A REVIEW OF THE FACTS
James Wang, Ph.D. Bill Chameides, Ph.D.
Are Humans Responsible for Global Warming?
The case for attributing the recent global warming to human activities rests on the following undisputed scientific facts: • Carbon dioxide (CO2) is a greenhouse gas that warms the atmosphere. • Since pre-industrial times, atmospheric CO2 concentrations have increased from about 280 parts per million (ppm) to over 380 ppm. Current concentrations of CO2 and other greenhouse gases are unprecedented in at least the last 650,000 years, based on records from gas bubbles trapped in polar ice. • Independent measurements demonstrate that the increased CO2 in the atmosphere comes from burning fossil fuels and forests. The isotopic composition of carbon from these sources contains a unique “fingerprint.” • Since pre-industrial times, global average temperatures have increased by about 0.7ºC, with about half of the warming occurring over the past few decades. • The only quantitative and internally consistent explanation for the recent global warming includes the intensified greenhouse effect caused by the increase in CO2 and other greenhouse gases. The U.S. National Academy of Sciences—the independent organization of the country’s most renowned scientists established by Congress to advise the nation on scientific and technical issues—has concluded: “The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action.” Some argue that the recent global warming is due to natural fluctuations and not to human activities. This argument and its fallacies are discussed below.
Argument 1: CO2 is not coming from human activities
CO2 has natural sources: volcanoes for example. All animals exhale it. How can human activities be affecting the concentration of CO2 on a global scale?
Natural processes emit large quantities of CO2 into the atmosphere, but they also remove it—at nearly identical rates. This balance maintained the concentration of CO2 at a stable level for thousands of years prior to the Industrial Revolution. In the case of global warming, the question is: What is causing the increase in CO2 concentrations? The answer turns out to be incontrovertible. The isotopic composition of carbon in atmospheric CO2 provides a unique “fingerprint” that tells scientists that the lion’s share of the additional CO2 accumulating in the atmosphere is from the burning of fossil fuels.
Argument 2: No one really knows why the climate varies
The global climate has fluctuated considerably over the Earth’s history, either for unknown reasons or because of “internal variability” in the climate system. We do not know enough about the climate system to attribute the present global warming to any specific cause.
It is true that the Earth’s climate has exhibited wide swings over geologic time due to natural processes. However, scientists have reasonable qualitative explanations for most of the significant variations in 2
climate over geologic time;1 they can be largely attributed to specific processes, not to unknown internal oscillations. Many of the major climatic changes can be traced to changes in the Earth’s orbit around the sun (Hays et al. Science, 194, 1976, pg. 1121). Others can be linked to specific events (such as the impact of a comet or meteorite or the assembly or breakup of supercontinents) that led to large changes in the concentration of atmospheric greenhouse gases. For more recent times (the past millennium), scientists have been able to quantitatively attribute the major temperature fluctuations to changes in solar activity, volcanic eruptions, and human-produced greenhouse gases and particulate pollution. These natural processes can not explain the current warming.
Argument 3: The Medieval Warm Period disproves global warming The current warming trend is analogous to the...