivalves and high latitude, Pteropods (tiny, free-swimming sea snails) are the example-groups of organisms that next to warm water corals are the most sensitive to conditions at present day warming and higher. These organisms are vulnerable to acidification, which disrupts their shell formation, and they can also be directly affected by the increased temperatures. Both groups constitute highly important parts of the foodwebs of the oceans, and declines in their populations are a serious threat to the functioning of these, and to fish and other animals that depend on them as food.

Finfish and krill are also already at moderate risk at present day warming, and expected to experience or approach high risks at 1.5°C. Like for most other groups, the mechanisms behind these risks are complex. For finfish, these could involve changes in net primary productivity, upwelling conditions, stratification, ocean circulation etc., and any associated changes in foodweb structure and food availability. Finfish can also be affected by ocean acidification, as for instance, evidenced by a recent study, demonstrating effects of elevated (end-of-century) CO₂-levels on Atlantic cod larvae (in combination with food limitation)⁵.

As finfish are mobile, for many species observed or predicted effects could imply geographic redistributions of fish populations, for instance through expansions to high latitudes and at the cost of populations and fisheries at lower latitudes. In fact, and as the IPCC report states, with reference to low and mid latitudes, “Reduced ocean upwelling has implications for millions of people and industries that depend on fisheries for food and livelihood”.

Seagrasses (mid latitude) and mangroves finally make up the two example groups where risks are expected to remain moderate at warming up to 1.5°C (but reach a high level for seagrasses between 1.5 - 2°C). Seagrasses are predicted to suffer in particular from temperature extremes, but also from indirect effects of turbidity. Mangroves are affected not only temperature extremes, but most notably also illustrate effects of sea level changes in combination with human activities. Sea level changes are crucial as mangroves are specialized to the conditions that prevail in the transition zone between the coast and adjacent terrestrial areas. Mangroves could gradually shift shorewards as a consequence of changes in sea level. However, the occurrence of shoreward refugia is seriously limited by coastal infrastructure, positioning mangrove communities in what is referred to as a “coastal squeeze” in the IPCC special report. Human activities furthermore reduce available areas for mangroves by negatively impacting sediment supply.

Given all these serious effects on organisms and ecosystems that have been observed already, or are predicted with further warming and acidification, it is not surprising that the IPCC special report also highlights the risks of these effects translating into severe implications for ecosystem services. These risks, which are summarized in the lower panel of Figure, contain both direct negative effects on specific livelihoods (e.g. fisheries, tourism) and inhibition of large scale, global services such as carbon uptake and net primary production. In addition, coastal protection for human conditions against e.g. intensifying storms, rising sea level and larger waves is impeded by the loss of structures such as coral reefs and mangroves.

Overall, a general conclusion that can be drawn from the IPCC special report is a great concern for the risks associated already with a 1.5°C global temperature rise. This fact stresses the urgency to limit greenhouse gas emissions rapidly and with a global commitment. It also underlines the need to act now to help human communities, many of which are in countries in economic transition, to adapt to ecosystem effects that are sadly already happening, and continue to be at serious risk even if the currently most ambitious aims could be met.

Marko Reinikainen

¹ Temperature rise refers to comparison with pre-industrial Global Mean Surface Temperature (GMST)
² See e.g. Figure SPM3. and Box B3 in the SPM SROCC
³The SR 1.5 Figure 3.18 is used here to illustrate effects of climate change as it incorporates coastal and marine organisms as well as ecosystem services and sectors (as opposed to the similar Figure SPM3. of the SPM SROCC, which, however, contains additional ecosystems and organisms). The SPM SROCC is commented on in the Editorial of this number of Acid News.
⁴Hughes et al. 2019. Nature vol. 568, pp. 387–390
⁵Stiasny et al. 2018. Global Change Biology vol. 25, pp 839-849.

Figure. Risks for specific marine and coastal organisms, ecosystems and sectors. The key elements are presented here as a function of the risk level assessed between 1.5 and 2°C (Average global sea surface temperature).