Climate change alone is expected to threaten with extinction approximately one quarter or more of all species on land by the year 2050, surpassing even habitat loss as the biggest threat to life on land. Species in the oceans and in fresh water are also at great risk from climate change, especially those that live in ecosystems like coral reefs that are highly sensitive to warming temperatures, but the full extent of that risk has not yet been calculated.
Climate change is a threat because species have evolved to live within certain temperature ranges, and when these are exceeded and a species cannot adapt to the new temperatures, or when the other species it depends on to live cannot adapt, for example its food supply, its survival is threatened.
The IPCC has predicted that by 2100, assuming that current trends in burning fossil fuels continue, the surface of the Earth will warm on average by as much as 6 degrees Celsius (around 11 degrees Fahrenheit) or more. It is not possible to predict how most species, including our own, and how most ecosystems, will respond to such extreme warming, but the effects are likely to be catastrophic.
To put an average surface warming of 6 degrees Celsius into context, consider the following:
All the changes we have seen to date that have been ascribed to global warming—the melting of glaciers, sea ice, and permafrost; the bleaching and dying of coral reefs; extreme storms and flooding, droughts, and heat waves; and major shifts in the ranges of organisms and in the timing of their biological cycles--have occurred with an average warming of the Earth’s surface since the late 19th Century, when this warming (and the Industrial Revolution) began, of less than 1 degree C.
The average temperature of the Earth’s surface during the peak of the last Ice Age, 20,000 years ago, when large areas of North America, northern Europe and northern Asia were under a sheet of ice 2 miles and more thick, was only about 6 degrees C. cooler than it is now.
Mother and cub polar bears on ice floes separated by large areas of open water. Photo by Martha de Jong-Lantink | Flickr.com