Figure 26: Conceptual diagram of the Greenhouse Effect

6.3 Climate Change

The increase in greenhouse gases within the atmosphere, largely caused by the activities of humans, is causing the earth to warm (Figure 26). The ECMP contains actions that will assist effective adaptation to a changing climate. Key threats arising from climate change and their resulting impacts are summarised below.

The impacts of climate change will continue to increase in intensity. While uncertain, modelling for northern NSW indicates that impacts will start to have significant environmental, social and economic impacts after around 20 years. Accordingly, it is important to understand the implications of climate change now and start more detailed planning during the first five-years of the Manning River ECMP.

Figure 26: Conceptual diagram of the Greenhouse Effect

6.3.1 Key Threats

The following are the key threats facing the Manning estuary and catchment due to climate change.

Threat 1 - Sea Level Rise: Sea levels are rising as the Earth warms, mainly due to the expansion of water in the oceans as they warm, and the melting of ice from glaciers and polar ice caps. Therefore, tidal levels in the estuary will also rise.

Threat 2 - More Intense Extreme Rainfall: A warmer atmosphere can hold more moisture, leading to more intense rainfall events.

Threat 3 - Overall Drier Catchment: The warming climate will also cause the catchment to become drier over time. It is expected that droughts will become more prevalent.

Threat 4 - Warmer Water: The water in estuaries will heat up in line with the warming of the planet. One degree (Celsius) of warming has already occurred, and it is possible that by 2040 the estuary will have warmed by a further degree.

6.3.2 Key impacts

Impacts on Tidal Inundation

Results of the flood modelling completed for MCC in 2016 demonstrate the tidal inundation that will occur for both +0.28m of sea level rise (which could occur by 2040-2050) and +0.98m for around the year 2100. These show that inundation impacts mostly affect low lying areas adjacent to the estuary, including land which has been previously drained for agriculture.

Analysis completed by the NSW State Government indicates that a sea level rise of +0.5m would result in tides inundating around 38km of “road”, although most of these “roads” are “tracks” and local roads.

The upstream freshwater reaches of the river may be impacted by changes in tide and inundation levels.

Impacts on Sediment Dynamics

A drier catchment along with more extreme rainfall events will result in more extreme erosion from the catchment.

Proposed entrance modifications at Harrington may cause more sand to be carried into the estuary from the Ocean. The effects would depend on the entrance configuration ultimately adopted. These are presently being considered by Transport for NSW.

Impacts on Water Chemistry

Water chemistry changes due to climate change will be a complex issue. Some of the key effects are:

• More dissolved CO2 will lead to a more acidic ocean.

• Changes in tide will affect the generation of acid from acid sulfate soils.

• Less runoff and higher water levels would increase salinity in the estuary.

• Stagnant pools of water on the floodplain (from more tide inundation) may result in algal blooms.

Long-term monitoring and review along with preparation for adaptive management are the most appropriate tools available to face this issue.

Impacts on Intertidal Habitats, Fish Communities and Ecosystems

Due to rising water levels and tides in the estuary, intertidal habitats, in particular saltmarsh, are expected to migrate upslope. It is also possible that habitats may be less robust in response to the changing environment.

In response to water temperature and sea levels changing, it is expected that fish habitats will change. This is likely to cause changes to fish communities in the estuary.

As the water gets warmer, sub-tropical and tropical species are expected to become more prevalent. Impacts could be mixed, with some new species threatening existing ecosystem functions, and some being of social or economic benefit.

The productivity of estuarine ecosystems is likely to change in response to habitat and fish community shifts. It is a complex issue and difficult to say whether the change will be positive or negative.

Impacts on floodplain drainage and Acid sulfate soil exposure

Glamore and colleagues (2016) assessed the impact of rising sea levels on the Manning estuary floodplain as projected for 2050 and 2100. Forecasted increases in high tides will reduce drainage, cause overtopping of levees, impact on backswamp connectivity, and affect agricultural productivity in some regions. The Lower Manning River Drainage Remediation Plan notes that the greater issue for land management will be elevated low tides, which will reduce drainage from low-lying backswamps29 .

Stakeholders include: MCC; LLS; DPI-Fisheries; Commercial fishery and aquaculture businesses; DPIE-EES; Floodplain land holders; NPWS; Tourism (e.g., recreational fishing, ecotourism, boating); TfNSW (navigation and waterway access); Crown Lands; Local Aboriginal Land Councils, the Aboriginal community (e.g., impact on heritage sites); research institutions; DPIE-Planning; Federal government departments including the current Department of Agriculture, Water and the Environment.

Photo: Christine Price. Extreme weather events will become more regular as the climate warms up

29 (Glamore, Ruprecht, & and Rayner, Lower Manning River Drainage Remediation Action Plan, 2016)

6.4 Loss and degradation of coastal wetlands

Coastal wetlands in the Manning estuary are under significant threat from development, modified hydrology and the impacts of climate change. Key activities and stressors include:

• agricultural land use and associated floodplain draining and stock access

• urban development and associated filling

• degradation by pollutants such as acid drainage, increased nutrients and sediment loads

• clearing and fragmentation of vegetation

• climate change (particularly sea level rise) and drought

• weed and pest invasion

• inappropriate fire regimes

• general ignorance of wetlands values30 .

Most estuarine habitats including mangroves and saltmarsh were rated poor or fair in the Rapid Site Assessment. 31 Mangrove extent and connectivity has been reduced to narrow, patchy bands, rarely exceeding 10m wide, providing limited bank protection from high flows and boat wash. Distribution of saltmarsh identified through the Rapid Site Assessments was very low.

Stock impact on coastal wetlands and fringing vegetation was found to be a widespread threat to estuary health. Stock frequently have access to estuarine shorelines and wetlands leading to soil degradation, erosion, reduced vegetation and poor water quality.

Weed abundance within mapped wetlands was relatively low. Environmental weeds such as Lantana, Bitou Bush, Tree Pear, Coastal Morning Glory and Cassia were becoming established in some wetlands, particularly on the floodplain. Several Swamp Oak and Mangrove Forests were observed to be dominated by an understorey of the invasive weed Juncus acutus (Sharp Rush) which should be prioritised for treatment.32 .

Sambar deer, foxes, hares and gambusia fish are present in Cattai Wetlands. Deer trample and graze on wetland vegetation. Foxes prey on native species, particularly impacting threatened and migratory shorebirds near the north and south entrances.

Stakeholders include: MidCoast Council, Hunter LLS, DPIE, DPI-Fisheries, NSW Government, NPWS, WRL; private landholders, MCC, Crown Lands, Recreational and Commercial Fishers, Community.

Related issues: Floodplain drainage and acid sulfate soils

30 (Eco Logical Australia Pty Ltd, 2019) 31 (Swanson, 2020) 32 (Eco Logical Australia Pty Ltd, 2019)