The environment of the polar regions

The polar regions can be defined in a number of ways, based on geographical, topographic and even political factors. However, geometrically, the Arctic and Antarctic are considered as the areas of the Earth poleward of the Arctic and Antarctic Circles, which are located at latitudes of 66° 33′ 39″ north and south of the Equator (see maps on the end papers). These areas experience at least one day each year when the Sun does not set, and one day when the Sun is always below the horizon. At the poles themselves there is only one sunrise and one sunset each year, which occurs on the equinoxes of 21 September and 21 March. Together the Arctic and Antarctic comprise about 8% of the surface area of the Earth.

The regions of perpetual summer sunlight and winter darkness are present because the Earth is tilted away from the plane of its orbit around the Sun by 23° 27′, resulting in the high latitude areas having periods when they are orientated away from or towards the Sun. The tilt of the Earth's axis changes over long periods of time (millennia), resulting in variations in the latitude of the Arctic and Antarctic Circles. The change in the tilt, along with slow variations in the Earth's orbit about the Sun, alter the amount of solar radiation arriving at different parts of the Earth, which is a major factor in long-term, millennial-scale climate variability.

Introduction

In earlier chapters we have given a summary of our current understanding of past climate change at high latitudes and provided scenarios of how the climates of the two polar regions may evolve over the next century under different levels of greenhouse gas emissions. We are at a critical period in the study of the Earth's climate. Improved data sets providing insight into past climate variability are constantly appearing from new ice and ocean coring initiatives. In addition, we are getting superior observations from satellite systems that give us an unprecedented coverage of the atmospheric, oceanic and cryospheric conditions at high latitudes, which allow us to better understand the mechanisms that are important there. In recent decades our ability to model the environment of the polar regions has also improved. However, there are still many questions unanswered regarding the reasons for past and current climate change and significant doubt about what may happen over the next century.

Regarding future climate changes, perhaps the largest uncertainty is over how greenhouse gas emissions will change over the coming decades. With the release of the IPCC's Fourth Assessment Report, there has been a greater willingness to accept that human activities since the start of the Industrial Revolution, and particularly over the last 100 years, have altered the climate of the Earth.

The last few years have seen an unprecedented level of interest in the climate of the polar regions. The discovery of the Antarctic ozone hole, the reduction in extent of Arctic sea ice, the disintegration of floating ice shelves around the Antarctic and the high levels of aerosols reaching the Arctic have all been reported extensively in the media. This has been coupled with climate model predictions showing that the high latitude areas will warm more than any other region on Earth over the next century if ‘greenhouse gas’ concentrations continue to rise. Yet some have pointed to rapid climatic fluctuations that have taken place in the polar regions over the last few centuries and millennia and questioned whether the recent changes that we have seen are not simply a result of natural climate variability. Hence the time is right for a reappraisal of our understanding of recent high latitude climate change in the context of increasing anthropogenic influence on the Earth and our greater understanding of the reasons for past climate variability.

This book seeks to assess the climatic and environmental changes that have taken place over the last century and set these in the context of our understanding of natural climate variability in the pre-industrial period. We will draw on many of the new climate data sets that have become available in recent years and also make use of the results of modelling experiments.

Introduction

In this chapter we describe the main types of data available for the study of climate change within the polar regions. In comparison with most other regions of the Earth the time-series of ‘traditional’, in-situ instrumental observations is relatively short, particularly in Antarctica where most stations have only been operating for about 50 years. With short records it is more difficult to determine whether recent trends are significant, particularly for regions where there is high natural climate variability, such as the Antarctic Peninsula. One way of extending climate records is to use ‘proxy’ climate data; for example, the commercial whaling expeditions in both the Arctic and Antarctic provide historical data about the position of the sea ice edge, where the greatest amount of hunting took place.

The inhospitable nature of the polar regions means that conducting science in such areas can be very expensive. Thus, there are relatively few surface meteorological stations compared with the mid latitude and tropical areas. This is illustrated in Fig. 2.1, which shows the coverage of surface, ship and aircraft observations assimilated into the European Centre for Medium-range Weather Forecasts (ECMWF) model at 00 GMT 12 July 2010. In recent years advances in technology have allowed the deployment of autonomous automatic weather stations (AWSs) and these are particularly useful in the polar regions as they can be sited in very remote locations. The majority of the synoptic reports in Antarctica situated away from the coast are from such AWSs.

Introduction

The Holocene is the period of approximately the last 11.7 kyr and covers the time from the end of the last ice age up to the present. It therefore includes the so-called anthropocene, which is the period when humankind has influenced the climate system. For most of this latter period there are instrumental meteorological records, and this era is dealt with in the next chapter.

The Holocene marks the return of warmer and more humid conditions after the cold and dry period of the Last Glacial Maximum (LGM) (see Section 4.2.5). The start of the Holocene coincided approximately with the end of the Younger Dryas event (12.8–11.5 kyr BP) (see Section 4.2.5), an abrupt return to cold conditions (stade) during the gradual warming at the end of the last Pleistocene glaciation. Temperatures derived from ice cores collected on the Greenland icecap (see Section 2.4.2) suggest that the transition to the Holocene was a rapid switch of mode, with the Younger Dryas ending abruptly over a period of about 50 years. However, in other parts of the world the transition was not so rapid.

