Each species evolves to thrive in its own particular ecological niche — to live in a particular "home" with specific living conditions (including temperatures ranges and other plant and animal species). Some species are more adaptable, or "opportunistic", than others. For example, rats and dogs can survive under many different conditions, but koalas can only live where there is eucalyptus. Human caused climate change will alter temperatures, precipitation and sea level - wiping out some habitats and shifting others faster than many species can migrate.
Unless we drastically reduce our greenhouse gas emissions, we can expect several factors to combine that will make the coming die out astonishingly severe. The climate is changing faster than at almost any time in our planet's history. Also, many ecosystems are already stressed by human activities — destructive logging, excessive grazing, over fishing, toxic pollution and the like. And, human development and habitat destruction impedes many species from migrating — superhighways effectively block land animals, for example.
A recent major study indicates that if global temperatures increase 3.2-3.6°F, which is considered a mid-range estimate, a million species would be threatened with extinction over the next fifty years. This can only be avoided by rapid emissions reductions in the next few decades. There is still time to save many species, but it is fast running out. Of course, if temperatures go even higher, more species will be lost.
Some examples of species and habitats at risk:
Coral bleaching is a condition that can seriously damage and kill entire coral reefs. Corals contain microscopic algae called zooxanthellae that provide the coral with food and give them their vibrant colours. Rising ocean temperatures cause corals to become stressed, and they expel the zooxanthellae and turn white or "bleach". If zooxanthellae do not return to the coral's tissue, the coral will die.
As little as a 1.8°F increase in temperature above the summer maximum can cause corals to bleach. Tropical sea temperatures have increased by 1° Celsius over the past 100 years and are predicted to continue rising.
An example of this problem is Australia's world famous Great Barrier Reef, which lies off the state of Queensland. At around 1,243 miles long it is the world's largest reef.
But in 2002 the reef experienced its worst ever case of coral bleaching, with over 60 percent of the reef being affected. Unless projected levels of climate change are slowed, much of the reef will be dead in decades. Deprived of their living homes, hundreds of species relying on the reef will also die out.
Arctic sea ice could disappear within 70 years, and wild polar bears with it.
Polar bears are the world's largest land predator. They can go for long periods, even months, without eating, but need to build up fat to live through lean times. The polar bear does this mostly by eating seals they catch on the ice. Without the ice they can't get to their prey. In fact, without sea ice, much of the Arctic ecosystem would change or collapse. Polar bears also use floating sea ice platforms for travel, and pregnant polar bears build snow dens for the winter, which they give birth in. In the last two decades, Arctic ice cover has retreated five percent and the ice that is left has lost at least 30 percent of its thickness; and an average of two weeks have been lost from the bear's hunting season.
Like animals and insects, plant species require specific climates. You don't find yellow birch trees growing next to Saguaro Cactus, for example. Changes in precipitation and temperature will mean that some species can no longer survive where they are now growing. Also, like animals plants, are vulnerable to competition. As warming occurs, species that have adapted to living in cooler climates can be pushed out of existence by newcomers better suited to the new temperatures.
Most plants can't migrate very quickly, compared to animals and insects. They are restricted by how far their seed or pollen can travel, and the climate will change too fast for many of them if current trends continue. Human barriers (such as farms and urban areas) will also impede plant migrations.
Many animals and insects need specific plants, or types of plants, as part of their habitat. So the loss of plant species will have a ripple effect - leading to more animal and plant extinctions.
PBS - Great White Bear
Global Warming and Terrestrial Biodiversity Decline - WWF report
United Nations Framework Convention on Climate Change