The role of water vapor and solar radiation in determining temperature changes and trends measured at Armagh, 1881-2000

A 120 year series of climate measurements at Armagh Observatory, a rural site in Northern Ireland, was analyzed to yield monthly, seasonal, and annual values of long-and short-wave irradiances which were then related to the measured changes in air temperature. Three quarters of the significant increase and large decadal variations in atmospheric long-wave radiation was associated with the concurrent changes measured in specific humidity; the remaining quarter was associated with increases in the concentrations of carbon dioxide and other anthropogenic radiatively active gases. Significant but smaller long-term decreases in short-wave solar irradiance reduced by half the net, all-wave radiation forcing at the surface. Together the changes in long-and short-wave irradiances at Armagh accounted for more than three quarters of the interannual variations in mean annual temperatures. Climate sensitivity to long-wave forcing at the surface, 0.121 degrees C per W m(-2), was 5 times greater than that to short-wave forcing, and two possible explanations for this difference, water vapor feedback and changes in atmospheric circulation, are discussed.