V.U.B. Master in Human Ecology, Academic year 2003-2004
The Effects of Global Warming on the Hydrologic Cycle
Thomas Deflo | Yuhua Bai | Ang Li | Katarina Tuharska
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1. The water molecule
In order to understand how the state of the hydrologic cycle will change as water
molecules are affected by a rise in temperature, it is advised to fully fathom the unique
behavior of the water molecule H2O.
Water molecules are polar molecules: they have two oppositely charged atoms. Within
each water molecule, hydrogen binds with oxygen atoms through regular electron
sharing. As a result, two hydrogen atoms have their positively charged proton core partly
exposed. The negatively charged electrons from the oxygen atoms bind to these free
protons. This combination is a so-called hydrogen bond, allowing water to remain in the
liquid, solid state at higher temperatures than any oth er liquid – up to 100°C.
Hydrogen bonds (in brown) give water its unique liquidness.
This doesn’t mean that water can not reach the gaseous state below its high boiling point.
When solar radiation hits water on the Earth surface, some water attains the gaseous
phase, because molecules near the surface have enough energy and freedom to overcome
the relatively weak hydrogen bonds of their neighbors. As surface temperatures rise, the
number of energetic molecules to jostle and escape the water’s surface tension will be
greater, and evaporation will be more rapid. Evaporation is also greater with more surface
area and air circulation, such as in oceans, because more space is available for the vapor
to turn away from the liquid phase.
The hydrogen bonds between water molecules give water a particularly large heat
capacity, as they allow a relatively great vibration between molecules. As a result, much
more latent heat is absorbed and released when water changes phase than with other
liquids. Latent heat first occurs inside water vapor from the ocean to the atmosphere.
When water vapor rises and cools, water molecules condensate into clouds. The amount
of energy that is ultimately released far from the source of evaporation, when clouds rain,
is exactly equal to the amount that was absorbed when the water vaporized.
This latent heat capacity of water, thanks to hydrogen bonds, is very important in Earth's
climate: liquid and gaseous water serves as a transporter of energy over the globe,
influencing local temperatures. We can say that the hydrogen bonds inside water
indirectly shape the Earth’s climate. Over one third of the Earth’s net radiation goes into
the phase change of liquid into gaseous water.
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2. The hydrologic cycle
Water molecules can take on three molecular phases of ice, liquid water, and water vapor.
The hydrologic cycle is the perpetual movement of water in all these phases on Earth.
This dynamic water transport is driven by radiation, thermodynamic circulation, and
gravity. All water now present on Earth has been recycled for billions of years, with
virtually no new water created or lost since.
Schematic representation of the hydrologic cycle.
2.1. Quantities of the hydrologic cycle
Earth's total water volume is 1 360 000 000 km³ (one km³ contains 106 liters). Of this
volume, the following storage can be distinguished (in km³):
oceans
glaciers, icecaps
groundwater
lakes and rivers
water vapor
1320 x 106
25 x 106
13 x 106
250 x 103
13 x 103
Annually, the hydrologic cycle moves 496 x 103 km³ of water around the Earth.
Evidently, the amount of water is in balance between the different components of the
cycle: input and output are equal. This is essential for the cycle to remain harmoniously
circular. For instance: if evaporation would not fully replenish the atmosphere, the latter
would dry up in ten days. Or if precipitation from the atmosphere into the oceans would
grind to a halt, the Earth’s sea level would lower by 1 meter per year. The mean residence
time of water molecules is about 10 days in the form of atmospheric vapor, but about
3 000 years in liquid water. The atmosphere has a very short memory compared with the
longer memory of the oceans.
