Climate changes of the past 1 000 and 10 000 years in the Canadian Arctic
By: K. Gajewski
This is an expanded summary of a presentation given to the Commission on Geosciences, Environment and Resources on 15 July 1999, Ottawa. Note that references are not included, but are available on request.
The instrumental climate record of the arctic extends only to the late 1940's, with scattered observations prior to that time. Further to the south, for example in the Hudson Bay area, the Hudson Bay Company has good data extending back a few centuries, but this needs to be extracted and analysed. Another problem in the arctic is that there are so few meteorological stations (Figure 1) so we have little knowledge of the spatial variability of arctic climate. Non-standard data can be accumulated to fill in the gaps, but this takes considerable effort and the data are irregularly available in time and space at best. One example of non-standard data that is available is the Polar Continental Shelf Project (PCSP) database.
PCSP provides logistic support for field research in the Canadian Arctic Islands. For the 1970's and 1980's, every field team working across the arctic was issued a thermometer and instructions about observing the weather. These twice-daily observations were radioed to the base and archived. There are literally thousands of observations from many sites, however, most of the series are short (ie 1-2 weeks) and there are few sites with records over more than one season (Figure 2). The data collected from the 1970's to the 1990's is an example of non-standard data that could be exploited to give us more detailed information of the spatial patterns of arctic climate.
Therefore we need to use so-called "proxy climate data" to study climate variability of longer time scales. Proxy climate data are natural indicators that respond to climate change and leave a fossil record. I will talk about 3 types of data. Some indications of the potential length of the record and the resolution are given.
1. Lake sediments, and enclosed microfossils, especially pollen
I will discuss the variability of the arctic climate at three different scales: the past 100 years, the past 1000 years and the past 10 000 years.
Last 100 years
At this time scale, there are the obvious inter-annual variations such as the El Nino-Southern Oscillation (ENSO) or the Arctic Oscillation (AO), etc. There are also decadal variations such as the droughts during the 1930's and 1950's. In the arctic, these may translate as periods of less or more snow, cold or warm summers, late or early spring, etc. This may impact the wildlife or fish populations; for example, there is a suggestion that a few winters of increased snow on Victoria Island may be influencing the large mammal populations.
There is a fair bit of work relating this variability in the arctic to the global climate. For example: How do variations in streamflow of large rivers (such as the Mackenzie) or ice extent or ocean salinity impact climate conditions to the south? This has been studied using observational and modeling efforts. The instrumental record can be used as the basic data, but in some cases tree-ring data have also been used.
Documenting decadal changes in the arctic is difficult.
Last 1000 years
Climatic variability occurs at many time scales. During the past 1000 years there have been alternating changes of climate such as the so-called Little Ice Age, Medieval Warm period, etc. Interesting questions not well understood include the nature of these climate variations, and the scale interactions with longer-term climate changes. For example, was the Little Ice Age a period of extended cool temperatures, or was it a period of increased frequency of cool decades or cool years? What is the spatial extent of the Little Ice Age, and was it cold everywhere? Was the Little Ice Age an isolated event, or have there been alternating periods of relative cold and warmth?
A major source of information about this time period is tree-ring records. Tree rings have been analysed from 1000's of sites from around the world. Many of these have been deposited in the International Tree-ring database, available on the web. However, some of the major groups working at treeline have not deposited their published data in the tree-ring database, so syntheses are difficult.
To give an idea of the spatial density of information, here is the number of sites currently available:
Tree-ring records in the arctic are rarely very long, and long-term trends are usually masked by the nature of the record and processing. Thus, you can reconstruct decadal climate variation, but identifying the Little Ice Age, for example, is difficult. However, the methodology for this is constantly improving. Tree-ring work is not logistically difficult (once you get to the site). Doing the analysis in the lab is time-consuming, but not excessively so. Several large labs and many smaller labs do this, and the set up for basic tree-ring analysis is not too expensive. Thus, it is possible to quickly accumulate new data, assuming you can get into the field to get the data. More complicated analyses, wood density for example, are more difficult, but it seems that there is significant information to be extracted. New methodologies are being developed that may make it easier (scanners, software to capture images, etc.).
