If you thought that the kelp forests only problem was the sea urchin you were wrong. Another huge if water temperature. Kelp can only grow in cool temperatures. Why well because in warmer waters there can be less inorganic nitrogen in the water then if it were cooler. Kelp require this to survive and without it they can shrink in size. Why might this be a problem now. Well because of global warming which is making our oceans even warmer. So during the summer time the kelp forests along the californian coast start to recede up. However it goes back down. But each time it can't go as far as the earth is slowly warming up.