Changes in the Earth's environment are now sufficiently complex that our ability to forecast the emergent ecological consequences of ocean acidification (OA) is limited. Such projections are challenging because the effects of OA may be enhanced, reduced or even reversed by other environmental stressors or interactions among species. Despite an increasing emphasis on multifactor and multispecies studies in global change biology, our ability to forecast outcomes at higher levels of organization remains low. Much of our failure lies in a poor mechanistic understanding of nonlinear responses, a lack of specificity regarding the levels of organization at which interactions can arise, and an incomplete appreciation for linkages across these levels. To move forward, we need to fully embrace interactions. Mechanistic studies on physiological processes and individual performance in response to OA must be complemented by work on population and community dynamics. We must also increase our understanding of how linkages and feedback among multiple environmental stressors and levels of organization can generate nonlinear responses to OA. This will not be a simple undertaking, but advances are of the utmost importance as we attempt to mitigate the effects of ongoing global change.
Climate Change, Ocean Acidification, and Ocean Warming
Ocean acidification is projected to lower the Ωar of reef waters by 0.3-0.4 units by the end of century making it more difficult for calcifying organisms to secrete calcium carbonate while at the same time making the environment more favorable for abiotic and biotic dissolution of the reef framework. There is great interest in being able to project the point in time when coral reefs will cross the tipping point between being net depositional to net erosional in terms of their carbonate budgets. Periodic in situ assessments of the balance between carbonate production and dissolution that spans seasonal time scales may prove useful in monitoring and formulating projections of the impact of ocean acidification on reef carbonate production. This study represents the first broad scale geochemical survey of the rates of net community production (NCP) and net community calcification (NCC) across the Florida Reef Tract (FRT). Surveys were performed at approximately quarterly intervals in 2009-10 across seven onshore-offshore transects spanning the upper, middle and lower Florida Keys. Averaged across the FRT, the rates of NCP and NCC were positive during the spring/summer at 62 ± 7 and 17 ± 2 mmol m-2 d-1, respectively, and negative during the fall/winter at -33 ± 6 and -7 ± 2 mmol m-2 d-1. The most significant finding of the study was that the northern-most reef is already net erosional (-1.1 ± 0.4 kg CaCO3 m-2 y-1) and mid-reefs to the south were net depositional on an annual basis (0.4 ± 0.1 kg CaCO3 m-2 y-1) but erosional during the fall and winter. Only the two southern-most reefs were net depositional year-round. These results indicate that parts of the FRT have already crossed the tipping point for carbonate production and other parts are getting close.
Cold-water corals, such as Lophelia pertusa, are key habitat-forming organisms found throughout the world's oceans to 3000 m deep. The complex three-dimensional framework made by these vulnerable marine ecosystems support high biodiversity and commercially important species. Given their importance, a key question is how both the living and the dead framework will fare under projected climate change. Here, we demonstrate that over 12 months L. pertusa can physiologically acclimate to increased CO2, showing sustained net calcification. However, their new skeletal structure changes and exhibits decreased crystallographic and molecular-scale bonding organization. Although physiological acclimatization was evident, we also demonstrate that there is a negative correlation between increasing CO2 levels and breaking strength of exposed framework (approx. 20–30% weaker after 12 months), meaning the exposed bases of reefs will be less effective ‘load-bearers’, and will become more susceptible to bioerosion and mechanical damage by 2100.
Most current cost−benefit analyses of climate change policies suggest an optimal global climate policy that is significantly less stringent than the level required to meet the internationally agreed 2 °C target. This is partly because the sum of estimated economic damage of climate change across various sectors, such as energy use and changes in agricultural production, results in only a small economic loss or even a small economic gain in the gross world product under predicted levels of climate change. However, those cost−benefit analyses rarely take account of environmental tipping points leading to abrupt and irreversible impacts on market and nonmarket goods and services, including those provided by the climate and by ecosystems. Here we show that including environmental tipping point impacts in a stochastic dynamic integrated assessment model profoundly alters cost−benefit assessment of global climate policy. The risk of a tipping point, even if it only has nonmarket impacts, could substantially increase the present optimal carbon tax. For example, a risk of only 5% loss in nonmarket goods that occurs with a 5% annual probability at 4 °C increase of the global surface temperature causes an immediate two-thirds increase in optimal carbon tax. If the tipping point also has a 5% impact on market goods, the optimal carbon tax increases by more than a factor of 3. Hence existing cost−benefit assessments of global climate policy may be significantly underestimating the needs for controlling climate change.
Understanding social-ecological system dynamics is a major research priority for sustainable management of landscapes, ecosystems and resources. But the lack of multi-decadal records represents an important gap in information that hinders the development of the research agenda. Without improved information on the long-term and complex interactions between causal factors and responses, it will be difficult to answer key questions about trends, rates of change, tipping points, safe operating spaces and pre-impact conditions. Where available long-term monitored records are too short or lacking, palaeoenvironmental sciences may provide continuous multi-decadal records for an array of ecosystem states, processes and services. Combining these records with conventional sources of historical information from instrumental monitoring records, official statistics and enumerations, remote sensing, archival documents, cartography and archaeology produces an evolutionary framework for reconstructing integrated regional histories. We demonstrate the integrated approach with published case studies from Australia, China, Europe and North America.