Lake sediments can also be used, and various microfossils can be extracted (pollen, diatoms, etc). To study century-scale variability, you need well-dated lake sediment records: varved sediments can give you precise annual dating, or high-resolution diagrams can give you dating with errors of a few years. It is hard to find sediment records with annual variations, but when they are found they provide excellent records of environmental change at high resolution. A positive aspect of lake sediments is a longer record than available from tree-rings, so you can study century-scale events for past 10 000 years. An important disadvantage is the time it takes to analyse the data. Extracting microfossils from lake sediments and counting the organisms under the microscope can take months to years for each site.
An example of the kinds of results that can be obtained from lake sediments comes from Devon Island (Figure 3). We showed recent changes in varve thickness and diatom abundance occurred in this century, and work of Douglas indicated that this change was the major one for the entire postglacial period.
An example of a synthesis of several records comes from the Overpeck et al. paper recently published in Science (Figure 4 & Figure 5). This illustrates some coherence among the different regions, and among the different kinds of proxy-climate data sources. There are, however, significant regional differences, as discussed in the paper. More recent work by Mann and Bradley is providing more extensive analysis.
There was little done on the problem of century-scale climate change in the 1980's but there has recently been a resurgence of interest on this time scale. The IGBP PAGES science plan had the past 2000 years as one of the time streams of interest. Based on this, there have been several groups searching for and analysing laminated sediments in the arctic, and elsewhere.
Last 10 000 years
Surprisingly, little is known about the arctic climate of the post-glacial period. Much of the work has been done using data such as the mollusc or whalebone distributions. Changes of the location of the major water masses have been interpreted from the range of different species of fossil molluscs. It is known that there were extensive peat deposits formed in areas where today there is little accumulation. Several syntheses have been attempted, and all point to large gaps in our knowledge of the climate of this time period.
There has been little done using lake sediments, which must be the major source of information about past climates in the region. Paleolimnology (the study of fossils of aquatic organisms) is in early stages. Much of the research effort is still going into basic research, such as which species are now growing in the arctic and where (diatoms). There has been some work done on processes of lake sedimentation, but the connection to climate change is not clear.
Pollen analysis is a major source of information about past climates. In the United States and southern Canada, there are 100's of pollen diagrams, and quantitative syntheses of the change through time of the vegetation have been attempted. I will discuss the current situation in 4 regions: Treeline, Baffin, Beringia and the rest of the arctic
There is a fair bit known about treeline variations. Major areas of research activity have been the MacKenzie Delta region and northwestern Québec, where we have a good idea of the postglacial position of treeline. American groups have analysed many sites in Alaska. Central northern Canada (old Keewatin) between Great Bear Lake and Hudson Bay has been studied, although more work is needed. The situation in Labrador is confusing. The main message from this is that treeline variations have been quite small in the past, and that different regions have different histories (Figure 6, Figure 7 & Figure 8). This is in contrast with GCM model results, which indicate large variations in treeline during the Holocene and this discrepancy needs to be resolved. There was an joint Russian-Canadian plan to reconstruct circumpolar treeline variations and relate this to climate and climate change, but this synthesis has stalled.
The US initiative, PALE, which evolved into the International project (CAPE) put a lot of money into arctic paleoecological work. After GISP/GRIP, this was a major initiative of NSF Polar Programmes. One of the major areas of activity is Baffin Island, with some work elsewhere in the Canadian arctic as well. There is extensive sampling and coring on Baffin Island followed by the analysis of pollen and other microfossils as well as work on processes of sedimentation. Work is being done on 2 timescales: the past 1000 and the past 10 000 years. However, publications are just beginning to come out of this project.
A lot of data has been accumulated, and there seems to be significant collaboration with Russians. The people working in this region are starting to make syntheses of these data (Figure 9).
Canadian Arctic Islands
New data are only slowly being accumulated. The arctic is one of the last places where there is little known, because the usual methods of pollen analysis were worked out for forested areas. It takes up to 10 times more time to get the data, meaning most sites are low resolution. Issues of pollen transport are confusing, foe example, nearly half of the pollen to arctic sites comes from tree pollen. This is either transported to the arctic from the south, but may also come from Tertiary deposits. It may thus be possible to determine past wind patterns from networks of these sites, as well as using pollen as tracers in pollution transport studies. Map shows the distribution of published sites and others where we are currently working (Figure 10).
