A good millennium? Editorial. Eric Wolff British Antarctic Survey, Cambridge

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1 Weati1t.r Vol. 55 January 2000 Editorial The year 2000 sees the 150th anniversary of the founding of the Royal Meteorological Society. There will be a number of special events and meetings conducted by the Society throughout this auspicious year. As Weather s contribution to this celebration there will be a Special Issue in April, the actual month in which the Society was founded 150 years ago. This will focus on the year 1850, snapshots in weather and the development of meteorology over the following 150 years, and a view of the current state of knowledge in that most perti- nent of fields to a meteorologist, frontal theory. To mark the arrival of a new millennium in conjunction with this Society anniversary we will also publish an invited article each month throughout the year looking both backward and forward in time through a meteorological theme. We begin this month with an article by Eric Wolff discussing the climate of the last millennium with particular reference to records from ice cores. We hope that you will enjoy Weather even more than usual during this sesquicentennial year! Grant Bigg A good millennium? Eric Wolff British Antarctic Survey, Cambridge At the end of a year, it is traditional to look back and assess the weather in the context of previous years, typically the last century. As we reach the end of the millennium, we can try to make the same judgement of the climate of the last 1000 years - how did it compare, especially to the last 100 millennia? Was it a warm or a cold millennium in comparison to the average? Did it contain a large number of extreme events and surprises? What is the state of the climate as we enter the next millennium? How will the millennium be viewed in a few thousand years time? Palaeoclimate - records of past climate Of course, the direct recording of climate in most parts of the world barely extends back out of the twentieth century, let alone the millennium. We therefore have to rely on less direct methods of obtaining information about past climate. Nearly all palaeoclimate records rely on some feature that is laid down in a monotonic chronological sequence, and that contains a measurable parameter that responds to changing climate. Suitable media include tree rings, marine sediments, pollen records in lake sediments, and ice cores. Tree-ring widths and densities respond to a variety of environmental factors, but by careful selection they can be used as a proxy for summer temperatures. By combining large numbers of tree-ring records with overlapping ages, chronologies of over years, with extremely accurate dating (to the year), can be created. It is largely fiom tree-ring records that we believe that the last decade of the millennium has been the warmest decade not only of the century, but probably of the millennium (Mann et al. 1999). At the other extreme, marine-sediment records generally have rather poor time resolution, but can give the climatic context over millions of years. They record primarily ocean conditions (for example, the isotopic content of deep-water organisms is related mainly to the salinity of the ocean, effectively a proxy of ice-sheet volume, and hence of sea-level); however, again careful choice of proxies allows determination of some atmospheric and surface water properties.

2 Weather Vol. 55 January 2000 Although I will mention the other types of palaeoclimatic data, this paper concentrates on the records available from ice cores. Ice cores In the central parts of polar ice sheets (and exceptionally at high-altitude glaciers at lower latitudes), there is very little snowmelt at the surface. At such sites, snow that falls is preserved in a strict chronological sequence; it is lost only by flow towards the coast where it may melt or be lost as icebergs. When the snow falls, it contains information about many aspects of the climate and atmosphere - the water molecules themselves (through their isotopic content) contain information about the temperature at the time of snowfall, the precipitation rate can be determined from the thickness of the preserved layers, and the concentrations of chemicals trapped in the snow indicate changes in sources and in atmos- pheric circulation. In addition, as further layers of snow build up, the pressure on the lower layers causes them to eventually form a solid network of ice, with air bubbles trapped between them; the air bubbles contain a record of the concentration of atmospheric gases such as carbon dioxide and methane. Ice cores can be retrieved by drilling through the ice sheets to retrieve cylinders of ice 10 cm or more in diameter. A whole host of physical properties and chemical concentrations can be measured on the solid or melted cores. They contain much information, including