A fundamental misconception about the role that carbon dioxide plays in glacial transitions has helped fuel the argument that the lag time between temperature and CO2 in the paleoclimate record casts doubt on carbon dioxide as an important greenhouse gas.
It’s crucial that media reporting on climate change understand an important distinction between the dual roles of greenhouse gases as both forcings and feedbacks.
In the geologic past, carbon dioxide and other greenhouse gases acted primarily as feedbacks to external climate forcings. Our current and basically unprecedented experience is that we as humans are directly emitting carbon dioxide and other greenhouse gases that affect climate change.
That distinction – greenhouse gases as both forcings and feedbacks – is critical in understanding the behavior of these gases in the paleoclimatic and present periods.
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The figure to the right shows changes in temperature and CO2 concentrations over the past 450,000 years. Four distinct ice ages occurred during that time. The strong correlation of the curves makes it immediately apparent that some relationship seems to exist between temperature and CO2.
During both the transition in to and out of a glacial period, CO2 concentrations appear to lag temperature changes by an average of between 600 and 1,000 years (though some recent research suggests that this lag may be shorter than previously thought).
If CO2 lags behind temperature changes, it stands to reason that some other mechanism is responsible for the initial temperature change. In fact, we do know just such a mechanism that does a reasonably good job accounting for the initial cause and end of ice ages: changes in orbital forcing known as Milankovitch cycles.
Milankovitch cycles refer to the effects of periodic variations in Earth’s orbit on the amount of solar radiation reaching parts of the Earth’s surface. Three cycles are particularly important: the eccentricity of the Earth’s orbit (e.g., how elliptical the Earth’s orbit is); the axial tilt of the Earth (known as obliquity); and the change in the direction of the Earth’s axis of rotation (known as precession). Each of these Milankovitch cycles has a recurring periodic variation, and the overlap of these periods combine to change the total solar forcing in a way that helps explain Earth’s periodic ice ages, as shown below.
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Initial temperature changes at the beginnings and ends of ice ages are caused by changes in orbital forcings. These temperature changes have effects on the natural carbon, nitrogen, and methane cycles. In particular, initial warming reduces ocean uptake of atmospheric carbon (because warmer water can absorb less CO2 from the atmosphere), and warmer temperatures increase the decay rate of vegetative matter. Similarly, cooling at the start of an ice age increases ocean uptake and reduces emissions from vegetative decay.
There are many other important interactions between temperature changes and the carbon cycle and many outstanding questions are only beginning to be answered by paleoclimatologists. However, the role of CO2 and other greenhouse gasses as a feedback to Milankovitch forcings during glacial and interglacial transitions provides a compelling explanation for observed changes. Jeff Severinghaus, professor of geosciences at Scripps Institution of Oceanography, succinctly explains:
The contribution of CO2 to the glacial-interglacial coolings and warmings amounts to about one-third of the full amplitude, about one-half if you include methane and nitrous oxide.
So one should not claim that greenhouse gases are the major cause of the ice ages. No credible scientist has argued that position (even though Al Gore implied as much in his movie). The fundamental driver has long been thought, and continues to be thought, to be the distribution of sunshine over the Earth’s surface as it is modified by orbital variations …
The greenhouse gases are best regarded as a biogeochemical feedback, initiated by the orbital variations, but then feeding back to amplify the warming once it is already underway.
Current climatic changes are substantially different from those that occurred in the past. For one thing, they are happening at a much faster rate than changes in past glacial periods. Significant climate changes are occurring over the course of decades and centuries, rather than millennia. Scientists know that Milankovitch forcings are not having a significant impact on changes observed over the past century, as they do not operate on such a short timescale, and scientists have good measurements of what their effect should be. For the first time, greenhouse gasses are primarily acting as forcings in the climate system instead of as a feedback to external forcing (though their role as feedbacks is still important, as illustrated in discussions of a potential methane feedback from melting arctic permafrost).
While the lag between temperature and greenhouse gas changes in the paleoclimate record is important in understanding the function of greenhouse gasses in the Earth’s climate, and has helped in estimating the effects of CO2 concentrations on radiative forcing, it in no way discredits the conventional knowledge that CO2 is forcing recent changes in the Earth’s climate.
As Eric Steig, a geochemist at the University of Washington who works extensively with ice cores, remarks, “the ice core data in no way contradict our understanding of the relationship between CO2 and temperature”.