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With courtesy
of NOAA, we quote the following
website information.
Please visit the below website address for
more details:
http://www.ncdc.noaa.gov/paleo/pubs/crowley.html
Abstract:
Recent reconstructions of
northern hemisphere temperatures and climate forcing
over the last 1000 years allow the warming of the 20th
century to be placed within a historical context and
various mechanisms of climate change to be tested.
Comparison of observations with simulations from an
energy balance climate model indicate that as much as
41-64% of pre-anthropogenic (pre-1850) decadal-scale
temperature variations were due to changes in solar
irradiance and volcanism. Removal of the forced response
from reconstructed temperature time series yields
residuals that show similar variability to control runs
of coupled models, thereby lending support to the
models' value as estimates of low-frequency variability
in the climate system. Removal of all forcing except
greenhouse gases from the ~1000 year time series results
in a residual with a very large late 20th century
warming that closely agrees with the response predicted
from greenhouse gas forcing. The combination of a unique
level of temperature increase in the late 20th century
and improved constraints on the role of natural
variability provides further evidence that the
greenhouse effect has already established itself above
the level of natural variability in the climate system.
A 21st century global warming projection far exceeds the
natural variability of the last 1000 years and is
greater than the best estimate of global temperature
change for the last interglacial.
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Links to Paper
Sources:
Published July 14, 2000
Science, 289: 270-277.
www.science.com
View the data plotted in
figure 1 ,
figure 2 ,
figure 3 , and
figure 4 .
Note that the Volcanic Forcing Time Series are not adjusted for
30% albedo of the earth-atmosphere system, whereas the solar forcing
numbers are for net radiative forcing (AFTER THIS 30% ADJUSTMENT).
April 2001 versions of estimates of volcano, solar, greenhouse
gas, and tropospheric (1000-1998) total forcing prior to accounting for
the planetary albedo affect can be
downloaded here.
Notes on April 2001 data file:
all units are in W/m**2. hl in volcanic time series
refers to the fact that eruptions of unknown origin have
been assigned a high latitude (hl) origin. There are
"tails" to most of the large eruptions that were
determined based on the estimated e-folding time of the
aerosols as being about 1 year. Sol.Be10 refers to the
Beryllium 10 measurements of Bard et al. scaled to the
Lean et al. changes over the last 400 years. GHG refers
to greenhouse gases. Aer refers to tropospheric
aerosols. |
Figure 1:
Comparison of decadally
smoothed Northern Hemisphere mean and annual temperature records for the
past millennium (1000-1993), based on reconstructions of Mann et al.
(Mn) (11) and CL (12). The latter record has been spliced into the
11-point smoothed instrumental record (16) in the interval in which they
overlap. CL2 refers to a new splice that gives a slightly better fit than
the original (12). The autocorrelation of the raw Mann et al. time
series has been used to adjust (adj) the standard deviation units for the
reduction in variance on decadal scales.
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Figure 2:
Forcing time series (W/m**2,
note scale changes for different panels) used in model runs: (A) ice core
millennial volcanism time series from this study; ice-core Robcock and
Free (19) reconstruction from 1400 to the present after adjustments
discussed in (9) and (25); and Sato et al. (28) Northern Hemisphere
radiative forcing, updated to 1998. (B) Example of splice for solar
variability reconstructions, using the 10Be based irradiance
reconstruction of (30) and the reconstruction of solar variability of Lean
et al.. (C) Comparison of three different reconstructions of solar
variability based on 10Be measurements (30), 14C
residuals (31), and calculated 14C changes based on 10Be
variations (30); (D)Splice of CO2 radiative forcing changes
1000-1850 (35) and post-1850 anthropogenic changes in equivalent
greenhouse gas forcing and tropospheric aerosols.
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Figure 3:
(A) Model response to
different forcings, calculated at a sensitivity of 2.0?C for a doubling of
CO2; (B) Example of the combined effect of volcanism volcanism
and solar variability (with 11-point smoothing), using the Bard et al.
(30) 14C index.
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Figure 4:
Comparison of model response
using all forcing terms (with a sensitivity of 2.0 C) against (A) the CL
(12) data set spliced into the 11-point smoothed Jones et al. (16)
Northern Hemisphere instrumental record, with rescaling as discussed in
the text and in the Fig. 1 caption; and (B) the smoothed Mann et al.
(11) reconstruction. Both panels include the Jones et al.
instrumental record for reference. To illustrate variations in the modeled
response, the 14C calculation from Bard et al. (30) has
been used in (A) and the 10Be estimates from (30) have been
used in (B).
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Figure 5:
Analysis of preanthropogenic
residuals in the paleo records. (A) Estimates of residuals using all
combinations of temperature reconstructions and total forcing (including
three different solar indices), with trend lines fitted for each of the
six residuals. (B) Control runs (detrended) from three different coupled
ocean-atmosphere models (46): the NOAA/GFDL, the HadCM3, and the
ECHAM3/LSG. For the sake of comparison with the paleo data, the GCM runs
have been truncated to the same length as the paleo residuals and have
been plotted using the arbitrary starting year of 1000.