a reasonably direct indicator of temperature (the oxygen and deuterium isotope records (Jouzel et al. 1997)); depending on the location (and especially the snow accumulation rate) they have a sub-annual resolution and can be well dated, and records extend back more than years in Greenland and probably over years in Antarctica. Probably the biggest limitation of ice cores is that the vast ' -400.I Q, Q 0" 5 > mc: E L.I- Q) CI e -580 Q, -35 Q" S? m i? -A Age (kyr BP) Fig. 1 Climate records of the last years (125 kytyr) before present (BP). Both the oxygen isotope ratio from the Greenland Ice Core hject (GRIP) and the deuterium content from Estok, Antarctica, are considered w be temperature proxies, with more negative values corresponding to lower temperatures. The isotope ratios (parts per thousand by are relative w normal standards for the appropriate element. 3

3 Weat1it.r Vol. 55 January 2000 majority of records come from the polar regions, and we have to relate more complex records from other media to them if we want to learn about mid-latitude or tropical climate. Climate in Greenland and Antarctica over the last years During the early 1990s, two projects (one American, one European) drilled ice cores through the highest point of the Greenland ice sheet, at Summit. The cores extended to a depth of just over 3000m. The very lowest layers were affected by flow disturbances that confused the chronology, but the past years are clearly recorded, and are very similar in both cores. At Vostok in Antarctica, a determined effort over the last two decades by Russian drillers and scientists, complemented more recently by additional French and American effort, has led to a less detailed record of the last years. Figure 1 shows the isotopic records (a proxy of temperature) from the European Greenland Ice Core Project (GRIP) (Johnsen et al. 1995), and from the Antarctic Vostok core (Petit et al. 1999). The overall pattern is very clear, and well known from other climate records. The last 11 millennia are the warm period known as the Holocene interglacial, while much of the previous 89 millennia form part of the last glacial period. If we could go back about another 30 millennia (i.e. to years ago), we would find ourselves in another warm period with temperatures similar to those of today (the end of that warm period is seen at the start of the plotted Vostok data in Fig. 1). Of course the principal characteristic of the glacial period was the presence of large ice sheets over North America and northern Europe, with an associated lowering of sea-level by about 120m. However, generally lower temperatures were seen throughout the globe. Temperatures in the last glacial maximum are believed to have been 6-10 degc lower than today in Antarctica, but in Greenland the latest estimates are that temperatures were over 20degC lower than today. Cooling was not so strong in the tropics, but recent estimates give typical figures of 4-5 degc. Model simulations and palaeoclimatic data suggest that in the last glacial period, precipitation rates were generally lower than today s, and the larger pole-equator temperature gradients led to a more vigorous circulation. So, the first-order answer to questions about the climate of the last millennium is that, on a global scale, it was mild, wet and calm in comparison to the average millennium of the last 100. Rapid variations: Dansgaard - Oeschger events Not only the state of climate, but also its stability and predictability, are important. Our knowledge of past climate variability on short (decadal) time-scales has increased dramatically with the availability of long, high-resolution datasets from a range of palaeoclimate media. Inspection of the GRIP record in Fig. 1 indicates that the last few millennia have been unusually stable. During the last glacial period, Greenland at least saw huge temperature excursions (Johnsen et al. 1992) of 10 degc or more (known as Dansgaard-Oeschger events). Some 23 of these events have been identified. The typical cycle consists of a cold period lasting from between one and a few millennia, followed by a rapid shift to warm conditions that lasted for a similar period; the warm periods tended to end less dramatically than they started, but still at a fast scale. Figure 2 shows three parameters from the Greenland ice core over the last 40 millennia. Each Dansgaard-Oeschger event shows not only the change in temperature but big changes in other parameters, such as snow accumulation rate, terrestrial dust, and methane concentration. Methane concentrations, for example, are believed to have altered in response to changes in tropical wetlands (Chappellaz et al. 1993), while calcium, as an indicator of terrestrial dust in the atmosphere, is probably responding to changes in climate at the source area (Fuhrer et al. 1999), which geochemical evidence suggests is most probably eastern Asia. Signals of the events are seen throughout much of the Northern Hemisphere at least. For example, the same pattern of change is seen clearly in marine-sediment records of climate from the Cariaco Basin, Venezuela, and 4

