Global areal protection targets have driven a dramatic expansion of the marine protected area (MPA) estate. We analyzed how cost‐effective global MPA expansion has been since the inception of the first global target (set in 1982) in achieving ecoregional representation. By comparing spatial patterns of MPA expansion against optimal MPA estates using the same expansion rates, we show the current MPA estate is both expensive and ineffective. Although the number of ecoregions represented tripled and 12.7% of national waters was protected, 61% of ecoregions and 81% of countries are not 10% protected. Only 10.3% of the national waters of the world would be sufficient to protect 10% of each ecoregion if MPA growth since 1982 strategically targeted underrepresented ecoregions. Unfortunately 16.3% of national waters are required for the same representative target if systematic protection started in 2016 (an extra 3.6% on top of 12.7%). To avoid the high costs of adjusting increasingly biased MPA systems, future efforts should embrace target‐driven systematic conservation planning.
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