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Figure 6:
Comparison of the GHG forcing
response (from Fig. 3) with six residuals determined by removing all
forcing except GHG from the two different temperature reconstructions in
Fig. 1. As in Fig.5, the three different estimates of solar variability
were used to get one estimate of the uncertainty in the response. This
figure illustrates that GHG changes can explain the 20th century rise in
the residuals; +/-2 standard deviation lines (horizontal dashed lines)
refer to maximum variability of residuals from Fig. 5A (inner dashes) and
maximum variability (outer dashes) of the original pre-1850 time series
(Fig 1). The projected 21st century temperature increase (heavy dashed
line at right) uses the IPCC BAU scenario (the "so called IS92a
forcing")(59) for both GHG and aerosols (sulfate and biomass burning,
including indirect effects), and the model simulation was run at the same
sensitivity (2.0 C for a doubling of CO2) as other model
simulations in this article.
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Exploring Climate Events and Human Development
The Past 300 Years:
Putting the 20th Century in Perspective
The animation above is from the
joint gateway of the Historic Land Use Estimation Efforts by the National
Institute of Public Health and the Environment (RIVM, Netherlands) and the
Center for Sustainability and the Global Environment (SAGE, USA). Below is
their description of the work they have been conducting.
One of the conclusions of the
recent IPCC Working Group I Third Assessment Report ' The Scientific Basis
', was that "Emissions of greenhouse gases and aerosols due to human
activities continue to alter the atmosphere in ways that are expected to
affect the climate". It clearly identified carbon emissions from land use
change as an important driver of global climate change. Nevertheless, there
have been relatively few comprehensive studies of global, long-term
historical changes in land cover due to land use. Here, we present two
recently developed historical databases of global land use change. Based on
historical statistical inventories (e.g. census data, tax records, land
surveys, historical geography estimates, etc) and applying different spatial
analysis techniques, an attempt has been made to reconstruct land cover
change due to land use for the last 300 years. The initiative for this
effort has its origin at a PAGES/LUCC meeting held in Bern in March 2000.
PAGES is 'The International Geosphere-Biosphere Programme (IGBP) Core
Project charged with providing a quantitive understanding of the Earth's
past climate and environment', while 'The Land Use and Land Cover Change
(LUCC) Project is a programme element of the International
Geosphere-Biosphere Programme (IGBP) and the International Human Dimensions
Programme on Global Environmental Change (IHDP)'.
Scientists from different disciplines were thus brought together to discuss
how to close the gap between the initiatives of these land use and
population research communities. It appeared that PAGES primarely looked at
periods of several hundred to thousands of years, while LUCC merely reported
on recent decadal land use changes. At the meeting in Bern, two databases
were presented: one by dr. Navin Ramankutty of SAGE, and the other one by
ir. Kees Klein Goldewijk of RIVM. Both databases attempted to fill the
timespan gap left by PAGES and LUCC. These two efforts blend perfectly in
with FOCUS 3 of PAGES called "Human Impacts on Terrestrial Ecosystems (HITE,
Activity 3)", which emphasizes the historical land use changes during the
past 300 years.
Return to
Climate History
100.
Exploring Climate Events and Human Development
The Past 100 Years:
The 20th Century's Human Climate Conundrum |
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The 20th Century has been like no
other. With human population jumping from 1.6 to 6 billion between 1900 and
2000, there have been more people vulnerable to climate change than ever
before. Moreover, during this same period, carbon dioxide concentrations in
the atmosphere have risen from 290 parts per million (ppm) to 369 ppm, with
strong
evidence
pointing to the burning of fossil fuels as a primary cause of these increases.
Many climate researchers and policy makers are concerned that increased in
population and rising standards of living will lead to ever higher levels of
carbon dioxide and other greenhouse gases. See the Intergovernmental Panel on
Climate Change (IPCC),
Etheridge, et. al., 1998 and
U.S. EPA
Global Warming.
During the 20th Century there
were two world wars, numerous hurricanes and typhoons, influenza breakouts,
droughts and famines... and at virtually every step of the way climate played
some role in the events.
There has also been increased
cloud cover, particularly in the
Northern
Hemisphere during the past century (Groisman,
1999). While water vapor is the most abundant greenhouse gas, low clouds
also shade and cool the surface. Currently the role that water vapor and
clouds play in warming or cooling the Earth's climate system is being
investigated by scientists.
The 20th century has experienced
catastrophic climate events. The worst tropical storm of the 20th century
occurred in
Bangladesh in November 1970 where 300,000-500,000 people were killed due
to winds coupled with a storm surge.
The most destructive climate-related event was likely the
Yangtze River Flood of 1931 that impacted over 51 million people (1/4 of
China’s population), including 3.7 million people who perished due to disease,
starvation or drowning. The flood was preceded by a prolonged drought in China
during the 1928-1930 period.
Droughts
have also caused severe problems in the United States during the 20th Century.
The "Dust Bowl" in the 1930s hit the midwest particularly hard. Another
drought in the 1950s impacted the Southwest, but compared with long-term paleo
records, neither of these droughts were as severe or long-lasting as other
droughts in prior centuries, as the graph to the right shows. (See
Paleo Perspective: North American Drought) for more on the human impact on
climate.)
Have swelling populations and
human activities such as the burning of fossil fuels lead to warmer
temperatures and global warming? Research shows that global temperatures have
in fact risen by .6 degrees Celsius over the past 100 years and the National
Academy of Sciences indicate that the increase is due in large part to human
activity. (See
Whitehouse briefing from June 11, 2001.) According
Mann et. Al,
1999, the 20th century warming counters a millennial-scale cooling trend
which is consistent with long-term astronomical forcing. Also see
Paleo Perspective Global Warming for more on human impact on climate.
Click here to view
Putting the 20th
Century in Perspective
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