Overestimate of committed warming.

Palaeoclimate variations are an essential component in constraining future projections of climate change as a function of increasing anthropogenic greenhouse gases. The Earth System Sensitivity (ESS) describes the multi-millennial response of Earth (in terms of global mean temperature) to a doubling of CO concentrations. A recent study used a correlation of inferred temperatures and radiative forcing from greenhouse gases over the past 800,000 years to estimate the ESS from present day CO is about 9°C, and to imply a long-term commitment of 3–7°C even if greenhouse gas levels remain at present-day concentrations.

Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica.

Here, we present direct measurements of atmospheric composition and Antarctic climate from the mid-Pleistocene (∼1 Ma) from ice cores drilled in the Allan Hills blue ice area, Antarctica. The 1-Ma ice is dated from the deficit in (40)Ar relative to the modern atmosphere and is present as a stratigraphically disturbed 12-m section at the base of a 126-m ice core. The 1-Ma ice appears to represent most of the amplitude of contemporaneous climate cycles and CO2 and CH4 concentrations in the ice range from 221 to 277 ppm and 411 to 569 parts per billion (ppb), respectively.

Constraints on the late holocene anthropogenic contribution to the atmospheric methane budget.

The origin of the late preindustrial Holocene (LPIH) increase in atmospheric methane concentrations has been much debated. Hypotheses invoking changes in solely anthropogenic sources or solely natural sources have been proposed to explain the increase in concentrations. Here two high-resolution, high-precision ice core methane concentration records from Greenland and Antarctica are presented and are used to construct a high-resolution record of the methane inter-polar difference (IPD).

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 CO(2) and CH(4) 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.