The effect of climate change on oceans is likely more extensive than you think. A study from the International Union for Conservation of Nature (IUCN) indicated that climate change and the ensuing hotter water reduced the amount of oxygen dissolved into oceans by 2 percent between 1960 and 2010. While that may seem like only a modest amount, oxygen levels in some tropical regions dropped by up to 40 percent. Moreover, scientists warned that even a slight dip could be particularly troublesome for big, energy-dependent fish like marlin, sharks and tuna. Lower oxygen levels drive them to shallower water, putting them at greater risk of overfishing.