Search Results

You are looking at 1 - 7 of 7 items for

  • Author or Editor: Thomas J. Crowley x
  • All content x
Clear All Modify Search
Thomas J. Crowley

Geologic studies provide a valuable perspective on the importance of greenhouse forcing for climate change. On both Pleistocene and tectonic time scales, changes in climate are positively correlated with greenhouse gas variations. However, the sensitivity of the system to greenhouse gas changes cannot yet be constrained by paleoclimate data below its present large range. Geologic records do not support one of the major predictions of greenhouse models—namely, that tropical sea surface temperatures will increase. Geologic data also suggest that winter cooling in high-latitude land areas is less than predicted by models. As the above-mentioned predictions appear to be systemic features of the present generation of climate models, some significant changes in model design may be required to reconcile models and geologic data. However, full acceptance of this conclusion requires more measurements and more systematic compilations of existing geologic data. Since progress in data collection in this area has been quite slow, uncertainties associated with these conclusions may persist for some time.

Full access
Thomas J. Crowley

Abstract

There have been numerous attempts to propose past warm time periods as “analogs” for a future greenhouse warming. In this paper it is argued that, although paleoclimate studies may provide important insights into process operating in the climate system, there may be no warm time period that is a satisfactory past analog for future climate. The future greenhouse warming may represent a unique climate realization in earth history. This conclusion is based on the following considerations: 1 ) comparisons with Holocene (9000 BP) or Eemian climates (120 000 BP) may be inappropriate because much of the variations in these climates can be explained in terms of seasonal rather than mean-annual forcing it has yet to be demonstrated that increased warmth for these intervals involved mew annual temperature increases that were globally synchronous, 2) comparisons with older and warmer climates (ex., Pliocene, Eocene, or Cretaceous) can be misleading because these warm periods had reduced polar ice cover, whereas future air temperatures will be very warm, but ice sheets will persist because of their large thermal inertia. Due to the different time scales for the atmosphere, deep ocean, and ice sheets, this significant nonequilibrium component to the future climate response is probably very different than the long time-averaged picture representative of past warm periods. Furthermore, changes in geography have probably significantly modified the atmosphere and ocean circulation during the earlier warm periods resulting in regional climates significantly different than what might occur in the future. It is therefore suggested that future discussions on geologic analogs be restricted to study of processes operating in the climate system and that continued use of the term for past warm time periods be abandoned.

Full access
Jiacheng Zhang and Thomas J. Crowley

Abstract

The principal results of studies on historical climate change from A.D. 1000 to the present in China are reviewed. The studies are based on analysis of local annals and court records. After discussing the methodology of transferring descriptive accounts into quantitative estimates of past climates, we summarize the main results, which are generally substantiated by multiple lines of evidence: 1) There were significant historical climate fluctuations in China, with a range of about 1.0°–1.5°C in recent centuries. 2) Significant decadal-scale warm fluctuations occurred during a cool interval broadly correlative with the Little Ice Age. 3) There was an increased frequency of both droughts and floods in some pans of China during the Little Ice Age. Increased frequencies of dust storms accompanied the dry phases of the cool periods. 4) The spatial pattern of some Little Ice Age precipitation changes appears to reflect a modified development of different phases of the summer monsoon. 5) As suggested by recent GCM studies, enhanced Little Ice Age aridity may be due to increased winter snow cover causing temperature and soil moisture feedbacks the following spun 6) Although there is some agreement between climate change in China and elsewhere, there are also indications that significant lap occur between the timing and direction of climate change in different regions. This pattern appears different from the warming trend of the past century, which is more uniform in both hemispheres.

Full access
William T. Hyde and Thomas J. Crowley

Abstract

A number of climate model predictions indicate that the global average temperature should rise another 0.1°–0.3°C in the next 10–15 yr, thereby rising above the noise level of natural variability. However, volcanic eruptions could mask the CO2 effect and complicate discussions on a greenhouse gas protocol. The authors quantify this possibility by creating a volcano eruption probability density function from a 600-yr-long record of Northern Hemisphere eruptions. Results indicate that there is a substantial probability of one or two eruptions with a radiative impact greater than −1 W m−2 (≈37% and 15%, respectively) in the next decade, and a 15%–25% chance of a single large eruption (−3 W m−2 or greater).

