The Heat Is Online

Study: 75 percent of Warming Due to Human Activity

Study Faults Humans for Large Share of Global Warming

By Andrew C. Revkin, The New York Times, July 14, 2000

A new analysis of the climate of the last 1,000 years suggests that human activity is the dominant force behind the sharp global warming trend seen in the 20th century.

The study, by Dr. Thomas J. Crowley, a geologist at Texas A&M University, found that natural factors, like fluctuations in sunshine or volcanic activity, were powerful influences on temperatures in past centuries. But he found that they account for only 25 percent of the warming since 1900. The lion's share, he said, can be attributed to human influences, particularly to rising levels of carbon dioxide and other heat-trapping "greenhouse gases" that come from the burning of fuels and forests.

"These twin lines of evidence provide further support for the idea that the greenhouse effect is already here," Dr. Crowley wrote in describing the work in today's issue of the journal Science. Several climate experts said his findings offer the most direct link yet between people and the 1.1 degree rise in average global temperature over the last 100 years.

The study has already sparked debate among camps of scientists who dispute the climate records used in Dr. Crowley's analysis and others who say the oceans play an underappreciated role in controlling warming and cooling of the planet.

But many climatologists said it marked a substantial move forward in understanding forces that have warmed the earth and are likely to continue warming it.

Resolving this puzzle -- the balance between human and natural influences -- has been something of a holy grail in atmospheric science, particularly because the answer could determine whether countries enact plans in coming years to reduce emissions of greenhouse gases.

Many scientists said that Dr. Crowley's work, while not definitive, was helping build a strong case.

"This seems to reinforce the notion that we're into a new mode beyond simple natural forcing, that something else is taking place," said Dr. Raymond S. Bradley, the director of the Climate System Research Center at the University of Massachusetts, Amherst.

"The logical conclusion is there is some role played" by human affairs, added Dr. Bradley, whose studies of past climates were one of the yardsticks used to measure the reliability of the new work.

There has been a slow shift toward this conclusion in the last few years. In April, for example, the United Nations Intergovernmental Panel on Climate Change, the international group whose work has largely shaped the debate on the issue for a decade, circulated a draft of its next report on global warming, saying "there has been a discernable human influence on global climate."

Scientists said the main value Dr. Crowley's study was his specific description of the limited role of natural forces in recent decades.

In a separate commentary, also published in Science, Michael E. Mann, a climate expert at the University of Virginia and a research partner with Dr. Bradley, said the new work also indirectly confirms that computer models that predict a continuation of the warming in coming decades are reasonably reliable.

The news cheered environmental officials at the White House who have backed action to control greenhouse emissions, particularly because diplomats will meet again this fall to debate ways to put into practice the provisions of the Kyoto Protocol, a 1997 agreement outlining a plan to cut greenhouse emissions.

Dr. Neal Lane, a physicist who is the presidential science adviser, said in an interview that it was important not to overplay the study, which relied on a fairly simple computer model and estimated variations in the sun's power that were, at best, rough approximations.

But the new study largely correlates with work by Dr. Mann and others that used different methods but reached similar conclusions.

"Scientists will debate whether this is the smoking gun," Dr. Lane said. "But it's clear that the changes we've seen in this century are unprecedented in the last thousand years. This argues for prudent action to stem the growth of greenhouse gases."

Critics of the global warming theory quickly took note of the work, but mainly to poke at its weaknesses. For example, Dr. S. Fred Singer, the president of Science & Environmental Policy Project, a private consulting group in Virginia, said Dr. Crowley had drawn precise conclusions from imprecise calculations.

But other scientists defended Dr. Crowley's approach.

The initial goal in the work, Dr. Crowley said, was to gain a better understanding of how natural influences were shaping past global temperatures, which have included a variety of shifts ranging from a medieval warming to a prolonged cooling from the 1600's to the 1800's that is called the Little Ice Age.

Two of the most important factors are changes in the radiation flowing from the sun, which is thought to follow many complex cycles, and volcanoes, which temporarily cool the earth by lofting a veil of fine sulfate droplets that reflect sunlight.

Dr. Crowley found that past variations in the radiance of the sun or bursts of volcanic activity, when fed into a computer model simulating the flow of energy to and from the earth, produced temperatures that match most of the ups and downs of the actual climate from the year 1000 to the mid 1800's.

