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Arctic Environmental Change of the Last Four Centuries

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Science  14 Nov 1997:
Vol. 278, Issue 5341, pp. 1251-1256
DOI: 10.1126/science.278.5341.1251

Abstract

A compilation of paleoclimate records from lake sediments, trees, glaciers, and marine sediments provides a view of circum-Arctic environmental variability over the last 400 years. From 1840 to the mid-20th century, the Arctic warmed to the highest temperatures in four centuries. This warming ended the Little Ice Age in the Arctic and has caused retreats of glaciers, melting of permafrost and sea ice, and alteration of terrestrial and lake ecosystems. Although warming, particularly after 1920, was likely caused by increases in atmospheric trace gases, the initiation of the warming in the mid-19th century suggests that increased solar irradiance, decreased volcanic activity, and feedbacks internal to the climate system played roles.

Global climate change is likely amplified in the Arctic by several positive feedbacks, including ice and snow melting that decreases surface albedo, atmospheric stability that traps temperature anomalies near the surface, and cloud dynamics that magnify change (1-3). The Arctic, in turn, influences climate change at lower latitudes through changes in river runoff and effects on global thermohaline circulation, impacts on atmospheric circulation, and modulation of atmospheric CO2and CH4 concentrations (1, 4). Although the Arctic is now one of the least disturbed regions on Earth, it may also be one of the most susceptible to both natural and human-induced climate change.

The instrumental record of Arctic climate change is brief and geographically sparse. The few records that extend back to the beginning of the 20th century suggest that the Arctic has warmed by about 0.6°C since that time, with peak temperatures (1.2°C more than in 1910) around 1945 (2). Although some areas of the Arctic have cooled recently—for example, as a result of dynamics within the North Atlantic (5)—the observed 20th-century Arctic-average temperature increase exceeds that of the hemisphere as a whole. The record is thus consistent with amplification of climate change through snow/ice radiation and other feedbacks (6,7).

In this article, we use the paleoenvironmental record to assess the climate events of this century from the perspective of the last four centuries. We build on previous work (8-10) by compiling a variety of complementary paleoenvironmental indicators of climate from around the entire Arctic. This perspective permits the visualization of natural subdecadal to century-scale climate variability in the circum-Arctic region and allows us to examine the role that natural forcing mechanisms play in driving Arctic climate. Given the growing focus on Arctic environmental dynamics (4), we also examine the centuries-long paleoenvironmental perspective to determine whether the climate changes we infer have resulted in changes to the natural Arctic ecosystem.

The Paleoclimate Perspective

Proxy data from lakes, wetlands, ice cores, and marine sources demonstrate that Arctic interannual to century-scale environmental variability of the last 400 years is superimposed on longer-term changes of the Holocene (the last 10,000 years). Most of the Arctic experienced summers warmer (1° to 2°C) than today during the early to middle Holocene, but the timing of this Milankovitch-driven warmth (that is, warmth resulting from greater summer insolation) differed geographically because of the effects of local sea surface temperatures (SSTs) and land-ice cover (11-13). Decreases in summer insolation—perhaps coupled with other, more abrupt changes in climate forcing—led to successively cooler summers in the late Holocene; this trend culminated in the Little Ice Age, a period that began before 1600 (14-17) and evidently ended sometime in the last century (11, 12, 14).

Our compilation of paleoclimate records provides a 400-year view of Arctic variability that is superimposed on lower frequency Holocene trends. This view is based (18) on more than 20 records from the North American Arctic and substantially fewer records spread out across the Eurasian Arctic (Table 1 and Figs. 1 and2). Although covariation among continental records in Alaska and western Canada (Fig. 2A) suggests that the Eurasian records may be broadly representative, a need for caution is highlighted by the differences among records from the eastern Canadian Arctic and Greenland (Fig. 2B), a region marked by geographic variability even in the instrumental period (2). Thus, although our compilation provides a much improved basis for understanding the patterns and causes of interannual to century-scale environmental variability in the circum-Arctic, future work will be needed to increase data coverage, particularly in Eurasia.

Figure 1

Map showing locations of proxy climate records used in this study and listed in Table 1. Circles around groups of sites indicate the groups averaged before calculating the Arctic-wide temperature average. Isolated single sites not circled were averaged directly into the Arctic-wide series (19).

