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monitoring and forecasting system with the United Nations

Educational, Scientific and Cultural Organization (UNESCO)

for sub-Saharan Africa. Another very important outcome is

the unique Princeton Global Meteorological Forcing Dataset

that is now widely used by scientific and drought forecasting

communities worldwide.

Wood’s and Sheffield’s system utilizes modern remote

sensing and ground monitoring capabilities to help fuse

state-of-the-art hydrologic science, much of which they

helped develop, with seasonal climate and shorter-term

weather studies in a way that enhances, fundamentally and

significantly, our understanding of land-atmosphere coupling

and the ability to monitor as well as quantify the space-time

variability of droughts, past and future. An important compo-

nent of this fusion is the bridging of scales between relatively

low-resolution climate models and hydrologic models

having much finer spatial and temporal scales of resolution.

Consequently, terrestrial hydrology can be simulated at fine

temporal (hourly) and spatial (12 km) scales over continental

domains for the long periods (50 years) necessary to create the

historical record required to fit probabilistic models.

Previous assessments of historic changes in drought over

the late twentieth and early twenty-first centuries expected

climate change to cause an increase in drought frequency

and severity due to a corresponding decrease in regional

precipitation and increase in evaporation. In a 2012 letter to

Nature

, the team effectively overturned this expectation by

demonstrating that it is based on an oversimplified potential

evaporation model. By contrast, their more comprehensive

approach indicates that there has been little change in drought

over the past 60 years. This explains why tree-ring drought

reconstructions diverge from earlier drought records, and it

alters our perspective on how global warming impacts hydro-

logical phenomena and extremes.

Climate change and the world’s water

There has been considerable work in previous awards that

relates to the problem of desertification. In 2012, Dr Kevin

Trenberth and Dr Aiguo Dai from the National Centre for

Atmospheric Research in the United States won our Surface

Water Prize for their ground-breaking work that provides

a powerful estimate of the effects of climate change on the

global hydrological cycle, with a clear explanation of the

global water budget. If we are going to tackle desertifica-

tion in the twenty-first century, one of the overwhelming

challenges is to understand hydrologic variability and the

impact that climate change is certain to have on global

water resources.

Trenberth and his team made a unique contribution

through the investigation of climate variability and trends

in the past, and through the use of models and other creative

efforts to reconstruct river discharge into the oceans across

the planet for almost 1,000 river basins. They used climate

models to understand likely changes in the future and the

uncertainty associated with those predictions, and explained

their findings using such popular indicates as the Palmer

drought index. As a result, they have provided an exemplary

account of the global water budget that is now being used in

textbooks and encyclopedias.

They have made pioneering contributions to under-

standing the past with real data, and evaluating the future

prospects within the context of what we know of the global

climate and hydrology. They have provided a much better

understanding of hydrologic responses to climate change,

Image: PSIPW

The PSIPW Sixth Award ceremony, December 2014

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