But that neat relationship among sunlight, eruptions and temperature broke down completely in the 20th century, he reported. The only "forcing" that remotely matched the jump in temperatures seen in the latter half of the century was the rise in emissions of greenhouse gases.

Dr. Crowley himself noted that his model was rough; for example, he said, past solar variation is only indirectly recorded, partly in traces of elements found in old glacial ice that were made as radiation from the sun and other stars bombarded the atmosphere, and partly in observations by Galileo and subsequent astronomers who charted sunspot activity from 1610 on.

Even so, the good correlation between his work and other recent reconstructions of past climates prompted a variety of climate experts to say the study was valid. A big step forward.

A separate study, to be published tomorrow in Geophysical Research Letters, an influential journal, largely echoes Dr. Crowley's assertion that human actions are dominating current climate shifts. But the second study cautions that other, natural factors could stall further warming.

One of the authors of that report, Dr. Michael E. Schlesinger, a climatologist at the University of Illinois at Urbana-Champaign, said it was particularly important for policy makers and the public not to assume that temperature trends will follow a smooth course.

He said that the relationship between the oceans and the atmosphere is so complex that alterations in it could easily cause temporary cool periods or other unpredictable -- and possibly abrupt -- changes that could create confusion and paralyze work to attack global warming.

Commentary in Science, July 14, 2000

Lessons for a New Millennium

Michael E. Mann*

A key factor hampering our ability to confidently assess the human influence on the warming of the past century is our limited understanding of the climate changes believed to have occurred in previous centuries. What caused the "Little Ice Age" of the 15th to 19th centuries or the putative "Medieval Warm Period" of earlier centuries (1, 2)? Might not the same, presumably natural, factors bear some responsibility for the dramatic warming of the 20th century (3-6)? On page 270 of this issue, Crowley (7) provides some convincing answers to these questions and makes a compelling case for the assertion that anthropogenic greenhouse gas increases are behind the continued warming of the globe.

Conventional approaches to understanding the factors underlying the recent warming have involved complex numerical models of the combined ocean-atmosphere system. Although highly suggestive of a detectable human influence on climate, these studies have been limited by intrinsic uncertainties in comparing model-predicted climate change patterns with the instrumental climate record. At roughly one century, the latter is too short to allow unambiguous attribution of changes to human influences (8).

Crowley's study circumvents this limitation by making use of empirical information about longer term climate variability. The author uses an Energy Balance Model (EBM), calibrated to exhibit a similar response to external radiative influences as more elaborate coupled ocean-atmosphere models. This allows an efficient investigation of forced changes in annual mean temperatures in the Northern Hemisphere over the past millennium. The model is driven with (admittedly uncertain) empirical estimates of the time histories of the most relevant factors affecting the atmosphere's radiative balance (solar radiative output, volcanic aerosol loading,

anthropogenic greenhouse gas concentrations, and industrial aerosols). Comparison of the predicted response with independent (although also uncertain) estimates of Northern Hemisphere annual temperature variations over the past millennium based on proxies such as tree rings, ice cores, and corals, which naturally record climate variations (9, 10) (see the figure), yields fairly close agreement (11). Of equal interest, however, is the level of disagreement: Within estimated uncertainties, the amplitude of the residual temperature variations not explained by the model agrees precisely with the typical amplitude of purely random or "stochastic" climate variability observed in coupled ocean-atmosphere models.

Temperature histories explained?

Comparison of proxy reconstructions of annual mean Northern Hemisphere (NH) temperature change (9) with the EBM results described by Crowley (7). The blue-shaded region represents the approximate uncertainty range in the empirical temperature estimates of (9). Two extratropical warm-season Northern Hemisphere temperature reconstructions (20, 21) are shown for comparison.

Crowley's report thus strengthens the case for a detectable human influence on 20th century global warming by establishing that (i) much of the climate history of the past millennium can be explained in terms of a few well-established, physically well-constrained radiative forcings, (ii) the dramatic warming of the 20th century can almost certainly not be explained by the natural forcings, but instead requires the emergent anthropogenic forcings of the 20th century, and (iii) more detailed climate models used to detect and attribute observed patterns of climate change to anthropogenic factors (8) appear to capture the unforced component of climate variability with sufficient accuracy. The last conclusion strengthens the independent conclusion drawn from simulations using more complex models that human-induced climate change is now detectable.

