Bjerknes Lecture: Global Dimming and its Masking Effect on Global Warming
Is the Planet dimmer now that it was during early twentieth Century? Solar radiometers around the world are indicating that surface solar radiation in the extra tropics was less by as much as 5% to 10% during the mid twentieth century, while in the tropics such dimming trends have been reported to extend into the twenty first century. The reality as well as the potential causes of these trends have been explored through numerous field campaigns and satellite studies and these have brought into sharper focus several long standing fundamental issues such as anomalous solar absorption, the masking effect of aerosols on global warming and the need for a theory that can explain why the planetary albedo is about 29%. To understand why this last topic is an important question, consider the following two examples. A global albedo of 32% would plunge the Earth into a climate similar to that of the last ice-age; while an albedo of 26% would be comparable to a six fold increase in the CO2 concentration to 1800 ppm. Is the albedo hovering around 29% , because of the need to maintain a habitable climate?; or is it sheer chance that the planet just settled into the 29% albedo? Except for the ill fated Gaia theory, I am not aware of any theory that has contemplated this issue seriously. I will conclude with insights into how to reconcile the large solar dimming with the observed global warming, with sobering implications for future changes in climate and water budget of the planet.