Bauer etal Assessing climate forcings for past Millennium GRL2003

Geophysical Research Letters, 2003

Climate Dynamics, 2007

A climate simulation of an ocean/atmosphere general circulation model driven with natural forcings alone (constant “pre-industrial” land-cover and well-mixed greenhouse gases, changing orbital, solar and volcanic forcing) has been carried out from 1492 to 2000. Another simulation driven with natural and anthropogenic forcings (changes in greenhouse gases, ozone, the direct and first indirect effect of anthropogenic sulphate aerosol and land-cover) from 1750 to 2000 has also been carried out. These simulations suggest that since 1550, in the absence of anthropogenic forcings, climate would have warmed by about 0.1 K. Simulated response is not in equilibrium with the external forcings suggesting that both climate sensitivity and the rate at which the ocean takes up heat determine the magnitude of the response to forcings since 1550. In the simulation with natural forcings climate sensitivity is similar to other simulations of HadCM3 driven with CO2 alone. Climate sensitivity increases when anthropogenic forcings are included. The natural forcing used in our experiment increases decadal–centennial time-scale and large spatial scale climate variability, relative to internal variability, as diagnosed from a control simulation. Mean conditions in the natural simulation are cooler than in our control simulation reflecting the reduction in forcing. However, over certain regions there is significant warming, relative to control, due to an increase in forest cover. Comparing the simulation driven by anthropogenic and natural forcings with the natural-only simulation suggests that anthropogenic forcings have had a significant impact on, particularly tropical, climate since the early nineteenth century. Thus the entire instrumental temperature record may be “contaminated” by anthropogenic influences. Both the hydrological cycle and cryosphere are also affected by anthropogenic forcings. Changes in tree-cover appear to be responsible for some of the local and hydrological changes as well as an increase in northern hemisphere spring snow cover.

Climate Dynamics, 2001

We analyse possible causes of twentieth century near-surface temperature change. We use an``optimal detection'' methodology to compare seasonal and annual data from the coupled atmosphere-ocean general circulation model HadCM2 with observations averaged over a range of spatial and temporal scales. The results indicate that the increases in temperature observed in the latter half of the century have been caused by warming from anthropogenic increases in greenhouse gases oset by cooling from tropospheric sulfate aerosols rather than natural variability, either internal or externally forced. We also ®nd that greenhouse gases are likely to have contributed signi®cantly to the warming in the ®rst half of the century. In addition, natural eects may have contributed to this warming. Assuming one particular reconstruction of total solar irradiance to be correct implies, when we take the seasonal cycle into account, that solar eects have contributed signi®cantly to the warming observed in the early part of the century, regardless of any relative error in the amplitudes of the anthropogenic forcings prescribed in the model. However, this is not the case with an alternative reconstruction of total solar irradiance, based more on the amplitude than the length of the solar cycle. We also ®nd evidence for volcanic in¯uences on twentieth century near-surface temperatures. The signature of the eruption of Mount Pinatubo is detected using annual-mean data. We also ®nd evidence for a volcanic in¯uence on warming in the ®rst half of the century associated with a reduction in mid-century volcanism.

