Eleventh Year Annual Report: Causes of Climate Variability
The Bender and Sarmiento Groups are using observations and models to investigate the causes of natural variablity in climate and the carbon cycle.
Search for old ice to be used in reconstructing greenhouse gas concentrations
Snow in polar regions traps air as it is buried and transformed into impermeable ice. This trapped air, sampled in deep ice cores, provides a record of CO2 variations during interglacial cycles back to 800,000 years before present. Extending this record further back in time would allow us to investigate the role of CO2 in the somewhat warmer climates of recent geologic time. However, the deepest ice cores yet drilled, targeted to encompass the longest possible continuous records, end at 800,000 years.
Michael Bender and colleagues have studied two Antarctic sites where, because of exotic local glaciology, older ice may be present near the surface. At one site, in the Allan Hills, they found ice much younger than expected, dating to only a little over 100,000 years. The researchers can specifically tie the ice to the last interglacial, and this deposit will be valuable because it provides easily accessible ice to study some aspects of climate during this period. At the second site studied, Mullins Valley, south of New Zealand near the coast, ice was found dating to about 1,300,000 years before present, buried by only about 15 m of shallower ice. This ice is not suitable for direct measurements of CO2, but the team is in the process of making proxy measurements that will give good estimates of its concentration. Their expectation is that CO2 will fall in its range of more recent times as measured in ice cores, but this remains to be seen. The discovery of 1,300,000 year old ice raises hopes that even older ice can be found, at relatively shallow depths, in glaciologically favorable regions of Antarctica.
Sensitivity of atmospheric CO2 and climate to explosive volcanic eruptions
An important part of natural carbon-climate variability is caused by volcanic eruptions both during recent decades and during the preindustrial period. While the direct radiative and dynamical effects of sulfate aerosols from volcanic eruptions on the physical climate system are relatively well known, less emphasis has been placed on investigating the impact of volcanic eruptions on the global carbon cycle and variability in the atmospheric CO2 record.
The Sarmiento Group assessed the impact of volcanic eruptions on the coupled climatebiogeochemical system by forcing a comprehensive, fully coupled carbon cycle-climate model with pulse-like stratospheric optical depth changes. The model simulates a decrease of global and regional atmospheric surface temperature, regionally distinct changes in precipitation, and a decrease in atmospheric CO2 after volcanic eruptions (Figure 15). The volcanic-induced cooling reduces overturning rates in tropical soils, which dominates over reduced litter input due to soil moisture decrease, resulting in higher land carbon inventories for several decades. The perturbation in the ocean carbon inventory changes sign from an initially weak carbon sink to a carbon source. Positive carbon and negative temperature anomalies in subsurface waters last up to several decades. The multi-decadal decrease in atmospheric CO2 yields an additional radiative forcing that amplifies the cooling and perturbs the Earth System on much longer time scales than the atmospheric residence time of volcanic aerosols.
The results have important implications for estimates of the carbon cycle-climate sensitivity γ, expressed as change in atmospheric CO2 per unit change in global mean surface temperature. On decadal time scales, modeled γ is several times larger for a Pinatubo-like eruption than for the industrial period and for a high emission, 21st century scenario. In a follow-up study the Sarmiento Group will investigate the extent to which initial conditions (i.e., season and phase of El Niño-Southern Ocean Oscillation) and internal variability influence the coupled climatecarbon cycle response to volcanic forcing, and how this affects estimates of the terrestrial and ocean carbon sink efficiency.