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Special Topic : Our Life, Our Water


Special Topic: Our Life, Our Water


How Will the Global Hydrological Cycle Change with "Global Warming"?

Dr. Yasunari, Program Director of Hydrological Cycle Research Program, FRSGC and FORSGC, who has received 2002 Meteorological Society of Japan's Fujiwara Award, will explain the effect of global warming on the global hydrological cycle.


Tetsuzo YASUNARI ( Program Director, Hydrological Cycle Research Program, FRSGC and FORSGC )



The hydrological cycle is an important part of the climate system, and its change and variability have great implications not only for feedback processes in the climate system, but also in terms of the impact on human life. Particularly in Monsoon Asia, where more than half of the world's population reside, people depend heavily upon rain and will be affected by its variability and extremes (i.e., floods, droughts etc.) in the monsoon season.


In the past several decades, particularly in the recent two decades (1980s through 1990s), the global or hemispheric mean surface temperature has remarkably increased, as reported in the Intergovernmental Panel on Climate Change (IPCC) report. Under some scenarios of the anthropogenically-induced greenhouse gas increase, most of the state-of-the-art climate models have predicted surface-temperature increases of 3 to 5ºc /decade by the end of 21st century. 

These models have also predicted an increase in global precipitation and facilitation of the hydrological cycle. The range of predicted values within the models, however, is far larger than that of the temperature, due to many unresolved problems and uncertainties in the hydrological processes, such as interactions between cloud/precipitation systems and land/ocean surface processes.



Figure 1 Time series of global mean CMAP (black bar) and GPCP (white bar) annual precipitation (November to October) for the period from 1979 to 1999. Linear trend values for CMAP and GPCP are plotted with solid and broken lines, respectively. (Yasunari, Igarashi, Tomita, and Masuda, 2002).



We have examined the recent change and trends of hydrological parameters using the two sets of nearly homogeneous precipitation data for the global domain. One data set is from the Climate Prediction Center Merged Analysis of Precipitation (CMAP), and the other is from the Global Precipitation Climatology Project (GPCP) for the same period from 1979 to 2000.

Time series of global-mean precipitation from the two data sets for the period from 1979 to 1999 were computed for the whole year, as shown in Figure 1. The significant decreasing linear trends are noticeable in the time series for CMAP data, whereas no decreasing or increasing trend is seen for GPCP time series.



Figure 2 Figure 2. (a) Interannual variability of summer water balance
components for the Lena River basin from 1979 to 1995. (b) As
in (a), except for the Yenisey basin. (c) As in (a), except for the
Ob basin (Fukutomi, Igarashi, Masuda, and Yasunari, 2002).

Caution may be needed for this type of time-series analysis when the data have been merged and edited by using different data source for a different period, butwe should also emphasize that both sets of data DO NOT show any increasing trends. The decreasing trend is mostly attributed to that over the tropical ocean areas (Yasunari et al., 2002). In a broad region of Siberia, where the recent warming trend is most remarkable, no increasing or decreasing trend in precipitation has been noted either in winter or summer, though a dominant period of 6 to 8 years is seen in summer as shown in Figure 2 (Fukutomi et al., 2002). 

In the Monsoon Asian region, large differences in trends and decadalscale changes are noticeable both in their regionality and seasonality, though further intensive study is really needed.

These discrepancies between the model predictions and the observational results strongly urge us to promote further monitoring of the global hydrological cycle by in-situ and satellite observations, intensive studies of hydro-meteorological processes in the atmosphere and at land/ocean surfaces, and development of high-resolution climate models with more sophisticated hydrological processes.

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