Implications for Ecosystems and People*
* Coordinating lead authors: O. Hoegh-Guldberg – University of Queensland, Australia; H. Poertner – Alfred-Wegener-Institute, Germany; R. Cai – State Oceanic Administration, China; D. Karl – University of Hawaii, Hawaii; I. Losada – University of Cantabria, Spain; P. Wong – National University of Singapore, Singapore.
THE latest assessment by the IPCC (AR5) demonstrates with very high confidence that increasing atmospheric carbon dioxide and other greenhouse gases (GHGs) are producing, and will produce fundamental changes to the physical, chemical, and biological properties of our centrally important ocean. These changes pose serious risks for human communities and industries everywhere.
There has been a growing appreciation of the impact of climate change on the ocean. In keeping with this, the recent IPCC AR5 consensus increases the focus on the ocean, significantly ramping up our understanding of the influence of climate change on the ocean. Various chapters exclusively dealing with ocean issues are found in the AR5. To aid a new and integrated understanding of the oceans under climate change, the authors of this communiqué have compiled a separate volume which contains the key ocean-related chapters and cross-chapter boxes from Working Group II (WGII) in the AR5.
The news contained in these chapters is significant and builds on the consensus from WGI that the chemical and physical conditions of the ocean are changing at rates which are, in some cases, faster than any seen during the past 65 million years. The contributions indicate with high confidence that many marine organisms and ecosystems are undergoing fundamental change as the world‘s oceans warm, acidify and lose oxygen. These changes emphasise the serious risks that the world‘s oceans face as we approach 2°C above the Preindustrial Period or atmospheric concentrations of carbon dioxide of 450 ppm and above.
Reducing these risks must be a global priority if we are to avoid grave consequences for ecosystems and society. The evidence, much of it held with high confidence, of these risks and the solutions are contained within this special collection of ocean-related reprints from WGII.
Conditions in the ocean are changing extremely rapidly
The ocean has absorbed approximately 30% of the average carbon dioxide from the atmosphere, as well as 93% of the extra energy from the enhanced greenhouse effect. The upper layers of the ocean (> 700 m) have warmed and acidified over the past 60 years, and ocean volume (sea level) has increased. These changes will take a long time to reverse. Returning ocean chemistry to its original condition by continental weathering and run-off, for example, is estimated to take at least 10,000 years.
Increasing heat content in the upper layers of the ocean has resulted in greater stability and hence less mixing of the water column, leading to a reduced supply of nutrients to the sunlit upper layers of the ocean, where half of the world‘s primary productivity occurs. Warming is reducing oxygen concentrations overall through temperature effects on solubility, while reduced mixing contributes to hypoxic zones exacerbated by nutrient pollution.
The sea level is rising at an average rate of 3.2 mm per year. The increased rate of sea level rise is a consequence of thermal expansion of a warming ocean, as well as water being added by the melting western Antarctic and Greenland ice sheets. The projected sea level rise under ‘business as usual’ conditions by 2100 is 0.52 to 0.98 m, with an accelerating rate of 8 to 16 mm per year. Sea level rise has the potential to reach several metres over the next (few) centuries, and is a major risk for coastal ecosystems, communities and infrastructure.
Differences in representative concentration pathways (RCP) for the ocean are largely indistinct until 2035. Projected temperatures of the surface layers of the ocean, however, diverge beyond that date and will be 1°C to >3°C higher under RCP2.6 versus RCP8.5 than across most ocean sub-regions by 2100. Our understanding of how ocean water column structure, and other features such as upwelling will change, is in its infancy but should be a priority for further research.
Marine organisms and ecosystems are being transformed
The rapid changes in physical and chemical conditions within the ocean are already affecting the distribution and abundance of marine organisms and ecosystems. Many marine organisms (particularly fish and plankton) have already moved to higher latitudes consistent with warming trends, while there is medium confidence that the timing of key life-history events such as plankton blooms, spawning of fish and invertebrates, and migratory patterns are also changing in ways that are consistent with climate change.
Ocean acidification which is a consequence of increased carbon dioxide entering the ocean is reducing rates of calcification for organisms such as corals and pteropods, and is reducing the viability of many invertebrate populations through impacts on both adult and reproductive stages. There is also abundant evidence that ocean acidification interacts with thermal stress to narrow the range of conditions tolerated by many organisms. Ocean acidification has been reported to reduce rates of calcification, growth, scope for activities, and reproductive potential, and increase decalcification and mortality in a range of crucially important organisms.
The reduction in sea ice and the warming of high latitude ocean waters is associated with an increase in phytoplankton as well as fisheries productivity. In contrast, there is strong evidence that many important fishery species at low latitudes are declining in abundance and may face local extinction, while disease and introduced species are on the move, with implications for wild caught fisheries and aquaculture. Positive trends in the higher latitudes, unfortunately, are very likely to be temporary as waters continue to warm well into the future.
Impacts of heat stress and mass mortality have been increasingly reported for coral reef ecosystems and intertidal communities. When present rates of warming are compared to the tolerance of reef building corals, scientific consensus is almost certain that reef building coral communities, and therefore coral reefs, would disappear by mid-century, putting at risk the livelihoods of hundreds of millions of people.
Ocean change has serious implications for human well-being
A changing ocean poses risks to people and infrastructure. Rising sea levels in combination with changing storm intensity and inundation are placing large numbers of people and valuable coastal infrastructure at risk. The loss of ecosystems such as coral reefs will increase the exposure of low-lying islands and coastal areas to wave and storm damage, as well as reducing the ability of these countries and regions to attract and retain tourism. The re-organisation of marine ecosystems also has implications for food security, human wellbeing and economic development across latitudes. Exposure and vulnerability to these changes are highly differential and exacerbated by poverty and inequality. Changes in the conditions associated with the ocean are also very likely to affect ocean services such as carbon sequestration, gas exchange and weather patterns – with consequences for the global and regional climate systems.
Conclusion and solutions
The ocean is changing rapidly. As a result, risks are escalating for people and industries. This special collection of ocean-related reprints from WGII describes the risks and also the solutions. It addresses the latest science on how coastal communities can accommodate rising sea levels and storm damage, how industries such as fishing and aquaculture can anticipate and adapt to changing stock levels, and how reducing non-climate stresses on ecosystems will build ecological resilience to climate change related impacts. Human adaptation capacity is, however, limited and options for adaptation will eventually disappear under unabated climate change. Consequently, mitigation of the underlying reasons for ocean change (i.e. rising greenhouse gas levels) is critically important as this century unfolds.