I recently wrote up a brief primer on the whole CO2 lag vs lead misconception for http://yaleclimatemediaforum.org/ which might help clear up any confusion. Also, feel free to point out any glaring errors on my part.
Common Climate Misconceptions: CO2 as both a Feedback and Forcing in the Climate System by Zeke Hausfather "The temperatures and carbon dioxide concentrations have been correlated, but we know for sure that the temperature was the cause and the concentration was its consequence, not the other way around... It follows that the CO2 greenhouse effect has not been important in the history and we shouldn't expect that it will become important in the future," writes (http://motls.blogspot.com/2006/07/carbon-dioxide- and-temperatures-ice.html) Harvard University string theorist Lubos Motl. Similarly, the advocacy site CO2Science argues (http:// www.co2science.org/scripts/CO2ScienceB2C/articles/V4/N14/C1.jsp) that this apparent lag "should put to rest the notion that atmospheric CO2 is a major driver of climate change." The argument that the lag between temperature and CO2 in the paleoclimate record casts doubt on the importance of CO2 in modern climate change has gained prominence in recent years. However, it is based on a fundamental misconception of the role that CO2 plays in glacial transitions. In the geologic past, CO2 and other greenhouse gases acted primarily as feedbacks to external climate forcings. Our current experience is largely unprecedented, as we are directly emitting CO2 and other greenhouse gases to catalyze climate changes. This distinction between the duel roles of greenhouse gasses as both forcings and feedbacks is crucial in understanding the behavior of these gasses in the paleoclimatic and present periods. The figure above shows changes in temperature and CO2 concentrations over the past 450,000 years, during which time four distinct ice ages occurred. It is immediately apparent that some relationship seems to exist between temperature and CO2, as the curves are strongly correlated. During both the transition to and out of a glacial period, CO2 concentrations appear to lag temperature changes by an average of around 600 to 1000 years (though some recent research (http:// rabett.blogspot.com/2007/09/there-goes-another-one.html) 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. Luckily for us, we 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 (http://en.wikipedia.org/wiki/Milankovitch_cycles) refer to the effects of periodic variations in the orbit of the Earth 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 eliptical the Earth's orbit is), the axial tilt of the Earth (known as obliquity), and the 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 the change total solar forcing in a way that helps explain Earth's periodic ice ages, as shown below. Initial temperature changes at the beginning and end 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 (since warmer water can absorb less CO2 from the atmosphere), and warmer temperatures increase the decay rate of vegitative matter. Similarly, cooling at the start of an ice age increases ocean uptake and reduces emissions from vegitative 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, succenctly explains (http://www.realclimate.org/index.php/ archives/2007/04/the-lag-between-temp-and-co2/): "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 than those that occurred in the past. For one thing, they are happening at a much greater rate than changes in past glacial periods. Significant climate changes are occuring over the course of decades and centuries, rather than millenia. We know that Milankovitch forcings are not having a significant impact on changes observed over the past century, as they do not opperate on such a short timescale and we have good measurements of what their effect should be. For the first time, greenhouse gasses are primarilly acting as forcings in the climate system instead of as a feedback to external forcing (though their role as feedbacks are still important; witness the discussion of a potential methane feedback from melting arctic permafrost). While the lag between temperature and greenhouse gas changes in the paleoclimate record is important to our understanding of the funciton 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 (http://www.realclimate.org/index.php/archives/2007/04/the-lag- between-temp-and-co2/), "the ice core data in no way contradict our understanding of the relationship between CO2 and temperature". --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups Global Change ("globalchange") newsgroup. Global Change is a public, moderated venue for discussion of science, technology, economics and policy dimensions of global environmental change. Posts will be admitted to the list if and only if any moderator finds the submission to be constructive and/or interesting, on topic, and not gratuitously rude. 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