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The hydrologic cycle involves evaporation, transpiration, condensation, precipitation, and
runoff of water. Let us follow through the quantitative fluxes of these components. We
can say that the cycle starts with evaporation of water into the atmosphere from the main
storage sinks on the planet’s surface: 85% of water vapor comes from the oceans (425 x
103 km³/y), 25% rises up from water bodies and transpiration by vegetation, a
combination called evapotranspiration (71 x 103 km³/y). A very small portion of water
evaporates through sublimation, when water changes from ice or snow into a gas. Most of
the evaporated water rains back down into the seas (385 x 103 km³/y), while (40 x 103
km³/y) starts its travel through the atmosphere by horizontal air advection and joins the
(71 x 103 km³/y) of evaporated water from inland water bodies. This water vapor (111 x
103 km³/y) then cools off as it rises in the atmosphere up to 50km, and because of this
cooling, a phase change in the water molecule appears from gas into liquid – the reverse
phase change from evaporation: water condensates into clouds. The condensed water
droplets inside the clouds become more heavy, and then fall down to the surface by the
effect of gravity. This precipitation can have different densities, dependent on the
temperature inside the clouds: rain, partially frozen raindrops, snow, or hail. Of this
precipitation, the majority (71 x 103 km³/y) infiltrates and percolates deep into the
ground, to later return to the oceans via groundwater flow – slowly but eventually. The
remaining (40 x 103 km³/y) of precipitation becomes land runoff, to be absorbed as land
moisture and evapotranspirate again, or to be returned back to the inland and oceanic
water bodies through rivers and streams.
2.2. Components of the hydrologic cycle
As water covers 70% of the Earth, the hydrologic cycle constitutes the planet’s major
biogeochemical system. Produced by the Sun, and together with features offered by
lithosphere and atmosphere, water transport shapes and sustains the Earth’s biosphere.
The transport of water irrigates surface vegetation, renews freshwater resources, sustains
vital food chains, provides sanitation, heating and cooking, produces energy, cleans up
bacteria, and transforms the land. Its influence on the global climate is both complex and
sensible to change. Water determines life on our planet, and the water cycle provides it.
2.2.1. Oceans
The hydrologic cycle is the main mechanism for the distribution of solar heat around the
globe, fundamentally shaping the global climate. Solar radiation is first and foremost
absorbed by the oceans, as oceanic heat capacity is much greater than that of land
(because of the water molecules’ hydrogen bonds). This oceanic heat capacity is then to
be transported, along ocean currents, to other latitudes around the globe. Ocean currents
thus moderate the surface of the Earth's overall temperature. Heat budgets in seawater
create fundamental variability in temperature conditions: due to the heat absorbing
capacity, land closer to the sea will have seasonally fewer differences of +/-10°C, while
temperatures far inland can fluctuate up to 60°C (e.g. in Siberia). Solar radiation and
ocean currents are thus in a constant combine to shape surface temperatures.
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Map of major ocean currents around the globe. These currents result from a transport of energy towards the poles in
atmosphere and water. Warmer currents originating from the tropics are apparent.
2.2.2. Water vapor
The next component in the hydrologic cycle occurs in the atmosphere. Evaporation
requires absorption of heat from the surrounding air and stores it as latent heat. By doing
so, the water cycle helps to cool off the surface of the Earth. This is of central importance
to our study topic – how the permanently residing of water vapor in the air, before the
condensation phase, is chiefly responsible for the natural greenhouse effect, rising earth
surface temperatures to a habitable level. Water vapor is the main greenhouse gas, as it is
highly absorptive to reflected infrared radiation. Distribution and absorption of heat in
the atmosphere by water vapor is, in other words, another crucial biogeochemical aspect
of the planet.
2.2.3. Precipitation
Next after evaporation, condensation into liquid water occurs by cooling convection. The
latent heat is released again by this condensation into the atmosphere, as the reverse
physical process of evaporation. The heat then becomes sensible with precipitation: at an
atmospheric saturation point in moisture levels (relative humidity 100%), rain falls out of
the denser clouds with gravity. Rain is more likely to fall above mountainous regions.
Consistent rainfall above maritime, or seasonally concentrated rainfall above continental
coordinates exist, dependent on the distance the condensed water can travel before it rains
down.
