insight review articles
234
NATURE | VOL 405 | 11 MAY 2000 | www.nature.com
Humans have extensively altered the global
environment,
changing
global
biogeochemical cycles, transforming land and
enhancing the mobility of biota. Fossil-fuel
combustion and deforestation have increased
the concentration of atmospheric carbon dioxide (CO2)
by 30% in the past three centuries (with more than half of
this increase occurring in the past 40 years). We have
more than doubled the concentration of methane and
increased concentrations of other gases that contribute to
climate warming. In the next century these greenhouse
gases are likely to cause the most rapid climate change that
the Earth has experienced since the end of the last
glaciation 18,000 years ago and perhaps a much longer
time. Industrial fixation of nitrogen for fertilizer and other
human activities has more than doubled the rates of
terrestrial fixation of gaseous nitrogen into biologically
available forms. Run off of nutrients from agricultural and
urban systems has increased several-fold in the developed
river basins of the Earth, causing major ecological changes
in estuaries and coastal zones. Humans have transformed
40–50% of the ice-free land surface, changing prairies,
forests and wetlands into agricultural and urban systems.
We dominate (directly or indirectly) about one-third of
the net primary productivity on land and harvest fish that
use 8% of ocean productivity. We use 54% of the available
fresh water, with use projected to increase to 70% by
20501. Finally, the mobility of people has transported
organisms across geographical barriers that long kept the
biotic regions of the Earth separated, so that many of the
ecologically important plant and animal species of many
areas have been introduced in historic time2,3.
Together these changes have altered the biological diver-
sity of the Earth (Fig. 1). Many species have been eliminated
from areas dominated by human influences. Even in
preserves, native species are often out-competed or con-
sumed by or