The Big Picture 2

The Big Picture 2

Let's return to the figure showing the broad histories of atmospheric CO₂ (with estimates from different techniques shown by different lines plus the shaded band at the bottom), and of ice on the planet (glaciers extending farther toward the equator are shown by longer bars hanging down from the top). Clearly, CO₂ and ice moved in opposite directions, with rising CO₂ occurring with melting ice. The figure has been “smoothed”, and so doesn’t show the details of the shorter-lived events discussed just above.

Estimates of the history of CO₂ over the last 400 million years (MA), with today on the right. The shaded band is output from a model calculating CO₂ from inputs such as volcanic eruptions and outputs such as fossil-fuel formation; the other curves represent estimates of atmospheric CO₂ from different techniques applied to sedimentary deposits. The bars hanging down from the top show the extent of ice at low elevation (very high mountains can have ice even in times with warm temperatures at low elevations where most people live, but low-elevation ice requires cooler temperatures in most places); times with no bar had no ice at low elevation.

Source: From CCSP, 2009: Past Climate Variability and Change in the Arctic and at High Latitudes. A report by the U.S. Climate Change Science Program and Subcommittee on Global Change Research [Alley, R.B., J. Brigham-Grette, G.H. Miller, L. Polyak, and J.W.C. White (coordinating lead authors)]. U.S. Geological Survey, Reston, VA, 257 pp. modified from IPCC (2007).

By themselves, the correlations just discussed between CO₂ and temperature do not prove that CO₂ caused the warmth. But, straightforward physics shows the warming effect of CO₂. And, although warming can raise CO₂ over short times, as at the start of the PETM or the ends of the ice ages, over long times warming lowers CO₂ by causing faster rock weathering and fossil-fuel formation. Thus, the prolonged high levels of CO₂ during warm times were not caused by the hot climate; instead, such high levels were caused by faster volcanism, or thicker soils slowing access of CO₂ to react with rocks, or other geological reasons.

The physics, and the lack of other plausible causes despite major efforts to find something, show that the warmth was caused by the CO₂. Testing our understanding by “retrodicting” what happened—starting with the causes and simulating the effects of the climate changes—shows that our models work well. If there is a problem, the world has changed a little more in response to CO₂ than expected from the models.