The importance of this final phase change in the hydrologic cycle, in which the
desalinated water becomes liquid again, can not be overstated. Life depends on it. It is
also essential in keeping the hydrologic cycle intact, as the 40 x 103 km³/y of oceanic
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water vapor that moved land-inward is returned through streams and rivers to its source,
thus keeping ocean levels equalized.
This does not mean that the amount of precipitation around the globe is equally
distributed. Average annual precipitation for the world is estimated to be 2,9 mm per day,
but large variations exist around the mean, mostly due to latitudinal differences in
insolation levels. Tropical latitudes next to the equator receive most of the solar radiation.
This has a direct effect on the amount of water movement upward into the atmosphere.
Indeed, precipitation levels are the highest above these latitudes. The remaining dried up
air then moves by advection and falls down above higher latitudes (30° N & S). These
arid zones receive the least of precipitation. Other precipitation-poor regions are the polar
areas, where cold air (because of low insolation levels) prevents longer stays of water
vapor.
Integration of mean observed
precipitation levels from 25
years up to 1997 in mm per day
[CMI 2003]. Tropical latitudes
have the highest precipitation
levels. Subtropical and polar
regions have the lowest. Indeed,
most of the world’s deserts occur
at 30°N and 30°S.
2.2.4. Groundwater and surface runoff
As indicated, most of the evaporation rains back into the oceans. Water that rains on the
land as freshwater provides its vital drinking water to the biosphere. It has a series of
destinations. River and lake beds can be replenished by rain. Rain can infiltrate into the
soil and reside in different materials as soil moisture, to be taken up by vegetation and
fauna, or to freeze (for at least two years) as permafrost. If it is drained deeper into the
ground, while being filtered through the soil layers, some water can return as throughflow
to the oceans or percolate deeper and become groundwater, to be contained in deep rocky
aquifers, well up to the biosphere in natural springs, or be retrieved by artificial pumping.
The horizontal movement of groundwater flow will find its way back to the oceans.
Surface runoff occurs when infiltration can not keep up with precipitation, and water
forms streams or rivers. This runoff flows back into bigger water bodies on land or
towards the sea. This runoff balances the hydrologic cycle by replenishing the oceans
with the exact amount of water that left land inwards as vapor. Excess of water over the
river beds leads to flooding. This is a normal phenomenon, and fertilizing to the
surrounding grounds, but exacerbated by anthropogenic land-use change such as
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deforestation, river straightening, wetland transformation, and global warming. We now
turn to the latter and then assess its influence on the described hydrologic cycle.
3. Global warming
The relationship between temperature and the hydrologic cycle is the central subject of
this study. As we have seen, the hydrologic cycle dominantly affects the temperature
distribution of the globe, over ocean, atmosphere, and land. Simultaneously, temperature
levels fundamentally influence the hydrologic cycle. Changes in the former are bound to
engender changes in the latter – and vice versa. By investigating the condition of the
current evolution in global temperature, we can deduce how the hydrologic cycle will
respond to it, and in turn influence it.
Of all insolation reaching the Earth, 51% is reflected or retained in the Earth’s
atmosphere, before 49% is absorbed by the surface. Light hitting the surface gets partly
retransmitted in the form of longer waves, or infrared radiation. Gases in the atmosphere
with dense molecular structures capture this radiation and scatter it through the air and
back to the surface. Infrared radiation that is trapped by these gases in the surfacetroposphere region (0-16km) warms up the Earth. This is the so-called ‘greenhouse’
effect, thanks to which the mean temperature is +14°C at the Earth’s surface instead of 19°C.
Atmospheres of neighboring planets contain much more (Venus), or much less (Mars) greenhouse gases.
Consequently, their surface temperatures are higher or lower.
Greenhouse gases are water vapor (H2O), carbon dioxide (CO2), methane (CH4), and
nitrous oxide (N2O). These gases are added to the natural state as byproducts of human
activity. CO2 is responsible for 60% of the contribution to the anthropogenic greenhouse
effect.
