The term Blue Carbon (BC) was first coined a decade ago to describe the disproportionately large contribution of coastal vegetated ecosystems to global carbon sequestration. The role of BC in climate change mitigation and adaptation has now reached international prominence. To help prioritise future research, we assembled leading experts in the field to agree upon the top-ten pending questions in BC science. Understanding how climate change affects carbon accumulation in mature BC ecosystems and during their restoration was a high priority.Controversial questions included the role of carbonate and macroalgae in BC cycling, and the degree to which greenhouse gases are released following disturbance of BC ecosystems. Scientists seek improved precision of the extent of BC ecosystems; techniques to determine BC provenance; understanding of the factors that influence sequestration in BC ecosystems, with the corresponding value of BC; and the management actions that are effective in enhancing this value. Overall this overview provides a comprehensive road map for the coming decades on future research in BC science.

pyrifera) and poleward range-extension of a key herbivore (sea urchin) and other 40 trophically important reef organisms has occurred. Although, evidence of changes on other 41 coastlines around Australia is limited, we suggest that this is due to a lack of data rather 42 than lack of change. Because of the east-west orientation of the south coast, most of 43 Australia's temperate waters are found within a narrow latitudinal band, where any 44 southward movement of isotherms is likely to affect species across very large areas. Future 45 increases in temperature are likely to result in further range-shifts of macroalgae and 46 associated species, with range-contractions and local extinctions to be expected for species 47 that have their northern limits along the southern coastline. While there is currently no 48 evidence of changes attributable to non-temperature related climate impacts, potentially 49 due to a lack of long-term observational data, experimental evidence suggests that ocean 50 acidification will result in negative effects on calcifying algae and animals. More 51 importantly, recent experiments suggest the combined effects of climate change and non-52 climate stressors (overharvesting, reduced water quality) will lower the resilience of 53 temperate marine communities to perturbations (e.g. storms, diseases, introduced species), 54 many of which are also predicted to increase in frequency and/or severity. Thus climate 55 change, both singularly and synergistically, imposes change to southern Australian coastal 56 3 species, including important habitat-forming algae and the associated ecological 57 functioning of temperate coasts. Management of local and regional scale stresses may 58 increase the resistance of temperate marine communities to climate stressors and as such, 59provides an attractive management tool for building resilience in temperate systems. 60 4

In recent decades, global climate change [1] has caused profound biological changes across the planet [2-6]. However, there is a great disparity in the strength of evidence among different ecosystems and between hemispheres: changes on land have been well documented through long-term studies, but similar direct evidence for impacts of warming is virtually absent from the oceans [3, 7], where only a few studies on individual species of intertidal invertebrates, plankton, and commercially important fish in the North Atlantic and North Pacific exist. This disparity of evidence is precarious for biological conservation because of the critical role of the marine realm in regulating the Earth's environmental and ecological functions, and the associated socioeconomic well-being of humans [8]. We interrogated a database of >20,000 herbarium records of macroalgae collected in Australia since the 1940s and documented changes in communities and geographical distribution limits in both the Indian and Pacific Oceans, consistent with rapid warming over the past five decades [9, 10]. We show that continued warming might drive potentially hundreds of species toward and beyond the edge of the Australian continent where sustained retreat is impossible. The potential for global extinctions is profound considering the many endemic seaweeds and seaweed-dependent marine organisms in temperate Australia.

Predictions about the ecological consequences of oceanic uptake of CO 2 have been preoccupied with the effects of ocean acidification on calcifying organisms, particularly those critical to the formation of habitats (e.g. coral reefs) or their maintenance (e.g. grazing echinoderms). This focus overlooks the direct effects of CO 2 on non-calcareous taxa, particularly those that play critical roles in ecosystem shifts. We used two experiments to investigate whether increased CO 2 could exacerbate kelp loss by facilitating non-calcareous algae that, we hypothesized, (i) inhibit the recovery of kelp forests on an urbanized coast, and (ii) form more extensive covers and greater biomass under moderate future CO 2 and associated temperature increases. Our experimental removal of turfs from a phase-shifted system (i.e. kelp-to turf-dominated) revealed that the number of kelp recruits increased, thereby indicating that turfs can inhibit kelp recruitment. Future CO 2 and temperature interacted synergistically to have a positive effect on the abundance of algal turfs, whereby they had twice the biomass and occupied over four times more available space than under current conditions. We suggest that the current preoccupation with the negative effects of ocean acidification on marine calcifiers overlooks potentially profound effects of increasing CO 2 and temperature on non-calcifying organisms.

Climate‐driven change represents the cumulative effect of global through local‐scale conditions, and understanding their manifestation at local scales can empower local management. Change in the dominance of habitats is often the product of local nutrient pollution that occurs at relatively local scales (i.e. catchment scale), a critical scale of management at which global impacts will manifest. We tested whether forecasted global‐scale change [elevated carbon dioxide (CO2) and subsequent ocean acidification] and local stressors (elevated nutrients) can combine to accelerate the expansion of filamentous turfs at the expense of calcifying algae (kelp understorey). Our results not only support this model of future change, but also highlight the synergistic effects of future CO2 and nutrient concentrations on the abundance of turfs. These results suggest that global and local stressors need to be assessed in meaningful combinations so that the anticipated effects of climate change do not create the false impression that, however complex, climate change will produce smaller effects than reality. These findings empower local managers because they show that policies of reducing local stressors (e.g. nutrient pollution) can reduce the effects of global stressors not under their governance (e.g. ocean acidification). The connection between research and government policy provides an example whereby knowledge (and decision making) across local through global scales provides solutions to some of the most vexing challenges for attaining social goals of sustainability, biological conservation and economic development.