Bette L. Otto-Bliesner

Recent warming reverses long-term arctic cooling.

Climate Reversal The climate and environment of the Arctic have changed drastically over the short course of modern observation. Kaufman et al. (p. 1236) synthesized 2000 years of proxy data from lakes above 60° N latitude with complementary ice core and tree ring records, to create a paleoclimate reconstruction for the Arctic with a 10-year resolution. A gradual cooling trend at the start of the record had reversed by the beginning of the 20th century, when temperatures began to increase rapidly. The long-term cooling of the Arctic is consistent with a reduction in summer solar insolation caused by changes in Earths orbit, while the rapid and large warming of the past century is consistent with the human-caused warming. A 2000-year-long Arctic cooling trend seen in a surface air temperature reconstruction was reversed during the last century. The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation

The covariation of carbon dioxide (CO2) concentration and temperature in Antarctic ice-core records suggests a close link between CO2 and climate during the Pleistocene ice ages. The role and relative importance of CO2 in producing these climate changes remains unclear, however, in part because the ice-core deuterium record reflects local rather than global temperature. Here we construct a record of global surface temperature from 80 proxy records and show that temperature is correlated with and generally lags CO2 during the last (that is, the most recent) deglaciation. Differences between the respective temperature changes of the Northern Hemisphere and Southern Hemisphere parallel variations in the strength of the Atlantic meridional overturning circulation recorded in marine sediments. These observations, together with transient global climate model simulations, support the conclusion that an antiphased hemispheric temperature response to ocean circulation changes superimposed on globally in-phase warming driven by increasing CO2 concentrations is an explanation for much of the temperature change at the end of the most recent ice age.

Simulating Arctic climate warmth and icefield retreat in the last interglaciation

In the future, Arctic warming and the melting of polar glaciers will be considerable, but the magnitude of both is uncertain. We used a global climate model, a dynamic ice sheet model, and paleoclimatic data to evaluate Northern Hemisphere high-latitude warming and its impact on Arctic icefields during the Last Interglaciation. Our simulated climate matches paleoclimatic observations of past warming, and the combination of physically based climate and ice-sheet modeling with ice-core constraints indicate that the Greenland Ice Sheet and other circum-Arctic ice fields likely contributed 2.2 to 3.4 meters of sea-level rise during the Last Interglaciation.

Transient Simulation of Last Deglaciation with a New Mechanism for Bølling-Allerød Warming

Model Behavior The initial pulse of warming during the last deglaciation, which defined the start of an interval called the Bølling-Allerød, occurred abruptly about 14,500 years ago. To date, the most detailed simulations used models of intermediate complexity, not with more sophisticated Coupled Global Climate Models (CGCMs) that can synchronously couple both oceanic and the atmospheric components. Overcoming practical and technical challenges, Liu et al. (p. 310; see the Perspective by Timmermann and Menviel) performed such a simulation using CCSM3, a state-of-the-art ocean-atmosphere CGCM. In contrast to previous studies, which indicated that the Bølling-Allerød was triggered by a nonlinear bifurcation between modes of deep ocean circulation in the Atlantic, the results suggest that the event was a transient response caused by the cessation of meltwater input into the surface ocean in the North Atlantic region. A coupled atmosphere-ocean general circulation model simulates the warming of the last deglaciation. We conducted the first synchronously coupled atmosphere-ocean general circulation model simulation from the Last Glacial Maximum to the Bølling-Allerød (BA) warming. Our model reproduces several major features of the deglacial climate evolution, suggesting a good agreement in climate sensitivity between the model and observations. In particular, our model simulates the abrupt BA warming as a transient response of the Atlantic meridional overturning circulation (AMOC) to a sudden termination of freshwater discharge to the North Atlantic before the BA. In contrast to previous mechanisms that invoke AMOC multiple equilibrium and Southern Hemisphere climate forcing, we propose that the BA transition is caused by the superposition of climatic responses to the transient CO2 forcing, the AMOC recovery from Heinrich Event 1, and an AMOC overshoot.

Last Glacial Maximum and Holocene Climate in CCSM3

Abstract The climate sensitivity of the Community Climate System Model version 3 (CCSM3) is studied for two past climate forcings, the Last Glacial Maximum (LGM) and the mid-Holocene. The LGM, approximately 21 000 yr ago, is a glacial period with large changes in the greenhouse gases, sea level, and ice sheets. The mid-Holocene, approximately 6000 yr ago, occurred during the current interglacial with primary changes in the seasonal solar irradiance. The LGM CCSM3 simulation has a global cooling of 4.5°C compared to preindustrial (PI) conditions with amplification of this cooling at high latitudes and over the continental ice sheets present at LGM. Tropical sea surface temperature (SST) cools by 1.7°C and tropical land temperature cools by 2.6°C on average. Simulations with the CCSM3 slab ocean model suggest that about half of the global cooling is explained by the reduced LGM concentration of atmospheric CO2 (∼50% of present-day concentrations). There is an increase in the Antarctic Circumpolar Current an...

Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise

Sea-level rise from melting of polar ice sheets is one of the largest potential threats of future climate change. Polar warming by the year 2100 may reach levels similar to those of 130,000 to 127,000 years ago that were associated with sea levels several meters above modern levels; both the Greenland Ice Sheet and portions of the Antarctic Ice Sheet may be vulnerable. The record of past ice-sheet melting indicates that the rate of future melting and related sea-level rise could be faster than widely thought.

