The world's climate is changing and will continue to change into the coming century at rates projected to be unprecedented in recent human history. The risks associated with these changes are real but highly uncertain. Societal vulnerability to the risks associated with climate change may exacerbate ongoing social and economic challenges, particularly for those parts of societies dependent on resources that are sensitive to changes in climate. Risks are apparent in agriculture, fisheries and many other components that constitute the livelihood of rural populations in developing countries. In this paper we explore the nature of risk and vulnerability in the context of climate change and review the evidence on present-day adaptation in developing countries and on coordinated international action on future adaptation. We argue that all societies are fundamentally adaptive and there are many situations in the past where societies have adapted to changes in climate and to similar risks. But some sectors are more sensitive and some groups in society more vulnerable to the risks posed by climate change than others. Yet all societies need to enhance their adaptive capacity to face both present and future climate change outside their experienced coping range. The challenges of climate change for development are in the present. Observed climate change, present-day climate variability and future expectations of change are changing the course of development strategies -development agencies and governments are now planning for this adaptation challenge. The primary challenge, therefore, posed at both the scale of local natural resource management and at the scale of international agreements and actions, is to promote adaptive capacity in the context of competing sustainable development objectives.

Abstract. A range of different statistical downscaling models was calibrated using both observed and general circulation model (GCM) generated daily precipitation time series and intercompared. The GCM used was the U.K. Meteorological Office, Hadley Centre's coupled ocean/atmosphere model (HadCM2) forced by combined CO2 and sulfate aerosol changes. Climate model results for 1980-1999 (present) and 2080-2099 (future) were used, for six regions across the United States. The downscaling methods compared were different weather generator techniques (the standard "WGEN" method, and a method based on spell-length durations), two different methods using grid point vorticity data as an atmospheric predictor variable (B-Circ and C-Circ), and two variations of an artificial neural network (ANN) transfer function technique using circulation data and circulation plus temperature data as predictor variables. Comparisons of results were facilitated by using standard sets of observed and GCM-derived predictor variables and by using a standard suite of diagnostic statistics. Significant differences in the level of skill were found among the downscaling methods. The weather generation techniques, which are able to fit a number of daily precipitation statistics exactly, yielded the smallest differences between observed and simulated daily precipitation. The ANN methods performed poorly because of a failure to simulate wet-day occurrence statistics adequately. Changes in precipitation between the present and future scenarios produced by the statistical downscaling methods were generally smaller than those produced directly by the GCM. Changes in daily precipitation produced by the GCM between 1980-1999 and 2080-2099 were therefore judged not to be due primarily to changes in atmospheric circulation. In the light of these results and detailed model comparisons, suggestions for future research and model refinements are presented. IntroductionThe present generation of global general circulation models (GCMs) and higher-resolution limited area models (LAMs) of the climate system are restricted in their usefulness for many subgrid scale applications (including those to hydrology) by their coarse spatial resolution and the uncertain reliability of their output on timescales of months or less, especially for variables pertaining directly to the hydrologic cycle [Carter et al., 1994]. As Hostetler [1994] has observed, the parameterizations used in GCMs and in hydrological models are least reliable on the scale(s) at which these models interface. Hydrological models are frequently concerned with small, subcatchment scale processes and must parameterize regionalscale ones, whereas atmospheric models deal most proficiently with fluid dynamics at the planetary scale and parameterize many regional and smaller-scale processes.Climate model resolution issues have important implications

The Upper Blue Nile river basin is the largest in Ethiopia in terms of volume of discharge, second largest in terms of area, and contributes over 50 per cent of the longterm river flow of the Main Nile. This paper provides a review of the nature and variability of the climate and hydrology in the source region of the Blue Nile -the central Ethiopian Highlands. Annual rainfall over the basin decreases from the south-west (>2000 mm) to the north-east (around 1000 mm), with about 70 per cent occurring between June and September. A basin-wide time series of annual rainfall constructed from 11 gauges for the period 1900 to 1998 has a mean of 142lmillimetres, minimum in 191 3 (1 148 mm) and maximum in 1903 (1 757 mm). Rainfall over the basin showed a marked decrease between the mid-1 960s and the late 1980s and dry years show a degree of association with low values of the Southern Oscillation Index (Sol). The October to February dry season in 1997198 was the wettest on record and responsible for widespread flooding across Ethiopia and also parts of Somalia and Kenya. Available river flow records, which are sparse and of limited duration, are presented for the Blue Nile and its tributaries upstream of the border with Sudan. Runoff over the basin amounts to 45.9 cubic kilometres (equivalent to 1456 m3s-') discharge, or 261 millimetre depth (1 961 -1 990), a runoff ratio of 18 per cent. Between 1900 and 1997 annual river flow has ranged from 20.6 cubic kilometres (1913) to 79.0 cubic kilometres (1909), and the lowest decade-mean flow was 37.9 cubic kilometres from 1978 to 1987. Annual river flow, like rainfall, shows a strong association with the SOL

