The world's oceans absorb approximately one-third of the carbon dioxide produced by human activity. Higher levels of carbon dioxide cause animals to work harder to extract oxygen from the water, making it challenging to eat, reproduce and survive. More carbon dioxide will also lead to increasingly acidic oceans, eventually making seawater corrosive to calcium carbonate-shelled animals. The Australian Antarctic Division predicts ocean acidity will rise by 320 percent by 2100. As global temperatures rise, ocean surfaces become warmer, with less oxygen dissolved, and low-oxygen zones -- which NASA identifies as "dead zones" -- created in the ocean depths.
Animals that rely on the existence of sea ice -- such as polar bears, walruses and seals -- face a severe threat from the warming seas of the Arctic, as they hunt from ever-diminishing sea ice platforms. In the Antarctic, the loss of sea ice along the coast of the continent has forced penguin populations to move south and caused a marked decrease in the availability of krill, a major food source for many Antarctic animals.
According to research completed by R. Colwell, et al., and published in "Science" in 2008, tropical regions saw a 1.4-degree Fahrenheit increase in average temperatures between 1975 and 2008. Wildlife can adapt to temperature increases by moving to higher elevations and cooler air, but to do so now, many animals would encounter territories entirely new to them or incapable of supporting them due to human development.
For wildlife in temperate zones -- the areas of the Earth between the tropics and the Arctic and Antarctic circles -- the effects of greenhouse gases will vary. Rising temperatures severely disrupt animals that hibernate, such as bats and bears, and many will attempt to disperse to cooler climates to survive. Extinction may be inevitable for some animals whose behavior patterns or diet prevent them from leaving their existing habitats, warns R.L. Peters in "The Effects of Global Warming on Biological Diversity."
For desert animals that developed highly specialized adaptations to cope with extreme environments, further changes in temperature or rainfall levels could push them beyond endurance. Higher temperatures could also mean a greater number of wildfires, causing a change in the number of plant species available as food.