Over the last half a billion years, the Earth has experiences five mass extinction events. The worst event occurred 251
million years ago, killing off an estimated 95% of all species. The vast majority of species that once inhabited the Earth
have disappeared, but new species have arisen to create the interconnected webs of biodiversity that now characterize
the biosphere. Yet scientists now predict the Earth is plummeting toward a "sixth extinction"-a mass die-off
event that could wipe out nearly a third of all species by 2100. Unlike past extinctions caused by chance collisions with
asteroids or other natural changes, this event will be a direct consequence of humankind's destructive impacts on the
Species have always gone extinct, evolving and eventually dying off when they are no longer well suited to their
environments or they evolve into a new species. From the fossil records, scientists believe the background level of
extinctions has historically been about one species per million per year, or about 10 to 100 species annually. Yet we
are now losing about 27,000 species each year, mostly due to tropical deforestation and habitat loss. In the last 400
years, 89 large mammal species have gone extinct and another 169 are critically endangered-almost 45 times the predicted
It is clear that humans are causing biodiversity loss in almost every corner of the globe, through land-use change; soil,
water, and air pollution; water diversion; habitat fragmentation; exploitation of certain species; introduction of non-native
species; and stratospheric ozone depletion.ii But how much of that loss can be attributed to climate change? According to
Thomas et al., quite a bit.iii In his groundbreaking paper, "Extinction Risk from Climate Change," published in
the journal Nature, Thomas and his colleagues predict that 17-37% of species will be "committed to extinction" by
the year 2050 on the basis of mid-range climate warming scenarios. The researchers sampled 1,103 species in terrestrial
regions from Mexico to Australia. Assuming that each species can only persist under a certain set of climate conditions,
nearly one-quarter of these species could disappear based on present models of climate change-"a loss that would exceed
that expected from habitat destruction." iv
Yet as biologists J. Alan Pounds and Robert Puschendorf warn, "these estimates may be optimistic." The impacts
of climate change will interact with other factors, such as land-use change and influxes of invasive species, compounding
the risk to native species. Further, the analysis conducted by Thomas and his colleagues only took into account temperature.
Other changes, such as precipitation and intensive storms, could intensify the threat.
These dire predictions are more than mere conjecture. In the late 1980s, scientists noticed that entire populations and
species of frogs and other amphibians were disappearing all over the world, but no one could offer a clear explanation for
the phenomena. The National Science Foundation, for example reported, "at least 110 species of brightly colored harlequin
frogs once lived near streams in the tropics of Central and South America, but about two-thirds vanished in the 1980s and 1990s." v
In 2006, researchers working in Costa Rica recognized the culprit: a climate-driven skin fungus fatal to amphibians. Using
sea and land temperature measurements, the scientists demonstrated that amphibian decline was occurring "in near lockstep
with climate change." vi Most of the 70-plus members of the harlequin frog genus Atelopus, endemic to Central
and South America, have already disappeared. Scientists now say these species are the earliest victims of climate change.
Others will soon follow their lead.
Polar bears and other arctic species, such as walruses and seals, could be next. Hunting these marine species is an ages-old
tradition for many Native Alaskans, yet unpredictable weather due to climate change is making the seasonal harvest difficult or
impossible. Many marine mammals depend on sea ice for mating, birthing, and rearing their young. Yet as ice disappears, these
species can't reproduce, pushing some, like the walrus and polar bear, on the path to extinction.
In 2008, Native King Islanders were unable to harvest a single walrus. This change in cultural food harvesting leads not only
to a shortage of nutritional meat, but depletes the availability of ivory tusks, which many Native Alaskans carve into jewelry,
sculptures, and trinkets as a main source of income.vii Compounded with rising fuel and food prices, this winter
could prove especially dangerous for Native Alaskans as they fight to preserve a lifestyle they have maintained for centuries.
- Public Broadcasting Service. 2001. "The Current Mass Extinction." Available online at
[accessed 30 October 2008].
- Intergovernmental Panel on Climate Change. 2004. Climate Change and Biodiversity
- Thomas, Chris D., et al. 8 January 2004. "Extinction risk from climate change." Nature vol. 426. pp. 145-8.
- Pounds, J. Alan and Robert Puschendorf. 8 January 2004. "Clouded futures." Nature vol. 426. pp. 107.
- National Science Foundation. 11 January 2006. "Climate Change Drives Widespread Amphibian Extinctions."
Press Release 06-008. Available online at www.nsf.gov/news/news_summ.jsp?cntn_id=105707.
[accessed 30 October 2008].
- Lyndersen, Kari. 24 August 2008. "Native Alaskans See Walruses Disappear with Sea Ice." National Housing
Institute. Available online at
[accessed 1 November 2008].