| A process of worldwide climate change is now under way. Projections of its course over the coming century range from the relatively manageable to the catastrophic, depending on the way the climate develops and the choices that people and their governments make. RFF does not work in the physical sciences. This summary outlines the understanding of climate science on which RFF researchers’ analytical work is based.
Human activity produces a number of gases that, emitted into the Earth’s atmosphere, increase its retention of heat and result in a warmer planet. The most important of these greenhouse gases is carbon dioxide, produced by burning the fossil fuels on which the world depends for most of its energy.
Concentrations of carbon dioxide in the atmosphere have risen by more than one-third since the beginning of the Industrial Revolution. Over the past century the average global temperature has risen about 0.7 degrees Celsius (1.3 degrees Fahrenheit). One question is whether the rising concentrations of greenhouse gases are the cause of this warming. Since the evidence is largely statistical, scientists state their conclusions in terms of probability. The International Panel on Climate Change has said that most of the warming over the last 50 years is “very likely” caused by greenhouse gases generated by human activity. “Very likely,” in the IPCC’s language, means a likelihood of more than 90 per cent.
Forecasting future climate change is extremely difficult. The IPCC has used a set of scenarios based on varying assumptions about population, economic growth, technical development and public policy. Under the scenario with the least impact, the IPCC estimates that the global average temperature would rise about 1.8 degrees Celsius (3.2 degrees Fahrenheit) from the late 20th century to the late 21st century. At the other extreme, a scenario assuming continued heavy dependence on fossil fuels produces an estimate rise of 4 degrees Celsius (7.2 degrees Fahrenheit). At the low end of the range, most human societies could probably adapt without great difficulty over a period as long as a century. But even the middle of the range would have dire implications for many forms of life and for human health and agriculture. The European Union has set a goal of holding the rise in temperature to 2 degrees Celsius (3.6 degrees Fahrenheit), on grounds that anything greater is likely to be dangerous.
All of these estimates are produced by models that assume gradual change. But there are other possibilities. The geological record shows that many times over the millennia the Earth’s climate has changed abruptly --- in periods not of centuries but of decades or even a few years. That can happen, apparently, when a feedback mechanism is suddenly triggered. The nature of these mechanisms is not yet known to science, nor where the thresholds may lie. There is no way at present to calculate the risk of catastrophically sudden climate change. But geology warns that the risk is not zero.
Carbon dioxide emitted to the atmosphere may eventually be absorbed in the oceans, soils or growing vegetation. Conversely, the destruction of vegetation, as in land clearing, increases the concentration of carbon in the atmosphere. That links the science of carbon sinks to land use and forestry management.
Currently, worldwide emissions of carbon dioxide are rising rapidly. To stabilize the concentration of carbon dioxide in the atmosphere, this trend will have to be reversed. To the extent to which the world succeeds in this endeavor, it will diminish the risk of damaging changes in the climate. To the extent that it fails it will have to adapt to life on a planet that is warmer, and possibly much warmer.
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