The natural carbon cycle releases CO2 into the atmosphere by biosphere and oceanic
sinks who then reabsorb it. This is a balanced cycle. Human activities add to this cycle by
fossil fuel combustion and by reducing the cycle’s sink potential through land-use
(mainly deforestation). The human burning of fossil fuels by industry, households and
transport releases up to 6,3 gigatonsi of CO2 per year, while human land-use change
prevents +/- 1,7 gigatons of CO2 uptake per year. This results in a net addition of up to
3,2 gigatons of CO2 in the atmosphere per year. As a result, atmospheric concentrations
i
Gigaton : billion metric tons.
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of CO2 since the last two centuries have not likely been exceeded for 20 million years,
and if present trends continue, their levels could quadruple by 2100.
Changes in solar radiation, volcanic dust emission, terrestrial magnetism, climatic
feedback mechanisms and rotational or orbital shifts, have been natural sources for
temperature variability for the past millions of years [Goudie 1992 pp. 255 – 276],
resulting in glacial and interglacial periods. But current warming trends are unusually
rapid and self-reinforcing. Anthropogenic greenhouse gases will exacerbate the natural
greenhouse effect by their radiative forcing, without any counteracting mechanism in
place. The opposite is true: the greenhouse effect is expected to reinforce itself by
positive feedback processes.
The ‘radiative forcing’ of a gas is its influence on the balance of energy in the
atmosphere, expressed in Watts per square meter (Wm-2), and a positive radiative forcing
means that more energy remains in the atmosphere than is emitted. The radiative forcing
by the anthropogenic greenhouse gases make the average temperature over the surface of
the Earth climb. Global meteorological data are concordant with this: parallel with the
increase in CO2, Earth’s mean surface temperature has increased by 0.6(±0.2)°C, likely to
be the greatest temperature rise in 1000 years [IPCC 2001]. From their current level of
14°C, temperatures will very probably rise with 1.4°C to 5.8°C by 2100, due to increased
positive radiative forcing.
Variations of the Earth’s surface
temperature since measurements
begun (a), and over the last
millennium from proxy data (b)
[IPCC 2001]. The temperature
increase in the 20th century is
likely to be unprecedented for at
least 1000 years.
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4. Effects on the hydrologic cycle
4.1. Oceans
4.1.1. Sea level
120 000 years ago, just before last ice age, the mean global temperature was higher than
today, and average sea level altitude was 5-6 meters higher. This was caused by the
natural melting of Greenland and Antarctic ice sheets, over a long period of time. Current
global warming trends are exceedingly faster. In one century, global average sea level
rose between 0,1 and 0,2 m, and could increase up to 0,94 m by 2100. After that, many
scenarios predict a continuing increase in sea levels.
4.1.1.1. Thermal expansion
First of all, global warming induces the thermal expansion of surface waters. When
surface water is heated, its molecules absorb thermal energy and express it as kinetic
energy. As does any other material, it will expand because of this. There is widespread
agreement that sea levels will rise as a result, but scientists are uncertain how the thermal
expansion will be distributed over the oceans. As thermal expansion has a lower effect in
cooler than in warmer water, sea level rise is expected to vary with geographical position,
e.g. northern Europe is less prone to thermal expansion than southern Europe [Houghton
1998].
Because of the large heat capacity of water, sea level rise is projected to continue for
hundreds of years, even after stabilisation of present greenhouse gas concentrations.
