Australia's first national coastal flood accounting totals $855 billion by 2100

Add the seven jurisdiction figures and they come to $855.3 billion ^[1]. That is worth checking, because it tells you the national headline is the state ledger summed rather than a top-down number apportioned afterwards ^[1][7][8]. Coastal set-backs, hazard overlays and development approvals are made at exactly that level.

The state ordering breaks from the property count. Western Australia carries the largest loss total at $230.5 billion while sitting third on property count with 51,366; Queensland has 93,157 properties at risk and $214.5 billion ^[7][8][9]. Divide dollars by properties and you get about $4.5 million in Western Australia against $2.3 million in Queensland, a ratio near 1.95 ^[4]. That ratio reflects asset mix rather than per-house damage. The $855 billion also covers 2 million hectares of land, farms, critical infrastructure and environmental assets ^[3], so what the Western Australian number is really reporting is that its dollars sit in assets other than dwellings. Three states hold roughly 80 percent of the property exposure ^[3].

The exclusions all lean the same way. Storm intensity is held constant and erosion losses are left out ^[11], and the possibility of rapid Greenland and West Antarctic ice sheet collapse, with up to 2 m of rise by the end of the century, is not in the economic losses at all ^[12]. The emissions path is IPCC SSP2-4.5, around 2.7 C of warming by 2100, described in the research as broadly consistent with a continuation of current policies and well above the 1.5 C Paris level ^[4]. The researchers also modelled a very high emissions scenario, and phys.org's account does not give that total ^[5].

The method runs in a fixed order, and that order matters as much as the detail within each step. Flood extents were mapped first, from sea-level projections, storm-surge estimates and Australia's elevation data; properties and land inside those extents were then identified and valued; statistical modelling estimated damage severity over time before physical damage was converted into economic loss ^[6]. Every step inherits the flood extent, so the count of nearly 270,000 properties is the figure to interrogate before the dollar figure.

The Gold Coast is the most exposed urban area at $84.4 billion, which is 39.3 percent of Queensland's total and 9.9 percent of the national one ^[10][2]. Against that, Andrew Watkins, a Climate Council councillor and co-author of the council's accompanying report, cites Tropical Cyclone Alfred costing the City of Gold Coast $35 million in beach repairs ^[16][13]. The two numbers measure different things: one is a realised council invoice for sand after a single storm, the other is a century of modelled asset damage.

The phys.org account omits an uncertainty band around the $855 billion, and it omits a high-emissions figure ^[1][5]. The $855 billion is a point estimate resting on the conservative side of its own assumptions, with two of the largest upside terms, erosion and ice-sheet collapse, sitting outside the model ^[11][12].