DoENI/SNIFFER Scoping Study of Climate Change Impacts in Northern Ireland

2.  FUTURE CHANGES IN NORTHERN IRELAND

Dr Nick Betts, School of Geography, Queen's University Belfast

Climate change scenarios

Since no single climate change scenario can capture the range of possible climate futures, four generalised climate scenarios for Northern Ireland are presented here: Low, Medium-low, Medium-high and High. These are derived from the UK Climate Impacts Programme (UKCIP) 1998 scenarios (UKCIP, 1998), which are in turn derived from the HadCM2 General Circulation Model (GCM), developed at the UK Meteorological Office Hadley Centre. The four scenarios are being used for all the UK regional climate impact assessment scoping studies, to ensure comparability. The names of the scenarios refer to respective warming rates. Note that these scenarios do not incorporate consideration of sulphate aerosol influences.

Climate changes are presented in these scenarios for three future thirty-year periods centred on the 2020s, the 2050s and the 2080s. The climate change for each of these periods is calculated as the change in thirty-year mean climates with respect to the 1961-90 average. Thus the scenarios for the 2020s are assumed to be representative of the period 2010-2039, the scenarios for the 2050s represent the period 2040-2069, and the scenarios for the 2080s represent 2070-2099.

Changes in carbon dioxide

Atmospheric carbon dioxide concentration for 1961-90 averages 334 ppmv (parts per million by volume). Dependent on the emissions scenario chosen, future estimated carbon dioxide values range between 398 to 447 ppmv in the 2020s, between 443 to 554 ppmv in the 2050s, and between 498 to 697 ppmv in the 2080s.

Temperature

Below are the range of possible changes in temperature (°C) for Northern Ireland, derived from the four UKCIP98 scenarios. These are compared with the 1961-90 mean.

Rainfal

Below are the range of changes in precipitation for Northern Ireland, derived from the four UKCIP98 scenarios. These are expressed as a per cent change from the 1961-90 mean. Note that patterns of precipitation change are less consistent (and hence presumably less reliable) between seasons and scenarios, than is the case for changes in temperature.

Mean seasonal climate

Results from the Medium-high scenario are presented below. This is not necessarily the best-guess estimate, but more modelling results are readily available for this scenario.

WINTER (DJF) sees a small decrease in diurnal temperature range of about 0.2°C by the 2080s. This change is consistent with an increase in cloud cover and precipitation. Vapour pressure increases 1.7 hPa by the 2080s, but, with increasing mean temperatures, relative humidity changes little. Incident short-wave radiation decreases by 1 Wm-2, a change consistent with increased cloudiness (1%), and precipitation. There is little change in mean wind speed over Northern Ireland by the 2080s. Winter potential evapotranspiration increases by 12%, mostly during the last decades of the century, being particularly sensitive to temperature.

SPRING (MAM) sees a reduction in diurnal temperature range of 0.4°C by the 2080s, which correlates with increased cloud cover (1%). Incident short-wave radiation decreases by 2 Wm-2. Vapour pressure increases of 1.7 hPa are accompanied by a slight rise in relative humidity (1%). Wind speeds show little change from the present. Potential evapotranspiration in spring increases 5% by the 2080s.

SUMMER (JJA) diurnal temperature range decreases some 0.2°C by the 2080s, in association with slight increased cloud cover (0.5%) and reduced short-wave radiation (2 Wm-2). Increases are estimated in vapour pressure (2.2 hPa) and relative humidity (0.5%). Mean wind speed increases 1.5 per cent by the 2080s. Potential evapotranspiration shows a 6% increase, which will compound the effect of the summer precipitation decreases (2-3 per cent)on water resources.

AUTUMN (SON) sees an increase in diurnal temperature range of 0.1°C, accompanied by an increase of short-wave radiation (3 Wm-2 ) and reduced cloud cover (2%). Vapour pressure increases of 2.2 hPa are accompanied by minimal changes in relative humidity. Increased wind speeds of about 4% combined with increases in radiation, means that potential evaporation rises 18% by the 2080s, with an 11% increase as early as the 2020s.

Climate variability and extremes

In terms of year to year variability, winter temperatures become less variable and summer temperatures more variable. Precipitation variability increases in every season.

The number of degree-days for a minimum temperature below 0°C decreases by more than 75% by the 2080s, while the degree-days for a maximum temperature above 25°C more than trebles. Degree-days for mean temperatures above 5.5°C (indicative of the plant growing season) increase by more than 5% per decade.

More frequent days with high summer wind speeds are predicted, but there is little change in winter daily wind extremes (mean wind speed more than 10 metres per second). In winter there exists a possibility that overall gale frequency declines in future, although very severe winter gales increase. Any changes in summer gale frequencies are likely to be modest (~10 per cent increase).

Precipitation intensities increase in both winter and summer, the most intense events becoming several times more frequent than at present. This is likely to lead to greater risk of flooding.

By the 2080s lightning frequency may increase by up to 20 per cent, consistent with the anticipated increases in precipitation intensities.

Related to changing airflow patterns over the whole of the British Isles, for the 2080s period, analysis suggests a tendency for autumns to experience windier conditions, with a reduction in northerly and easterly airflows and an increase in southwesterly and westerly flow. Summers become slightly more anticyclonic in character with more westerly and northwesterly flow, while winter and spring become slightly less cyclonic. (See Figure 9 for a comparison of present-day precipitation patterns associated with differing airflows.)

It needs to be emphasised that even under the changes in average thirty-year climates, the year-by-year evolution of climate over Northern Ireland will have substantial variability, with periods of opposite trend to those presented here for the expected thirty-year mean changes (see UKCIP, 1998).

It should also be noted that models developed subsequent to the UKCIP98 scenarios (e.g. the HadCM3 GCM), indicate some differences to the UKCIP98 scenarios which have been analysed here, notably with respect to enhanced summer dryness and significant increase in mean wind speed.

Sea-level change

Estimates of the change in mean sea level around Northern Ireland coasts by the 2050s resulting from global climate change range between 13 cm to 74 cm, dependent upon scenario. These values must be modified in relation to natural vertical land movements.

Reference
UK Climate Impacts Programme (1998). Climate Change for the UK: Scientific Report. UK Climate Impacts Programme Technical Report No. 1, Climatic Research Unit, Norwich. 80 pp. Available from http://www.ukcip.org.uk