4.1.1.2. Melting
a) Glaciers: The amount of water evaporating and precipitating over land is surmounted
by runoff quantities from melting freshwater bodies. The hydrologic cycle is thus
quantitatively disturbed, with more water put into the oceans. Sea levels rise because of
this. Northerly continents have more to fear from melting of glaciers and ice sheets than
from thermal expansion. Globally, glaciers are projected to lose about 25% of their mass
by 2050. In the European Alps, about half of the original ice volume has already been lost
since 1850, while almost all existing European glaciers could disappear over the next 100
years, with many of the smaller ones already disappearing within decades [Haeberli &
Hoezle 1995]. Glacial ice in the Austrian Alps is more reduced today than any time
during the past 5 000 years. Similar melting is visible in Spain, Russia and many other
regions with glaciers: in Mt. Kenya's largest glacier has almost disappeared. Since 1912,
82% of Kilimanjaro’s ice has melted, with about one-third melting in just the last dozen
years. At this rate, all of its ice will be gone in about 15 years.
b) Ice sheets: Melting ice sheets will contribute even more to sea level rise. In the Arctic
Ocean, satellite measurements indicate that ice cover has decreased by about 7% per
decade since 1978. The potential for sea level rise from melting ice sheets is dramatic.
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There are predictions that if Antarctic and Greenland ice sheets would melt in the next
hundred years or more, it would change global sea levels up to 5 meters [Houghton
1998]. Indeed, the Larsen ice sheet of Antarctica is only one of the ice sheets that started
to melt. West Antarctic ice sheets has been steadily melting since the end of the last ice
age. Global warming will speed the process, and will continue to steer it for thousands of
years after greenhouse gas concentrations will have been stabilised.
4.1.2. Thermohaline circulation
The danger with global warming is that surpassing physical threshold levels could lead to
the climate entering a new state. Global warming will also lead to nonlinear responses in
the oceanic system. The oceanic body seems vulnerable to temperature triggers
overarching a certain capacity. By 2100, a quadrupling of CO2 concentrations, with
subsequent temperature climbing of 1,4°C to 5,8°C, could lead to the collapse of the
thermohaline circulation.
The Great Ocean Conveyor Belt is a large oceanic current, driven by differences in ocean
density. It originates in the central Pacific, travels above the north coast of Australia,
rounds the southern tip of Africa, to move into the North Atlantic. The current becomes
the Gulf Stream by the time it passes the Gulf of new Mexico. The conveyor belt cools
down as it releases heat above the European continent by evaporation and precipitation.
As it loses water to evaporation from its surface layer, it becomes increasingly salty.
Around the latitude of Iceland, the stream becomes so dense that it sinks and flows back,
along the ocean floor, rounding South-Africa and Australia, into the Pacific, where it is
warmed and pushed back up to the surface to restart.
Global circulation pattern of the Great Ocean Conveyor Belt, with warmer waters on the surface, and cold
waters deeper down.
Due to global warming, Arctic sea ice is bound to melt, and precipitation above the
northern oceans is bound to increase. This adding of freshwater into the oceans will
interfere with the conveyor belt, as it slows down in the North-Atlantic. The system
depends on high salt concentrations near the water surface. Melting the ice dilutes the salt
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and weakens the pumping effect of the stream. By further diminishing the density of the
North Atlantic water, the Gulf Stream could be halted. Most global climate models show
how this could lead to the cooling of the European climate, with temperatures dropping
by 5°-10°C, leading to a Little Ice Age, as in the 15th century.
General Circulation Model reflecting changes in surface air temperature, caused by a modelled halt in deep,
saline water formation in the North Atlantic [Rahmstorf 2002].
Furthermore, the Great Conveyor Belt could be altered more widely. As more tropical
water vapor moves towards the north pole and precipitates above the northern
hemisphere, the input of fresh water into the North Atlantic ocean could increase up to a
point where the thermohaline ocean belt would be lacking so much saline, heavy water,
that it would be cut through, prevented to return southwards, and collapse altogether
[Marotzke 2000]. This would definitely further change the regional patterns of
temperature, evaporation and precipitation. These changes could lead to a completely
different climate in many parts of the world.
