March 31, 2008 – Original Source: Climate Progress
Earlier this week Alexandra Kougentakis, Fellows Assistant at the Center for American Progress, attended one of the American Meteorological Society’s seminars discussing the latest in climate science (which are great resources for policymakers, as they tend to take place on Capitol Hill). She has kindly reported on the event below:
One of the favorite, though well-debunked, claims of the global warming skeptics is that the recent warming is due to the recent up tick in solar activity. The current solar cycle has indeed seen higher-than-average sunspots, but what most strengthens skeptics’ argument is the lack of knowledge about what this means. In that light, the American Meteorological Society’s recent seminar, “Solar Radiation, Cosmic Rays and Greenhouse Gases: What’s Driving Global Warming?” was especially illuminating.
The core of the skeptics’ argument is to take legitimate scientific fact and distort it to serve a false premise. Solar activity is among the external factors listed by the IPCC whose variation could be a source of radiative forcing, which is the net change in solar ray penetration between the two atmospheric layers closest to the earth. In other words, solar activity is potentially a cause of climate change. The historic correlation between solar activity and climatic shifts seen in the paleoclimate record provides evidence to this effect. Since the 17th century, the record of 11-year cycles of solar irradiance, or brightness, charted through the analysis of tree rings and ice cores makes clear that solar irradiance has increased. A correlation has been observed between solar activity and climate shifts, at least up until the mid-20th century, when the connection became sharply weaker.
One of the strongest arguments against attempts to link solar activity to current warming has to do with inconsistencies in the solar signal.
As climate scientist Dr. Caspar Ammann pointed out at the AMS seminar, the atmosphere is divided into different layers between which the impacts of ozone of pollution vary. While the models to predict the layers’ temperatures are not yet perfect, it is understood that solar irradiance would warm both the troposphere, the layer closest to the earth, and the stratosphere, the next outer layer. The warming that the troposphere is experiencing now is accompanied by a cooling of the stratosphere, which signifies that the current flux in solar irradiance is not a complete explanation for current climate shifts.
Climate experts widely recognize that the sun is a minor player at best in the current warming trend, and this is due in part to the scope of its impact seen in the past. At the AMS seminar, solar physicist Dr. Judith Lean further made the point that it is not realistic to separate anthropogenic and solar effects of warming since they are occurring simultaneously.
There are a lot of different factors affecting climate, some natural, some anthropogenic, and likely some that haven’t been clearly identified yet. The interaction between these factors makes it difficult to establish a linear connection between temperatures and a single variable. Addressing the specific argument of the skeptics, however, the solar-climate connection alone cannot account for the level of warming that has occurred since the mid-20th century.
The major take-home message of the AMS seminar, highlighted by both Dr. Lean and Dr. Ammann, was that both the empirical record and modeling demonstrates that current warming trends are most directly explained by anthropogenic greenhouse gas emissions, more than any other factor. This will probably do little to silence the ranting of global warming skeptics, but it is important to understand that it is only too convenient to blame nature for the problems that we bring upon ourselves.