The new Law Dome ice core and firn air results (see Auxiliary Material) extend the existing Law Dome records by 1000 years to AD 0 and increase the data density between AD 1000 and 1980. Additionally, new N2O measurements have been made for this period. The results show significant atmospheric variability on decadal, centennial and millennial timescales.
3.1. Industrial Period Trace Gas Variations (AD 1800 to Present)
 The trace gas records show significant increases in atmospheric concentrations during the last 200 years (29% CO2, 150% CH4 and 21% N2O), most of which occurred during the 20th century (Figure 1). The increased data density adds definition to the observed variations during this period.
 The stabilization of atmospheric CO2 concentration during the 1940s and 1950s is a notable feature in the ice core record. The new high density measurements confirm this result and show that CO2 concentrations stabilized at 310–312 ppm from ∼1940–1955. The CH4 and N2O growth rates also decreased during this period, although the N2O variation is comparable to the measurement uncertainty. Smoothing due to enclosure of air in the ice (about 10 years at DE08) removes high frequency variations from the record, so the true atmospheric variation may have been larger than represented in the ice core air record. Even a decrease in the atmospheric CO2 concentration during the mid-1940s is consistent with the Law Dome record and the air enclosure smoothing, suggesting a large additional sink of ∼3.0 PgC yr−1 [Trudinger et al., 2002a]. The δ13CO2 record during this time suggests that this additional sink was mostly oceanic and not caused by lower fossil emissions or the terrestrial biosphere [Etheridge et al., 1996; Trudinger et al., 2002a]. The processes that could cause this response are still unknown.
 The CO2 stabilization occurred during a shift from persistent El Niño to La Niña conditions [Allan and D'Arrigo, 1999]. This coincided with a warm-cool phase change of the Pacific Decadal Oscillation [Mantua et al., 1997], cooling temperatures [Moberg et al., 2005] and progressively weakening North Atlantic thermohaline circulation [Latif et al., 2004]. The combined effect of these factors on the trace gas budgets is not presently well understood. They may be significant for the atmospheric CO2 concentration if fluxes in areas of carbon uptake, such as the North Pacific Ocean, are enhanced, or if efflux from the tropics is suppressed.
3.2. Late Preindustrial Holocene (LPIH) Changes (0 to 1800 AD)
 One of the most notable features in the records is the abrupt decrease in concentrations during the 16th and 17th centuries. Following AD 1550 gas concentrations decreased by 10 ppm CO2, 40 ppb CH4 and ∼5 ppb N2O, with minima at ∼AD 1600 (Figure 2a). The lower concentrations coincide with a period of generally cooler Northern Hemisphere temperatures commonly termed the Little Ice Age, LIA (AD 1300–1850 [e.g., Jones and Mann, 2004]). Several reconstructions of northern hemisphere temperature anomalies of the last 2000 years [e.g., Mann and Jones, 2003; Moberg et al., 2005] show their coldest periods around AD 1600 (Figure 2b).
Figure 2. The Law Dome trace gas records: AD 0 to 1800. (a) CO2, CH4 and N2O records shown with published Law Dome records. Spline fits (40 year smoothing for CO2 and CH4, 100 year smoothing for N2O) and uncertainties are graphically shown. (b) Northern Hemispheric temperature anomalies [Mann and Jones, 2003; Moberg et al., 2005].
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 Previous interpretations of the Law Dome CO2, CH4 and the δ13C isotope records suggests that reduced emissions from the terrestrial biosphere were the likely cause of the lower gas concentrations during the LIA [Francey et al., 1999; Trudinger et al., 2002a; Ferretti et al., 2005]. We note that there are no Law Dome δ13CO2 measurements at AD 1600, so it is unclear whether the initial CO2 decrease was driven by the terrestrial biosphere, and some changes to oceanic carbon exchange are possible. Also, a temperature-dependent terrestrial plant CH4 source recently identified by Keppler et al.  could help explain lower LIA emissions. However, the total pre-industrial CH4 source (∼200 Tg yr−1) does not support a plant source in the range estimated by Keppler et al.  (62–236 Tg yr−1) without a major revision of the other known sources (wetlands, biomass burning, fossil, animals) or of the sink strength. We can also add information about the LIA using the N2O record. Emissions of N2O from soils under natural vegetation (the largest natural source) would decrease under the cold, and possibly dry [Jones and Mann, 2004, and references therein], conditions dominating in the northern hemisphere during the LIA. Changing sea surface temperatures may also affect N2O emissions by changing the biogeochemical cycle of nitrogen or the solubility of N2O in the surface water column (0.04 ppb for 1°C change in the surface 100m [Flückiger et al., 2002]), although this is not expected to be significant during the LIA. Therefore, we conclude that the lower gas concentrations during the LIA were dominantly driven by changes in emissions from the terrestrial biosphere due to colder temperatures.
 Previous studies have also shown that trace gas concentrations responded to changes in the climate system during the LPIH causing small climate feedbacks [Etheridge et al., 1996; Crowley, 2000; Bauer et al., 2003], rather than being a significant climate driver [Ruddiman, 2003]. Gerber et al.  calculated that a change of 1°C in Northern Hemisphere temperature during the LPIH would result in a global CO2 change of 12 ppm. The quality of previous CO2 data and an assumption of unchanging ocean circulation are uncertainties in this relationship. The new measurements increase the certainty of the CO2 changes in the Law Dome record and as previously mentioned, the δ13CO2 suggests that the gas response during the LIA is mainly the result of a cooling of the terrestrial biosphere rather than of the ocean. We apply the Gerber relationship to the Law Dome CO2 record through the LIA, and calculate an overall Northern Hemispheric cooling of 0.4°C between AD 1550 and 1700. Similarly, the abrupt 10 ppm CO2 decrease between AD 1550 and 1610, if driven by the terrestrial biosphere, would indicate a temperature decrease of 0.8°C. This is larger than the temperature change derived by Mann and Jones  but more comparable to that derived by Moberg et al. . The temperature proxies evident in the combined trace gas records could be further exploited. We find no evidence for the 30 ppm variation during the 13th century indicated by the stomatal CO2 proxy of van Hoof et al. (2005).
 The CH4 concentration increased by ∼100 ppb between AD 0 and 1800 (Figure 2a). We discuss this increase in terms of CH4 sources, as changes to the CH4 sink are unlikely during the LPIH [Thompson, 1992]. The LPIH CH4 budget was largely composed of emissions from natural sources (wetlands and natural biomass burning). Emissions from these sources are dependent on climatic factors, particularly temperature and precipitation. Warm, wet conditions enhance emissions from wetlands, while warm, dry conditions enhance emissions from biomass burning. Northern Hemisphere temperature records (Figure 2b) show long term cooling during this period, which does not support increased natural emissions. Smaller contributions to the CH4 budget come from anthropogenic sources, including rice agriculture, ruminant livestock, domestic waste, wood-fuel and biomass burning. The anthropogenic sources are strongly linked to human population, which increased by over 400% during the LPIH [McEvedy and Jones, 1979], making it plausible that anthropogenic emissions contributed to the CH4 rise before 1800.
 The most notable N2O preindustrial variation was a 10 ppb increase between AD 670 and 810. There is some evidence of this change in the Dome C N2O record [Flückiger et al., 2002], but more measurements are needed to verify and resolve it. There is no evidence of changes in the other trace gases or climate proxies at this time. It appears that the change in N2O was not caused by changes in climate (or that the hemisphere-scale climate proxies did not record a climate event at the time) or by changes in biogeochemical factors that also affect CO2 or CH4.