Global temperature and climate are largely determined by the balance of incoming energy from the sun, minus outgoing radiation. Incoming light radiation from the sun has short-wavelengths and can readily pass through the atmosphere, but after being absorbed and re-radiated from Earth’s surfaces the out-going infra-red radiation has longer wave-lengths and is less able to pass through the atmosphere. The so-called “greenhouse gases” absorb and re-radiate a portion of the outgoing long-wave radiation back toward earth, acting like a heat-trapping blanket. Even slight changes in the ratio of incoming and outgoing solar energy have significant influence on our global climate system. Even though greenhouse gasses make up less than 1% of Earth’s atmosphere, our global climate is quite sensitive to changes in their concentration. 

Ice-core data from Greenland and Antarctica tells us that atmospheric levels of carbon dioxide (CO₂₎ vary somewhat predictably with cycles of ice ages and warm inter-glacial periods. The ice cores also show that atmospheric CO₂ is increasing almost 100 times faster today than during past climate cycles, and that current concentrations of  CO₂ are higher than at any time in at least the last 800,000 years. Given the difficulty of rapidly changing our resource-intensive lifestyles, we’ll be lucky if global atmospheric CO₂ concentration merely doubles. More likely it will go much higher before we control our appetite for fossil fuels and land exploitation. 

While CO₂ is of primary concern among greenhouse gasses, there are others such as methane (CH₄) that contribute to global warming¹.  CO₂ is unique in that is has a very long, approximately 100 year, “residence time” in the atmosphere².  Concentrations of CO₂ in the atmosphere will likely remain far above “normal” for centuries, because the millions of tons of CO₂ released to the atmosphere during the agricultural revolution, the industrial revolution, and the automobile revolution will not reach a new equilibrium until biological and geophysical processes (in the oceans and on land) have a chance to capture and store most of the “extra” carbon. 

We have a moral obligation to leave future generations with choices and opportunities for survival. We must avoid irreversible harm to the planet’s life support systems including a livable climate and function ecosystems that sustain life.

[1] Water vapor also has a significant influence on climate, but it has a very short residence time in the atmosphere so it is better thought of as a “feedback” than a “forcing.” http://www.realclimate.org/index.php?p=142 Warming is expected to increase water vapor in the atmosphere but the effects on climate are very complex and remain unclear. Water vapor can act as both a greenhouse gas with a warming influence (positive feedback), and it can have a cooling influence via cloud formation and increased albedo (negative feedback). Scientists are keenly interested in this issue and continue to study the role of water vapor and clouds in future climate scenarios.