Chapter #01: Climate and water cycle:

Water is involved in all components of the climate system (atmosphere, hydrosphere, cryosphere, land surface and biosphere). Therefore, climate change is expected to depend upon several independent and inter-connected mechanisms in its overall impacts. Obviously, any future projections will also be dependent on these inter-relations.

Climate warming observed over the years is consistently associated with changes in several components related to water such as: changing precipitation patterns, intensity and extremes of precipitation, wide-spread melting of snow and ice (both on land locked glaciers and the polar region), increasing atmospheric water vapour, increasing evaporation, changes in soil moisture patterns, and changes in runoffs and river discharge. Even though there is significant natural variability- on inter-annual to decadal time scales, there is still substantial uncertainties in trends of the hydrological variables due to various factors (both observational, and coverage in space and time). There are significant local factors which impact some of the estimates (like in runoff). Based on analyses presented in IPCC-WG1-AR4 and IPCC-WG2-AR4 reports, following observations and trends can be seen on an average basis::

1.0 Observations::

• Precipitation and water vapour
Using GPCP analyses, there is a clear trend that precipitation has generally increased over land in the 20th century between 30and 85N but in the tropical belt (10S to 30N) there is a general decrease over past 30-40 years.(Ref WG1, fig. 3.15). There is also a signal in sea water freshening as salinity decrease in North Atlantic and south of 25S reaffirming the GPCP analysis. In the northern tropical and extra tropical region, there is a marked increase in precipitation from 1900 to about 1950 and a decline after around 1970.

The global changes are not showing any specific trend most probably because the decadal variation overtakes all other trends (WG1 fig 3.12). The annual precipitation at 5x5 averages show an increase over 105 years (1901 to 2006) over most of North America and Eurasia. In an interim period (around 1979-2005, there is a complex pattern indicating regional drying (e.g. south-west North America) and a mixed pattern in distribution over Eurasia. There are indications that a latitudinal change is taking place in the monsoon performance all over the globe. (WG1 3.3.2.2, fig 3.13). There is a very clear signal of large negative trend since 1901 in annual precipitation in western Africa and Sahel, where precipitation has increased since 1979. Over North-west India, even though the annual precipitation trend over 1901-2005 shows a clear increase, from 1979- there is an overall decrease. North-western Australia shows a wetter trend over all periods while far south-west shows a drying trend from 1975 onwards. (Theoretical global simulations based on greenhouse warming fail to capture this behaviour)

There is a widespread increase in heavy precipitation events (e.g. above 95 percentile) even in places where total precipitation has decreased (specifically over Europe, North America). The tropical regions have been clearly marked by increase in strengths of convective systems and storms, especially after 1970. (WG13.8.2.2), this increase is seen well correlated with increase in SST. (These trends are consistent with the theoretical simulations using global warming).

The water vapour content of the troposphere has an increasing trend (consistent with global warming) by 1.2±0.3% globally per decade from 1988 to 2004 matching the SST variation pattern. Even though annual to decadal variations are dominant, there is a significant upwards trend very clearly seen over Northern Hemisphere.

• Cryosphere
The cryosphere (consisting of snow, ice and frozen grounds and polar regions) has about 75% of the world’s freshwater reservoir. Therefore, it plays a very significant role in impact assessment scenario for climate change. Question is: Is there a significant change in ice-storage across the world?

Snow cover shows a decreasing trend over most of Northern Hemisphere (using satellite observations 1966-2005), especially in spring and summer. There is a stepwise drop of about 5% is seen in annual mean in the late 1980s. The decrease is very marked in the lower altitudes (e.g. in Himalayas and Alps and the mountains of western North America).

There is marked degradation of permafrost and seasonally frozen grounds. The top temperature on permafrost grounds have been seen to increase up to 3C since 1980s in the Arctic region. The seasonally-frozen grounds in non-permafrost regions have shown a clear decrease in coverage area by about 7% in the Northern Hemisphere from 1901 to 2002 and the depth of ice has decreased by about 20 cm in Eurasia. There have been changes in freeze up and break-up periods for rivers and lakes in the seasonal freezing zones. Over the past 150 years, the Northern Hemisphere is showing a delay on an average at a rate of 5.8±1.6 days per century while the break-up has been accelerated at a rate of 6.5±1.2 days per century (WG1 4.3).

