With the buzz surrounding Al Gore’s "An Inconvenient Truth" (the movie, the book and even his face on Entertainment Weekly), or Tom Brokaw’s Discovery Channel special on "Global Warming: What You Need to Know," it is an easy jump—a leap often taken by the media—to view recent weather as evidence of global warming. This inference is far from certain because climate is the long-term average of weather, while newsworthy weather is the extreme deviation from this mean. But a recent National Research Council report concluded that global warming is real, clearly indicated in the best available climatological data. After two decades, it now it seems the political debate has moved forward to whether we’re responsible and if so what we can do about.

Knowledge of the solar system and general astronomical principles brings some perspective. The Sun radiates energy that warms the Earth, following the humped-back energy distribution of the Planck's Law for temperature of 5800 °K, peaking at about 500 nm in the yellow part of the spectrum (from Wien’s Law). The Earth follows the same law, emitting as much as is receives, but at a much lower “effective” temperature of 255 °K (or -18 °C, calculated using the Stefan-Boltzmann formula), predominantly as infrared radiation. We, trees, rocks, everything on this planet glows dimly, but quite predictably, in the infrared, with a peak around 10,000 nm.

Atmospheric constituents absorb at certain, long wavelengths and alter the balance of outgoing versus incoming energy, known as radiative forcing. Heat is trapped in the lowest layer of the atmosphere (the troposphere, between the surface and most clouds), providing us a much more comfortable planet, at an average of 15 °C or 59 °F. An extreme example of this greenhouse effect, Venus sizzles at a lead-melting 471 °C because the dense atmosphere, nearly 100x that of Earth’s and mostly carbon dioxide CO₂. On the other hand, Mars’ atmosphere is too thin to warm it much: it is consequently very cold (-15 °C or -67 °F) and dry. 

Small changes in radiative forcing due to clouds, aerosols, carbon dioxide, water vapor and other “greenhouse gases” have direct and predictable effects on this energy balance, and ultimately temperature and climate. However, there are many processes, feedbacks and uncertainties to consider in our complicated climate system. Although small changes will likely have linear (that is, proportional) effects, there is concern the “business as usual” scenario will soon induce non-linear responses, popularly known as “tipping points.”

What does climate change mean to the star gazers of Earth? Climate is not weather, so it’s difficult to tell. Observing is to seeing deep sky objects like fishing is to catching, and observing success is “threatened” by realities of modern life besides the weather and clouds: neighbor’s porch lights, a new shopping center down the road, accelerated development of farmland and forests. These are all factors we as individuals can do something about and hopefully make a difference. Amateur (from the French word for love) astronomers need to be advocates for the sky, and as a group, our club certainly has a role here. In the same way, we as sky-gazing individuals—recognizing our diversity of opinions and perspectives—have a unique perspective on the geology and climate of this planet as well. With that knowledge comes some responsibility to act to ensure the sky we enjoy and experience is accessible to future generations and fellow lovers of the sky around our planet.

Also recommended: Tim Flannery's "The Weather Makers". and Joe Buchdahl's "Global Climate Change Student Guide". Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 23 July 2006. Text and images copyright © 2006 by Douglas C. Miller, All Rights Reserved. This material may not be reproduced in any form without prior permission.