The Sarmiento and Bender groups examine climate change in the context of long-term natural variability. Sarmiento and colleagues are working to separate anthropogenic signatures on atmospheric carbon from the background noise of natural events, while the Bender group looks to ancient sea ice to better understand the carbon cycle.


Atmospheric CO2 response to volcanic eruptions

In previous work, the Sarmiento group has shown that tropical volcanic eruptions are one of the most important natural factors that significantly impact the climate system and the carbon cycle on annual to multi-decadal time scales. The three largest explosive volcanic eruptions in the last 50 years – Agung, El Chichón, and Pinatubo – occurred in spring-summer in conjunction with El Niño events and left distinct negative signals in the observational temperature and CO2 records. However, confounding factors such as seasonal variability and El Niño-Southern Oscillation (ENSO) may obscure the forcing-response relationship.

In this study, Thomas Frölicher in the Sarmiento group determined for the first time the extent to which initial conditions, i.e. season and phase of the ENSO, and internal variability influence the coupled climate and carbon cycle response to volcanic forcing, and how this affects estimates of the terrestrial and ocean carbon sinks. Ensemble simulations with the Earth System Model CSM1.4- carbon predict that the atmospheric response is ~60% larger when a volcanic eruption occurs during El Niño and in winter than during La Niña conditions (Figure 5). The simulations suggest that the Pinatubo eruptions contributed 11 ± 6% to the 25 Pg terrestrial carbon sink inferred over the decade 1990-1999 and -2 ± 1% to the 22 Pg oceanic carbon sink.

Recent studies have indicated a possible positive trend in the airborne anthropogenic CO2 fraction, suggesting a worrying decrease in the efficiency of the ocean and land carbon sinks. In contrast to these claims, this study indicates that accounting for the decadal-scale influence of explosive volcanism and related uncertainties removes the positive trend in the airborne fraction of anthropogenic carbon. The results highlight the importance of considering the role of natural variability in the carbon cycle for interpretation of observations and for data-model intercomparison.

Figure 5: Temporal evolution of global ensemble monthly mean differences in (a) atmospheric surface temperature and (b) atmospheric CO2. The eruptions started in El Niño winter (black), La Niña winter (red), El Niño summer (green) and La Niña summer (blue) conditions. Shadings indicate one standard deviation confidence interval of the ensemble simulations. Dashed black lines indicate observation-derived temperature and atmospheric CO2 changes after the Mt. Pinatubo eruption (the calculation of the observation-derived atmospheric CO2 changes must be viewed as a tentative attempt – Frölicher et al. 2013).

Extending the record of ancient CO2 levels

The deepest ice cores yet drilled, targeted to encompass the longest possible continuous records, end at ice 800,000 years old. To extend this record, Michael Bender and colleagues have been searching Antarctic sites where, because of complex glacier flows influenced by the Transantarctic Mountains, older ice may be present near the surface (Figure 6). In the Allan Hills, the researchers have discovered million year old ice that will enable Bender’s group to extend back ice core records of CO2 and climate beyond the oldest ice otherwise available. The team plans to do additional prospecting for even older ice in the region. The hope is that this work will yield ice that can be used to extend the climate record back into the “40 k world” of shorter, less intense climate cycles that prevailed from about 2.5-1 million years ago.

Bender has also written a book, Paleoclimate, to be published by Princeton University Press in 2013. The book covers natural climate change and CO2’s role in climate throughout geologic time.

Figure 6: Camp and ice core drill (on sled) in the Allan Hills where the Bender team is seeking even more ancient ice.