Monday, August 23, 2010

Forcing the Climate

In an editorial for the Houston Chronicle earlier this month, Walter Cunningham (a geophysicist and former Apollo astronaut) claimed that global warming is a natural phenomenon that is unrelated to human activities. "Scientists have long known that the sun, oceans and variations in the Earth's orbit are the principal drivers of climate change", he wrote. It is actually true that changes in the Earth's orbit can explain the periodic ice ages and warm periods of the past. But all of the known sources of natural variation cannot account for the warming observed in the last few decades.

Three properties of the Earth's orbit are known to change on long timescales, from 26,000 to about 100,000 years. These include how non-circular the orbit is (the eccentricity), how titled the rotation axis is compared to the orbital plane (the obliquity), and the direction of that tilt at a given point in the orbit (precession). The effect of these variations on the length of the seasons and the amount of sunlight received by the Earth throughout the year was worked out in the 1930's by a Serbian mathematician. Comparing the predictions of this natural variation in solar energy received by the Earth with the reconstructed surface temperature over the past 250,000 years shows a clear correlation, at least until recently.

What about the Sun itself? Could variations in its energy output be responsible for recent changes in the Earth's climate? In fact, the magnetic field of the Sun flips every 11 years and during this time the total energy output changes by about 0.1 percent -- the Sun emits more energy during periods of high magnetic activity. However, the change in energy output from one magnetic cycle to the next can only account for about 10 percent of the warming over the past century, and it has actually been going down since the 1960's while the temperature of the planet has continued to rise dramatically.

Climate scientists have tried to account for global warming using only these natural contributions, but they can only match the recent increases in temperature when they also consider human activities. The most significant of these activities is the release of heat-trapping pollution like carbon dioxide and methane, but the scientists also consider the cooling effects of aerosols and modern changes in land use patterns. On balance, considering all of the natural and human-induced contributions to climate change, the conclusion is unambiguous. The planet is warming because of us.

It is human nature to prefer a reassuring lie over an inconvenient truth. We want an easy way to escape the difficult situation we have created for ourselves. In a rebuttal to Cunningham's editorial, Nobel prize winner Robert Curl put it simply. "How much does the present owe to the future? This is a hard philosophical question. Neither Cunningham nor I will live to see how this turns out, but I expect my grandchildren to. I prefer that the planet they inherit is not a world in distress."