2002

1] Using a coupled atmosphere/ocean general circulation model, we have simulated the climatic response to natural and anthropogenic forcings from 1860 to 1997. The model, HadCM3, requires no flux adjustment and has an interactive sulphur cycle, a simple parameterization of the effect of aerosols on cloud albedo (first indirect effect), and a radiation scheme that allows explicit representation of well-mixed greenhouse gases. Simulations were carried out in which the model was forced with changes in natural forcings (solar irradiance and stratospheric aerosol due to explosive volcanic eruptions), well-mixed greenhouse gases alone, tropospheric anthropogenic forcings (tropospheric ozone, wellmixed greenhouse gases, and the direct and first indirect effects of sulphate aerosol), and anthropogenic forcings (tropospheric anthropogenic forcings and stratospheric ozone decline). Using an ''optimal detection'' methodology to examine temperature changes near the surface and throughout the free atmosphere, we find that we can detect the effects of changes in well-mixed greenhouse gases, other anthropogenic forcings (mainly the effects of sulphate aerosols on cloud albedo), and natural forcings. Thus these have all had a significant impact on temperature. We estimate the linear trend in global mean near-surface temperature from well-mixed greenhouse gases to be 0.9 ± 0.24 K/century, offset by cooling from other anthropogenic forcings of 0.4 ± 0.26 K/century, giving a total anthropogenic warming trend of 0.5 ± 0.15 K/century. Over the entire century, natural forcings give a linear trend close to zero. We found no evidence that simulated changes in near-surface temperature due to anthropogenic forcings were in error. However, the simulated tropospheric response, since the 1960s, is $50% too large. Our analysis suggests that the early twentieth century warming can best be explained by a combination of warming due to increases in greenhouse gases and natural forcing, some cooling due to other anthropogenic forcings, and a substantial, but not implausible, contribution from internal variability. In the second half of the century we find that the warming is largely caused by changes in greenhouse gases, with changes in sulphates and, perhaps, volcanic aerosol offsetting approximately one third of the warming. Warming in the troposphere, since the 1960s, is probably mainly due to anthropogenic forcings, with a negligible contribution from natural forcings.

Journal of Climate, 2004

Ensemble simulations are run with a global coupled climate model employing five forcing agents that influence the time evolution of globally averaged surface air temperature during the twentieth century. Two are natural (volcanoes and solar) and the others are anthropogenic [e.g., greenhouse gases (GHGs), ozone (stratospheric and tropospheric), and direct effect of sulfate aerosols]. In addition to the five individual forcing experiments, an additional eight sets are performed with the forcings in various combinations. The late-twentieth-century warming can only be reproduced in the model with anthropogenic forcing (mainly GHGs), while the early twentieth-century warming is mainly caused by natural forcing in the model (mainly solar). However, the signature of globally averaged temperature at any time in the twentieth century is a direct consequence of the sum of the forcings. The similarity of the response to the forcings on decadal and interannual time scales is tested by performing a principal component analysis of the 13 ensemble mean globally averaged temperature time series. A significant portion of the variance of the reconstructed time series can be retained in residual calculations compared to the original single and combined forcing runs. This demonstrates that the statistics of the variances for decadal and interannual time-scale variability in the forced simulations are similar to the response from a residual calculation. That is, the variance statistics of the response of globally averaged temperatures in the forced runs are additive since they can be reproduced in the responses calculated as a residual from other combined forcing runs.

Climate Dynamics, 1999

Numerical experiments have been carried out with a two-dimensional sector averaged global climate model with a detailed radiative scheme in order to assess the possible impact of solar and volcanic activities on the Earth's surface temperature at the secular time scale from 1700 to 1992. Our results indicate that while the general trend of the observed temperature variations on the century time scale can be generated in response to both the solar and volcanic forcings, these are clearly not su$cient to explain the observed 20th century warming and more speci"cally the warming trend which started at the beginning of the 1970s. However, the lack of volcanism during the period 1925}1960 could account, at least partly, for the observed warming trend in this period. Finally, while Schlesinger and Ramankutty (1994) assumed that random forcing could not be a possible source of the 65}70 year oscillation they detected in the global climate system, our results indicate that the volcanic forcing over the past 150 years could have introduced an oscillation of around 70 years in the Earth's surface temperature.