Figure 2

(A) Standardized 400-year proxy climate records reflecting surface air temperature for sites from Arctic Europe east to western Canada (Fig. 1 and Table 1) (18). Red indicates temperatures greater than one standard deviation warmer than average for the reference period (1901–1960), whereas dark blue indicates at least one standard deviation colder than this average. (B) Same as (A) but for sites in Canada east to Greenland. All series are presented as 5-year averages except for sites 8 and 29, which are plotted at their original lower resolution. All time series represent surface air temperature except for site 29, which represents SST. All time series shown will be available athttp://www.ngdc.noaa.gov/paleo/paleo.html.

Table 1

Paleoclimate site information.

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The most notable pattern of change revealed by most records across the Arctic is the near ubiquitous transition from anomalously cold conditions of the 19th century to peak warm conditions of the 20th century. This event can be seen even more clearly in a new proxy record of average Arctic temperature change over the last 400 years (19) (Fig. 3). Where quantitative estimates are available, it appears that the 19th- to 20th-century warming was 1° to 3°C locally (8, 15,20-25) and averaged about 1.5°C across the Arctic. We thus confirm that the 20th-century warming observed in the instrumental record was only part of a more circum-Arctic climate event that began in the mid-19th century and marked the end of the Little Ice Age in the Arctic (8, 26, 27).

Figure 3

Comparison of hypothesized external climate forcing (colored lines) and standardized proxy Arctic-wide summer-weighted annual temperature [gray lines, plotted as sigma units (18, 19)] for (A) atmospheric CH4 (66), (B) atmospheric CO2 (67), (C) solar irradiance (32), and (D) Greenland (GISP2) ice core volcanic sulfate. Eruptions known to be overrepresented in the GISP2 record are marked with an asterisk (37,38).

Another striking aspect of Arctic temperature change over the last 400 years is that most of the Arctic experienced cooling in the first part of the 19th century, resulting in the coldest temperatures of the Little Ice Age. In addition, although much of the Arctic was colder than today during parts of the 17th century, several of the records show temperatures nearly as warm as today during the 18th century (9). Arctic climate change before 1800 was more regional in nature than after this time. For example, many sites around the Arctic were warmer at some time between 1700 and 1820 than they were later in the 19th century, but the timing and duration of this warmth varied from region to region, as did the preceding 17th-century period of generally colder conditions.

The annually dated record of Arctic climate variability encompassing the last 1000 years has less spatial coverage than does the multiproxy record of the last 400 years. Sediment, ice core, historical, and tree ring data for this earlier period indicate that although Arctic summers of the 20th century were generally the warmest of the last 400 years, they may not have been the warmest of the last millennium (21,28-30). The few time series of climate change spanning the last millennium also suggest that the Arctic was not anomalously warm throughout the so-called Medieval Warm Period of the 9th to 14th centuries (31).

Climate Forcing Mechanisms

The paleoclimate record of the Arctic reveals large variability over all of the last 400 years. Natural variability in most regions was as large before the 20th-century buildup of atmospheric trace gases as afterward (Figs. 2 and 3). Natural climate forcing mechanisms must therefore be considered before attributing some or all of the recent changes in the Arctic to human influences. The similarities of our new Arctic climate reconstruction (Fig. 3) to earlier reconstructions for the Northern Hemisphere (8,10), and the coincidence of high inferred solar irradiance and warm Arctic temperatures in both the 18th and 20th centuries, suggest that solar forcing played a role (32-36). The strong recovery of high temperatures after the 1840s may also have been a partial response to high solar irradiance at that time. On the other hand, the lack of a distinct prolonged cold period associated with the 17th- to early 18th-century Maunder sunspot minimum period (34) argues against a dominant role for solar forcing over the past 400 years, as does the observation that changes in solar irradiance evidently did not lead climate warming into the 20th century.

