The Little Ice Age (LIA) and Medieval Warm Period (MWP) environments (the
most recent analogs for conditions cooler and warmer, respectively, than the
present century) can be characterized by interpreting the multi-parameter GISP2
series (Figure 1). The LIA appears to span the period 
AD1350 or 1450
to 
AD1900, depending upon measurement type (since each may respond
to climate change differently), and the MWP includes the milder few centuries
prior to the LIA.
GISP2 temperature modeled from oxygen isotopes reveals a relatively subdued temperature effect at this site for the LIA period. However, year-to-year correlations between the GISP2 isotopic record and sea surface and land temperatures over the North Atlantic, covering the period AD1840-1970, are related to changes in atmospheric circulation patterns such as the seesaw pattern of the North Atlantic Oscillation [ Barlow et al., 1993]. Accumulation rate, which is indicative of transport distance from the open ocean plus temperature en route, is generally lower during the LIA than the MWP [ Meese et al., 1994b].
Initial measurements of CO
in air bubbles of the GISP2 core [ Wahlen
et al., 1991] indicate that between AD1530 and AD1810 atmospheric CO
levels remained relatively constant at 280
ppmv. After this period,
concentrations rise rather abruptly and smoothly connect to the atmospheric
observations at Mauna Loa.
Non seasalt (nss) sulfate (reflecting primarily volcanic source SO
; Figure
1) does not appear to be a major forcing agent on multi-decadal-scale climate,
but does affect climate on the 1- to 3-year scale. Individual volcanic event
signatures (not identified on Figure 1) have been studied in the GISP2 core by
the measurement of electrical conductivity, volcanic source sulfate, and
particles. Examples of specific events that have been described include: local
eruptions (e.g., the AD1362 Oraefajokull (Iceland) eruption, Palais et al.
[1991]), intrahemispherically distributed eruptions (e.g., the AD1479 Mt. St.\
Helen's (Washington) eruption, Fiacco et al. [1993]), and
interhemispherically distributed eruptions (e.g., the AD1259 eruption possibly
produced by El Chichon (Mexico), Palais et al. [1992]). Zielinski
et al. [1994] provide a complete description of the Holocene volcanic event
history developed from continuous, high resolution sampling of sulfate in the
GISP2 record.
Transport and/or sources of dust (e.g., particles, calcium, magnesium, potassium) and species of marine origin (e.g., sodium, chloride, methanesulfonate) to central Greenland increased during the LIA. Nitrate sources (e.g., lightning, soil exhalation) decreased during the LIA. Ammonium outliers in Summit ice cores have been interpreted as northern high-latitude biomass-burning events [ Taylor et al., 1992; Whitlow et al., 1994] based on their association with other chemical products of biomass-burning [ Legrand et al., 1992]. Ammonium peaked both at the onset and at the end of the LIA.
Biannual and finer sampling of the major ions in the GISP2 record, coupled with
newly developed signal analysis techniques, have provided insight into the state
of the environment during the LIA and MWP [ Mayewski et al., 1993a].
Two important tracers in this work are calcium and sodium. Calcium is mostly
present as CaCO
and reflects aerosol transport of calcite and dolomite from
Northern Hemisphere continents to Greenland. Sodium derives from injection
of seasalt into the atmosphere and its subsequent transport to Greenland. Based
on changes in trend, continental air masses (e.g., calcium) appear to reflect the
onset of LIA conditions 
200 years before marine sources (AD1391
compared to AD1587) and to reflect the end of the LIA 
70 years earlier
(AD1846 compared to AD1914) than marine sources (e.g., seasalt sodium).
Evidence of such lagging may be useful for elucidating processes controlling
climate change and/or the continental versus marine responses to climate change.