P6

Question 1

The impacts of climate change on the hydrological cycle

Answer 1

Most scientists agree that climate change (global warming and oscillations such as ENSO) will result in an intensification, acceleration or enhancement of the global hydrological cycle.
As the impacts of climate change vary around the world, with differential amounts of temperature increase and varying changes to totals and distribution of rainfall, there will be differential changes in the way the hydrological cycle (open system operates within the world’s drainage basins. There will be different impacts on drainage basins in different climate zones of the world, for example, caused by differential precipitation and run-off predictions. Therefore, decision makers in various countries will have to cope with changes in water budgets, and this will impact on the way climate change is managed to secure water resources for the future.

Question 2

Modelling climate change trends tends to be complex for several reasons:

Answer 2

• Climate dynamics - the way the atmosphere, ocean, terrestrial, cryosphere and biosphere systems all interact with each other - is still only a partially understood science
• As a result of teleconnections, in some instances it is difficult to distinguish between the impacts of oscillations such as ENSO and climate warming.
• Global records are very incomplete; in many parts of the world there is insufficient depth or detail of evidence to establish reliable trends for the impact of climate change or to make firm predictions about the future.

Question 3

Figure 2.17 How climate warming modifies the hydrological cycle

Answer 3

Question 4

Summary of key findings relative to trends in water cycle components

Answer 4

Precipitation input

Evaporation and evapotranspiration
Soil moisture
Run off and stream flow (a 1°C rise in temperature could increase global run off by 40%)

Groundwater flow

Reservoir, lake and wetland storage

Permafrost
Snow
Ice
Oceans

Question 5

Precipitation input

Answer 5

The mode of the precipitation may be more important than the mean precipitation in determining hydrologic impacts.
Widespread increases in intense rainfall events have occurred although overall amounts remained steady or even decreased.
Areas of precipitation increase include the tropics and high latitudes, with decreases of 10-30° north and south of the equator. At the same time, length, frequency and intensity of heat waves has increased widely, especially in southern Europe and southern Africa. This has led to an increase in drought occurrence. With climate warming more precipitation now falls as rain, not snow, in northern regions. All this is consistent with a warmer atmosphere with a greater water-holding capacity.

Question 6

Evaporation and evapotranspiration

Answer 6

Some research suggested that in large areas of Asia and North America actual evaporation is increasing, although increased cloud cover from increased water vapour may work against this.
Transpiration is linked to any vegetation changes, which are linked to any changes in soil moisture and precipitation as well as increasing transpiration, which makes vegetation more productive.

Question 7

Soil moisture

Answer 7

Results are ambiguous here - the amount of soil moisture is related to many factors, of which climate change is only one. Where precipitation increases, it is likely that soil moisture will also increase.

Question 8

Run-off and stream flow (a 1°C rise in temperature could increase global run-off by 40%)

Answer 8

Evidence is developing to suggest that, along with more climate extremes, there will be an increase in hydrologic extremes, with more low flows (droughts) and high flows (floods). An accelerated cycle with more intense rainfall will increase run off rates and reduce infiltration. There are marked decreases in the continental interiors of the Mediterranean, Africa and the US South West.

Question 9

Groundwater flow

Answer 9

Evidence is again limited, with no definitive link between groundwater amounts and climate change, as human abstraction is the dominant influence on supplies, especially for agriculture.

Question 10

Reservoir, lake and wetland storage

Answer 10

Regional variations in lakes and reservoirs have been linked to regional changes in climate, for
example in Lake Chad.
Changes in wetland storage are occurring, but they cannot be conclusively linked to climate change. Wetlands are affected where there are decreasing water volumes and higher temperatures.

Question 11

Permafrost

Answer 11

temperatures.
Permafrost
Changes in the physical climate at high latitudes, primarily increasing air and ocean temperatures, are leading to permafrost degradation in northern areas. With the deepening of the active layer this has an impact on groundwater supplies and also releases methane from thaw lakes, which leads to positive feedback and accelerating change.