Comparison of our temperature record with an Arctic volcanic sulfate record recently developed from the Greenland Ice Sheet Project Two (GISP2) ice core (37, 38) indicates that most peaks in reconstructed atmospheric volcanic sulfate loading correspond to mean circum-Arctic cooling (Fig. 3). The repeated coincidence of high sulfate loading with the onset of Arctic cold events suggests that eruptions entrain positive ocean feedbacks capable of enhancing and prolonging Arctic cooling. For example, the anomalous early 19th-century period of frequent large sulfur-producing eruptions seems to have helped precipitate the drop into the coolest period of the Arctic Little Ice Age (37, 39). These findings agree with earlier evidence for volcanic forcing of Arctic temperatures over the last 100 years (40), as well as the more regional 200-year record of volcano-climate linkages for sub-Arctic North America (41) and earlier assertions that volcanic activity plays a role in modulating the climate of the Northern Hemisphere in general (10, 42). It is difficult to attribute the lack of a clear solar-climate correspondence, particularly during the period of the Maunder sunspot minimum (1650 to 1710), to counterbalancing by volcanic forcing. However, it is quite likely that the period of uncommonly low volcanic activity from 1935 to 1960 may have contributed to the peak Arctic summer temperatures at this time (43, 44), reducing the implied potential sensitivity of the climate system to solar forcing during this same period (36).

Variability internal to Earth's climate system, particularly in the ocean's thermohaline circulation, also can modulate climate over decades to centuries (45, 46). The variability evident in our 400-year Arctic compilation, although an order of magnitude smaller in scale than that driven by internal climate system processes over the last deglaciation (47), may relate to decadal-scale fluctuations in the state of the North Atlantic Oscillation (NAO) or to the transport of heat northward by thermohaline circulation. Such a forcing mechanism would leave a diagnostic pattern most obvious in, and downwind of, the North Atlantic (46, 48). The Nansen Fjord record from southeast Greenland (Fig. 2B), a proxy for North Atlantic SSTs (49), indicates that the range of North Atlantic variability over the past 400 years exceeds that of the recent instrumental record, and that the last century was marked by warming in the North Atlantic. However, comparison of this long proxy record of North Atlantic SSTs with terrestrial temperature proxies from sites across the Arctic (Fig. 2) does not reveal clear evidence for strong Atlantic forcing of Arctic climate. The full role of NAO and thermohaline forcing of Arctic climate remains to be evaluated as more multicentury time series from the circum–North Atlantic region become available.

Half of the post-1840 warming (about 0.75°C) took place from 1840 to 1920, during a period in which concentrations of atmospheric CO2 and CH4 increased only by about 20 ppm and 200 parts per billion by volume (ppbv), respectively (Fig. 3). Given the current best estimates of climate sensitivity to trace-gas forcing [1.5° to 4.5° global mean warming for a 280-ppm increase in CO2, and less for a doubling of CH4(6, 46)], trace-gas forcing alone might be able to explain only a small part (0.1° to 0.4°C) of the pre-1920 warming. The first half of the unprecedented 19th- to 20th-century increase in Arctic temperatures appears to be a natural readjustment as volcanic forcing weakened, and irradiance (and, to a lesser extent, greenhouse gases) increased, between 1840 and 1920. After 1920, both anomalously high solar irradiance and low volcanic aerosol loading likely continued to influence Arctic climate, but exponentially increasing atmospheric trace-gas concentrations probably played an increasingly dominant role (36). More recently, the observed slowdown in warming from 1950 to 1970 (2) (Fig. 3) may have been influenced by the increase in Arctic tropospheric aerosols that occurred after 1950 (6, 50).

The Response to Arctic Climate Variability

With the emergence of a coherent picture of Arctic climate change over the last several centuries comes the realization that interannual to century-scale Arctic climate variability is the norm. This variability has affected many aspects of the Arctic environment. The primary implication is that today's Arctic cryosphere (glaciers and frozen ground) and biosphere (terrestrial, lacustrine, and marine) are not at steady state; they have changed and will continue to change in response to an evolving Arctic climate.

