Geoscience Reference
In-Depth Information
lakes are used to store water, while dams and other structures are used to control water
flows in rivers. Water is heated and cooled and, as a strong solvent, it is polluted in many
areas.
Many physical scientists have tended to ignore the latter aspects and deal mainly with
the climate system either in its ''natural'' state or as changed by human activities by mainly
accounting for increased greenhouse gases and changing atmospheric particulates (IPCC
2007
). Even in this somewhat simplified framework, it has been challenging to simulate the
hydrological cycle. For example, global reanalyses of most existing observations have
substantial shortcomings in representing the hydrological cycle (Trenberth et al.
2011
).
Such shortcomings arise because, while observations are assimilated to ensure a realistic
representation of atmosphere and some Earth surface processes, the analysis increment
ensures that water is not conserved and sources of moisture for precipitation may come
from the increment and not evapotranspiration. Models generally have a lifetime of water
in the atmosphere that is too short, and this affects their ability to transport water vapor
onto land while they tend to recycle moisture locally more than observed.
The main impacts of a warmer climate on global water cycle include the following:
• With warming, higher atmospheric temperatures increase the water holding capacity of
the atmosphere by about 7 % per degree Celsius (e.g., Trenberth et al.
2003
).
• Over the ocean where there is ample water supply, the relative humidity remains about
the same and hence the observed moisture goes up at about this rate: an increase in total
column water vapor of about 4 % since the 1970s (Trenberth et al.
2007b
).
• Over land the response depends on the moisture supply.
• With more heat in the Earth system, the evaporation is enhanced resulting in more
precipitation. The rate of increase is estimated to be about 2 % per degree Celsius
warming (Trenberth
2011
).
• Locally this means increased potential evapotranspiration, and in dry areas this means
drying and more intense and longer lasting droughts.
• Larger warming over land versus the ocean further changes monsoons.
• Precipitation occurs mainly from convergence of atmospheric moisture into the
weather system producing the precipitation, and hence increased water vapor leads to
more intense rains and snow, and potentially to more intense storms.
• More precipitation occurs as rain rather than snow.
• However, higher temperatures in winter over continents favor higher snowfalls.
• Snow pack melts quicker and sooner, leading to less snow pack in the spring.
• These conditions lead to earlier runoff and changes in peak streamflow. Hence, there is
a risk of more extremes, such as floods and droughts.
The pattern of observed changes, so far, indicates wetter conditions in higher latitudes
across Eurasia, east of the Rockies in North America, and in Argentina, but drier conditions
across much of the tropics and subtropics (IPCC
2007
; Dai et al.
2009
,
2011
; Trenberth
2011
), and this pattern is referred to as ''The rich get rich and the poor get poorer''
syndrome (the wet areas get wetter while the arid areas get drier). This pattern is projected
to continue into the future (IPCC
2007
), including an increase in probability of the water-
related extremes (IPCC
2012
).
Over land there is a strong negative correlation between precipitation and temperature
throughout the tropics and over continents in summer, but a positive correlation in the
extratropics in winter (Trenberth and Shea
2005
). The latter arises from the baroclinic
storms that advect warm moist air ahead of and into the storm, combined with the ability of
warmer air to hold much more moisture. The former arises from the nature of the
