In my recent blog post on Global Warming and Uncertainty, which you can read here, I explained that, although carbon dioxide (CO2) is the main driver behind human-induced global warming, feedbacks will ultimately determine how much the global temperature increases. If you are unfamiliar with the concept of feedback and how it relates to global warming, I suggest you read that earlier blog before continuing here.

Many climate models show that, although the effect of a doubling of CO2 concentrations in the atmosphere from the pre-industrial level of 280 parts per million (ppm) to 560 ppm will cause a temperature increase of somewhere between 3.5 and 8.1 degrees F, only a portion of that (about 2-3 degrees F) can be directly attributed to CO2 itself. The extra temperature rise is due to how the models calculate feedback. Those who say that temperatures will rise by less than 2-3 degrees F are arguing that Earth’s climate engine has negative feedbacks built into it which will prevent the initial effects of CO2 warming from being amplified. At this point, it is very difficult for either side to “prove” whether the feedbacks will be positive, leading to more warming, or negative, leading to less warming. So we are left with hypotheses, or educated guesses. I have studied the literature carefully, and I personally believe that positive feedbacks will rule the roost, leading to an amplified warming. Let me give you some of my reasons.

The primary feedback from an initial warming caused by an increase in CO2 is increased evaporation from the oceans, which adds water vapor (H2O) to the atmosphere. But water vapor is a greenhouse gas, so this leads to more warming. The extent of this warming is unclear, primarily because water vapor gathers in the form of clouds, and clouds themselves have an ambiguous effect in terms of feedback. On the one hand, since their tops are white, clouds tend to reflect solar radiation back to space. This is a negative feedback, preventing warming of the atmosphere. On the other hand, clouds hold heat in near Earth’s surface, especially at night, leading to more warming – a positive feedback. Those who expect that the negative feedback from clouds will prevail are making a very large assumption, particularly since we already know that the presence of water vapor in the atmosphere leads to increased warming. So while I am not denying that an element of negative feedback exists, I am just not convinced that it will equal or exceed the other positive elements.

The other major feedbacks, similar to the case of clouds, involve changing color – this time of Earth’s surface itself. Since the initial effect of CO2 is warming, we are witnessing the retreat of glaciers, sea ice and snowpacks, particularly in the northern hemisphere. This leaves a darker surface, both at sea (dark blue) and on land (mostly green and brown). Replacement of white surface with dark water and land reduces the amount of solar radiation that reflects back into space, and causes a great deal more heat to be absorbed on Earth’s surface. I think it is hard to overstate the importance of this phenomenon, especially since it played such an important role at both ends of previous ice ages. As more snow piled up due to initial cooling caused by changes in Earth’s orbit, the colder Earth became, leading to ice ages. As snow receded near the end of the ice ages, more dark surface was exposed to the sun, allowing for an acceleration of the warming that led to the climate we have today.

So there you have it. If the doubling of CO2 itself without feedbacks is expected to increase global temperatures by about 2-3 degrees F from pre-industrial levels by 2100, it looks to me like we can expect an overall warming of greater than 3 degrees. Some climate models put that number at about 8.1 degrees F, a warming that would completely transform Earth in ways that can only be described as catastrophic. But then again, perhaps the positive feedbacks, even though they do rule the roost, will not lead to that much warming. In fact, up until now climate models have tended to over-predict the increase in global temperatures by about 30-40 percent, possibly because they do not sufficiently account for some negative feedback. Given all these factors, my best estimate for a doubling of CO2 by 2100 is about 4 degrees F. This will mean a climate warmer than any Earth has seen in the last 7 million years, a period far longer than humans have been around. At a minimum, the challenges our descendants will face from this will be enormous. But there is still time to take action to prevent a doubling of CO2 from occurring. That presents an enormous challenge to US.

Tom Blaine is an Associate Professor with OSU Extension, Community Development. Feel free to contact him to present a global warming update to your group.