Greenhouse gasinduced warming

The IPCC (1992, 1995) developed several scenarios of future greenhouse gas and aerosol emissions based on assumptions of population growth, economic development, land use, technological changes, energy availability and

Box 7.1 Future climate change and glaciation

As the understanding of past global climates and mechanisms of climatic change increases, the need to be able to predict future climate change and the growth and decay of ice sheets becomes increasingly important. Such predictions must, however, be based on geological, climatic and astronomical factors. The history of Late Cenozoic climate change and glaciation demonstrates that the Earth is now experiencing an interglacial phase which began about 10,000 years ago. In Europe, interglacials have been defined when deciduous forests replaced coniferous forests. If Pleistocene interglacials are defined by the presence of deep sea sediment oxygen isotope stage 5e, the interglacial periods lasted in general not longer than 10,000-12,000 years. Based on this evidence, we may be approaching a cooling phase with more glacial conditions. However, if the Little Ice Age temperature depression is repeated in the next centuries, it will compensate in part for anthropogenic warming. Short-term climatic trends give only restricted information. As an example, mean annual global surface temperatures have been rising since the termination of the Little Ice Age, except for a cooling phase from the

1940s to the 1960s. Different proxy data suggest that mean annual temperatures may have dropped 1.5-2°C since the mid-Holocene thermal optimum.

What, then, is the possibility of mountain glaciation occurring in the near future? It has been suggested that initial ice-sheet growth at the start of the last major glaciation (ca. 120,000 yr bp) in high northern latitudes occurred under similar climate conditions as at present (Miller and Vernal, 1992). Optimal ice-growth conditions include a warm northern ocean, strong meridional atmospheric-ocean circulation, and low summer temperatures. Greenhouse warming is suggested to be most significant in the Arctic and during the winter season (Berger, 1978) combined with decreased summer insolation. This may cause snowline depression and glacier growth in high northern latitudes. Heavy winter precipitation in the late 1980s and early 1990s in western Scandinavia has caused the largest glacier advance recorded on maritime glaciers since the termination of the Little Ice Age (e.g. Nesje et al, 1995). Between 1992 and 1997, Briksdalsbreen, a western outlet glacier from Jostedalsbreen, advanced 322 metres, giving a mean daily advance rate of 18 cm. The largest annual advance at Briksdalsbreen was recorded in 1993/94 (80 m) (Nesje et al, 1995).

fuel mix during the period from 1990 to 2100. The emissions can be used to estimate atmospheric concentrations of greenhouse gases and aerosols and the perturbation of natural radiative forcing. Then coupled atmosphere-ocean climate models can be used to project future climate. Large uncertainties remain, however, and these have been taken into consideration in the range of projections of future mean global temperature change. For the 'best estimate' scenario, including the effects of future increases in aerosols, models project an increase in mean global surface air temperature relative to 1990 of about 2°C by ad 2100. This estimate is about 30 per cent lower than the 'best estimate' in 1990. The corresponding high and low temperature estimates are 3.5°C and 1.0°C, respectively. The average rate of warming will probably be among the greatest during the last 10,000 years. However, the annual to decadal changes will include significant natural variability, and regional temperature changes may differ substantially from the global mean. All model simulations indicate greater surface warming over the land than in the oceans in winter, with a maximum surface warming in high northern latitudes in winter, little surface warming over the Arctic in the summer, and increased precipitation in high latitudes in winter. Most simulations also show a reduction in the strength of the North Atlantic thermohaline circulation and a reduction in the diurnal temperature range (IPCC, 1995). Higher temperatures will probably cause an intensified hydrological cycle, with more/less severe droughts and floods. Several models suggest an increase in precipitation intensity, with the possibility of more extreme rain-/ snowfall events. This could lead to increased winter precipitation on maritime glaciers, causing positive net balance and glacier expansion, such as in western Scandinavia during the 1990s (Nesje et al, 1995).

Ice sheets and glaciated regions play an important role in modulating the global environment. Their growth and retreat change the surface topography and albedo and therefore influence global temperatures and wind patterns. Their behaviour is characterized by feedback mechanisms which may amplify small climatic variations. The almost cyclic behaviour of the ice sheets through time is characteristic of a natural system attempting to establish equilibrium without obtaining stability.

Different views exist about the current state of ice sheets and their possible response to global warming. The Antarctic ice sheet consists of two different parts, which would increase sea-level by 65 m if melted. The East Antarctic ice sheet is continent-based and forms a dome over 4000 m in altitude, drained by a series of ice streams. The West Antarctic ice sheet consists of three domes and rises to about 2000 m. A considerable part of the ice sheet is grounded below present sea-level. The ice sheet is drained by ice streams, several of which flow into ice shelves, at velocities of several hundred metres per year.

It has been suggested that the East Antarctic ice sheet formed some 50-60 million years ago during the Tertiary, and that it became a large ice sheet 38-18 million years ago, after which it has been rather stable. The presence of marine diatoms (the Sirius formation) in a glacial till in the Trans-Antarctic Mountains challenge this view of long-term stability. The diatoms have ages of 2.5-3.0 million years and they have been interpreted to have originally grown in a marine environment in East Antarctica (e.g.

Barrett et al, 1992). This means that the ice sheet must have been much reduced in size at that time and that the ice sheet was more unstable that previously thought. The two models are dramatically different. The former reconstruction suggests that there is little risk of melting the Antarctic ice sheet under possible future global warming. The other, however, implies that the ice sheet may become unstable under future global warming scenarios, and contribute many metres to sea-level rise.

