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Causes of Climate Change Over the Past 1000 Years




Thomas J. Crowley 
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.


Links to Paper Sources:

Published July 14, 2000 Science, 289: 270-277.

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.
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.


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.
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).
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.
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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14 July 2000


Exploring Climate Events and Human Development

The Past 300 Years: Putting the 20th Century in Perspective landuse_a  
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.

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Climate History 100.
Exploring Climate Events and Human Development
The Past 100 Years: The 20th Century's Human Climate Conundrum

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