CRed - The Community Carbon Reduction Project at UNC-Chapel Hill

After passing through the atmosphere, the sun's heat becomes trapped near the surface of the earth, resulting in a relatively stable global climate allowing life to exist. Without greenhouse gases, nearly all the heat gained by the earth from sunlight would simply be lost (re-radiated or reflected) directly back into space. Recently, however, the composition of the atmosphere has begun to change due to the activities of humans, especially from the burning of fossil fuels. As greenhouse gases accumulate in the atmosphere, they more effectively trap the sun's heat, increasing the energy absorbed in the atmosphere, oceans, etc. This will, in the long run, result in increased mean global temperatures, although there will be variability across the surface of the earth, with perhaps some areas cooling while others heat up.

The CRed program focuses on one particular greenhouse gas: carbon dioxide. The link between human activities and the accumulation of carbon dioxide in the atmosphere has clearly been established by the Intergovernmental Panel on Climate Change (IPCC). It was not until the Industrial Revolution of the 18th and 19th centuries that atmospheric concentrations of carbon dioxide began to increase appreciably due to the burning of fossil fuels. This may also have been accompanied by an increase in global average surface air temperatures, although this increase has not been fully established by the scientific community. The increase in average surface temperature across the globe that has occurred over the past 150 years appears to be on the order of 0.6º C ± 0.2º C during the 20th century; best scientific estimates are that this increase could become larger by several degrees C by 2100. It is important that countries and individuals understand how their local climate might be affected in order to prepare for the political, economic, and social impacts of climate change.

Policy and planning decisions are always made under conditions of variability and uncertainty, and those associated with climate change are no exception. Variability refers to the fact that the degree of climate change, and the resulting impacts on society and ecosystems, will differ around the globe. Some areas will become warmer while others cool. Some will see more rainfall while others see less. Warming in some areas may be desired, while it will be a detriment to life in others. This is why it is important to ask about the effects of climate change on a region (such as England or North Carolina), rather than to pose more general questions about the effects globally. The scientific consensus is that mean global temperatures will increase, and that fluctuations in climate (including storms) will become more significant, but beyond this it is best to ask about climate change in your specific geographic area and to realize that it may not be the same as the global average.

Uncertainty refers to the state of our knowledge about the potential effects of increasing carbon dioxide in the atmosphere. All decisions in life (where to invest in the stock market; which house to buy; how restrictive an environmental regulation must be to protect health) are fraught with uncertainty. It is essential to understand this uncertainty without paralyzing the ability to make decisions, and perhaps to choose a course of action that is reasonable in light of that uncertainty. While there is uncertainty about the degree of reductions in carbon dioxide emissions needed to make climate changes sustainable, uncertainty that humans will bring about significant changes in the climate through greenhouse gas emissions over the next century is now largely confined to scientists at the fringe of serious analysis, and with ties to industries with evident self-interest in magnifying uncertainty to prevent action. What we can say about uncertainties is this:

1. There is little uncertainty in the scientific community that human activity, largely the burning of fossil fuels, is significantly increasing concentrations of carbon dioxide in the atmosphere. This concentration will almost surely double (over that present before the industrial revolution) during the 21st century without changes in the rates of emission. However, the exact amount by which the concentration will increase under any given scenario of global emissions (e.g. if society continues on its present course), is subject to significant uncertainty due to our incomplete knowledge of how plants will respond as atmospheric carbon dioxide levels increase (plants may grow faster and increase their absorption of carbon dioxide, lowering the atmospheric levels); how the oceans will respond; how effective reactions in soil will be at removing or producing carbon dioxide; etc. One way to characterize this uncertainty is to consider the range of predictions by the IPCC as shown in Figure 5 of their report, Summary for Policy Makers.
2. There is larger uncertainty in predictions of the effects of this increase in carbon dioxide on global mean temperatures. Again, there is strong scientific consensus that there will be an increase, but whether it will be a few degrees C or as high as 10 degrees C during this century is not well known. Again, this uncertainty can be characterized by considering the range of predictions of temperature, also shown in Figure 5 of the IPCC report, Summary for Policy Makers.
3. There is even larger uncertainty as to the implications of this increase in temperature on human and ecosystem health. The underlying belief in the Kyoto Protocol is that the effects can be managed if the temperature increase globally is below about 4 degrees C, based on the knowledge that past societies have adapted to changes of this magnitude. There is little scientific doubt that the effects above 4 degrees C will be profound, although again variable around the globe. Beyond this, the uncertainty in the magnitude of these effects can't even be characterized at the moment.
4. Finally, we have little idea as to how society will evolve in the near future, or how it will respond to policies that might be put in place. Population growth, changes in per capita energy consumption, and the rate at which more carbon-neutral technologies are introduced are all highly uncertain, especially when we consider the growth of economies in the developing world. As a result, it is necessary to consider a variety of scenarios of this development, and to report the range of predictions of carbon dioxide concentration, temperature and effects under these different scenarios.

Some idea of the overall uncertainty can be gleaned from the variation in simulations by the IPCC of atmospheric carbon dioxide concentrations, temperature rise, and sea level rise (as a measure of effects) over time under a variety of possible scenarios of development in society, and a variety of climate change models. From Figure 5 of the IPCC report, Summary for Policy Makers, you can see that the predictions of carbon dioxide concentration as of 2100 may be accurate to within about 25% (with concentrations of anywhere from 550 to 900 parts per million or ppm); predictions of temperature rise may be accurate to within about a factor of 2 (with increases of between 1.5 and 6 degrees C); and predictions of sea level rise may be accurate to within about a factor of 2 (with increases of between 0.1 and 0.7 meters).

Due to this unceratinty, the initial focus on local, national and international policy has been on "no regrets" policies: policies that make sense economically and environmentally even if the effects of climate change are not as bad as expected. Policies justified by principles of resource conservation, energy security, etc., might be the first to move forward, with the idea that these are needed from a variety of perspectives.