An obvious impact of recent climate change is the widespread retreat of glaciers throughout the Arctic over the last century (51). On Baffin Island, retreats in glacial equilibrium-line altitudes (52) coincided with the 19th- to 20th-century warming observed in sediment, tree ring, and ice core records. Evidence for similarly large post–Little Ice Age glacial retreats can also be found farther east in Greenland, Iceland, Spitsbergen, and Scandinavia (14, 53), as well as to the north on Devon Island (27) and to the west in Alaska (25). Although it has been suggested that climate warming may be accompanied by an increase in snowfall sufficient to expand ice sheets (54), it appears probable that most Arctic glaciers will continue to melt if the Arctic continues to warm, as forecast by model simulations of the climate response to increasing greenhouse gas concentrations (6,55). This inference may not apply to the Greenland Ice Sheet, whose modern mass balance is poorly known (51).

High-latitude permafrost conditions have also changed during the last 200 years. Where available (that is, North America), ground temperature records reconstructed from borehole temperature logs support the notion that large-scale warming has occurred since the 19th century, but these records also indicate that high-latitude permafrost conditions have changed, and are likely to continue changing, as the Arctic warms (24). This future change in frozen ground will, in turn, likely affect construction, transportation, hydrology, ecology, and trace-gas fluxes in the Arctic (4).

The paleoenvironmental record of the last four centuries supports recent ecological studies (56) and indicates that, even if climate warming takes place at the low end of the range suggested by climate model simulations of the next 100 years (6), such warming is capable of driving changes in the Arctic biosphere that will likely rival any changes driven by nonclimate processes, including human land use. Tree growth correlates positively with temperature in many parts of the Arctic (Fig. 2), just as Arctic tree growth forms and fire disturbance regimes change with climate (57). Studies of recent seedling establishment and viability north of the present treeline (58, 59), together with evidence from annually dated fossil pollen records, suggest that Arctic plant species abundances and ranges have varied in response to varying temperatures (60).

The limnology of Arctic lakes will also likely continue to respond to climate change (61). Sediments from shallow ponds and lakes on Ellesmere (62) (Fig. 4) and Devon islands (63) show that warming since the mid-19th century caused acute shifts in diatom algal floras, including the recent establishment in some lakes of diatom populations previously too light-limited by perennial ice for their subsistence. Freshwater algae are sensitive recorders of environmental change and may also be harbingers of more profound ecological reorganizations, as changes in their populations may influence the structure of higher trophic levels. The paleoenvironmental record of the Arctic makes it clear that we should expect changes to occur in both terrestrial and aquatic ecosystems if the present Arctic warming persists.

Figure 4

Lake sediment records from climatically and limnologically contrasting regions of Ellesmere Island (62, 68), each showing abrupt changes in the composition of fossil diatom assemblages deposited within approxi- mately the last 150 years. These biostratigraphic changes are unrelated to differential silica preservation and represent the greatest floristic shifts of the middle to late Holocene. Taxonomic diversification with greater representations of littoral and periphytic taxa (A to C), increased diatom algal biomass (C), and recent diatom recolonization (D) are all consistent with the abrupt 19th- to 20th-century shift toward longer summer growing seasons, reduced lake-ice severity, and greater habitat availability. The limnological consequences of the 19th- to 20th-century warming appear to be unprecedented in the context of pre–18th century natural variability. Except where noted, tick marks on horizontal axes represent 10% relative frequency intervals.

Finally, our records confirm the hypothesis, originally based on a single high-Arctic ice core record of summer temperature variations (27), that the repeated failure of Europeans to find the Northwest Passage during the 19th century was likely a result of exceptionally severe summertime air temperature and sea ice extent (64). Conditions more amenable to Arctic sea travel, and to the discovery of the Northwest Passage, arose naturally after the turn of the century (27).

Implications for the Future

Our reconstruction of past environmental change in the Arctic suggests that natural variability is large in this region and is working together with human forcing (through increased concentrations of atmospheric trace gases) to drive unprecedented changes in the Arctic environment. The complexity of natural and anthropogenic forcing highlights the probability that assumptions of climate stability, or efforts to simply extrapolate past patterns of change into the future, will ultimately fail to anticipate future Arctic climate change and its impacts. Reliable predictions of future change will require climate system models that prove effective in simulating past changes such as those reconstructed here. Even as these models are being developed and tested, however, the Arctic environment is likely to continue its pace of change.

  • * To whom correspondence should be addressed. E-mail: jto{at}ngdc.noaa.gov

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