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atmospheric circulation interactions with land. In cyclonic conditions, increased cloud and
rain provide more soil moisture and thus partitions the decreased surface energy more into
latent energy (higher evaporation) instead of sensible heat (lower temperatures). Anticy-
clonic conditions favor sunshine (more available energy), less rain and soil moisture, and
the larger surface energy raises temperatures instead of evaporating moisture. The result is
more likely either hot and dry or cool and wet conditions, but not the other options.
On global land, there is large variability in precipitation from year to year and decade to
decade associated especially with the El Ni ˜o-Southern Oscillation (ENSO) but there has
been an increase overall in land precipitation (Fig.
2
). The two wettest years are 2010 and
2011. In particular, major flooding in Pakistan, Australia, and Colombia was associated
with record high sea surface temperatures (SSTs) in the second half of 2010 into 2011
(Trenberth
2012
) and led to a dramatic drop in sea level of about 5 mm (Fig.
3
). The
prospects for more intense precipitation but longer dry spells lead to the increased risk of
flooding and drought, which pose major challenges for the society at large and those who
have to manage water resources for food, fiber and energy production, and human con-
sumption and leisure. We therefore view observing, understanding, modeling and pre-
dicting the global water cycle as a grand science challenge.
4 Grand Challenges
A Grand Challenge should inspire the community to want to be involved; it needs to be
specific and focused while identifying barriers and ways to advance the science, and it
must capture the imaginations of funding agencies, science program managers and the
public. It should also provide a vehicle to encourage the different WCRP panels to interact
in pursuing a common goal. It must provide a way forward that is tractable, perhaps via
new observations (e.g., from satellites), computer and model advancements, and ideas. It
must matter, as shown by answers to questions on possible benefits to society by providing
Fig. 2 Annual mean anomalies in global land precipitation from 1900 to 2011 in mm; from NOAA.