There is some uncertainty relating to the West Antarctic ice sheet, which is grounded below sea-level in several areas. According to Mercer (1978), Hughes (1981) and van der Veen (1987), the ice sheet may be buttressed by the surrounding ice shelves, and a minor sea-level rise or increased melting rates could trigger ice calving, surge and collapse of the ice sheet. Such a collapse, which is known from the Quaternary record, would raise global sea-level by 5-7 m. In West Antarctica, future developments depend on the shape of the subglacial topography. If the calving front retreats into deeper water, the rate of calving will increase, leading to progressive collapse.

The response of the Greenland ice sheet to warming scenarios is also uncertain (e.g. Reeh, 1989). Based on the observed retreat of outlet glaciers in West Greenland over the last century (Weidick, 1984), it is reasonable to expect enhanced melting of the outlet glaciers. Satellite observations show, however, that during the period from 1978 to 1986 the southern part of the ice sheet increased in thickness (Zwally, 1989).

Our understanding of the behaviour of the largest ice masses is poor, because the largest ice sheets are quite inaccessible and so large that accurate calculations of their mass balance are extremely difficult. In addition, the ice sheets are part of a complex system with links to the atmosphere, oceans, biosphere and lithosphere. To be able to predict how they respond to a given forcing, it is necessary to understand the relative importance of the feedback mechanisms to the components in the system. According to Saltzman (1985), we should rather concentrate on using field evidence from terrestrial and adjacent marine areas to constrain the

Surface temperature (°C)

Figure 7.2 (a) The relationship of ablation and accumulation to annual surface temperature for an ice sheet, (b) The dependence of net annual mass balance on temperature changes from positive to negative. This complicates the response of ice sheets to climate change. (Adapted from Warrick and Oerlemans, 1990)

Surface temperature (°C)

Figure 7.2 (a) The relationship of ablation and accumulation to annual surface temperature for an ice sheet, (b) The dependence of net annual mass balance on temperature changes from positive to negative. This complicates the response of ice sheets to climate change. (Adapted from Warrick and Oerlemans, 1990)

models. Therefore field evidence should be used to develop models and theories.

Ice sheets may respond to warming in three main ways: (1) by warming of the ice; (2) by changing the surface mass balance; and (3) by changing sea-level or sea temperature. Ice accumulating on the ice-sheet surface may, however, take several tens of thousands of years to pass through the system. After warming of the air temperature, warmer ice accumulating on the surface will gradually replace colder ice, eventually leading to a lower ice sheet with a reduced volume. The response time for this adjustment is so long and the effect so small, however, that it is insignificant on a human time-scale. A change in the mass balance is critical to the response of an ice sheet. Commonly for glaciers, more warming leads to more melting. For ice sheets it is not that simple. Figure 7.2 shows how ablation increases from areas where the mean annual temperature is — 12°C, to values of over 4myr_1 where the mean annual temperature is close to 0°C. In cold regions, accumulation is related to the ability of air to hold water vapour. In the interior of ice sheets the mass balance is limited by the snowfall. As a result, in environments colder than approximately -10°C, warming will increase the mass balance as precipitation increases, while in milder regions, increased melting will reduce the mass balance. In East Antarctica and northern Greenland, with temperatures mostly below —10°C, the mass balance is likely to increase with minor warming. Southern regions of the Greenland ice sheet are likely to be reduced by increased melting. A complicating factor is, however, that melting could lower the marginal areas at the same time as the interior is thickening due to increased snowfall. The result is a steeper surface profile with increased flow velocities, which may counteract the thickening of the interior. In addition, an increase in water depth can trigger iceberg calving, an effect known to produce the highest rates of retreat of marine Quaternary ice sheets (the Laurentide ice sheet over Hudson Bay and fjord glaciers along the coast of Norway). Ice sheets are an important element of the global environmental system, with implications for climate and sea level change. This system is so complex, and has so many feedback loops, that it is extremely difficult to predict the response to future human-induced environmental changes.

The minor mountain glaciers and ice caps contain a small part of the total ice mass on Earth. The retreat of these glaciers subsequent to the Little Ice Age is well documented (e.g. Grove, 1988). The most comprehensive study was made by Meier (1984). He calculated that they contributed 2-4 cm of the sea-level rise during the last century. Since small alpine glaciers respond quickly to climatic change, they are likely to be major contributors to the first phase of possible global warming.

Zuo and Oerlemans (1997) used an ice-flow model to simulate past and future frontal fluctuations of the Pasterze glacier in Austria. Their model experiments suggested that the glacier has been in a non-steady state most of the time, and that the glacier has a response time of 34-50 years. Furthermore, the model shows that the Pasterze glacier will retreat significantly if warming is taking place, of the order of 2-5 km by the year 2100.

Oerlemans (1997) used historic glacier-length variations to constrain a computer model for future frontal variation of Nigards-breen, an eastern outlet glacier from Jostedals-breen in western Norway. When calibrated with past changes, the model predicts an 800 m advance of the glacier front if the mass balance remains as it was between 1962 and 1993. For a uniform heating rate of 0.02 K a-1, Nigardsbreen will advance slightly until the year 2020, after which a significant retreat will occur, reducing the glacier to 10 per cent of the 1950 volume.

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