4.1.3. Ocean-atmosphere interactions
Meteorological oscillations between ocean and atmosphere are bound to be influenced by
global warming. One such interaction between water and air is the inter-annual
occurrence of El Niño. This climate phenomenon is a shift in air pressure above the
Pacific ocean: high pressure develops above the southern Pacific and low pressure above
the central Pacific. This alters trade wind patterns and dependent ocean currents. Heat
that is normally blown westward is concentrated at Eastern and central Pacific parts of
the ocean. Of the 29 El Niños that have occurred since 1700, the 1982-83 El Niño was
the strongest. It caused droughts in Africa, Australia, India, Indonesia, and the
Philippines, flooding in Peru and Ecuador, and devastating storms in California.
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Normal situation (above) and El Niño phenomenon (below) over the Pacific Ocean [SOEST 2004]. Trade
winds heading west collapse or even reverse. The warm water of the western Pacific flows back eastward,
and sea surface temperatures increase significantly off the western coast of South America.
The steep rise in temperature the past century has probably lead to an exacerbation of the
El Niño phenomenon, whose behavior is stronger and more frequent from 1976 than
since the last 100 years. The year 1997 gave another record El Niño event, when global
temperature records were broken for sixteen consecutive months. Unusual floods, strong
cyclones and droughts are the result. The circulation of heat away from the tropics is a
natural phenomenon, regularly concentrated in the El Niño event. But as global warming
pushes more and faster evaporation upwards from the tropics, with its accompanying
latent heat, more ‘fuel’ would be delivered for El Niño to develop. Even if global
warming would have little influence on the El Niño amplitude, it is likely to worsen the
meteorological phenomena that occur when El Niño appears (more heavy rainfall, and
risk of severe droughts and floods).
4.2. Water vapor
Although measurement problems hinder the analysis of long-term water vapor changes,
recent studies confirm that evaporation increases. Knowledge about changes in water
vapor at upper tropospheric and lower stratospheric levels is of great importance because
positive radiative forcing can result from them.
As sunlight evaporates water, heat is released from the water in the form of infrared
radiation. A small part of this radiation escapes into space, but most of it remains trapped
inside the water vapor and radiates back to the surface of the Earth. This effect is chiefly
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responsible for the natural greenhouse effect. Indeed, water vapor is the most important
greenhouse gas.
Absorptivity of infrared electromagnetic radiation by H2O. Water molecules absorb much of the radiation
between wavelengths of 0,7 to 20> µm.
As surface temperatures increase with global warming, more evaporation of water will
occur over the oceans. More evaporation is related to increasing temperatures. Hence, the
amount of water vapor in the atmosphere will increase. By its greenhouse potential,
additional water vapor will further exacerbate global warming effects. Evaporation will
lead to more evaporation; the process intensifies itself.
Water
evapora
tion
Radia
tive
forcing
Surface
warming
Clockwise scheme of the positive feedback effect on water vapor by global warming.
Warmer air can also hold more vapor: the amount of water needed to saturate air
humidity increases exponentially with temperature. Global warming will thus allow
higher humidity levels in the atmosphere. This has been shown to stimulate
evapotranspiration.
4.3. Clouds
Between evaporation and precipitation, water molecules first condensate into clouds.
Cloud formation has an important influence on the radiative balance of the Earth,
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particularly in the atmosphere: by keeping solar radiation from reaching the surface,
clouds keep the Earth at a cooler temperature. Simultaneously though, clouds prevent
reflected solar radiation from escaping into space and have a warming effect. Overall, the
cooling effect is estimated to be dominant: clouds reflect about 50 Watt/m2 of incoming
solar radiation back to space (half the planet’s albedo), while their absorption of heat
keeps 20 Watt/m2 in the atmosphere. This is another crucial climatologic feature of the
hydrologic cycle, and varies with cloud typology.
The formation of clouds is part of a negative feedback system. It is likely that there has
been a 2% increase in cloud cover over mid- to high latitude land areas during the 20th
century. This translates well into an observed decrease in daily temperatures. The extra
formation of clouds by global warming can be assumed to have a net cooling effect on
the Earth’s climate. The process of global warming thus reduces its own origin. But as we
said, cloud typology determines their radiative effect. Global warming can also drag
cloud formation into a positive feedback system. As temperatures rise, an increase in high
clouds will result, which are efficient at reflecting long-wave radiation back to the Earth’s
surface, while letting short-wave radiation through. This will lead to further warming of
the atmosphere.