4 Weather Vol. 55 January 2000 I' I" ' I 0 1 m 2 m 3 m 4 m Age (years before present) Fig. 2 Records of the last years $-om the Greenland Ice Core Project. The oxygen isotope ratio is a proxy for temperature (Johnsen et al. 1995); the calcium represents terrestrial dust (Fuhrer et al. 1999) and is at high concentration in cold periods; methane (parts per bdion by volume (p.p.b.v.)) is high in warm periods (Chappellaz et al. 1993). The numbers (14) refer to Dansgaard-Oeschger warm events. The oxygen isotope ratio (parts per thousand by volume (p.p.t.v.)) is relative to normal standards. in records (considered to represent ocean circulation changes) from Santa Barbara Basin, California. The most recent climate oscillation (between about and years ago, see Fig. 2) is seen in estimates of ice-front position in Iceland, and of temperatures derived from beetle remains in the British Isles. A recently published lake pollen study from Italy (Allen et al. 1999) clearly records rapid climate variations that appear to correlate with the Greenland changes. In Antarctica, much more subdued changes are seen during the same period, and some of these seem to be out of phase with the Greenland events. Current theories suggest that the events are related to major changes in ocean heat transport and circulation. Inputs of freshwater from melting andor iceberg calving from the Lawentide (North American) ice sheet are believed to have been the cause of reduced ocean overturning. The marine-sediment record shows that at least six major iceberg calving events (known as Heinrich events, and identified by large-particle 'ice-rafted' debris) occurred during the last glacial period. Although such events are almost certainly linked with more frequent Dansgaard-Oeschger events (Bond et al. 1993), the exact cause and sequence of events remain unclear. The evidence suggests that there was considerable variability in circ- 5

5 Wiurlitr Vol. 55 January 2000 ulation and climate, and that it was felt particularly keenly around the North Atlantic. It is therefore probable that each Dansgaard- Oeschger event impacted strongly on Britain and northern Europe. The palaeorecords show that there were big changes from millennium to millennium, and that huge and unheralded changes could occur within a single millennium. The Greenland warmings (of order 10 degc) were generally completed within a few decades (less than a human lifetime), and dust concentrations in the atmosphere changed sometimes by a factor of 2-3 in just a couple of years (to give concentrations lower than had been seen for the previous 1000 years). The system then stayed latched in the new climate for the next 1-3 millennia. These changes, particularly around the North Atlantic, must have had a profound effect on ecosystems (as indeed is seen in the pollen records), and on the human populations of the time. The last millennium in perspective In contrast to the turbulent climate of the last glacial, the last 11 millennia have appeared rather stable. There certainly have been variations; for example, the mid-holocene warm period, around 6000 years ago, saw summer temperatures in the Northern Hemisphere probably a degree or two higher than those of today. However, these recent millennial changes have, at least in high latitudes, been rather gentle once the ice sheets had completed their retreat (although there are signs of major changes in hydrology, for example in parts of the tropics, in recent millennia). When the climate of the last millennium is viewed in the perspective of the last 100, it appears mild and uneventful. It also appears relatively similar to the millennia that immediately preceded it. Of course, the big question is whether it will also appear similar to the millennium that follows it. Keen-eyed palaeoclimatologists of the future will notice one very unusual fact about the second millennium AD. The factors that ultimately control mean global temperature are very few: (i) solar input, determining how much solar radiation reaches the planet, and controlled by solar output and by 6 the earth s orbit around the sun; (ii) global albedo, determining how much solar radiation is reflected away, which is determined by the material at the earth s surface and by the aerosol and cloud in the atmosphere; and (iii) greenhouse gas concentrations determining how much radiation is absorbed and retained in the atmosphere. The solar input vanes quite smoothly and minimally on millennial timescales. Albedo is very low in comparison to most of the last 100 millennia because of the absence of ice sheets on the northern continents, but it is not so