On an average, glaciers and ice-caps show a moderate (but consistent) decrease in mass balance over Northern Hemisphere (WG1 4.5.2, fig 4.15) 0.37±0.16 mm/yr of equivalent sea-level, SLE) between 1960 t0 1990 and 0.77±0.22 mm/yr of SLE during 1990 to 2004. There is strong likelihood that this glacier melts have led to formation of lakes in Steep Mountain ranges.

• Sea level
There is very high confidence in stating that global mean sea level is increasing at an average rate of 1.8±0.5 mm/yr for 1961-2003 and 3.1±0.7 mm/yr for 1993-2003, leading to century average of 1.7±0.5 mm/yr. Spatially, the change is highly non-uniform (WG1 5.ES).

(The theoretical simulations using coupled models suggest that for the period 1993-2003, the contribution from thermal expansion is 1.6±0.5 mm/yr, while from glacier and ice melt is 0.77±0.22 mm/yr with Antarctic contributing 2.8±0.7 mm/yr. Even though, the theoretical estimates may be higher, trend wise it is considered to be consistent with the observations).

• Evapo-transpiration
There are very limited observations for direct evapo-transpiration over global land mass region and almost non over the oceanic regions. The indirect estimates are based on the analysis using atmospheric and land surface models and other proxy observations. In general, despite very limited information, reported analysis (Qian et al. 2006, WG1 3.3.3) have shown that the global evapo-transpiration closely follows variations in land precipitation.

• Soil moisture
Historical records of soil moisture content are available only from few regions and that too very limited period. (WG1 3.3.4). There are about 600+ stations in the globe covering variety of climatic variations, Robock et al (2000) have identified that in the top (1m) soil, the moisture content during summer have increasing trends over Russia, Europe, China and central USA. More information is needed for regions like Africa, Australia and Asia.

• Runoff and river discharge
The river discharges and the runoff of the precipitation is a very significant factor in altering the ocean salinity contrasts and the associated air-sea interaction driving the global circulation process. A large number of studies exist to obtain potential trends in river discharge during 20th century. However, the analyses have been mostly statistical in nature and are inconclusive to determine relations with trends in temperature or precipitation. The trends in river runoff are not always expected to be consistent with precipitation due to limitation of observations and human interference (e.g. reservoir impoundment).

At the global scale, there is evidence of a broadly coherent pattern in annual river runoff. In the higher latitudes and large parts of USA, where river reservoirs are not very abundant, there is seen a significant increase in river runoff. In the other parts such as West Africa, southern Europe and southernmost South America, there is decrease in runoff (Milly et al 2005, WGII 1.3.2). More work is indicated in this direction.

There is, however, a robust and widespread indication that the timing of river flows in many regions (especially those which are fed by winter precipitation as snow fall) has significantly altered. The advance in river flow has been seen forward by 1 to 2 weeks during 1936 to 2000 winter time, although there is no significant change during summer time.

• Tele-connections
The El Nino-Southern Oscillation (ENSO, where El-Nino is sea level anomaly in the eastern Pacific, and Southern Oscillation, SO, is defined as difference in mean sea level pressure anomalies between Tahiti, in east Pacific, and Darwin, in western Pacific), is considered as the most significant mode of inter-annual variation of global climate. ENSO is associated with an east-west shift in tropical Pacific precipitation, and modulation of main tropical convergence zone (TCZ). ENSO has impact on the atmospheric circulation outside tropical region (Pacific-North America and Pacific-South America) leading to major regional climate effects. The strength and frequency of ENSO varies on decadal scale in association with Inter-decadal Pacific Oscillation (IPO) (WG1 3.6.2).

