Causes of Climate Change Over the Past 1000 Years: Readme file --------------------------------------------------------------------- World Data Center for Paleoclimatology, Boulder and NOAA Paleoclimatology Program --------------------------------------------------------------------- NOTE: PLEASE CITE ORIGINAL REFERENCE WHEN USING THIS DATA!!!!! NAME OF DATA SET: Causes of Climate Change Over the Past 1000 Years LAST UPDATE: 4/2001 (Addition of file forc-total-4.12.01.txt) CONTRIBUTORS: Thomas J. Crowley IGBP PAGES/WDCA CONTRIBUTION SERIES NUMBER: 2000-045 SUGGESTED DATA CITATION: Crowley, T.J., 2000, Causes of Climate Change Over the Past 1000 Years, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series #2000-045. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA. ORIGINAL REFERENCE: Crowley, T.J., 2000, Causes of Climate Change Over the Past 1000 Years, Science, v.289, pp 270-277, July 14, 2000. GEOGRAPHIC REGION: Global PERIOD OF RECORD: 1000-1998 AD FUNDING SOURCES: National Oceanic and Atmospheric Administration NA96GPO415 (USA), Department of Energy (USA) through Battelle/Pacific Northwest Laboratory. LIST OF FILES: Readme.Crowley2000.txt (this file), Crowley.fig1.data.txt, Crowley.fig2.data.txt, Crowley.fig3.data.txt, Crowley.fig4.data.txt, forc-total-4.12.01.txt (April 2001 versions of estimates of volcano, solar, greenhouse gas, and tropospheric (1000-1998) total forcing prior to accounting for the planetary albedo (Tab-delimited ASCII format). DESCRIPTION: 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. Fig. 1 Decadally smoothed time series of Crowley-Lowery time series spliced into smoothed Jones et al instrumental record after 1860, the 11-point smoothed Mann et al. record, a slight modification (labeled CL2) of the original Crowley and Lowery reconstruction to 1965, and ± 2 s.d. values from Mann et al. The autocorrelation of the Mann et al time series has been used to adjust (adj) the standard deviation units for the reduction in variability for decadal time scales. Fig.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. NOTE THAT VOLCANIC FORCING TIME SERIES HAS NOT BEEN 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 file forc-total-4.12.01.txt contains estimates of volcano, solar, greenhouse gas, and tropospheric (1000-1998) total forcing, prior to accounting for the planetary albedo. 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. Fig.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. Fig.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).