The Holocene can be split into a number of stages, and in this chapter we will divide it into the Early Holocene (11.7–5 kyr BP), the Mid Holocene (5–3 kyr BP) and the Late Holocene (3 kyr BP to present) (see Table 5.1).

Greenland ice core data suggest that temperatures during the Holocene have been about 12 °C higher than during the Pleistocene.

Introduction

The climates of the polar regions are characterised by long periods of continuous sunlight in summer and perpetual darkness in winter that lead to large annual cycles in many aspects of the environment. The temperatures are very low in winter and only moderate in the summer because of the low elevation of the Sun and the highly reflective nature of the snow and ice surfaces. In fact the cryosphere is a major factor in defining the climates of the high latitude areas and the interactions of the ice and snow with the ocean and atmosphere will be discussed extensively in the following sections.

Many factors are responsible for high latitude climate variability and change, which can occur on a range of timescales. On long timescales, major changes in global climate are driven by orbital and solar variability. These affect the seasonal and latitudinal distribution of energy received from the Sun. Oxygen isotope data from ocean floor sediments indicate periods when the polar ice sheets were significantly more expansive than at present, particularly in the Northern Hemisphere (glacials) and when they were of similar size to the present (interglacials). Changes in the output of the Sun can result in high latitude climatic fluctuations with periods of reduced irradiance, such as the Maunder Minimum of the seventeenth and eighteenth centuries, being detectable in some aspects of the polar climates.

Introduction

The period examined in this chapter extends back one million years, a time span that was chosen as encompassing the mid and late Quaternary and being slightly longer than the oldest Antarctic ice obtained at the time of writing: this was drilled at Dome C and extends back to ~800 kyr BP (years before AD 1950) (Parrenin et al., 2007). It also comprises the modern half of the Pleistocene (‘most recent’) epoch, a term originally coined by English geologist Charles Lyell in 1839. Throughout this period there has been the cyclic growth and decay of major Northern Hemisphere ice sheets through the exchange of mass between these ice sheets and the oceans.

On the notation used

Examining climate on this long-term timescale, using oxygen-isotope (δ18O) stratigraphy analysis of marine sediment cores and ice cores, has led to the development of a particular notation that allows direct comparison between records with differing sedimentation/accumulation rates. Emiliani (1955) introduced marine isotope stages (MIS) based on δ18O records derived from deep sea sediment cores. These MIS are time periods with boundaries at the mid-point between isotopic temperature maxima and minima of successive stages (Fig. 4.1). Beginning with MIS 1, which characterises the Holocene, odd and even stages represent interglacial and glacial periods further back in time, respectively: the exception is MIS 3, which was incorrectly identified as an interglacial when first defined and actually forms part of the last glacial with MIS 2 and MIS 4.

Introduction

Analyses of the conventional surface meteorological observations indicate that the near-surface air temperature of the Earth as a whole has increased by about 0.6 °C over the last century (IPCC, 2007). However, the patterns of surface change across the Earth in the instrumental era are complex and sensitive to the period examined. Many studies highlight that some of the largest environmental changes have taken place at high latitudes.

In this chapter we are concerned with high latitude atmospheric, oceanic and cryospheric changes over the period for which there are a reasonable number of in-situ instrumental records. This is obviously shorter than for the more populated mid latitude regions and covers only about the last 100 to 150 years in the Arctic, and about 50 years in the Antarctic. The first long meteorological records started in Europe during the seventeenth century at locations such as Paris and London, but measurements from the Arctic generally began during the nineteenth century. However, as will be discussed later, there are several Arctic or near-Arctic temperature records that extend back to 1840–1860, such as those from Murmansk, Russia and Reykjavik, Iceland, and around a dozen starting from the second half of the nineteenth century. The greatest increase in the number of Arctic meteorological records came over 1930–40, and later in this chapter we discuss the temperature records from 59 stations in the high latitude areas of the Northern Hemisphere that provide reasonable longitudinal coverage over the areas around the Arctic Ocean.

Introduction

Many of the high latitude climatic changes discussed in earlier chapters occurred because of natural climate variability, associated with fluctuations in the orbit of the Earth around the Sun, changes in the amount of solar radiation emitted by the Sun, volcanic eruptions that injected large amounts of dust into the atmosphere, changes in the ocean circulation and exchanges of heat between the ocean and atmosphere. However, from about the middle of the eighteenth century, at the start of the Industrial Revolution, humankind began to influence the climate system through the emission of increasing amounts of greenhouse gases. At first the impact was very small, but in the last decade of the nineteenth century Svante Arrhenius (Arrhenius, 1896) suggested that the increasing blanket of greenhouse gases above the Earth could raise temperatures in the troposphere. However, it was only in the second half of the twentieth century that there was widespread interest in climate change as decade on decade the mean temperature of the Earth started to increase and record high temperatures were registered with increasing frequency.

The occurrence in recent decades of high profile severe weather events, such as droughts, severe hurricanes and heat waves, resulted in major debates on the role of humans in these events. In the early years of the twenty-first century it was hard to open a newspaper or switch on a television without encountering discussions on the reasons for recent climate change, with environmentalists and global warming ‘sceptics’ presenting opposing views.