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does climate change today compare with climate
change in the past?
Climate scientists reconstruct the
Earth's climate history by studying proxy records
including ice cores, corals, and tree rings. These
records show natural variability in the Earth's climate
such as distinct colder glacial periods and warmer
interglacial periods. One particularly well-documented
example is the Younger Dryas. Recent human-induced
climate change is outpacing
examples of historical climate change.
Studying the Climate
Climate scientists glean information
about past climates by studying ice cores, sediments
from the ocean, corals, tree rings, and the geologic
record. From these proxy records, they reconstruct
Earth's climate history and compare climate changes
of the past with observations of recent climate changes
and their projections of climate change in the twenty-first
Earth's Dynamic Climate
|Figure 1. Temperature
changes in the past 400.000 years.Source:
Petit, J.R., J. Jouzel, D. Raynaud, et al.
"Climate and atmospheric history of the past
420,000 years from the Vostok ice core, Antarctica."
Nature 399(6735), 429-436, 1999.
Scientists have long known that the
Earth's climate is dynamic, fluctuating between colder
and warmer periods generally referred to as glacial
and interglacial states (Figure 1). Despite these
swings, the Earth has supported life for the past
four million years. Historically, climate changes
have usually occurred on timescales of thousands
of years, although there well-documented exceptions
such as the Younger Dryas when climate change has
occurred more quickly.
Due to the rapid addition of greenhouse gases to
the atmosphere, especially since the Industrial Revolution,
climatologists expect to observe greater changes
in the Earth's climate over the next century than
have been recorded over approximately the past 12,000
The transition from the peak of the
last ice age around 20,000 years ago to the start of
the modern interglacial period (the Holocene) around
10,000 years ago, was interrupted by a sharp return
to near-glacial conditions during the Younger Dryas
period. This period lasted from about 13,000 years
ago to 11,500 years ago. The onset of this cold period
took a couple of hundred years, but its termination
was extremely rapid (over decades). The strongest cooling
during this period occurred in the areas around the
North Atlantic Ocean (Greenland, Europe, Western Canada),
but its impacts were felt globally, as far away as
the Californian coast and China.
|Figure 2. Temperature reconstruction
for the past thousand years. Data shown in
red are observations from instruments, blue
are from tree rings, corals, ice cores and
historical records. Source: Intergovernmental
Panel on Climate Change (IPCC), Technical
Summary of Working Group I Report, Third
Assessment Report. 2001.
At the end of the Younger Dryas, ice sheets that
had extended down to the New York metropolitan region
began to retreat and the climate shifted from a glacial
to an interglacial state.
The Past 1500 Years
Temperatures began increasing in northern latitudes
around 600 AD and warming occurred in other regions
up until about 1250 AD due to natural variability
in the Earth's orbit. Areas of Greenland were colonized
that were previously too cold for food cultivation.
This period is sometimes referred to as the Medieval
Warm Period, although the warming did not occur
on a global scale.
Between 1250 and 1850, a period of cooler temperatures
known as the Little Ice Age occurred (Figure 2).
Although temperatures tended to be cooler than
average, the Little Ice Age was not a continuous
time period of colder than normal temperatures
because summers were often very warm. Between 1250
AD and the early 1500s, the climate cooled slowly
and then appeared to be stabilizing with the exception
of the North Atlantic where temperatures began
to decrease rapidly. Glaciers that had been receding
re-advanced and large bodies of water remained
frozen through the summers. Volcanic activity contributed
to the cooling during the Little Ice Age. The average
temperature during the Little Ice Age was about
1.8°F (1°C) cooler than it is today.
|Figure 3. Temperature changes
over the past one hundred and twenty years. Source:
NASA Goddard Institute for Space Studies
(NASA GISS) 2001. http://www.giss.nasa.gov/data/update/gistemp/graphs/
Climate Change in the Twentieth
Century and Beyond
In the twentieth century, global temperature averaged
over land and ocean areas warmed by 1.08°F (0.6°C)
(Figure 3). The rise was not uniform in time or space.
For example, temperatures increased in the first
part of the 20th century and then decreased between
about 1940 and 1970. Also, temperature rise has
been greater in the northern high-latitudes. Between
1970 and today, temperatures have warmed, and the
1990s saw some of the hottest years of the twentieth
If the current trend continues, temperatures will
be more than ~1.8°F (1°C) warmer by 2100
than they were in 1900. However, climate models used
by the Intergovernmental Panel on Climate Change
(IPCC) project that the climate will warm 2.25 -
(1.4 - 5.8°C) in the twenty-first
century, outpacing the warming trend of the twentieth
century, and at the higher end comparable in magnitude
to the temperature swing in the Younger Dryas, when
temperatures were only 9-12.6°F (5 - 7°C)
cooler than the global temperature today. Therefore,
the IPCC projections for the future are within a
range of temperatures that could lead to a major
Intergovernmental Panel on Climate Change (IPCC),
2001. "Working Group I Third
Assessment Report." Cambridge University Press.
National Academy of Sciences (NAS). 2001. "Climate
Change Science: An Analysis of Some Key Questions."
National Academies Press. 42 pp.
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