Nonetheless, Crowley's study does not explain the entire climate history of the past millennium. The model does not, for example, reproduce the cooling of the late 19th century that is seen both in proxy-based climate reconstructions (9, 10) and the early instrumental record (12); the warming, in essence, begins too soon in the model. One possible explanation offered by Crowley is that both the reconstructions and the instrumental record may independently underestimate the hemispheric temperatures during this period, for example, because of sparse spatial sampling. A better explanation, however, also noted by Crowley, is that a potentially important surface radiative forcing not included in his simulations--land usage changes, which affect Earth's surface albedo--may be responsible for the observed cooling. A recent study (13) indicates that anthropogenic large-scale land usage changes should have culminated in an annual mean cooling of more than 0.3ºC in the 19th century. This additional anthropogenic forcing is not only large enough to explain the discrepancy between observation and Crowley's EBM results, it has also been implicated (14) in another residual discrepancy, namely the observed differences between conventional proxy-based estimates of past hemispheric temperature changes (9, 10) and ground surface temperature estimates from borehole profiles (15).

Crowley's study also does not explain the regional complexity of surface temperature trends during the past millennium. There is little doubt that the temperature anomalies associated with the Little Ice Age and the Medieval Warm Period were far more prominent in some regions (such as Europe) than in others. These large regional anomalies vary in amplitude, timing, and sign and thus average out to yield more modest variations for the Northern Hemisphere on the whole (9, 10). In recent decades, Europe has warmed faster than the Northern Hemisphere on the whole, whereas certain regions in the North Atlantic have actually cooled in the face of widespread warming. This is a result of a combination of regional temperature overprints by the North Atlantic Oscillation (NAO) and related, but distinct, patterns of multidecadal variability associated with the thermohaline circulation of the North Atlantic (16, 17).

It is quite reasonable to assume that similar factors were associated with the pronounced temperature changes in Europe in past centuries that accompanied more modest hemispheric-wide temperature changes. Keigwin and Pickart (18) have shown evidence that a heterogeneous temperature pattern in the North Atlantic region consistent with the NAO coincided with the European Medieval Warm Period and Little Ice Age. There is evidence that the aforementioned multidecadal variations in the North Atlantic can couple to variations in solar radiative output that occur on similar time scales (19).

Could a similar mode of North Atlantic variability resonate with solar radiative variations at millennial time scales, imprinting a regional pattern of enhanced anomalies on top of the more modest hemispheric-scale warming that Crowley's study attributes in part to solar forcing at these time scales? Only further, more detailed modeling studies and expanded networks of paleoclimate indicators will further elucidate the spatial and temporal patterns of climate change in past centuries.

The author is in the Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22902, USA. E-mail:

Science, 14 July, 2000 v. 289

Causes of Climate Change Over the Past 1000 Years

Thomas J. Crowley

Recent reconstructions of Northern Hemisphere temperatures and climate forcing over the past 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. Comparisons of observations with simulations from an energy balance climate model indicate that as much as 41 to 64% of preanthropogenic (pre-1850) decadal-scale temperature variations was 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 those of 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 past 1000 years and is greater than the best estimate of global temperature change for the last interglacial.

Department of Oceanography, Texas A&M University, College Station, TX 77843, USA. E-mail:

The origin of the late-20th-century increase in global temperatures has prompted considerable discussion. Detailed comparisons of climate model results with observations (1) suggest that anthropogenic changes, particularly greenhouse gas (GHG) increases, are probably responsible for this climate change. However, there are a number of persistent questions with respect to these conclusions that involve uncertainties in the level of low-frequency unforced variability in the climate system (2) and whether factors such as an increase in solar irradiance or a reduction in volcanism might account for a substantial amount of the observed 20th-century warming (1, 3-10). Although many studies have addressed this issue from the paleoclimate perspective of the past few centuries (3-10), robust conclusions have been hampered by inadequate lengths of the time series being evaluated. Here I show that the agreement between model results and observations for the past 1000 years is sufficiently compelling to allow one to conclude that natural variability plays only a subsidiary role in the 20th-century warming and that the most parsimonious explanation for most of the warming is that it is due to the anthropogenic increase in GHG.