different from the last 10 millennia (although deforestation and other factors could alter that). However, our future palaeoclimatologist will notice (from measurements of the gas bubbles in ice cores) that carbon dioxide and methane concentrations grew in the last century of the millennium to levels that were unprecedented in the last years (indeed for much longer than that). Methane concentrations today are twice as high as they have ever been in the last 100 millennia, and carbon dioxide is about 30% higher than anything in the ice-core record. Today s climate scientists are using the most sophisticated computer climate models to try to assess the effect of these changes on the climate of the next millennium. Future scientists will have the benefit of hindsight; they will see the end of this millennium as containing an extraordinarily rapid and unprecedented event in trace gases, and they will certainly want to relate that to whatever they see unfolding in climate records of the early centuries of the third millennium. References Allen, J. R. M., Brandt, U., Brauer, A., Hubberten, H.-W., Huntley, B., Keller, J., Kraml, M., Mackensen, A., Mingram, J., Negendank, J. F. W., Nowaczyk, N. R., Oberhansli, H., Watts, W. A., Wulf, S. and Zolitscha, B. (1999) Rapid environmental changes in southern Europe during the last glacial period. Nature, 400, pp Bond, G., Broecker, W., Johnsen, S., McManus, J., Labeyrie, L., Jouzel, J. and Bonani, G. (1993) Correlations between climate records from North Atlantic sediments and Greenland ice. Nature, 365, pp Chappellaz, J., Blunier, T., Raynaud, D., Barnola, J. M., Schwander, J. and Stauffer, B. (1993) Syn-

6 Weather Vol. 55 chronous changes in atmospheric CHI and Greenland climate between 40 and 8kyr BP. Nature, 366, pp Fuhrer, K., Wolff, E. W. and Johnsen, S. J. (1999) Timescales for dust variability in the GRIP (Greenland) ice core in the last years. J. Geophys. Res. (In Press) Johnsen, S. J., Clausen, H. B., Dansgaard, W., Fuhrer, K., Gundestrup, N., Hammer, C. U., Iversen, P., Jouzel, J., Stauffer, B. and Steffensen, J. P. (1992) Irregular glacial interstadials recorded in a new Greenland ice core. Nature, 359, pp Johnsen, S. J., Dahl-Jensen, D., Dansgaard, W. and Gundestrup, N. (1995) Greenland palaeotemperatures derived fkom GRIP bore hole temperature and ice core isotope profiles. EUus, 47B, pp Jouzel, J., Alley, R. B., Cuffey, K. M., Dansgaard, W., Grootes, P., Hofhann, G., Johnsen, S. J., Koster, R. D., Peel, D., Shuman, C. A., January 2000 Stievenard, M., Stuiver, M. and White, J. (1997) Validity of the temperature reconstruction fkom water isotopes in ice cores. J. Geophys. Res., 102, pp Mann, M. E., Bradley, R. S. and Hughes, M. K. (1999) Northern hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophys. Res. Lett., 26, pp Petit, J. R., Jouzel, J., Raynaud, D., Barkov, N. I., Barnola, J. M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V. M., Legrand, M., Lipenkov, V. Y., Lorius, C., Pepin, L., Ritz, C., Saltzman, E. and Stievenard, M. (1999) Climate and atmospheric history of the past years fkom the Vostok ice core, Antarctica. Nature, 399, pp Correspondence to: Dr E. WOE, British Antarctic Survey, High Cross, Madingley Road, Cambridge CB3 OET. Impacts and consequences of the ice storm of 1998 for the North American north-east Lesley-An n D u pig n y-gi ro ux University of Vermont, USA Along Interstate 89 in New Hampshire, as one drives from Sunapee to Sutton, slender white birch trees bend towards the highway forming an eerie arch that reminds the onlooker of the ice storm of According to meteorologists at the National Weather Service, Burlington International Airport, the month of January in this Vermont city tends to be cold and snowy, with about 93% of the hydrometeors falling as snow and at times sleet. January 1998 was a month of exceptions in more ways than one. Before this month would end, much of northern Vermont, New York, New Hampshire and Maine, as well as the Canadian provinces of Quebec, Ontario and New Brunswick, would have experienced what has popularly been called the Great Ice Storm of Ice storms are an inherent feature of winter weather; 14 severe ice storms were reported for the New England region between 1832 and In Canada severe ice storms have been recorded in Ottawa in December 1986 and in Montreal in February 1961 (Environment Canada 1998). However, this ice storm was remarkable, not only in its spatial extent and duration but also in terms of the severity of its impacts in the short and long term on the human population, flora, fauna, agriculture, the infrastructure and financial resources of the afflicted regions. In this article, a closer look will be paid to the meteorological conditions that spawned this disastrous storm, as well as the many impacts and consequences that have resulted. The precipitation itself may have retreated 7