Current techniques

We can learn a great deal about sea ice from satellite and aircraft surveys – its extent, its type, its surface features. But its thickness is hard to measure by remote sensing, because the brine cells in the ice give it a high electrical conductivity such that electromagnetic waves do not easily penetrate. The radio-echo sounding methods which have been used to measure the thickness of terrestrial ice sheets and glaciers cannot therefore be used for sea ice.

So far, five direct techniques have been commonly employed for measuring ice-thickness distribution. In decreasing order of total data quantity, they are:

1. submarine sonar profiling;

2. moored upward sonars;

3. airborne laser profilometry;

4. airborne electromagnetic techniques;

5. drilling.

Submarine sonar profiling

Most synoptic data to be published so far have been obtained by upward sonar profiling from submarines. Beginning with the 1958 voyage of Nautilus (Lyon, 1961; McLaren, 1988), which was the first submarine to the North Pole, many tens of thousands of kilometres of profile have been obtained in the Arctic by US and British submarines, and our present knowledge of Arctic ice-thickness distributions derives largely from the analysis and publication of data from these cruises. Problems include the necessity of removing the effect of beamwidth where a wide-beam sonar has been employed (Wadhams, 1981), and the fact that the data are sometimes obtained during military operations, which necessitates restrictions on the publication of exact track lines. For the same reason, the data set is not systematic in time or space.

Introduction

The dynamical response of sea ice to climate change depends on a complex interplay between mechanical and thermodynamic processes driven by radiation, temperature, wind and oceanic forcing. Because of ice deformation, a typical 100 km2 patch of sea ice will contain a variety of ice thicknesses. These thicknesses range from open water to very thick ice, including pressure ridges extending possibly 30 m or more below the surface. On top of this matrix there is often a relatively thin snow cover. Although thin, this snow cover can cause substantial insulation of the ice and reduce its growth rate.

While the main driving forces that move this ice cover come from wind and currents, ice does not just move as a passive tracer. Instead, ice has a motion and, more notably, deformation that is significantly affected by the ice interaction. Far from shore, the effects of interaction are more subtle, but still considerable in that excessive ice buildup is prevented by ice pressure. In addition, deformation typically takes place in the form of long intersecting leads; a fracturing pattern that is common in the brittle failure of many materials. From models and observation, ice pressure is comparable in magnitude to the buildup of surface pressure in the ocean by sea-surface tilt. Ice stresses averaged over the ice thicknesses that are typically used in large-scale models, for example (Hibler, 2001), are of the order of 2–5 × 104 N/m2, which is approximately equivalent to the bottom pressure of a 2–5 m high column of water.

In 2000, Tony Payne and I organized a session at the annual congress of the European Geophysical Society on the mass balance of the cryosphere. It was clear from the impressive scientific breakthroughs presented at this meeting and also in the recent literature that major progress has been achieved in this subject over the last decade. This is a result of advances in both observational technology through new satellite and airborne hardware, and our modelling capability, and through improvements in computational power and physical understanding. As a consequence, it was timely and fitting to embark on producing a comprehensive review of what we know about the theory behind measuring and modelling mass balance and the actual results from the latest observations and model simulations. In this respect, the book is unique, in that it combines both the theory and the results in a single text. Twenty-three expert authors have contributed to seventeen chapters covering sea ice, glaciers and ice sheets in five thematic sections. Although this is an edited volume, each chapter is extensively cross-referenced and forms part of a fully integrated text. In addition, the chapters were externally peer-reviewed to ensure the highest scientific standards. Part I of this book is designed to offer a comprehensive, yet compact, reference text on the theory and practice of measuring mass balance.

Summary of findings

Sea ice

A variety of indicators suggest that during the latter half of the twentieth century the Arctic has undergone substantial climate change (Serreze et al., 2000). One of the key indicators is sea-ice extent and thickness, both of which have shown a measurable and disturbing decrease during the last half of the twentieth century (see Chapter 8). Interestingly, the rate of decrease appears to be at a maximum during summer (see Figure 12.8). The pre-satellite time series (before 1972) has a larger error bar on it, but nonetheless paints a consistent and compelling picture. Extrapolation forward in time suggests that there could, potentially, be no summer sea ice in the Arctic within 50 years. This will have major impacts on energy and moisture exchange, and consequently on the climate of the northern hemisphere. The consequences of such dramatic changes in sea-ice cover are the subject of a number of general circulation model (GCM) studies, and it is currently too early to say what the implications of these changes might be.

The sea-ice record for the Southern Ocean is less temporally extensive and, essentially, limited to the satellite era. Additionally, the seasonal variation in extent is much greater than in the Arctic, increasing the noise on any long-term signal that may be present. In contrast to the Arctic, there is no measurable trend in sea-ice mass balance, although some regional variations have been noted from passive microwave data covering the last 20 years (1979 to 1999; see Parkinson (2002)).