The data used in this study include physically based reconstructions of Northern Hemisphere temperatures and indices of volcanism, solar variability, and changes in GHGs and tropospheric aerosols.

Northern Hemisphere temperatures. Four indices of millennial Northern Hemisphere temperature have been produced over the past 3 years (11-14). The analysis here uses the mean annual temperature reconstructions of Mann et al. (11) and of Crowley and Lowery (CL) (12), because the energy balance model used in this study calculates only this term [the other records (13, 14) are estimates of warm-season temperature at mid-high latitudes]. The Mann et al. reconstruction was determined (8) by first regressing an empirical orthogonal function analysis of 20th-century mean annual temperatures against various proxy indices (such as tree rings, corals, and ice cores). Past changes in temperature are estimated from variations in the proxy data (15). The Mann et al. reconstruction has a varying number of records per unit of time (although the number in the earlier part of the record is still greater than in CL). The CL reconstruction is a more heterogeneous mix of data than the Mann et al. reconstruction, but the number of records is nearly constant in time. It is a simple composite of Northern Hemisphere climate records and was scaled (12) to temperature using the instrumental record (16) in the overlap interval 1860-1965. The instrumental record was substituted for the proxy record after 1860 for two reasons: (i) there were too few proxy data in the CL time series after 1965 to reconstruct temperatures for this interval, and (ii) the original CL reconstruction indicated a "warming" over the interval 1885-1925 that is at variance with the instrumental record. This difference has been attributed (11, 17) to an early CO2 fertilization effect on tree growth. The significance of this decision will be further discussed below; model-data correlations presented in the study include both the original proxy record and the substituted instrumental time series.

Despite the different number and types of data and different methods of estimating temperatures, comparison of the decadally smoothed variations in each reconstruction (Fig. 1) indicates good agreement (r = 0.73 for 11-point smoothed correlations over the preanthropogenic interval 1005-1850, with P < 0.01). Both records [and the Jones et al. (13) and Briffa (14) reconstructions] show the "Medieval Warm Period" in the interval ~1000-1300, a transition interval from about 1300-1580, the 17th-century cold period, the 18th-century recovery, and a cold period in the early 19th century. Even many of the decadal-scale variations in the Medieval Warm Period are reproducible (12), and both reconstructions [and (13, 14)] indicate that peak Northern Hemisphere warmth during the Middle Ages was less than or at most comparable to the mid-20th-century warm period (~1935-1965). This result occurs because Medieval temperature peaks were not synchronous in all records (12). The two temperature reconstructions also agree closely in estimating an ~0.4°C warming between the 17th-century and the mid-20th-century warm period (18).

Fig. 1. Comparison of decadally smoothed Northern Hemisphere mean 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. [View Larger Version of this Image (34K GIF file)]

Volcanic forcing. There is increasing evidence (3, 7-10) that pulses of volcanism significantly contributed to decadal-scale climate variability in the Little Ice Age. Although some earlier studies (9, 10) of forced climate change back to 1400 used a composite ice core index of volcanism (19), which has a different number of records per unit of time, the present study primarily uses two long ice core records from Crete (20) and the Greenland Ice Sheet Project 2 (GISP2) (21) on Greenland, with a small augmentation from a study of large eruptions recorded in ice cores from both Greenland and Antarctica (22). This approach avoids the potential for biasing model results versus time because of changes in the number of records. Because Southern Hemisphere volcanism north of 20°S influences Northern Hemisphere temperatures, the ice core volcano census samples records down to this latitude. The volcanism record is based on electrical conductivity (20) or sulfate measurements (21), and a catalog of volcanic eruptions (23) was used to remove local eruptions (24) and identify possible candidate eruptions in order to weight the forcing according to latitude. Eruptions of unknown origin were assigned a high-latitude origin unless they also occurred in Antarctic ice core records (22).