4.4. Precipitation
[IPCC]states that ‘the influence of warmer temperatures and increased water vapor in the
atmosphere are not independent events, and are likely to be jointly related to increases in
heavy and extreme precipitation events’.
Overall, global warming will lead to an increase in global precipitation levels – about
+2% since the beginning of the 20th century – as more water vapor will be available to
condensate and deliver water in liquid form, back to the surface, as rain and snow.
However, this increase is unevenly distributed across the climatologic areas of the planet,
and over time, as well as varying with altitude. Topographical factors such as mountains
and soil types locally shape hydrologic cycles. Some areas will receive more, others less
rainfall. What is certain, is that global warming will reshape the distribution of
precipitation patterns, likely to become much more variable, with more deviations from
the mean levels. As a result, a less steady input of water into the ground is apparent over
many areas, leading to reduced flow regimes.
For the northern hemisphere, [IPCC 2001] considers it very likely that global warming
has increased precipitation with 10% since the turn of the century, with a growing 2 to
4% of extreme rainfall. In 1998, the northern hemisphere (55°N and higher) had their
wettest year on record. Over the tropics, more extreme precipitation patterns are observed
in the past century. The Asian southwest monsoon, for instance – one of the most
important climate systems in the world – is more extreme. This is because it is generated
by the heat capacity contrast between land and sea, and global warming extrapolates this
contrast, as the heat capacity of water is greater than that of land.
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But rainfall has decreased over the sub-tropics (10°N to 30°N) by about 0.3% per decade,
leading to more droughts and desertification, especially in summer. This is explained by
an exacerbation of the Hadley cell; the main atmospheric current of warm, humid air
rising above the tropics and drying up before reaching subtropic latitudes. This will
worsen by the overshadowing of precipitation levels by evapotranspiration levels in arid
areas.
Global annual
precipitation trends
during the previous
century [IPCC 2001].
It is noteworthy that at the stage of desertification, a positive feedback system sneaks in:
desertification means a decrease in vegetation, which provides the atmosphere with less
evapotranspiration, hence less precipitation, which further worsens local droughts.
Deforestation in the Amazon, for instance, shows this evolution towards lower
precipitation levels because of dryer land cover [Pagano & Soroosh 2002] and less
available moisture. Once an area has been made into a desert, it is not likely to change
without external aid, on the contrary: it is more likely to dry up further.
4.5. Groundwater
Groundwater in arid lands is considered to be a stable water source, if its drainage or
evaporation rate doesn’t exceed its natural recharge rate. The combined effect of higher
evaporation and less precipitation from global warming on the land surface, means that
more water leaves the basins than is replenished by rain and runoff. At the same time,
social drainage continues, which can make the systematic input of water into the water
body crash. Recharge of aquifers, which occurs primarily through seepage of water from
streams, is exceeded by evaporation, because of recurrent heat waves. Similarly, soil
moisture and streamflow are more rapidly depleted.
Water bodies in centrally located, continental areas are very vulnerable to this imbalance:
with a 1° to 2°C increase in annual surface temperature, with increased evaporation, and
with a 10% precipitation reduction, a 1,5 to 2-fold decrease in water resources is
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envisaged. Indeed, many great lakes across the world manifest a decrease in net basin
levels, and continents such as North-America have suffered from the most severe
droughts in a 100 years.
4.6. Surface runoff
As a consequence of global warming, extreme weather events like floods and droughts
are projected to increase, in all parts of the world. Regions that did not suffer from these
events will likely be affected. Regions already suffering from these events will not be
spared from more extremities. Floods and droughts are the result of the hydrologic cycle
becoming less regular and quantitatively more unevenly distributed. Continents were
recently affected with both unusual droughts as well as unusual floods.
