Connecting Earth's water cycle to climate change

Aiguo Dai, NCAR's Climate and Global Dynamics Division

Nov 1, 2007 - by Staff

Photograph of Aiguo Dai
Aiguo Dai (Photo by Carlye Calvin, UCAR.)


Aiguo Dai is now an associate professor in the Department of Atmospheric and Environmental Sciences, University at Albany, State University of New York. (December 2012)

After graduating from high school, Aiguo Dai applied to the math department at China's Nanjing University.

"They didn't want me, so they put me in the meteorology department instead," Aiguo says. "But I think it turned out very well for me."

Today, Aiguo is a climate scientist at NCAR. "As a scientist, I get to do work that I really enjoy," he says. "The most interesting thing is that you can always learn something you didn't know yesterday."

Climate change is changing the water cycle

Aiguo focuses on how climate change is affecting Earth's water cycle. As average temperatures rise around the globe, the warmth triggers changes to precipitation, humidity, streamflow, and other water-related variables. Aiguo uses observational data and climate models to study the relationship between rising temperatures and these aspects of the hydrological cycle.

One of his current projects involves analyzing changes in continental discharge (the flow of water from Earth's land masses into the ocean) over the past 50 years. Preliminary results show an upward trend in water discharged into the Arctic Ocean, but no significant increase globally.

"This is in contrast to some earlier studies that show discharge has been increasing as part of an intensified hydrological cycle under global warming," Aiguo says.

The reason for the new findings, he explains, could be that while global warming is causing an increase in precipitation over high latitudes, precipitation has not been increasing over a large part of the subtropics and middle latitudes. The upward trend in discharge in the Arctic may be due to melting permafrost rather than precipitation.

The growing threat of drought

His precipitation research extends to the condition that results when there isn't enough of it—drought. In 2004, Aiguo published a study showing that the percentage of Earth's land area stricken by serious drought more than doubled from the 1970s to the early 2000s, with rising global temperatures likely a major factor. Widespread drying occurred over much of Europe and Asia, Canada, western and southern Africa, and eastern Australia.

Now, Aiguo is following this research up by using model-simulated climate changes from the Intergovernmental Panel on Climate Change (IPCC) as the basis to create drought scenarios for the 21st century.

"We want to extend historical analysis into the future to show how drought areas will evolve, and which parts of continents will get more severe droughts," he says.

Aiguo's favorite things about his job are the freedom to follow his interests in tackling different scientific problems and the opportunity to prove or disprove his ideas. "I'm always curious," he says. "I can write code, process data, and see if my idea turns out as I expected—that's the most exciting thing. And when I want to move to new ideas and problems, I can do that."

Painstaking work, useful results

It can be frustrating, however, when the ideal data or tools necessary to attack a problem aren't available. For example, historical streamflow data is scarce for many rivers in Africa that Aiguo includes in his global analyses. A French research agency maintains a Web site with daily plots, but they can't be downloaded in the form that Aiguo needs. As a result, he must print the plots and then manually reconstruct the data. Since there are hundreds of plots, this is quite time consuming.

"But it's important because we need the dataset to improve our analysis, especially for the Congo and Niger rivers," he says.

Another challenge for Aiguo is balancing the various duties of being a scientist, which include attending meetings, sitting on committees, reviewing papers for professional journals, and responding to requests for datasets. "Sometimes trying to find the time to sit down and do the research is hard," he says.

This array of activities also offers exciting opportunities. As a contributing author to the IPCC assessment reports, he is part of a group of researchers around the world, including many at NCAR, whose activities led to the IPCC sharing in the 2007 Nobel Peace Prize with former U.S. vice-president Albert Gore.

Color diagram of the land, atmosphere and sun
This illustration of the hydrological cycle shows water changing phase, from liquid to solid to gas and back to liquid, as it moves through the Earth system. The estimated global volume of water stored in each part of the system or exchanged with another part during a year is given in cubic kilometers (1 cubic km = 0.244 cubic miles). Aiguo and colleagues estimated these long-term means, or averages, based on available data, which comes mostly from the latter half of the 20th century. Aiguo is examining the impact of climate change, including global warming, on water storage and exchange. [ENLARGE] (Illustration from Trenberth et al., 2007, Estimates of the global water budget and its annual cycle using observational and model data, Journal of Hydrometeorology.)


Aiguo grew up in a small village in the Jiangxi province of southern China. His inspiration as a student was Chen Jingrun, one of the leading mathematicians of the 20th century and one of the most influential mathematicians in China's history. "When I was in middle and high school, he was my motivation to be a mathematician," Aiguo says.

The mysteries of meteorology

He didn't know much about meteorology when he entered the department at Nanjing University and was pleased to discover that the subject included lots of math and physics. After finishing his undergraduate work, he earned a master's degree in atmospheric science at the Chinese Academy of Science, completing a thesis on methane emissions from rice paddies. This was followed by a doctorate at Columbia University in New York.

The years at Columbia were especially formative for Aiguo. "During my graduate studies, I was fortunate to work with really good scientists who had a profound influence on my style of thinking and research," he says.

It was during this time, in the late 1980s and early '90s, that Aiguo became convinced that climate change was poised to become a major issue for science and society. "People were starting to realize that methane, carbon dioxide, and other greenhouse gases were affecting the atmosphere, and they started talking about global warming, but I don't think the human influence was really recognized yet," he says.

Concern for society in a warming world

Aiguo came to NCAR as a postdoctoral researcher in 1997, which is when he began to develop his focus on precipitation and climate change. Looking ahead, he plans to continue his research on how global warming is changing Earth's water cycle. He appreciates being able to study phenomena that have a large impact on society, such as droughts, floods, and intense rainfall events.

"You can't always link everything to daily life, but you can try to link your research to society," he says.

He'll get a major boost in 2013, when NASA launches a new satellite system, Global Precipitation Measurements, for measuring precipitation from space. GPM will have better spatial coverage than current satellite systems, which focus on low latitudes, and will be able to measure snow at high latitudes. "It's still several years away, but we'll have the most advanced space measurements of precipitation ever," Aiguo says. "We'll still need historical data records, but hopefully these satellite observations will accumulate into the future."

by Nicole Gordon

November 2007, Updated December 2012

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