Outside the tropics, variability of atmospheric circulation on intra-seasonal scale is dominated by variation in strength and location of jet-streams and associated storm tracks) known as Northern Annular Mode (Quadrelli and Wallace, 2004). NAM is closely associated with North Atlantic Oscillation (NAO, characterized by ou-of-phase pressure anomalies between temperate and high latitude sectors of Atlantic). NAO has its strongest signature in boreal winter when its +ve (-ve) phase exhibits an enhanced (diminished) Iceland Low and Azores High. (Hurrell eta l 2003). It has very strong influence on winter time temperature in most of Northern Hemisphere and precipitation over Europe and North Africa. There is a polewards (Equatorwards) shift in precipitation in its +ve (-ve) phase (Cook et al 2002, Jones et al 2003). There has been a short period (1960-1990) when NAO changed phase (minimum to strongly positive) and after that it is more or less near its long-term mean. Its relation with global warming is still not clearly established. North Atlantic SSTs show a warm phase (1930-1960) and a cool phase (1905-1925, 1970-1990) (Schlesinger and Ramankutty, 1994). This multi decadal Oscillation (AMO) may be related to the thermohaline circulation of the ocean (Delworth and Mann, 2000; Sutton and Hudson, 2003;Knight et al 2005). AMO is also believed to be a driver for multi-decadal variation in Sahel rainfall, precipitation in Caribbean and summer time climate of both North America and Europe, and ice melting in Greenland sea (WG1 3.6.6).

Like NAM, the Southern Annular Mode (SAM), is associated with synchronous pressure variations of opposite sign in mid- and high latitudes in the Atlantic, reflecting changes in the main belt of sub-polar regions around Antarctica. SAM affects the spatial pattern in precipitation variability over Antarctica and southern South America. Model simulations suggest that SAM is affected by increase in greenhouse gases and depletion of ozone (WG1 3.6.5).

2.0 Climatic impact on hydrology:

The most dominant climatic variables connected with hydrosphere and its exploitation as resources for development are: precipitation, temperature and evaporative demands (essentially dependent on net radiation at the ground, relative humidity, wind speed and temperature), runoff especially in catchments area) and river flows. The temperature is particularly important in snow-dominated regions and in the coastal areas. Temperature also manifests as change in steric sea level rise due to thermal expansion, affecting coastal areas. In terms of water availability as resources, the major impacts come through precipitation and runoff (the later having an impact on infrastructure built-up) and ‘evaporation-minus-precipitation’ in the ocean regions to estimate changes in salinity and associated density driven currents.

• Ground water
The rate of precipitation affects ground water recharge (for renewable resource component) and the water table. Though this process of recharging is still quite poorly quantified, there are certain trends which have been seen over some regions.

In the high latitude regions, thawing of permafrost causes changes in both level and quality of groundwater, due to increase coupling with surface water. Increased precipitation variability may decrease groundwater recharge in very humid areas because more frequent heavy precipitation event may happen and result in lesser percolation due to saturation of infiltration capacity of the soil in a shorter time it rains. In arid and semi-arid region, however, the infiltration may increase during heavy rainfall (because the absorption will take place faster than evaporation). In the areas, where water table is already high, increased recharge may cause soil salinization and water-logging. Most probably, in a global sense, groundwater recharge may increase but at a rate slower than runoff. (Doll & Florke, 2005).

• Floods
As discussed in the projected scenario of climate change, heavy precipitation events are likely to become more frequent throughout 21st century, there will be higher risk of flash flooding and urban flooding.

Multi-model ensemble analysis (WGII 3.4.3,;Palmer & Raisanen,2002) have projected that due to increase in boreal winter precipitation, high latitude regions are expected to be open for flood hazards. The monsoon regions (like in Asia), the control 100-year peak volume of river flow is projected to increase substantially and more often. Warming-induced snow-melt is expected to increase flooding probability from glacier fed rivers.

• Drought
Since there is a tendency for drying of mid-latitude continental areas during summer, there are possibilities of increased drought events taking place in these regions. This is expected to be brought about due to two competing factors: first, the decrease in precipitation and second increase in surface temperature leading to more evaporative demands and hence loss of soil-moisture.(Christensen et al 2007)

A projected earlier snow melt and reduction in snow cover is also expected to cause increase in the risk of drought in the mid-latitude and extra-tropical regions. The most affected regions will be around Andes in Southern America, Himalayas and part of northern North America).

• Water quality
Higher water temperatures, increased precipitation intensity and longer periods of low-flow are expected to exacerbate many forms of water pollution (including sediments, nutrients, dissolved organic carbon, salt, etc.). This will promote algae bloom (Kumagai et al, 2003) and increase the bacterial and fungal content, requiring substantial efforts in water purification for human usage. The more frequent spell of heavy rainfall in the high-latitude and tropical regions, will exert additional strain in the urban sewage system, which will increase risks of degradation in water quality.