Solar influence on climate during the past millennium: Results from transient simulations with the NCAR Climate System Model

The potential role of solar variations in modulating recent climate has been debated for many decades and recent papers suggest that solar forcing may be less than previously believed. Because solar variability before the satellite period must be scaled from proxy data, large uncertainty exists about phase and magnitude of the forcing. We used a coupled climate system model to determine whether proxy-based irradiance series are capable of inducing climatic variations that resemble variations found in climate reconstructions, and if part of the previously estimated large range of past solar irradiance changes could be excluded. Transient simulations, covering the published range of solar irradiance estimates, were integrated from 850 AD to the present. Solar forcing as well as volcanic and anthropogenic forcing are detectable in the model results despite internal variability. The resulting climates are generally consistent with temperature reconstructions. Smaller, rather than larger, long-term trends in solar irradiance appear more plausible and produced modeled climates in better agreement with the range of Northern Hemisphere temperature proxy records both with respect to phase and magnitude. Despite the direct response of the model to solar forcing, even large solar irradiance change combined with realistic volcanic forcing over past centuries could not explain the late 20th century warming without inclusion of greenhouse gas forcing. Although solar and volcanic effects appear to dominate most of the slow climate variations within the past thousand years, the impacts of greenhouse gases have dominated since the second half of the last century.

Transient simulations of Holocene atmospheric carbon dioxide and terrestrial carbon since the Last Glacial Maximum

[1] Conflicting hypotheses are investigated for the observed atmospheric CO2 increase of 20 ppm between 8 ka BP and pre-industrial time. The carbon component of the Bern Carbon Cycle Climate (Bern CC) model, which couples the Lund-Potsdam-Jena Dynamic Global Vegetation Model to an atmosphere-ocean-sediment component, is driven by climate fields from time-slice simulations of the past 21 ka with the Hadley Centre Unified Model or the NCAR Climate System Model. The entire Holocene ice core record of CO2 is matched within a few ppm for the standard model setup, and results are broadly consistent with proxy data of atmospheric 13 CO2, mean ocean d 13 C, and pollen data, within their uncertainties. Our analysis suggests that a range of mechanisms, including calcite compensation in response to earlier terrestrial uptake, terrestrial carbon uptake and release, SST changes, and coral reef buildup, contributed to the 20 ppm rise. The deep sea d 13 C record constrains the contribution of the calcite compensation mechanism to 4–10 ppm. Terrestrial carbon inventory changes related to climate and CO2 forcing, the greening of the Sahara, peat buildup, and land use have probably influenced atmospheric CO2 by a few ppm only. The early Holocene CO2 decrease is quantitatively explained by terrestrial uptake and calcite compensation in response to terrestrial uptake during the glacial-interglacial transition. The recent hypothesis by Ruddiman [2003] that anthropogenic land use caused a 40 ppm CO2 anomaly over the past 8 ka, preventing the climate system from entering a new glacial, would imply an anthropogenic emission of 700 GtC and a decrease in atmospheric d 13 Co f 0.6 permil. This is not compatible with the ice core d 13 C record and would require an upward revision of land use emission estimates by a factor of 3 to 4. INDEX TERMS: 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 1610 Global Change: Atmosphere (0315, 0325); KEYWORDS: carbon cycle modeling, carbon dioxide, Holocene

Global climate evolution during the last deglaciation

Deciphering the evolution of global climate from the end of the Last Glacial Maximum approximately 19 ka to the early Holocene 11 ka presents an outstanding opportunity for understanding the transient response of Earth’s climate system to external and internal forcings. During this interval of global warming, the decay of ice sheets caused global mean sea level to rise by approximately 80 m; terrestrial and marine ecosystems experienced large disturbances and range shifts; perturbations to the carbon cycle resulted in a net release of the greenhouse gases CO2 and CH4 to the atmosphere; and changes in atmosphere and ocean circulation affected the global distribution and fluxes of water and heat. Here we summarize a major effort by the paleoclimate research community to characterize these changes through the development of well-dated, high-resolution records of the deep and intermediate ocean as well as surface climate. Our synthesis indicates that the superposition of two modes explains much of the variability in regional and global climate during the last deglaciation, with a strong association between the first mode and variations in greenhouse gases, and between the second mode and variations in the Atlantic meridional overturning circulation.

Coupled Climate Simulation of the Evolution of Global Monsoons in the Holocene

Abstract Evolution of global monsoons in the Holocene is simulated in a coupled climate model—the Fast Ocean Atmosphere Model—and is also compared with the simulations in another coupled climate model—the NCAR Climate System Model. Holocene climates are simulated under the insolation forcing at 3000, 6000, 8000, and 11 000 years before present. The evolution of six major regional summer monsoons is investigated: the Asian monsoon, the North African monsoon, the North American monsoon, the Australian monsoon, the South American monsoon, and the South African monsoon. Special attention has been paid to the relative roles of the direct insolation forcing and oceanic feedback. It is found that the responses of the monsoons to the insolation forcing and oceanic feedback differ substantially among regions, because of regional features of atmospheric and oceanic circulation and ocean–atmosphere interaction. In the Northern Hemisphere, the coupled models show a significant enhancement of all of the monsoons in th...