As Earth’s atmosphere warms, so does the ocean. Scientists have demonstrated how rising ocean temperatures and carbon dioxide levels can stress marine organisms. But a new model developed by the Massachusetts Institute of Technology reveals a surprising conclusion: If global temperature trends continue, by the end of this century half the population of phytoplankton that existed in any given ocean at the beginning of the century will have disappeared and been replaced by entirely new plankton species. “That’s going to have impacts up the food chain,” says Stephanie Dutkiewicz, principle research scientist at M.I.T.’s Program in Atmospheres, Oceans and Climate.

Rising temperatures will force all kinds of sea creatures to adjust. Tiny phytoplankton, a major food source for fish and other sea creatures, could perish as temperatures rise in an ocean region. Most at risk are the organisms in the coldest waters, which lack the resilience to adapt to warmer homes. In theory, the phytoplankton could evolve to alter their body chemistry or they could migrate elsewhere, perhaps closer to the poles. Either way, such immense change may leave species higher up the food chain unable to feed themselves.

The new model does not specify precisely how phytoplankton will respond or which fish populations might flourish or flounder, but it is sufficiently detailed to indicate that the new ocean conditions will likely lead to widespread replacement of the phytoplankton now in place. Dutkiewicz’s model accounts for 100 different phytoplankton species whereas most other models include just three or four. “With such finer resolution,” Dutkiewicz says, “we can see how significantly ecosystem structures will change.”

The results depict a complex picture. As the temperature rises, many phytoplankton produce more offspring. But less mixing occurs between deep cold waters and warm surface waters—a phenomenon known as stratification. Most nutrients that phytoplankton rely on well up from the deep, so less mixing means less sustenance for the microorganisms. Oceans at low latitudes—already considered the deserts of the sea—will provide even fewer nutrients for microorganisms, leaving even less food for the fish that feed on them.

At higher latitudes, Dutkiewicz says, higher temperatures and less mixing could force phytoplankton to stay closer to the surface, where at least some nutrients are available. More sunlight in that top layer, however, could again change the mix of micro critters. “There is a huge range in size and type of phytoplankton, which can affect the fish that graze on them,” she says.

Dutkiewicz is now beginning to lend additional realism to the model by adding more factors, such as changing levels of nitrogen and iron. Ocean acidification is also high on her list—a chemical variable that could alter competition among phytoplankton, some of which are far more adaptable to changing pH levels than others. Any of these dials on the dashboard could significantly affect the fate of whole ecosystems.