Interestingly, rising temperatures during winter might improve the water quality in the mid-latitude and extra-tropical regions due to earlier ice break-up and higher oxygen levels available.

In arid and semi-arid regions, increased evapotranspiration will increase salinisation of shallow groundwater. As stream flow is projected to decrease in these regions, the salinity of rivers is expected to increase. (WGII 3.4.4)

In coastal areas, rising sea-level may have negative effects on storm-water drainage and sewer disposal system as well as possibility of intrusion of saline water from ocean into the coastal aquifers, adversely affecting the water quality. Any decrease in ground water recharge (as in mid-latitude), the negative impact of sea-level rise on the water resources will exemplify.

3.0 Impacts of climate change (precipitation vs. warming) on freshwater availability

Hydrological changes brought about by climate change may have negative as well as positive impacts. For example, u\increased runoff may be beneficial for a variety of upstream and out-of-stream water users by increasing renewable water resources, but may simultaneously generate problems by increase in the flood risk for the downstream users. In recent decades, a trend in wetter atmospheric conditions in parts of southern South America has increased the area of inundation, but has also improved crop yields in Argentina (Magrin et al 2005).

In shallow water table area, increased runoff would mean a sharper increase in water table and therefore may be threatening to agriculture pattern as well as urban properties due to prolonged water logging. (for example, Russia, Kharkina et al 2004).

Increased runoff may also lead to change in turbidity level of water reservoirs and therefore, pose problems for supply of drinkable water.

Higher temperature and increased variability of precipitation would, in general, lead to increased water demands through irrigation system (especially during crop growing season). Most of the climate model simulations use no irrigation based water availability, therefore, a long term climatic impact through these hydrological changes are an yest-to-be determined factor in assessing future climate projections and associated changes in hydrology (WG2 3.5.1)

Several studies in North America (Eheart and Tornil, 1999), China and Asia (Doll et al 2003) have shown that the overall impact of CO2 increase in terms of increase in warming and changes in pattern of precipitation are mostly off-set in terms of agriculture yield through use of water from irrigation system. That would mean that the profit-maximising irrigation water use responds more strongly to changes in precipitation than any other factor like CO2 increase (WG2 3.5.1)

Global estimates of population affected by water stress are vastly different. Nevertheless, it is significant factor in leading to demographic, socio-economical and technological changes. (WG2 3.5.1). Using per capita water availability as index for overall water usages, there is hardly in significant change in the water stressing on global scale. It is also seen that water stress is also leading to increase in non-climatic drivers (like water use efficiency, water productivity and industrial production), which reduces the overall impact on hydrological cycle not included in climate change scenario.

The uncertainties in climate change impacts on water resources are mainly due to uncertainties in precipitation input rather than due to uncertainties in greenhouse gas emissions. The other way round, i.e. the feedback from adaptations measures due to changes in hydrology on climate changes, are yet to be fully considered for future climatic projection scenario.

4.0 Impacts on eco-system and biodiversity
Temperature and moisture are the key physical variables, which determine the growth and productivity on biological side. The global climate change models project an overall increase in temperature, concentration of precipitation in terms of intense events, longer periods of low precipitation (i.e increase in dry days), and an earlier thawing of permafrost regions are some of the factors, which are going to determine the response of biota to climate change.

As mentioned above, major physical impact of climate change is through changes in hydrological cycle. The response of ecosystem to changes in hydrology is a complex phenomenon involving both biota and non-biotic processes and is mostly non-linear and involves varying time-lag from specie to specie. This might lead to a change in competition between species, altering the existing food chain and may eventually cause disintegration of some of the ecological communities (probably to extinction) (WG2 1.3.5.5). Since, freshwater availability is expected to get affected very significantly, the freshwater aquatic eco-systems are most threatened (WG2 3.5.1)

For the lake and stream system, the high-latitude regions are expected to experience increase in water levels of lakes, while those in mid-latitude and lower latitudes are expected to have lower water levels and some might disappear. The decrease in ice-cover period of lakes in mid- and high-latitude and increase in temperature of water may reduce oxygen content as well as increase specific nutrients from lake bottoms causing spread of algae and reduce the general specie diversity. In the humid regions, where flows are less variable and biological control strongest, drying or seasonal flooding of streams, lakes and extended water bodies can lead to reduction in eco-system productivity and might eventually lead to reduction biodiversity.