Climate data for studies within the SWURVE (Sustainable Water: Uncertainty, Risk and Vulnerability in Europe) project, assessing the risk posed by future climatic change to various hydrological and hydraulic systems were obtained from the regional climate model HadRM3H, developed at the Hadley Centre of the UK Met Office. This paper gives some background to HadRM3H; it also presents anomaly maps of the projected future changes in European temperature, rainfall and potential evapotranspiration (PET, estimated using a variant of the Penman formula). The future simulations of temperature and rainfall, following the SRES A2 emissions scenario, suggest that most of Europe will experience warming in all seasons, with heavier precipitation in winter in much of western Europe (except for central and northern parts of the Scandinavian mountains) and drier summers in most parts of western and central Europe (except for the north-west and the eastern part of the Baltic Sea). Particularly large temperature anomalies (> 6°C) are projected for north-east Europe in winter and for southern Europe, Asia Minor and parts of Russia in summer. The projected PET displayed very large increases in summer for a region extending from southern France to Russia. The unrealistically large values could be the result of an enhanced hydrological cycle in HadRM3H, affecting several of the input parameters to the PET calculation. To avoid problems with hydrological modelling schemes, PET was re-calculated, using empirical relationships derived from observational values of temperature and PET.

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This paper updates precipitation series for England and Wales, Scotland and Northern Ireland for 1990 to 1995. A new precipitation series for the whole of Ireland for 1840 to 1995 is developed. Recent changes in seasonal precipitation totals in all three regions are discussed. In all regions, recent precipitation in winter (DJF) has increased and summer (JJA) precipitation has decreased. This trend is most evident in Scotland, where the November to April total for 1986 to 1995 is 30 per cent more than corresponding values in 1951 to 1980. For England and Wales the 1995 summer was the driest on record, with 7 mm less rainfall than in 1976, the previous driest summer. Over Scotland and Ireland the 1995 summer was also dry but not as exceptional. © 1997 by the Royal Meteorological Society. Int. J. Climatol. 17, 427–438 (1997).

The relationships between regional daily precipitation time series in the British lsles and 3 indices of air flow are examined with a view to assessing their potential for use in GCM downscaling.These indices, calculated from daily grid-point sea-level pressure data, are as follows: total shear vorticity, a measure of the degree of cyclonicity; strength of the resultant flow; and angular direction of flow. The 3 indices, particularly vorticity, exert a strong control over daily precipitation characteristics such as the probability and amount of precipitation. There are significant regional differences in the relationships with precipitation, particularly between the England and Wales series and the Scotland and Northern Ireland series. Comparison of the relationships between air flow indices and regional and 2 single site precipitation series in England shows they are similar, although at the site-scale local factors may play an important role in affecting the relationships with the indices. Two models for generating daily precipitation series from vorticity are presented and evaluated by their ability to reproduce the following characteristics of precipitation over an independent validation period: annual totals and interannual variability, wet day probability and spell duration, and size of daily precipitation amounts. Model 1 is based on empirical relationships between vorticity and precipitation. Model 2 is based on user-defined categories of vorticity. The results for 2 sites (Durham and Kempsford) show that both models reproduce key characteristics of the observed daily precipitation series. Differences in model structure and number of parameters affect their accuracy in simulating the interannual variability and daily characteristics of precipitation. The air flow indices represent a significant advantage over traditional weather types because they are continuous variables. Previous downscaling techniques have relied upon classification techniques that impose artificial boundaries to define classes that may contain a wide range of conditions and no information about the intensity of development of the weather system concerned. As the 3 air flow indices are the basic determinants for describing the day's weather in many parts of the world, there is significant potential to apply this technique to other such regions. An example is shown of the relationships between daily precipitation in Switzerland and the air flow indices. The models may also be applied to the development of future daily precipitation scenarios using the coarse-scale output of GCM pressure fields.

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