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Oceans Suffocating as Huge Dead Zones Quadruple Since 1950, Scientists Warn (theguardian.com)
Ocean dead zones with zero oxygen have quadrupled in size since 1950, scientists have warned, while the number of very low oxygen sites near coasts have multiplied tenfold. From a report: Most sea creatures cannot survive in these zones and current trends would lead to mass extinction in the long run, risking dire consequences for the hundreds of millions of people who depend on the sea. Climate change caused by fossil fuel burning is the cause of the large-scale deoxygenation, as warmer waters hold less oxygen. The coastal dead zones result from fertiliser and sewage running off the land and into the seas. The analysis, published in the journal Science, is the first comprehensive analysis of the areas and states: "Major extinction events in Earth's history have been associated with warm climates and oxygen-deficient oceans." Denise Breitburg, at the Smithsonian Environmental Research Center in the US and who led the analysis, said: "Under the current trajectory that is where we would be headed. But the consequences to humans of staying on that trajectory are so dire that it is hard to imagine we would go quite that far down that path." "This is a problem we can solve," Breitburg said. "Halting climate change requires a global effort, but even local actions can help with nutrient-driven oxygen decline." She pointed to recoveries in Chesapeake Bay in the US and the Thames river in the UK, where better farm and sewage practices led to dead zones disappearing.
You are assuming a linear projection of something that is highly unlikely to scale linearly with either time or population growth. Even if you only look at current data, it is appears that we have already passed an inflection point (the second derivative is now negative).
The major cause of "dead zones" is agricultural fertilizer run off. Fertilizer running off the land is inherently wasteful, so farmers already have a financial incentive to fix this problem ... and they are doing so. Modern "no-till" farming can dramatically reduce run off. Data driven systems can also optimize fertilizer application with soil condition and weather. In the future, robotic application of fertilizer directly to either foliage or crop root zones will dramatically reduce fertilizer volume by delivering the nutrients only to the crop and not to weeds and bare soil.
The solution to "dead zones" is already in the pipeline, and within a decade the zones will be in decline.