Most wetland processes are dependent on catchments-level hydrology and therefore, will have most pronounced effects due to altered precipitation pattern and more frequent intense disturbances expected to be induced by climate change. As a supplementary response to changing water management practices, induced by changing pattern of freshwater availability, is also expected to alter the wetland hydrology. Consequently, the biodiversity in these regions is also expected to get very significant impact (WG2 4.4.8).

In the coastal regions, the changes and the timing of freshwater runoff will affect salinity, nutrient availability and moisture distribution. These factors are expected to alter the biological productivity of the region. Since the coastal inundation expected from sea-level rise will depend upon the coastal landforms, there will be varying impact on salinity ingress, sedimentary deposits and changes in biodiversity. For example, climate models indicate that in later half of 21st century, the river discharges in Arctic, northern Argentina, south Brazil, Indian sub-continent and China will increase while those in southern Argentina, Chile, western Australia, western and southern Africa and the Mediterranean will decrease. Consequently, in the regions with decreasing river flows, salinity is expected to advance upstream, thereby altering the coastal biodiversity, amount of nutrients delivered and sediment deposits (WG2 6.4.1.3)

In the mountain system, the ecosystem is primarily controlled by temperature and soil moisture along the mountain gradient. The snow-melt dominated watersheds will have an earlier peaking of water flows followed by prolonged period of low precipitation. The altered water supply, is expected to have significant impact on the biological production and biodiversity.

Since the productivity of agricultural, forestry and fisheries system depends critically on temporal and spatial distribution of precipitation and freshwater (in case of agriculture), there is expected to be profound impact on these systems or management practices associated with these systems. Even though, the decrease in precipitation in specific regions may induce more dependence on irrigation system and better water management, the increase in intensity of precipitation may pose problems by affecting agriculture directly and through changes in soil properties. The available more dry days may damage to plant growth if the breaks are occurring at critical juncture.

Similarly, for forest eco-system, the water availability being a key factor, the changing pattern of precipitation will significantly alter the incidences of forest fire (will increase due to lesser moisture available), water stress will increase (specifically in mid- and lower latitudes) and increase in pest attacks (WG2 13.4).

5.0 Specific Impacts on Asian continent
Asia has a large variability in water distribution. According to available data (FAO 2004), among the 43 countries in the region, 23 have renewable water resources estimated at per capita in excess of 3 K cu m, 11 between 1 K to 3K cu m, and 6 below 1K cu m, while there is no data from remaining 6 countries. There are large arid and semi-arid regions (west China and Mongolia to west Asia). Even in the humid and sub-humid zones of Asia, water stress due to uneven distribution of water is one of the constraints for sustainable development. At the same time, the population growth in Asia is one of the highest in the world, the water stress is expected to be quite significant factor in the socio-economic development strategy of the region.

Inter-seasonal, inter-annual and spatial variability in rainfall has been observed over the past few decades across Asia. There has been a decreasing trend in annual rainfall over most of Russia, north-east and north China, coastal belts and plains of Pakistan, parts of north-east India, Indonesia, Philippines and some part of Japan. There has been increasing trend in precipitation over western China, south-eastern China, Arabian Peninsula, Bangladesh, and western coasts of Philippines. In south-east Asia, frequency of extreme weather events (heavy rainfall, very intense cyclones, very low rainfall) have been increasing (Zhai, 2004; Shreshta et al 2000; WG2 10.2.3). There is substantial inter-decadal variability seen in the monsoons of India and East Asia.

The permafrost regions of Asia (largely in Russia) have shown a rapid thawing and decreasing depth consistent with global trends. The glaciers in Asia are melting at an almost constant rate since 1960s (a few at faster rate in Himalayas and few have negative rate as in central Karakorum due to increased precipitation,: Hewitt, 2005).

Increase in temperature and decrease in precipitation (along with increase in water use due to demand increase connected with population growth and industrialization), have led to water shortages and drying of lakes and rivers in many parts of Asia.