On June 9, New York magazine published an article titled "The Uninhabitable Earth."  The article predicted flooding, famine, economic collapse, and death from direct heat as a consequence of global warming.  Wild stuff.  The article was panned by climate scientists, both those who think global warming is a major threat and those who predict more modest warming.

How worried should we be about global warming?  It's helpful to think about the issue in geological terms.  During the last ice age, Earth was an estimated six to seven degrees Celsius cooler.  During the ice age, the Laurentide Ice Sheet covered Chicago along with the northern half of Illinois.  As you can see, a mere six to seven degrees of warming is sufficient to produce massive change.

The Intergovernmental Panel on Climate Change projects that if we don't attempt to limit the release of greenhouse gases, mainly CO2, the climate will warm by between 3.7 and 4.8 degrees Celsius over the 1850-1900 mean temperature.  This amounts to roughly a 2.85- to 3.95-degree increase in average temperature over the current temperature, because the planet has warmed by roughly 0.85 degrees Celsius since the late 19th century.

What will the effect of such a temperature shift be?  By 2100, the IPCC projects 0.8 meters of sea level rise, intensification of existing rainfall patterns, and a reduction in fish catches due to ocean acidification.  The IPCC projects that these changes would threaten food supply and water supply and flood low-lying coastal areas.

Climate change could make it more difficult to feed Earth's hungry and growing population.  Although projected to occur more slowly, sea level rise will eventually threaten cities such as Miami and New York.  It could also inundate large sections of Florida and Louisiana.

How certain are the IPCC's estimates?  That depends on water vapor.  Experts generally believe that holding everything else equal, doubling atmospheric carbon dioxide produces a one-degree rise in temperature.  They disagree on the feedback effect produced by this one degree of temperature rise.

As the Earth warms, snow melts, reducing the amount of light energy reflected into space.  Further, water vapor acts as a greenhouse gas, and as the atmosphere warms, it absorbs more water vapor.  The conventional wisdom among climate researchers holds that one degree of warming from CO2 produces a net warming of three degrees due to these positive feedbacks.

However, a significant minority of researchers believe that the net effect of CO2 is much smaller.  Water vapor produces cloud cover, and increased water vapor should produce more clouds, which would reflect light back into space.  These researchers project one degree or less of warming per doubling of CO2.

So how concerned should we be about climate change?  That depends on how sensitive our climate is to CO2.  If doubling CO2 raises global temperature by three degrees Celsius, then man-made climate change is a major concern.  If doubling CO2 raises global temperature by one degree Celsius, it is likely a mild and manageable problem.

If the conventional wisdom is correct, that doubling CO2 results in three degrees Celsius of warming, we still have close to a century to pursue mitigation and adaptation.  We have a century to pursue nuclear energy, our only off-the-shelf alternative to fossil fuel, and a century to adapt to sea level rise.  We have cause for concern, cause for action, but we do not have cause for panic.