Results to date: contrary to older views, it is possible to determine how the arctic climate changed in the past from pollen. But much work is still needed to understand what is happening. The early Holocene was warmer and there has been a cooling in the past several thousand years (Figure 11 & Figure 12). But the timing of this is not secure, and dating of arctic sediments is a major problem.
Where are we going? Figure 13 shows a map of a quantitative reconstruction of the climate at 6 000 years ago for North America. These quantitative maps can be used to verify climate model simulations, for example. Research is continuing to improve these. But these methods seem difficult to apply in the arctic. This is to be done in the next year or two.
A goal of the near future is to combine the regional-scale studies such as I have shown to produce a reconstruction of the climate of the circumpolar arctic for the past (Figure 14). This requires the usual international co-operation to produce this synthesis, as well as the resources to collect data from under-sampled areas.
I have tried to indicate how we are learning about arctic paleoclimates and some of the results that have been obtained. International co-operation is required for an understanding of arctic climate change and this must be encouraged. But regional efforts must also be encouraged as these provide the data for larger-scale syntheses. Logistic considerations, i.e., getting to the sites, is a major expense. But the laboratory work is also time-intensive and needs to be supported as well. Some unresolved technical issues remain, such as dating of sediments. Decadal to century-scale climate variability is probably the most relevant to society, the least known at this time, and one where there is currently considerable interest. We should ensure that arctic paleoenvironmental research of this time scale is encouraged.
Figure 1. Climate stations in the Canadian Arctic. These have been in place only in the postwar period.
Figure 2. Location of sites with non-standard climate data for the period 1974-1993. Many of these are very short records of a week or less, but a few were in place for several years. (From PhD thesis of D Atkinson, U Ottawa).
Figure 3. Example of a proxy-climate datum from the Canadian Arctic which records changes of the past 1000 years with high resolution. The thickness of the varves depicted on the right (upper panel) seems to co-vary with the record of ice melt from the Devon Island Ice sheet (bottom panel). The diatoms (aquatic algae; middle panel) have greatly increased in abundance in the 1920's and the 1950's. (From Gajewski et al., 1997).
Figure 4 & Figure 5. A synthesis of a number of high-resolution proxy climate data from around the arctic illustrate climate changes of the past 400 years. Note both the regional similarities and differences. Note also that many of the records record warming in this century. (From Overpeck et al., 1997).
Figure 6, Figure 7, & Figure 8. The postglacial history of latitudinal treeline. Figure 6 shows the present day location of treeline. Figure 7 shows the spruce pollen percentages from three sites that are today in the tundra. When spruce percentages are greater than 20%, the site is thought to be surrounded by trees. This suggests that treeline had advanced north of the present day position between 10 000 and 7 000 years ago in the Mackenzie Delta, around 5 000 to 4 000 years ago in central Canada, but never in north-western Quebec. Figure 8 illustrates the movement of treeline position during the past 9 000 years. (From Gajewski and MacDonald, in prep).
Figure 9. A synthesis of the available data from Beringia. This illustrates the site density at 6 000 years ago, and the types of results being obtained. (From the PALE Beringia website).
Figure 10. Location of sites from which lake sediment cores have been raised of pollen diagrams published. Notice the large areas with no data.
Figure 11 & Figure 12. An example of the changes in vegetation and climate at one site on Somerset Island. Figure 11 shows the % of several pollen types of herbaceous plants. There has not been much change in relative abundance of these types in the past 10 000 years. However, Figure 12 illustrates the change in pollen concentrations during this time period, suggesting a thinning of the vegetation during the past several 1000 years. The inset shows the modern pollen concentration along a latitudinal transect and illustrates that today, pollen concentrations are greater in the mid-arctic to the south. This kind of data is used to interpret the pollen core. The dating of this core is problematic, however. (From Gajewski, 1995).
Figure 13. An example of the state-of-the-art in paleoclimate reconstruction. These reconstructions for North America are based on pollen diagrams. Statistical calibration of the relation between modern pollen and climate is applied to pollen assemblages from 6 000 years ago to make maps of the climate for that time period. (From Gajewski et al., in press)
Figure 14. Potential for a circumpolar climate reconstruction for 6 000 years ago. This is shown simply to illustrate the site density. (From Bigelow et al., in prep).
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