Geoscience Reference
In-Depth Information
4.2 Grand Challenge on Water Extremes
How does a warming world affect climate extremes, especially droughts, floods and heat
waves, and how do land area processes, in particular, contribute?
A warming world is expected to alter the occurrence and magnitude of extremes such as
droughts, heavy rainfalls and floods, as well as the geographic distribution of rain and snow.
Such changes are related to an acceleration of the hydrological cycle and circulation changes
and include the direct impact of warmer conditions on atmospheric water vapor amounts,
rainfall intensity and snow-to-rain occurrence. How well are models able to handle extremes
and how can we improve their capability? New improved and updated data sets at high
frequency (e.g., hourly) are needed to properly characterize many of these facets of Earth's
climate and to allow for assessment against comparable model data sets. New research
activities are needed to promote analyses quantifying which changes are consistent with our
expectations and how we can best contribute to improving their prediction in a future climate.
Confronting models with new observationally based products will lead to new metrics of
performance and highlight shortcomings and developmental needs that will focus field
experiments, process studies, numerical experimentation and model development. New
applications should be developed for improved tracking and warning systems, and assessing
changes in risk of drought, floods, river flow, storms, coastal sea-level surges and ocean waves.
There is major concern that the occurrence, character and intensity of extremes will
change in the future as the climate changes due to human activities, and this will have
enormous consequences for society and the environment. Yet addressing changing
extremes satisfactorily is a daunting task, and it will be difficult to keep up with society's
expectations. As noted above, huge improvements in near-global spatial and temporal
coverage for precipitation, soil moisture and other hydrological variables provide oppor-
tunities for new datasets, products, improved models and model applications, making it an
opportune time to fully address extremes.
The climate system does not neatly package such extremes. Extremes may be highly
localized in time and in space. Drought in one region frequently means heavy precipitation
not that far away. The worst extremes are generally compound events which often are
consequences of a chain-of-events that may be related at the global scale despite their
regional implications. Flooding may be accentuated due to saturated soils from previous
storms and/or from snowmelt. Furthermore, coastal flooding may involve storm surge
effects, local precipitation and remote snowmelt signals.
Because of its importance, there are many efforts focusing at least in part on extremes
within WCRP. One focus is on drought, although there is certainly interest in other hydro-
meteorological extremes and related issues, such as statistical analyses. WCRP, mainly
through CLIVAR, also addresses tropical and extratropical cyclones and associated marine
storms as well as extreme sea-level variability and change that is connected to storm surges.
GEWEX with its focus on the water cycle and on land surface processes with strong obser-
vational capabilities from global to local and with numerous links with society is a natural
'home' for addressing many types of extremes. The question is what is missing and what can
be done within GEWEX to move ahead? The main GEWEX focal point is to increase efforts
on hydrometeorological extremes including drought, heat waves, cold outbreaks, floods,
storms and heavy precipitation events including hazardous winter snowfalls and hail.
The specific questions that will be addressed over the next 5-10 years include
•
What are the short-term, mid-term and strategic requirements for the existing
observing systems and datasets, and which observations are needed to accurately
