Acceptance of the fundamental process of global warming has grown over time and is increasingly widespread, albeit with some notable exceptions within the political community. The most important statement of the problem was given in 2001, when the United Nations’ Intergovernmental Panel on Climate Change (IPCC) concluded that human activities are the cause of the changes in the concentration of GHGs in the atmosphere and that these changes are likely to cause global average temperature to increase. The IPCC also projected that emissions of GHGs would increase without government action, and that the resulting increase in atmospheric concentrations will likely lead to higher temperatures and climate changes that are increasingly disruptive and harmful to humans. These facts are not disputed by any credible source.
The only reasonable solution to the problem of human-induced climate change is for society to reduce emissions of GHGs, thereby lowering atmospheric concentrations. Unfortunately, there are many obstacles to doing so.
The obstacles to finding an easy solution to the problem largely derive from its fundamental characteristics, which can be summarized in the following five factors that result in what is probably the most challenging problem ever faced by humanity.
- Uncertainties – Despite progress in climate science, significant uncertainties still remain regarding the environmental impacts from increasing concentrations and the timing and location of these impacts. There are also uncertainties that cannot be significantly reduced due to the long time frame of the problem, such as the path of future emissions, the cost of future mitigation of those emissions, and the role technological innovation will be able to play.
- Actor heterogeneity – Although the problem is one involving a global commons, it is also characterized by a geographically heterogeneous distribution of impacts and a far from unanimous assessment of the distribution of causal responsibilities. The impacts of climate change will most likely be severest in developing countries, both because of their greater geographical vulnerability (i.e., vulnerable to sea-level rise, etc.) and because of their lower adaptive capacity. In contrast, the build-up of GHGs in the atmosphere to date has largely resulted from the activities in industrialized countries.
- Long-term problem – GHGs have long atmospheric lifetimes (e.g., from 12 to 50,000 years), which allows them to continuously accumulate in the atmosphere. Action to address the problem, therefore, will be required long before significant benefits (in the form of avoided damages) are accrued.
- Global problem – The atmosphere is a global commons: the impacts of emissions are completely independent of the geographic location where they occur. This characteristic of the problem necessitates a collective solution with broad if not global participation; it must confront the incentives for each individual actor to free-ride on the efforts of others. Worse still, non-participants may actually gain a competitive advantage in international trade by staying out of a climate mitigation regime.
- Broad issue – The breadth of activities affected by this effort will almost certainly be vast. The major sources of GHG emissions are related to the consumption of fossil fuels, which currently supply 80% of the energy humans use. Dramatically reducing GHG emissions from energy consumption is likely to lead to significant costs in the form of economic restructuring and changes in lifestyles for many. Plus, it threatens to limit the economic advancement of many more by limiting their access to low cost energy resources.
Each of these factors necessitates society to do things we are not often good at:
- taking measures collectively that will only show the largest benefits for future generations;
- cooperating with friends and foes around the world in a deep and substantive way that changes all aspects of our lives and economy;
- Doing these things in a situation where we are not sure that the costs or benefits will be; and
- accepting that there will be winners and losers and taking action even in the face of objections from the losers.
I am curious if any of you know of another problem that presents similar challenges. Maybe some public health issues?
 IPCC Fourth Assessment Report.
 IPCC Fourth Assessment Report
 Other solutions have been proposed, such as the use of space mirrors, seeding clouds with reflective particles, scrubbing CO2 from the atmosphere, ocean fertilization, shooting sulphates into the atmosphere to mimic a continuous volcanic eruption, and moving out the orbit of the Earth from the Sun. None of these proposals has been shown to be practical.
 Free-riders are normally thought of as actors whose individual contribution does not make a significant difference to the common good, but reduces their own individual cost. In this case, however, some players have a more significant impact than others. For instance, if the United States tries to free-ride, the problem will not actually be solvable. See Mancur Olson, Jr. (1965). The Logic of Collective Action, Harvard University Press.
 This is because any climate mitigation regime is likely to impose extra costs on producers of goods that involve greenhouse gas emissions. As such, the production costs of members of the climate regime will likely rise relative to non-members’ costs.
 International Energy Agency. “Key World Energy Statistics”