The relative amplitude of volcanic peaks was converted to sulfate concentration by first scaling the peaks to the 1883 Krakatau peak in the ice cores. Although earlier studies (9, 10) linearly converted these concentration changes to radiative forcing changes, subsequent comparison (25) of the very large 1259 eruption [eight times the concentration of sulfate in ice cores from Krakatau and three times the size of the Tambora (1815) eruption (21)] with reconstructed temperatures (11-14) failed to substantiate a response commensurate with a linearly scaled prediction of an enormous perturbation of ~25 W/m2 (26). Calculations (27) suggest that for stratospheric sulfate loadings greater than about 15 megatons (Mt), increasing the amount of sulfate increases the size of aerosols through coagulation. Because the amount of scattered radiation is proportional to the cross-sectional area, and hence to the 2/3 power of volume (or mass), ice core concentrations estimated as >15 Mt were scaled by this amount (25). Aerosol optical depth was converted to changes in downward shortwave radiative forcing at the tropopause, using the relationship discussed in Sato et al. (28). There is significant agreement (29) between the 1000-year-long volcano time series and the concentration-modified Robock and Free (19) times series (Fig. 2A). Both proxy records show the general trends estimated from ground-based observations of aerosol optical depth (28): the pulse of eruptions in the early 20th century and the nearly 40-year quiescent period of volcanism between about 1920-1960. Because volcano peaks are more difficult to determine in the expanded firn layer of snow/ice cores, updated estimates of Northern Hemisphere radiative forcing from Sato et al. were used to extend proxy time series from 1960 to 1998.

Solar forcing. There has been much discussion about the effect of solar variability on decadal-to-centennial-scale climates (
3, 6, 8-10). An updated version of a reconstruction by Lean et al. (5) that spans the interval 1610-1998 was used to evaluate this mechanism [for reference, Free and Robock (10) obtained comparable solar-temperature correlations for the interval 1700-1980 using the Lean et al. and alternate Hoyt and Schatten (4) solar reconstructions]. The Lean et al. time series has been extended to 1000 by splicing in different estimates of solar variability based on cosmogenic isotopes. These estimates were derived from ice core measurements (30) of 10Be, residual 14C from tree ring records (31), and an estimate of 14C from 10Be fluctuations (30). The justification for including the latter index is that neither of the first two splices yields a Medieval solar maximum comparable to that of the present. Because of concerns about biasing results too much by the latter period, which has much more information than the former, the Bard 14C calculation was included so as to obtain a greater spread of potential solar variations and to allow testing of suggestions (32) that solar irradiance increases could explain the Medieval warming.

Once the splices were obtained, the records were adjusted to yield the potential ~0.25% change in solar irradiance on longer time scales (33). Because two of the solar proxies indicate that minimum solar activity occurred in the 14th century, the 0.25% range was set from that time to the present rather than from the 17th century, as was done by Lean et al. [the adjustment is very small for the different solar indices in the 14th century (~0.05 W/m2)]. The 20th-century increase in estimated net radiative forcing from low-frequency solar variability is about 10 to 30% greater than estimated from an independent method (34). An example of one of the splices is illustrated in Fig. 2B, and the three composites (Fig. 2C) show the pattern of potential solar variability changes used in this study.

Anthropogenic forcing. The standard equivalent radiative forcing for CO2 and other well-mixed trace gases (methane, nitrous oxides, and chlorofluorocarbons) is used after 1850 (Fig. 2D). Pre-1850 CO2 variations, including the small minimum from about 1600-1800, are from Etheridge et al. (35). Radiative forcing effects were computed based on updated radiative transfer calculations (36). The well-constrained change in GHG forcing since the middle of the last century is about four times larger than the potential changes in solar variability based on the reconstructions of Lean et al. (5) and Lockwood and Stamper (34).

Tropospheric aerosols consider only the direct forcing effect (that is, no cloud feedback), whose global level has been estimated as being about -0.4 W/m2 (37), with the Northern-to-Southern-Hemisphere ratio being in the range of 3 to 4 (38). Because there is an approximate offset in the radiative effects of stratospheric and tropospheric ozone (37), and its total net forcing is on the order of +0.2 W/m2 (37) and is applicable only to the late 20th century, this GHG was not further considered. Other anthropogenic forcing was not included because evaluations by the Intergovernmental Panel for Climate Change (IPCC) (37) indicate that the confidence in these estimates is very low.


