In this section, risk refers to residual risk, or the risk that a pressure will still result in an impact after management actions are applied. Pressures in the assessment summary are partitioned into those that are managed, and those that are unmanaged or for which impacts cannot be predicted. This assessment is qualitative rather than quantitative, and assumes that current management practices continue. Quantitative risk assessments are best practice and are increasingly implemented in coastal management at local and regional scales, such as the threat and risk assessment framework for the New South Wales marine estate (MEMA 2015), but are beyond the scope of this report.

The most significant risks are those associated with climate change. Sea level rise, and changes in climate and weather fall within the critical zone of the risk matrix, as both are almost certain and will have catastrophic impacts. Moreover, Australia has relatively little means to manage these risks, because of the global scale of both the driver and consequent pressures. Sea level rise will not only cause inundation of low-lying coast, it will also lead to extensive erosion inland and the destruction of coastal habitats. Increased storm activity will have a similar effect in exacerbating loss of coastal land and habitats.

Invasion by exotic species is another risk that is almost certain to occur in the future, given ongoing shipping between Australia and other regions. Invader establishment and spread are highly unpredictable, because they are determined by the combination of invader traits and density, and the recipient biotic and abiotic environment. Most exotic organisms that enter Australia do not become invaders, but those that do can be environmentally and economically devastating. Globally, the annual cost of introduced species exceeds US$1.4 trillion—5 per cent of GDP (Pimentel et al. 2001), and in Australia alone the cost is more than $7 billion per year (CSIRO 2011). Australia has a national ballast water management system in place, but currently there is no enforced national system for managing biofouling. Some states, such as Western Australia, have implemented biofouling regulations, and extending these nationally would go some way towards reducing risk. Domestic ballast water regulations are another measure that would reduce spread, but are currently only implemented in Victoria.

Most risks from managed pressures are considered to have high likelihood (i.e. are likely or almost certain to occur), but vary in the severity of their impact. Of these, coastal river and estuary pollution are predicted to have the most severe impacts, and both are almost certain to continue as coastal population grows. Current management is only partly effective in reducing their impact, particularly for diffuse sources.

Pressures considered low risk include tourism and recreation, desalination and artificial structures. Although almost certain to occur, they are generally well managed and/or small in extent. Desalination plants and artificial structures are both growing in number around Australia, but have highly localised effects. A 5-year study of the impacts of the Sydney Desalination Plant found that effects on marine invertebrate recruitment were limited to the vicinity of the outlet, and were undetectable within months of plant shutdown. Some species of fishes even appeared to benefit from the effluent, likely because of increased turbulence and food availability. Similarly, artificial structures benefit some fishes, but have severe but localised impacts on other taxa (e.g. burial of soft-sediment infaunal communities).

Risks associated with fishing differ between the commercial and recreational sectors because of differences in monitoring and management. Impacts from both activities are considered moderate, but better data for commercial fishing allow the risk to be considered likely, whereas a lack of data for recreational fishing means it must be considered possible. Unlike most other countries with coastal fishing waters, it is widely believed that impacts of recreational fishing in Australia exceed those of commercial fishing, but more data are needed to test this.

Microplastics are a major unknown risk, as their use and introduction to the coastal environment has far outpaced research into their potential impacts. Microscopic plastic fibres are embedded into many clothing and consumer products, and are introduced into waterways when these items are washed and fibres enter waste water. Nanoparticles are a similar issue, where research and environmental legislation have not kept up with emergence of novel pollutants.