Question 12

Snow

Answer 12

Most studies suggest that the length of the snow-cover season has decreased, especially in the northern hemisphere and, in the last 50 to 100 years, spring melt has occurred earlier, possibl accelerating in the last decade (with corresponding changes in river regimes).

Question 13

Ice

Answer 13

There is strong evidence that glaciers have retreated globally since the end of the ‘Little Ice Ag with downwasting (thinning of a glacier due to the melting of ice) accelerating in most areas sir the 1970s. This is the result of rapid temperature increase and changes in the precipitation tyi (more rain/less snow). Tropical high-altitude glaciers, e.g., in the Andes, have shown the most rapid changes, leading to low flow from a dwindling cryosphere supply.

Question 14

Oceans

Answer 14

Work on measuring sea surface temperatures has lagged behind land-based research, but in areas of ocean warming increased evaporation will occur, and there is limited evidence that mc cyclones are generated.

Question 15

Future trends - more global deluges and global drought
Floods and the future

Answer 15

Scientists generally agree that the hydrological cycle will intensify and that extremes will become more common.
The moisture-holding capacity of the atmosphere has been increasing at a rate of about seven per cent per degree Celsius of climate warming, creating the potential for heavier precipitation. There have likely been increases in the number of heavy precipitation events in many land regions.
Figure 2.18 shows the situation globally for 2010-11, with 2010 being the wettest year ever recorded (Figure 2.19).
Questions are inevitably asked after such freakish weather: is climate change to blame, and is there worse to come? Heavy precipitation events have certainly led to spectacular flooding, with economic losses rising ten-fold between 1990 and 2010. The fact that hydrological disaster losses have grown much more rapidly than precipitation or economic growth suggests that a climate change factor may be involved, although socio-economic factors such as land-use changes and greater use of vulnerable areas also play a huge part.
However, documented flood figures show no clear evidence of trends in either increasing frequency or magnitude of flood events globally. One study did show increasing frequency of ‘large’ floods in sixteen large basins across the globe during the twentieth century, with more 1-in-100-year events, but another study on global change in river flows only showed positive trends in Europe, and only for late autumn and winter floods.
Some floods from melting snow had actually decreased as the snow has disappeared.

Question 16

Low flows and drought in the future

Answer 16

Climate change is expected to influence precipitation, temperature and potential evapotranspiration and, through their combined effects, the occurrence and severity of droughts. In the past 30 years droughts have become more widespread, more intense and more persistent. The problem is that it is difficult to disentangle the possible impacts of climate change from those of human influences, such as land-use changes, and usual multi-decadal climate variability (see the Sahel case study on page 26).
However, more intense droughts have affected more people, and links to higher temperature and decreased precipitation have occurred (see the Australia case study on page 27).
Research studies have proved inconclusive globally but some regions, such as southern Europe, south-western USA and the Sahel, do seem to have a higher incidence and intensity of drought. A further issue is that the occurrence of the droughts seems to be determined largely by changes in sea surface temperatures, especially in the tropics, through associated changes in atmosphere circulation and precipitation amounts. In the western USA, the recent drought seems to be associated with diminishing snow pack in the mountains and the resultant reductions in soil moisture.
A further complicating factor for the occurrence of both floods and droughts is the ENSO (see page 25), which is associated with both extreme flooding in some areas and extreme droughts in others. Figure 2.20 shows the pattern of droughts and floods characteristically associated with an El Niño event. The impacts of the ENSO therefore also have to be considered when analysing future trends.
Other oscillations, such as the North Atlantic Oscillation (NAO), may also contribute to the complex picture.

Question 17

potential impacts of short-term climate change on water supply.

Answer 17

In conclusion, decision makers concerned with water planning and disaster management have to factor in the possibility of more extreme weather events - but there is no definite link yet made to climate warming as sufficient research has not yet been done. Countries developing their water-management strategies for the future need to consider a number of drivers of pressures on water, of which climate change is just one (Figure 2.22).
Alongside natural forces affecting water resources are many new human forces that affect the planet’s water system, often related to human activities and economic growth. Water is unique among natural resources as it is vital for both ecosystem and human well-being.