SENSING CLIMATE-CHANGE USING THE GLOBAL POSITIONING SYSTEM

Using simulated atmospheric data from the National Center for Atmosphe ric Research (NCAR) community climate model (CCM), we test the hypothe sis that the global positioning system (GPS) can be used to detect glo bal and regional climate change. We examine how the fundamental GPS va riables (wet and total delays and vertical profiles of refractivity) a s well as precipitable water as estimated by ground-based GPS receiver s would change in a climate with 2 times the present concentration of carbon dioxide (CO2). Because of the higher water vapor content in the doubled CO2 simulation the wet delay and the precipitable water show a significant increase in the tropics and middle latitudes. Refractivi ty also shows an increase in the lower troposphere. We also simulate t he changes in the GPS signal delay in a doubled CO2 climate as would b e measured by a radio occultation technique using low Earth-orbiting ( LEO) satellites equipped with GPS receivers. Increases in temperature and water vapor in the lower troposphere of the model atmosphere produ ce opposite effects on the occultation delay. Increased temperature te nds to decrease the delay, while increased water vapor increases the d elay. Amplified by the long ''lever arms'' of the LEO-atmosphere-GPS l ink, a strong ''greenhouse warning'' signal is simulated, with increas es in occultation delay of nearly 100 m using the globally averaged da ta. This increase indicates that globally the effect of increased wate r vapor dominates. However, significant regional differences are prese nt in the occultation delay response. In the tropics, where the temper ature increase is smallest and the water vapor increases are largest, increases in delay of about 300 m are simulated. In contrast, in the p olar regions where the increased temperatures are greatest and the inc reases in water vapor are smallest, the temperature effect dominates a nd a decrease in occultation delay of nearly 70 m is simulated. When c ompared to expected errors in measuring the occultation delay of about 1 m, these results indicate that monitoring trends in occultation del ays would be a practical way to detect global and regional climate cha nge.