Full access
Marisa Montoya, Hans von Storch, and Thomas J. Crowley

Abstract

The ECHAM-1 T21/LSG coupled ocean–atmosphere general circulation model (GCM) is used to simulate climatic conditions at the last interglacial maximum (Eemian, 125 kyr BP). The results reflect the expected surface temperature changes (with respect to the control run) due to the amplification (reduction) of the seasonal cycle of insolation in the Northern (Southern) Hemisphere. A number of simulated features agree with previous results from atmospheric GCM simulations (e.g., intensified summer southwest monsoons) except in the Northern Hemisphere poleward of 30°N, where dynamical feedbacks in the North Atlantic and North Pacific increase zonal temperatures about 1°C above what would be predicted from simple energy balance considerations. As this is the same area where most of the terrestrial geological data originate, this result suggests that previous estimates of Eemian global average temperature might have been biased by sample distribution. This conclusion is supported by the fact that the estimated global temperature increase of only 0.3°C greater than the control run has been previously shown to be consistent with CLIMAP sea surface temperature estimates. Although the Northern Hemisphere summer monsoon is intensified, globally averaged precipitation over land is within about 1% of the present, contravening some geological inferences but not the deep-sea δ 13C estimates of terrestrial carbon storage changes. Winter circulation changes in the northern Arabian Sea, driven by strong cooling on land, are as large as summer circulation changes that are the usual focus of interest, suggesting that interpreting variations in the Arabian Sea sedimentary record solely in terms of the summer monsoon response could sometimes lead to errors. A small monsoonal response over northern South America suggests that interglacial paleotrends in this region were not just due to El Niño variations.

Full access
William T. Hyde, Thomas J. Crowley, Kwang-Yul Kim, and Gerald R. North

Abstract

The Sensitivity of a linear two dimensional Energy Balance Model (EBM) to altered surface albedo and insolation over the last 18 000 years is compared to simulators made with the NCAR Community Climate Model (CCM). The two-dimensional EBM is a more general form of that described in North et al. and allows for regionally varying albedos of ice sheets and sea ice. It is shown that the EBM's hemispherically averaged land and sea seasonal temperature departures agree excellently with the CCM's in the Northern Hemisphere. In the Southern Hemisphere the seasonal comparisons are legs favorable, although the annual-averaged oceanic temperature departures at glacial maximum agree to within 0.3°C. Since the CCM used prescribed SSTs (from CLIMAP), whereas the ERMs are calculated, our results suggest that the hemispherically averaged glacial- interglacial SST change estimated by CLIMAP is consistent with the altered energy balance requirements of the earth-atmosphere system. Results also suggest that on the largest scales the seasonal temperature field at the earth's surface may be linearly dependent on change in orbital forcing and surface albedo. We conclude that the EBM performs well enough to justify its use as an exploratory tool for investigating the effect of altered boundary conditions on the earth/s annual temperature cycle.

Full access
Gabriele C. Hegerl, Thomas J. Crowley, Myles Allen, William T. Hyde, Henry N. Pollack, Jason Smerdon, and Eduardo Zorita

Abstract

Climate records over the last millennium place the twentieth-century warming in a longer historical context. Reconstructions of millennial temperatures show a wide range of variability, raising questions about the reliability of currently available reconstruction techniques and the uniqueness of late-twentieth-century warming. A calibration method is suggested that avoids the loss of low-frequency variance. A new reconstruction using this method shows substantial variability over the last 1500 yr. This record is consistent with independent temperature change estimates from borehole geothermal records, compared over the same spatial and temporal domain. The record is also broadly consistent with other recent reconstructions that attempt to fully recover low-frequency climate variability in their central estimate.

High variability in reconstructions does not hamper the detection of greenhouse gas–induced climate change, since a substantial fraction of the variance in these reconstructions from the beginning of the analysis in the late thirteenth century to the end of the records can be attributed to external forcing. Results from a detection and attribution analysis show that greenhouse warming is detectable in all analyzed high-variance reconstructions (with the possible exception of one ending in 1925), and that about a third of the warming in the first half of the twentieth century can be attributed to anthropogenic greenhouse gas emissions. The estimated magnitude of the anthropogenic signal is consistent with most of the warming in the second half of the twentieth century being anthropogenic.

Full access