Introduction

Throughout the history of modern science, glaciers and ice caps have not only been a source of fascination but also a key element in discussions about Earth evolution and climate change. The discovery of the Ice Age in the late eighteenth and the nineteenth centuries significantly contributed to the understanding of the evolutionary development of the Earth; it also demonstrated the possibility of important climatic changes involving dramatic environmental effects at a global scale. Today, glaciers and ice caps clearly reflect secular warming at a high rate and at a global scale; they are considered key indicators within global climate-related observing systems for early detection of trends potentially related to the greenhouse effect (Figure 15.1; IPCC, 2001). This chapter discusses the historical background, the observational data basis and related monitoring strategies. It also gives some examples, predominantly from low latitude glaciers. More detailed treatment of the theoretical and methodological background can be found in the Chapters 2, 4 and 6. An example of measurements in the Arctic is given in Chapter 14.

Historical background of world-wide glacier monitoring

The internationally co-ordinated collection of information about on-going glacier changes was initiated in 1894 with the foundation of the International Glacier Commission at the Sixth International Geological Congress in Zurich, Switzerland. It was hoped that the long-term observation of glaciers would provide answers to the questions about global uniformity and terrestrial or extra-terrestrial forcing of past, on-going and potential future climate and glacier changes (Forel, 1895).

Introduction

The ice caps and glaciers outside the Antarctic and Greenland ice sheets account for only about 4% of the area and 0.5% of the volume of ice on land, and would yield a global rise in sea level of about 0.5 m if they were to melt completely (Dyurgerov and Meier, 1997a; Meier and Bahr, 1996). However, these ice masses, individually of up to 104 km2, may be more significant contributors than the great ice sheets to sea-level rise today, and may remain so over the next century at least (Meier, 1984; Warrick et al., 1996). This is a function of both the climatic sensitivity of the geographical areas in which they are located and of their relatively rapid response time to environmental changes (Johannesson, Raymond and Waddington, 1989).

The ice caps and glaciers of the Arctic islands make up about 45% of the 540 000 km2 or so of ice outside Antarctica and Greenland (Dyurgerov and Meier, 1997a, 1997b). If the small glaciers and ice caps on Greenland and Antarctica are included, there is a global total of 680 000 km2 and about 180 000 km3 of ice, excluding the great ice sheets (Meier and Bahr, 1996). In either case, the glaciers and ice caps of the Arctic islands form a significant area and volume of the world's ice (Figure 14.1).

Introduction

Glacier variations have been of interest for hundreds of years because they can be sensitive indicators of changes in climate. More recently, the role of glacier runoff on the hydrology of mountain regions and the impact of glacier wastage on global sea level have become active areas of scientific effort.

We analyse observational data and the state of health of mountain and sub-polar glaciers for the last several decades, and connection of their changes to climate fluctuations and the global water cycle. We deal here with all glaciers on Earth, excluding the Greenland and Antarctic ice sheets. This analysis is mainly based on our most recently updated time series of mass balance components (Dyurgerov (2002); see http://instaar.colorado.edu/other/occ_papers.html). Every effort has been made to include data from all global sources of information, to check data quality and to eliminate errors.

The quantity and quality of data are far better for the northern hemisphere (especially Europe, Canada, USA and the former Soviet Union (FSU), than for the southern hemisphere. About 70% of the measurements have been carried out in Scandinavia, the Alps, the mountains of the USA, Canada, and the FSU, and the other 30% are sparsely distributed in many other mountain and sub-polar regions (Figure 16.1).

Introduction

Glaciers respond dynamically to external forcings, such as climate variations, which cause ice masses to approach a new equilibrium compatible with the new environmental conditions. For example, it has been suggested that greenhouse warming may result in increased snow fall in the interior of Antarctica and increased ablation in the coastal regions of this ice sheet. Model simulations of the ice-sheet response indicate a thickening in the interior and surface lowering near the margins. Thus, the slope of the ice surface becomes steeper, resulting in greater discharge velocities to redistribute excess mass from the interior toward the margins to compensate for mass loss from ablation. Generally, the response of ice sheets to forcings may be complicated because feedback processes become operable that may amplify or mitigate the ice sheet's adjustment to forcing or because of internal instabilities that may cause rapid changes in ice volume due to changes in the dynamical flow regime. To model ice-sheet evolution adequately, it is therefore necessary to identify the important physical controls and processes affecting the flow of glaciers.

In most models, whether numerical time-evolving or analytical, simplifying assumptions are commonly made to allow a solution to be found. Such simplifications are permissible provided the essential physics are retained. A model aimed at simulating the evolution of the Greenland ice sheet over the last few glacial cycles need not explicitly calculate deformation of each individual ice crystal.

