『Abstract
　This paper examines the historical change in the pH of natural rainwater due to increased atmospheric CO2 from 1800 until 2007, giving predicted change in 2100. During 1800-2007, the rainwater pH at 25℃ and 1 atm is calculated to decrease by 0.06 units, from 5.68 to 5.62. In 2100, the predicted rainwater pH is calculated at 5.49 using the projected pCO2 (700 ppmv; IS92a) at 25℃ and 1 atm. Equilibrium calculations were made in an attempt to elucidate the calcium carbonate (calcite) dissolution by rainwater. From 1800 to 2007, the dissolution of calcite with pCO2 of those time at 25℃ and 1 atm increase the dissolved calcium concentration from 466 to 516μmol kg^-1. This value is calculated to reach 633 μmol kg^-1 in the year 2100. Rainwater is found to become more acidic with decreasing temperature. In the year 2007 (pCO2 = 384 ppmv), a total difference of 0.08 units in rainwater pH is calculated between areas at 0℃ and 30℃. The equilibrium pH with respect to calcite was found to increase with decreasing temperature. At lower temperatures, rainwater pH is found to decrease, whilst CaCO3 dissolution increases. Limestone landmarks and buildings might be affected through the dissolution of calcium carbonate by rainwater acidification. The effects of rainwater acidification on overall chemical weathering may result in influences on agriculture, forestry, landslides and flooding.

Keywords: Acid rain; pH; Limestone; Carbon dioxide; Carbonate dissolution』

1. Introduction
2. Methods
　2.1. Atmospheric CO2 data after the 19th century
　2.2. Calculation of equilibrium pH in the CO2-CaCO3-H2O system
3. Results and discussion
　3.1. Historical rainwater pH
　3.2. The change of rainwater pH with temperature
　3.3. Historical change in calcite dissolution
4. Conclusions
Acknowledgements
References