Freshman Seminar:
Introduction to the Science of Climate Change
Prof. Jordi Miralda-Escudé, F 9:30

Lecture 9: Solutions to Global Warming.

How we can limit greenhouse emissions

To mitigate global warming in the future, what we need to do is to reduce the radiative forcing that is causing the warming. At present, about half of the anthropogenic greenhouse radiative forcing is caused by carbon dioxide, and another half by methane, nitrous oxide, and halocarbons. In addition, particles of black carbon (or soot) released to the atmosphere when fossil fuels or biomass is burned can also cause warming by absorbing sunlight (reducing the albedo of the Earth). Efforts should be made to reduce the emissions of all these agents.

The emission of carbon dioxide is associated with the burning of fossil fuels that provide most of the energy used by our society today. Carbon dioxide is made by the reaction of carbon in the fossil fuels with oxygen during the burning. It can therefore not be eliminated by using ``clean burning'' technologies: the amount of carbon dioxide produced is fixed for any given mass of coal, oil, or natural gas that is burned. The only ways to reduce carbon dioxide emissions are to burn fewer fossil fuels (especially less coal, which produces more carbon dioxide than oil and gas per unit of energy obtained), or to capture the carbon dioxide and store it underground instead of letting it go in the air.

To reduce carbon dioxide emissions, we have these options available:

There are also strategies for reducing other greenhouse gases and black carbon. Some of these have other benefits as well: for example, reducing methane and black carbon emissions can also reduce tropospheric ozone, smog and pollution in cities. A particularly interesting strategy for the overall reduction of radiative forcing has been put forward by Dr. James Hansen, explained in his Scientific American article of March 2004 .

International Treaties on Emissions.

The United Nations Framework Convention on Climate Change was a document agreed to in 1992 whereby the signatory nations commit to the ultimate objective of stabilization of greenhouse gas atmospheric concentrations at a level that would prevent dangerous anthropogenic interference with the climate system.

The Kyoto Protocol was proposed in 1997 as an international treaty by which nations would agree to gradually reduce their emissions. The treaty asks nations to account for their emissions of all greenhouse gases, and to reduce them by a certain amount in whichever way is most economically feasible (for example, a nation can choose to pursue a greater reduction of carbon dioxide, or of methane, or other gases depending on which is cheaper to do). Nations are also allowed to trade permits for emissions, so one country could emit more than its commitment if it purchases emission permits to another country that cuts its emissions below its commitment. Initially only developed nations are required to reduce their emissions by a specified amount before 2012. Developing nations, which at present still have much lower per capita emissions, will only be required to reduce emissions after 2012 under new negotiation.

At present the Kyoto Protocol has an uncertain future because the United States, Russia and Australia have not agreed to sign it. The Kyoto Protocol would achieve a reduction of world emissions relative to the situation with no treaty of about 5% by 2012, and the cuts can be increased under new negotiation after 2012.

A case study: The Ozone Problem and the Montreal Protocol.

In the late 1970s and early 1980s scientists realised that some chemicals containing chlorine (chlorofluorocarbons) used by industry and released to the atmosphere were reducing the protective layer of ozone in the stratosphere through chemical reactions catalyzed by chlorine. The stratospheric ozone layer absorbs ultraviolet light from the Sun, so its reduction implies an increase of sunburns, skin cancers and cataracts on humans and other negative effects on plants and agriculture due to increased exposure to ultraviolet light. Concern increased when it was discovered in 1985 that a large ozone hole appeared over Antarctica every spring, where conditions were particularly favorable for the destruction of ozone by chlorine. In 1987 the was approved, to reduce emissions of chlorofluorocarbons. The original 1987 Montreal Protocol would have led to only small decreases in the rate of emission of chlorofluorocarbons. The signing of the protocol led to investments into finding alternative technologies for industrial processes that could avoid the release of the chemicals threatening the stratospheric ozone layer. This made it possible to agree to much larger emission reductions in later ammendments to the Montreal protocol. At the present time, emissions are much smaller than before the Montreal protocol was brought into force. Thanks to this rapid reduction in emissions, the concentration of chlorine in the stratosphere has now stopped increasing. Because the chlorine that is already in the stratosphere takes a long time to be naturally removed, ozone levels are still low and it is thought that they will only gradually recover over the 21st century.

The Montreal protocol on stratospheric ozone is therefore an example of a successful international treaty to solve an environmental problem, but it also shows how the impacts of altering the atmosphere by the human society can persist for a long time in the future after remedial action is taken, and be increasingly dangerous as the necessary remedial actions are delayed.

Proposals to compensate greenhouse warming by artificially increasing sunlight reflection

There have been several ideas to compensate the global warming effect of greenhouse gases by the deployment of structures that would increase reflection of sunlight to space. These ideas include: releasing a lot of specially designed aluminum dust in the stratosphere; launching a large number of tiny hydrogen balloons with reflecting metal surfaces on their tops to the stratosphere; or building a 1000 kilometer diffracting grid located between the Sun and the Earth at a special place called the Lagrangian point where objects remain stationary.

These ideas sound rather fictional because of the enormous scale they involve. To compensate for the warming from greenhouse gases over the next century, we would need to block about 2\% of the sunlight reaching the entire planet Earth. Nevertheless, they have been shown to be feasible in a report by the scientists E. Teller, L. Wood and R. Hyde Lawrence Livermore National Laboratory . There are serious ethical and political concerns about attempting to directly manage the climate of the Earth. Nevertheless, if we realise at some point during the next century that climate change and sea-level rise induced by global warming due to greenhouse gases are going to be too severe to allow for an adaptation strategy, by that time reducing greenhouse emissions alone may no longer be a solution, because it takes more than 100 years for carbon dioxide to be absorbed from the atmosphere after emissions are stopped. In that case, this type of large-scale engineering of planet Earth would be the only solution left.


  • Mitigating the danger from global warming implies reducing the radiative forcing, which comes from carbon dioxide, methane and other greenhouse gases, and black carbon particles.
  • Reducing carbon dioxide emissions can be achieved by: improving energy efficiency, increasing nuclear energy, expanding renewable energy (hydroelectric, solar, wind, biomass), sequestering the carbon dioxide in underground deposits and soils.
  • The Kyoto Protocol is the present effort for an international treaty to reduce greenhouse emissions, and its political future is now in doubt. The Montreal Protocol is an example of a successful international treaty to deal with a global environmental problem.
  • Large engineering projects to reduce sunlight are ethically and politically difficult, but may end up being our only solution if action to reduce greenhouse emissions is delayed and climate change is later found to be too severe and dangerous for reasonable adaptation.