Page 295 - Earth's Climate Past and Future

PART V

Historical and Future

Climate Change

uring the last 12,000 years, a new era in Earth and millennial time scales, amounting to 0.5 C at most
°
history emerged. Prior to that time, scattered on a global basis. Within the last 125 years,
Dbands of Stone Age people lived a migratory industrialization has had a rapidly accelerating effect on

hunter-gatherer-fisher existence. Then agriculture was global climate, which has warmed by 0.7 C. Our
°
discovered, and humans began to live in permanent growing overprint on climate seems destined to
dwellings. A new hypothesis suggests that early farmers continue for centuries to come, and many parts of
began to alter climate thousands of years ago through Earth’s surface (particularly the Arctic) will be totally
increasing emissions of greenhouse gases. By 2000 years transformed by future warming.
ago, something like modern life had emerged: iron tools, To evaluate the extent of both natural and human
domesticated livestock, towns and cities, writing, and causes of climate change during the last century and in
religions. By 100 years ago, still early in the industrial the future, we explore these issues:
era, we had already moved more rock and soil than had

all the water, ice, and wind in the climate system.
Climate changes over this interval are reconstructed
• When did humans begin to play a role in climatic
from a wide array of archives and proxy indicators. The
change?
best archives for intervals spanning a few thousand
• What caused climate change during the last 1000 years?
years are annually layered ice cores and lake sediments.
• Was the warming since the late 1800s caused by
For recent centuries, annually layered tree rings and
humans or natural factors?
corals provide additional coverage, and historical
• What is the sensitivity of Earth’s climate system to the
observations are useful in a few regions. For the last
by-products of the Industrial Revolution, including
100 years, measurements made by instruments have
greenhouse gases (CO and CH ) and sulfur dioxide
2 4
become the major source of climatic histories. (SO )?
2
Natural climate changes over the last 1000 years
• What kinds of climate changes lie in Earth’s future?
were much smaller than those over tectonic, orbital,