Introduction

Mathematical modelling represents a vital tool for understanding and predicting the current and future behaviour of the Antarctic ice sheet. Above all, modelling tries to overcome the limitations of space and time associated with making direct observations. The dynamical timescales associated with many components of the Antarctic ice sheet are far larger than the limited period for which measurements are available. Models also generate information over the entire ice sheet and can yield insight into many processes that are often inaccessible for direct observation such as at the ice-sheet base. In addition, models are the only tools we have at our disposal to forecast the future evolution of the ice sheet.

Today, the Antarctic ice sheet contains 89% of global ice volume, or enough ice to raise sea level by more than 60 m (Table 13.1). Hence, only a small fractional change of its volume would have a significant effect on the global environment. The average annual solid precipitation falling onto the ice sheet is equivalent to 5.1 mm of sea level, this input being approximately balanced by ice discharge into floating ice shelves, which experience melting and freezing at their underside and eventually break up to form icebergs.

Changes in ice discharge generally involve response times of the order of 102 to 104 years. These timescales are determined by isostasy, the ratio of ice thickness to yearly mass turnover, processes affecting ice viscosity and physical and thermal processes at the bed.

Introduction

Measurement of the mass balance of larger glaciers, ice sheets, ice caps and ice fields requires different field techniques than for the smaller valley glaciers. These larger glaciers are an integral part of the Earth's interactive ice–ocean–land–atmosphere system, and may also provide valuable insight into the cause of changes of the Earth's climate system (Meier, 1998).

In this chapter, we deal with in situ measurement techniques. However, we include measurements based on aerial photography, since such measurements for more than half a century have been used in combination with field studies. Modern, mainly satellite-based, remote-sensing techniques for measuring glacier mass balance are presented in Chapter 4.

Mass balance equations

In glacier context, the term ‘mass balance’ is traditionally used in two ways with different meanings. At a specific point of the glacier, the local mass balance designates the sum of accumulation (supply of mass mainly by snow deposition) and ablation (loss of mass mainly by melting of snow/ice). The local (specific) mass balance may be positive or negative depending on whether accumulation or ablation dominates. However, the sign of the specific mass balance does not say anything about the local change of ice thickness or the local change of mass in a vertical column through the glacier. This is because the specific mass balance may be compensated for, or even be overruled by, mass input/loss due to a gradient of the horizontal ice flux.

Introduction

The cryosphere covers a vast expanse of the polar oceans and land surfaces. The area of the Southern Ocean covered by sea ice fluctuates between about 3.4 and 19.1 × 106 km2. over the period of one year. The Antarctic ice sheet covers an area of some 13 × 106 km2., greater than the conterminous USA. The number of glaciers on the planet is not well known, but certainly exceeds 160 000. Monitoring such large areas, often in remote and hostile environments, is ideally suited to satellite-based observations, which provide the only practical means of obtaining synoptic, timely coverage. Due to the importance of satellite remote sensing to observations of the cryosphere, we provide here a brief introduction to the subject, covering the satellites and sensors most commonly employed and describing how they can be used to derive information on mass balance. These instruments and techniques are referred to extensively in the subsequent chapters covering observational data on mass balance. This section is a primer in the subject. For comprehensive coverage of the general principles of remote sensing of the environment, the reader is referred to a number of excellent textbooks on remote sensing, referenced in this chapter. In section 4.2 we provide an overview of the general principles of satellite remote sensing, which are common to both land- and sea-ice measurements. In section 4.3 the characteristics and pertinent operating principles of the satellites and sensors relevant to cryospheric studies are reviewed.

The regions of the great ice caps in the Arctic and Antarctic are places of stunning beauty. Also, being tantalizingly remote and largely unspoilt by human interference, they hold compelling fascination and interest. However, these are not the only reasons for their study. Compared with the rest of the Earth's surface, they are of importance far beyond what might be expected from their comparative size. The changing balance in the cryosphere between the accumulation and ablation of ice has dominated the Earth's climatic history through the quasi-regular ice ages of the last million years – extending also to earlier epochs about which rather less is known. The world's coastal regions have been enormously affected as this changing balance has led to large excursions of sea level. For instance, at the end of the last ice age, 20 000 years or more ago, the sea level was lower than today by about 120 metres.

The long-term driving influence on the mass of ice in the polar regions, either in the form of sea ice or locked in the ice caps, has been the regular oscillations in key features of the Earth's orbit around the Sun, namely its eccentricity, the tilt of the Earth's axis and the time of year when the Earth is closest to the Sun. These features change with periods varying from about 20 000 years to about 100 000 years, and combine to cause substantial variations in the amount of solar energy that reaches the polar regions at different times of year, most particularly in the northern summer.