4.6.1. Floods
Global streamflow trends are directly proportional to global precipitation trends. Global
warming has led to increased precipitation at higher latitudes, especially in winter
periods. In these regions, river beds are liable to overflow. Small increases of water to
already saturated soils can cause large increases in runoff, resulting in floods.
Many low latitude rivers reach their maximum during raining periods over equatorial rain
forests. This makes them vulnerable to floods. Even with existing dams and reservoirs,
the increasing frequency in heavy rains could not be handled by them.
Per year, 20 billion tons of soil and rock are eroded to be exported to the oceans [Deléage
2001]. As floods worsen, more land would be eroded, which would in turn reduce the
river beds, allowing more flooding – one of the many positive feedback mechanisms
induced by global warming. River deltas and their surrounding land are under menace of
permanent overflow. As an example, 20% of the Nile river delta is under threat of being
flooded [Titus 1989].
A projected increase in extreme rainfall events during winter and spring, when soils are
saturated, could increase the frequency and severity in floods. An increase in large-scale
precipitation might also lead to increased flood risks of large river basins.
4.6.2. Droughts
The developing global warming scenario shows a radical decrease in rainfall in several
other parts of the world, especially in the summer periods. Warmer global temperatures
will cause an intensification of evaporation and melting over land, leading to greater
drying of soils and vegetation. Although the projections for rainfall vary considerably by
region, droughts are expected to be longer-lasting and more severe because of higher
temperatures and increased evaporation.
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In arid, continental regions, river runoff will be profoundly reduced by global warming.
With a 1° to 2° C increase in annual air temperature and a 10% precipitation reduction, a
40 to 70% reduction of annual river runoff can be expected in these regions.
Snow and ice contribute fundamentally to major rivers. We have seen that changes in the
cryosphere, with warmer temperatures and melting of glaciers, is happening fast. Over a
short time, major rivers like the Rhine could decline in flow by 10% during the summer,
as less snow is replenished. The Nile, which is very dependent on mountain snow from
Ethiopia, could also be altered drastically in its flow regime.
Seasonal rivers are unlikely to see their input of water maintained, with lowering patterns
of precipitation and even more extensive usage of their waters, as droughts occur
elsewhere.
5. Conclusion
Understanding of hydrologic processes and their incorporation in temperature models
have improved over the years. However, because of the nonlinear nature of the global
warming effects, precise biogeochemical dynamics are difficult to predict. What is
undisputed, is that the hydrologic cycle will be both quantitatively as well as
qualitatively affected, undiscriminating over all continents. In time, global warming
could give the planet a new hydrologic distribution. Presently, the clearest changes will
result in more extreme events.
While considerable caution exists to give an exhaustive picture of all components of the
hydrologic cycle under global warming conditions, certain trends are already visible.
Overall, the intensity of the cycle is strengthening, but its regularity is weakening. A
patchwork of hydrologic anomalies at several stages exist, dependent on latitude and
local climate conditions. General conclusions are hard to shape, but we can safely
conclude that sea level, evaporation, precipitation and runoff have increased in most parts
of the world, but that this has not happened in a regular fashion, effectively leading to
more drinking water shortages and reduced flow regimes.
Anthropogenic radiative forcing is most worrying by its positive feedback processes. By
concentrating on temperature effects on the hydrologic cycle, we have discovered clear
examples of this. Temperature and water systems are engaged in a positively correlated
relationship, of which the long-term result is uncertain but potentially very destabilizing
for the Earth’s climatic baseline conditions. Think of the possible shutdown of the
thermohaline circulation of the world’s oceans.
As for hydrology, most water management planning is made more fragile due to the
introduction of more extremities from global warming in the hydrologic cycle. Unusually
wet or dry events will affect reliability of models to estimate water fluxes. Predicted
magnitudes cannot take future extreme events into account when assessing risk scenarios.
The instability of the hydrologic cycle will make hydrographic practice more difficult.
18
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