A linear upwelling/diffusion energy balance model (EBM) was used to calculate the mean annual temperature response to estimated forcing changes. This model (39) calculates the temperature of a vertically averaged mixed-layer ocean/atmosphere that is a function of forcing changes and radiative damping. The mixed layer is coupled to the deep ocean with an upwelling/diffusion equation in order to allow for heat storage in the ocean interior. The radiative damping term can be adjusted to embrace the standard range of IPCC sensitivities for a doubling of CO2. The EBM is similar to that used in many IPCC assessments (40) and has been validated (39) against both the Wigley-Raper EBM (40) and two different coupled ocean-atmosphere general circulation model (GCM) simulations (41). All forcings for the model runs were set to an equilibrium sensitivity of 2°C for a doubling of CO2. This is on the lower end of the IPCC range (42) of 1.5° to 4.5°C for a doubling of CO2 and is slightly less than the IPCC "best guess" sensitivity of 2.5°C [the inclusion of solar variability in model calculations can decrease the best fit sensitivity (9)]. For both the solar and volcanism runs, the calculated temperature response is based on net radiative forcing after adjusting for the 30% albedo of the Earth-atmosphere system over visible wavelengths.


The modeled responses to individual forcing terms (Fig. 3A) indicate that the post-1850 GHG and tropospheric aerosol changes are similar to those discussed in IPCC (42). CO2 temperature variations are very small for the preanthropogenic interval, although there is a 0.05°C decrease in the 17th and 18th centuries that reflects the CO2 decrease of ~6 parts per million in the original ice core record (35). Solar variations are on the order of 0.2°C, and volcanism causes large cooling (43) in the Little Ice Age (3-7, 9, 10). Averaged over the entire preanthropogenic interval (Table 1), 22 to 23% of the decadal-scale variance can be explained by volcanism (P 0.01). However, over the interval 1400-1850, the volcanic contribution increases to 41 to 49% (P 0.01), thereby indicating a very important role for volcanism during the Little Ice Age.

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 and solar variability (with 11-point smoothing), using the Bard et al. (
30) 14C index.

Table 1. Correlations of volcanism (volc.) and solar variability (sol.) for the preanthropogenic interval, with percent variance shown in parentheses. The different solar time series reflect the three different solar indices used in this study. The Mann et al. time series (11) has been smoothed with an 11-point filter. CL was smoothed in the original analysis (12). Different abbreviations for solar forcing refer to the different indices discussed in the text: 10Be and 14C calculations are from Bard et al. (30); 14C residuals are from Stuiver et al. (31).

Volc. vs. Mann et al. (1000-1850)

0.48 (23%)

Volc. vs. CL (1000-1850)

0.47 (22%)

Volc. vs. Mann et al. (1400-1850)

0.70 (49%)

Volc. vs. CL (1400-1850)

0.64 (41%)

Sol (10Be) vs. Mann et al.

0.45 (20%)

Sol (14C Bard) vs. Mann et al.

0.56 (31%)

Sol (14C Stuiver) vs. Mann et al.

0.37 (14%)

Sol (10Be) vs. CL

0.42 (18%)

Sol (14C Bard) vs. CL

0.67 (45%)

Sol (14C Stuiver) vs. CL

0.30 ( 9%)

The sun-climate correlations for the interval 1000-1850 vary substantially by choice of solar index (Table 1), with explained variance ranging from as low as 9% (P 0.01) for the 14C residual index (31) to as high as 45% (P 0.01) for the Bard et al. (30) 14C solar index, which reconstructs a Medieval solar warming comparable to the present century but only about 0.1°C greater than predicted by the other solar indices (Fig. 3A). The large range in correlations for the solar records emphasizes the need to determine more precisely the relative magnitude of the real Medieval solar warming peak.

The joint effects of solar variability and volcanism (Fig. 3B) indicate that the combination of these effects could have contributed 0.15° to 0.2°C to the temperature increase (Fig. 1) from about 1905-1955, but only about one-quarter to the total 20th-century warming. The combined warmth produced by solar variability and volcanism in the 1950s is similar in magnitude but shorter in duration than the warmth simulated by these mechanisms in the Middle Ages. The variations in the past few decades resulting from the combination of solar variability and volcanism is 0.2°C less than the 1955 peak.

