Read here and here. Seems some jellyfish are growing in size and population but no one has done the research to determine why these changes are happening. The press has decided the global warming and climate change are the culprits, without any evidence to back up those semi-hysterical claims. It could be that warming has indeed caused an increase, and then again, it could be as one writer states: "Increasingly polluted
waters—off China, for example—boost growth of the microscopic plankton
that "jellies" feed upon, while overfishing has eliminated many of the
jellyfish's predators and cut down on competitors for plankton feed."
Regardless of the slime and stinging of a growing population of jellyfish, there are benefits they bring to the climate, such as sequestering huge amounts of CO2 as they descend to bottom of ocean in death, much like another sea creature named thaliaceans.