Precise timing and characterization of abrupt climate change 8200 years ago from air trapped in polar ice
Introduction
Future climatic change associated with anthropogenic greenhouse gases is a primary concern for human society. Climate models and paleoclimatic records suggest that future climate change may involve large and abrupt changes (Manabe and Stouffer, 1993; Stocker and Schmittner, 1997; IPCC, 2001; National Research Council (US). Committee on Abrupt Climate Change, 2002). One way to quantify possible impacts of future abrupt climate change amid model uncertainties is to look back to the past, and find the closest possible analog (National Research Council (US). Committee on Abrupt Climate Change, 2002; Alley and Agustsdottir, 2005). The abrupt climate change ∼8200 years ago, the largest abrupt climatic event in the past 10,000 years (Fig. 1), occurred at a time when the background climate was much like the present (the Holocene, a relatively warm period after the last glacial period, Fig. 1). The event was characterized by generally cool, dry, and windy climate, which affected ecosystems and early human societies (Alley et al., 1997a; Weiss, 2000). The widespread evidence for the 8.2 ka event, combined with a rapid decrease of atmospheric methane concentration (Spahni et al., 2003; this study) suggests that the event was at least hemispheric or “near-global’ in its geographical extent (Alley and Agustsdottir, 2005; Morrill and Jacobsen, 2005; Wiersma and Renssen, 2006). The cause of the event may have been the largest proglacial lake (Lake Agassiz) outburst of the last deglaciation (Clarke et al., 2004), which has been dated to 8470±300 (1σ) BP (Barber et al., 1999; Rohling and Palike, 2005). This hypothesis holds that a massive outflow of fresh water ran out the Hudson Strait, to the North Atlantic, causing a slowdown of the meridional overturning circulation, which enabled wintertime sea ice cover to expand with consequent hemispheric cooling and drying especially surrounding the North Atlantic area (Alley and Agustsdottir, 2005; Ellison et al., 2006). An alternative hypothesis is that a millennial-scale cooling trend started a few centuries earlier than the 8.2 ka event (Rohling and Palike, 2005). A minor solar minimum coinciding with the 8.2 ka event (Muscheler et al., 2004) as a third hypothesis, may have forced the system to cross a threshold and may have triggered the 8.2 ka event (Bond et al., 1997, Bond et al., 2001).
In this study, we address the detailed timing and evolution of the 8.2k event by measuring nitrogen isotope ratios and methane concentration in trapped air in the GISP2 ice core. Atmospheric methane concentration can be viewed as a qualitative indicator of integrated terrestrial hydrological conditions in methane-producing regions, owing to the dominant methane source from wetland areas (∼75% of natural emissions, Houweling et al., 2000). Nitrogen isotopes in trapped air in an ice core provide a signal of local temperature changes in Greenland (Severinghaus et al., 1998; Goujon et al., 2003; Landais et al., 2004). This method (Severinghaus et al., 1998; Severinghaus and Brook, 1999) provides an opportunity to precisely and directly assess the timing of abrupt climate change in Greenland with respect to changes in atmospheric methane, by comparing two gases in the same core. We also provide an improved estimate of the magnitude of the temperature change in central Greenland.
Section snippets
Materials and methods
We used the GISP2 ice core for our analyses. The resolution of nitrogen isotope data is 1 m (∼10 year) from 1359.95 to 1458.95 m depth, corresponding to a gas age range of 7600–8600 BP (see below for the basis for this chronology). Replicate analyses (2–3 for each depth) were conducted for the entire record. Additional replicates were done in the intervals 1412.95–1416.95 m (6 replicates per depth) and 1417.97–1423.95 m (4 replicates per depth) to increase the confidence level of the temperature
Magnitude of Greenland cooling
The magnitude of temperature changes is often used as the most important climatic indicator and as a target for climate model simulations (LeGrande et al., 2006). There have been many attempts to quantify the magnitude of the temperature change for the 8.2 ka event in central Greenland. A conventional method uses oxygen isotopes of ice (δ18Oice) calibrated by the empirical modern spatial relationship of temperature and δ18O of precipitation. This slope of the temperature–isotope relationship is
Methane concentration and emission history
How the 8.2 ka event evolved with time and space is important for understanding the mechanism of the event. The atmospheric methane concentration record provides a hint about the temporal evolution of the 8.2 ka event, because methane emissions are strongly affected by climate (Brook et al., 2000). The methane record thus can help to interpret the regional climatic signals found in other paleoclimate records. However, the ice-core methane record does not directly reflect atmospheric methane
Timing of the 8.2 ka event
Although many high-resolution proxy data for the early Holocene reveal an abrupt event at around 8.2 ka, the apparent age and duration are different in different locations, probably owing to the difficulties of all paleoclimate records with age uncertainty, continuity, and regional climatic influences (Alley and Agustsdottir, 2005; Morrill and Jacobsen, 2005; Rohling and Palike, 2005). Therefore, it is difficult to assess the relative timing of climatic events between different regions inferred
Conclusion
A large number of paleoclimatic records over a hemispheric area show a large and abrupt climate change around 8200 years BP. However, the duration and general character of the event have been ambiguous. Here, we provide a precise characterization and timing of the event using methane and nitrogen isotopes in trapped air in a Greenland ice core. Climate change in Greenland and at a hemispheric scale was simultaneous (within ±4 years) as supported by climate model results (LeGrande et al., 2006).
Acknowledgments
We thank N. Caillon and R. Beaudette for help with development of the copper method, R. Alley and K. Cuffey for accumulation and age data, E. Bauer for providing the land temperature and precipitation data, and S. Gille, Y. Lenn, K. Kawamura, and D. Lal for important discussions. Funding for this work came from NSF OPP (0512971), and the Gary Comer Science and Education Foundation.
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