Tellus A, 2002

Seventy-one sensitivity experiments have been performed using a two-dimensional sectoraveraged global climate model to assess the potential impact of six different factors on the last millennium climate and in particular on the surface air temperature evolution. Both natural (i.e, solar and volcanism) and anthropogenically-induced (i.e. deforestation, additional greenhouse gases, and tropospheric aerosol burden) climate forcings have been considered. Comparisons of climate reconstructions with model results indicate that all the investigated forcings are needed to simulate the surface air temperature evolution. Due to uncertainties in historical climate forcings and temperature reconstructions, the relative importance of a particular forcing in the explanation of the recorded temperature variance is largely function of the forcing time series used. Nevertheless, our results indicate that whatever the historical solar and volcanic reconstructions may be, these externally driven natural climate forcings are unable to give climate responses comparable in magnitude and time to the late-20th-century temperature warming while for earlier periods combination of solar and volcanic forcings can explain the Little Ice Age and the Medieval Warm Period. Only the greenhouse gas forcing allows the model to simulate an accelerated warming rate during the last three decades. The best guess simulation (largest similarity with the reconstruction) for the period starting 1850 AD requires however to include anthropogenic sulphate forcing as well as the impact of deforestation to constrain the magnitude of the greenhouse gas twentieth century warming to better fit the observation. On the contrary, prior to 1850 AD mid-latitude land clearance tends to reinforce the Little Ice age in our simulations.

Geophysical Research Letters, 2006

1] Temperature observations from the Northern Hemisphere reveal a warming since 1861 which is larger in winter than in summer. Possible explanations for a decline in seasonal spread are discussed using the Earth system model CLIMBER-2. Simulations forced by natural and anthropogenic factors (Milankovitch forcing, solar variability, volcanism, atmospheric CO 2 concentration, deforestation) generate specific seasonal responses. While the Milankovitch forcing increased the millennial seasonal spread, and solar variability and volcanism proved ancillary in reducing the spread on the centennial timescale, the anthropogenic factors appear the primary agents to attenuate the seasonal spread. The climatic effect of the anthropogenic factors is amplified by seasonally varying feedbacks related to the albedo of changing sea-ice and snow cover. Citation: Bauer, E., and M. Claussen , Analyzing seasonal temperature trends in forced climate simulations of the past millennium, Geophys. Res. Lett., 33, L02702,

2010

A long-standing task in climate research has been to distinguish between anthropogenic climate change and natural climate variability. A prerequisite for fulfilling this task is the understanding of the relative roles of external drivers and internal variability of climate and the carbon cycle. Here, we present the first ensemble simulations over 5 the last 1200 years with a comprehensive Earth system model including a fully interactive carbon cycle. Applying up-to-date reconstructions of external forcing including the recent low-amplitude estimates of solar variations, the ensemble simulations reproduce temperature evolutions consistent with the range of reconstructions. The 20th-century warming trend stands out against all pre-industrial trends within the ensemble. Volcanic 10 eruptions are necessary to explain variations in pre-industrial climate such as the Little Ice Age; yet only the strongest, repeated eruptions lead to cooling trends that stand out against the internal variability across all ensemble members. The simulated atmospheric CO 2 concentrations exhibit a stable carbon cycle over the pre-industrial era with multi-centennial variations somewhat smaller than in the observational records. Early 15 land-cover changes have modulated atmospheric CO 2 concentrations only slightly. We provide a model-based quantification of the sensitivity (termed γ) of the global carbon cycle to temperature for a variety of climate and forcing conditions. The magnitude of γ agrees with a recent statistical assessment based on reconstruction data. We diagnose a distinct dependence of γ on the forcing strength and time-scales involved, thus 20 providing an explanation for the systematic difference in the observational estimates for different segments of the last millennium.

2004

In a recent paper 1 (herein MM03), we developed an updated version of the climate proxy data set used by Mann et. al. 2 (MBH98) to compute a Northern Hemisphere (NH) temperature index. The most significant changes were the replacement of obsolete versions of proxy data used in MBH98 with current versions from the World Data Center for Paleoclimatology (WDCP) and the use of conventional principal component analysis (PCA) to reduce networks of tree ring chronologies to regional aggregates using the maximum period in which all sites were available. Applying the methodology of MBH98 to the new data yielded an NH temperature index in which the values in the 15 th century exceeded those in the late 20 th century, thereby contradicting the conclusions in MBH98 of a unique 20 th century climate warming. In their response, 3 Mann et. al. highlighted the influence of three (of 22) proxy series in their data that extend back to 1400. The three proxies are: a ring width series from the site at Twisted Tree Heartrot Hill (TTHH) in northern Canada; the first principal component (PC1) of