Polar Ice Decline
The polar ice caps have the effect of holding part of the Earth's water sequestered in massive sheets of ice on the poles of the planet. The poles at the top and the bottom of the Earth are habitats for a great many species of animals, who either live on the great ice sheets or live part of their lives beneath them. Some cetaceans rely on the frigid temperatures of the Arctic waters located beneath the ice sheets of the North Pole for mating, or even live there exclusively. Global warming causes this ice to melt and the water to warm, decimating the habitat for these animals.
Shipping, Fishing and Whaling
As these ice caps melt, more of the ocean is available for ships to travel on. Greater shipping area has the potential of harming cetaceans and their young through encounters with motor propellers and the hulls of ships. Other threats from polar ice-cap reduction include increasing the mobility of whaling ships, which would then have greater access to hunt both whales, dolphins and their young. Cetaceans are also vulnerable to the nets cast by fishing vessels, whose access to fish populations will also increase with the reduction in the polar ice caps.
Reduction In Food Source
Large cetaceans like whales rely on tiny ocean creatures called krill for food. An increase in overall ocean water temperatures will impact the krill populations of the ocean, potentially destroying a major food source for ocean mammals. The effect of starvation on these creatures will have the secondary effect of causing an overall decline in health for the animals, which increases their susceptibility to disease.
Changes In Water Chemistry
Some dolphins live in fresh water habitats like rivers. Warming temperatures in the ocean will cause an increase in the amount of carbon dioxide dissolved in ocean waters, which has the effect of increasing the acidity of river water. Another change in the chemistry of ocean water would come from the spills and clean-ups from oil and gas mining. As more areas around the poles open up from the reduction in polar sea ice, deposits of fossil fuels become available to mine. If spills occur, it affects the quality of water that cetaceans live, breed and eat in.