Geoscience Reference
In-Depth Information
2.1 Global Water Budget
A long-standing on-going challenge is to provide a reliable estimate of the annual mean
global water cycle. An estimate made by Trenberth et al. (
2007
) using observational data is
shown in Fig.
1
. However, there are large uncertainties in many of the estimated numbers,
not least because quantitative knowledge of the various components and their variability is
still fairly limited because of a lack of reliable data for surface evaporation, oceanic
precipitation, terrestrial runoff and several other fields. A further challenge is to determine
the interannual and longer-term variability of this cycle, particularly those aspects that may
be associated with climate change.
Global warming is likely to intensify the global hydrological cycle and bring changes to
the water vapour transport, changing the magnitude and spatial distribution of freshwater
fluxes between atmosphere, land and sea ice into the ocean (e.g., Held and Soden
2006
).
This, in turn, will modify the salinity distribution at the surface of the ocean and could
potentially alter the ocean circulation through effects on its density (Durack et al.
2012
).
An appropriate representation of these fluxes by general circulation models (GCMs) is
essential in order to make confident projections of future characteristics of the thermo-
haline circulation, in particular its stability, under global warming conditions. It is there-
fore very important to investigate the skill of climate models to simulate large-scale water
vapour transports and their mechanisms.
Fig. 1 The hydrological cycle. Estimates of the main water reservoirs, given in plain font in 10
3
km
3
, and
the flow of moisture through the system, given in slant font (10
3
km
3
year
-1
). From Trenberth et al. (
2007
),
their Fig. 1. Copyright 2007 AMS (
http://www.ametsoc.org/pubs/cr_2005.html
)
