In a previous blog post, I wrote an update on global climate change. One of the things you may notice now is the title of the current piece is on global warming, as opposed to global climate change. It has become the trend in recent years to replace the former with the latter, but I am going against that trend for the moment to make a point, and hopefully to get the reader to understand why there is so much uncertainty on this topic.
It was in the middle of the 19th century when scientists first discovered in the laboratory that carbon dioxide physically blocks the movement of infrared energy (think: heat). Scientists also pointed out that industrialization leads to higher concentrations of CO2 in the atmosphere. Within a hundred years (by the 1950s) scientists were getting very good at obtaining precise measures of atmospheric CO2 concentrations, and these measurements have been showing a constant increase every year since then. This is the evidence upon which the concern about global warming exists. If CO2 concentrations continue to increase due to our use of fossil fuels, this means that more heat gets trapped in the atmosphere, and therefore global temperatures rise (this is referred to as the greenhouse effect).
Now we get back to the difference between global warming and global climate change. If temperatures rise because of an increase in atmospheric CO2, that is not the end of the story. Earth’s atmosphere is full of “feedback mechanisms.” Feedback occurs when you have a change in a system that then causes something else to change. Imagine inside your home. You have a thermostat. In winter, you set the thermostat to “heat.” If the temperature in the house falls due to cold weather outside, the thermostat causes the heat to turn on, and the temperature rises. This is an example of “negative” feedback. “Positive” feedback occurs when a change in the system causes the initial change to be accented, so that the final status is more extreme than after the first round. Think about what happens when an electric guitarist starts playing very close to a speaker. The sound from the speaker adds to what initially goes through the amplifier, and so the volume skyrockets – ouch, it hurts my ears just thinking about it.
So the big question in global warming research is whether Earth’s climate system is dominated by positive or negative feedbacks. The effects of a doubling of CO2 concentrations alone may be in the neighborhood of 2-3 degrees F. But the real question is “then what?” That is one of the reasons why we have seen the shift from a discussion of global warming to global climate change.
One result of higher temperatures is more evaporation from the oceans, which puts more water vapor into the atmosphere. But water vapor is a greenhouse gas also. This feedback therefore would appear to be positive, and would lead to even more warming. But more water vapor means more clouds. And the effect of clouds on temperatures and climate is extremely complex. On the one hand, clouds tend to reflect sunlight high in the atmosphere, and therefore cause global cooling (a negative feedback). On the other hand, clouds trap heat in the lower atmosphere, especially at night (a positive feedback).
Scientists are now working to determine the kinds of climate change that are occurring or will soon occur as Earth’s temperatures increase due to higher concentrations of CO2 in the atmosphere. Much of this work is related to the kinds of feedbacks that are present in Earth’s climate engine. I understand that certain feedbacks are positive while others are negative, but it is hard for me to believe that the net feedback can be anything but positive. That is, it seems to me that the ultimate effect of a doubling of CO2 will lead to an increase in Earth’s temperature of more than 3 degrees F. More on why that is the case in my next blog entry.
(Submitted by Thomas W. Blaine, PhD, Associate Professor)