Retreating glaciers are evidence of a warming planet. This is Shepard Glacier in Glacier National Park (Montana). The top photo was taken in 1913. The bottom photo was taken in 2005 and illustrates the retreat of the glacier over the 92 years after the top photo was taken. The glaciers in Glacier National Park may be gone by 2030. Visit the U.S. Geological Survey's Climate Change in Mountain Ecosystems page for more about Glacier National Park's retreating glaciers and global climate change. U.S. Geological Survey photos by L. Alden (1913) and Blase Reardon (2005).
For the past 2 million years, great sheets of ice have advanced and retreated from both poles during the ice ages. The last ice advance reached its maximum about 20,000 years ago; ice subsequently retreated back to the poles. We live on an "icehouse" Earth during an "interglacial" period following the last ice advance.
However, today our climate is warming. As simply stated by the Intergovernmental Panel on Climate Change:
"Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global average sea level." (IPCC, 2007)
The scientific consensus is that this warming is very likely (more than 90% certain) human-caused. The burning of fossil fuels—releasing long buried energy from the sun—is the source of increased greenhouse gas concentrations, and hence global warming. Humans are not just along for the ride on this warming and changing planet, we are quite literally stepping on the gas! For more information on modern climate change causes and impacts, visit the NPS Climate Change Response Program.
So what is the big deal if climate is warming? It is true that the Earth has been much warmer in the past—many fossil sites provide evidence of these "greenhouse" climates. However our entire evolutionary history as humans is tied to an Earth with ice on both poles. Our modern society, with its infrastructure, cities, and agricultural areas, takes advantage of sea levels and climatic conditions over the past few thousand years, on a relatively cold Earth.
The fossil record provides clues not only to past climates, but also illustrates how animals, plants, and ecosystems responded to changes. Over time, all three components of ecosystems change: the underlying geologic landscape, the climate, and the communities of organisms that live there. Living things are adapted to a particular "comfort zone" of landscape, climate, and community. When conditions change outside of their comfort zone, animals or plants must migrate to more favorable conditions, adapt to the changes, or they will not survive. Fossils record these responses. They also provide clues for changes happening now and in the near future.
As climate continues to change, all living things—including humans—will face these same options: move to a more hospitable location, adapt to the changes, or face extinction. Unlike during past periods of natural climate change, there are now more than 6.8 billion humans living on Earth. Our expanding and immobile infrastructure of cities, transportation corridors, agricultural fields, and dammed rivers, has altered the landscapes within and between ecosystems. This infrastructure creates barriers to populations of animals and plants attempting to travel in response to climate changes. Evolution and adaptation requires lots of time. However, climate change is expected to be very rapid. Barnosky and Kraatz (2007) suggest that the rate of climatic change over the next 100 years will likely be faster than rates experienced by mammals over their Cenozoic evolutionary history. As climate warms, both local and global extinctions will reduce the diversity of life found on Earth.
Because global warming is very likely human-caused, that means there could be a human-powered solution! Humans are the only living things that can mitigate the coming changes. For more information about how national parks are responding to climate change and what YOU can do to help, visit the National Park Service Climate Change Response Program.