Combining all forcing (solar, volcanism, GHG, and tropospheric aerosols) results in some striking correspondences between the model and the data over the preanthropogenic interval (Fig. 4). Eleven-point smoothed correlations (44) for the preanthropogenic interval (Table 2) indicate that 41 to 64% of the total variance is forced (P < 0.01). The highest correlations are obtained for the CL time series, which has slightly more Medieval warmth than the Mann et al. reconstruction, and for the forcing time series that includes the largest solar estimate of Medieval warmth. Forced variability explains 41 to 59% of the variance (P 0.01) over the entire length of the records. Although simulated temperatures agree with observations in the late 20th century, simulations exceed observations by ~0.1° to 0.15°C over the intervals 1850-1885 and 1925-1975, with a larger discrepancy between ~1885-1925 that reaches a maximum offset of ~0.3°C from ~1900-1920. However, decadal-scale patterns of warming and cooling are still simulated well in these offset intervals. A sensitivity test (45) comparing forcing time series with and without solar variability indicates that changes caused by volcanism and CO2 are responsible for the simulated temperature increase from the mid- to late 19th century to the early 20th century, thereby eliminating uncertainties in solar forcing as the explanation for the temperature differences between the model and the data. Also shown in Fig. 4A is the CL reconstruction with the "anomalous" warm interval (~1885-1925) discussed above. For this reconstruction, 55 to 69% of the variance from 1005-1993 can be explained by the model (P 0.01).

Fig. 4. Comparison of model response (blue) 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).

Table 2. Correlations between model runs with combined forcing and the Mann et al. (11) and CL (12) time series. Correlations have been subdivided into the following three categories: Top set: Correlations for all the preanthropogenic interval 1005-1850 of model response to combined forcing ("All") with different solar indices (Table 1) and the 11-point smoothed Mann et al. time series and CL2 record spliced into the 11-point smoothed Jones et al. (16) time series. Middle set: Correlations over the entire interval analyzed. Bottom set: Correlations and variance explained for the interval 1005-1993 using the original CL2 reconstruction from 1005-1965, with the smoothed Jones et al. (16) record added from 1965-1993.

Summary of pre-1850 correlations, with variance shown in parentheses

All 10Be (solar) vs. Mann (sm11)

0.64 (41%)

All 14C Brd (solar) vs. Mann (sm11)

0.68 (46%)

All 14C Stv (solar) vs. Mann (sm11)

0.65 (42%)

All 10Be (solar) vs. CL2.Jns11

0.69 (48%)

All 14C Brd (solar) vs. CL2.Jns11

0.80 (64%)

All 14C Stv (solar) vs. CL2.Jns11

0.68 (47%)

Summary of correlations for 1005-1993, with variance shown in parentheses

All 10Be (solar) vs. Mann (sm11)

0.68 (46%)

All 14C Brd (solar) vs. Mann (sm11)

0.73 (53%)

All 14C Stv (solar) vs. Mann (sm11)

0.67 (45%)

All 10Be (solar) vs. CL2.Jns11

0.66 (43%)

All 14C Brd (solar) vs. CL2.Jns11

0.77 (59%)

All 14C Stv (solar) vs. CL2.Jns11

0.64 (41%)

Summary of correlations for 1005-1993 against unfiltered CL time series, with 11-point smoothed Jones et al. (16) record spliced in from 1965-1993

All 10Be (solar) vs. CL2.Jns 11

0.75 (57%)

All 14C Brd (solar) vs. CL2.Jns11

0.83 (69%)

All 14C Stv (solar) vs. CL2.Jns11

0.74 (54%)

Another means of evaluating the role of forced variability is to determine residuals by subtracting the different model time series from the two paleo time series over the preanthropogenic interval (Fig. 5A). The trend lines for three of these residuals are virtually zero, and there is only about a ±0.1°C trend for the other three residuals. Because the pre-1850 residuals represent an estimate of the unforced variability in the climate system, it is of interest to compare the smoothed residuals with smoothed estimates of unforced variability in the climate system from control runs of coupled ocean-atmosphere models. There is significant agreement (Fig. 5B and Table 3) between the smoothed standard deviations of the GCMs (46) and paleo residuals (47). These results support a basic assumption in optimal detection studies (1) and previous conclusions (48) that the late-20th-century warming cannot be explained by unforced variability in the ocean-atmosphere system. However, a combination of GHG, natural forcing, and ocean-atmosphere variability could have contributed to th