# 8 CO2 lags temperature 3.8%
skepticalscience.com
CO2 lags temperature
The skeptic argument...Joe Barton to Al Gore: "You display a timeline of temperature compared to CO2 levels over a 600,000 year period. You indicate this is conclusive proof of the link of increased CO2 emissions and global warming. A closer examination reveals something entirely different. I have an article from Science magazine that explains a rise in CO2 concentrations actually lagged temperature by 200 to 1000 years. CO2 levels went up after the temperature rose. Temperature appears to drive CO2, not vice versa. You’re not just off a little. You’re totally wrong." (Source: Office of Congressman Joe Barton)
What the science says...
Does temperature rise cause CO2 rise or the other way around? A common misconception is that you can only have one or the other. In actuality, the answer is both.
Milankovitch cycles - how increased temperature causes CO2 rise
Looking over past climate change, scientists have observed a cycle of ice ages separated by brief warm periods called interglacials. This pattern is due to Milankovitch cycles - gradual, regular changes in the earth's orbit and axis. While there are several different cycles, the dominant climate signal is the 100,000 year eccentricity cycle as the Earth's orbit changes from a more circular to a more elliptical orbit (Petit 1999, Shackleton 2000).
The eccentricity cycle causes changes in insolation (incoming sunlight). When springtime insolation increases in the southern hemisphere, this coincides with rising temperatures in the south, retreating Antarctic sea ice and melting glaciers in the southern hemisphere (Shemesh 2002). As temperature rises, CO2 also rises but lags the warming by 800 to 1000 years (Monnin 2001, Caillon 2003, Stott 2007).
How does warming cause a rise in atmospheric CO2? As the oceans warm, the solubility of CO2 in water falls (Martin 2005). This causes the oceans to give up more CO2, emitting it into the atmosphere. The exact mechanism of how the deep ocean gives up its CO2 is not fully understood but believed to be related to vertical ocean mixing (Toggweiler 1999).
The greenhouse effect - how increased CO2 causes temperature rise
When there's more CO2 in the atmosphere, the earth absorbs more heat. Shortwave radiation from the sun passes straight through our atmosphere and is absorbed by the earth. The earth reemits it as longwave (infrared) radiation which is partially absorbed by atmospheric CO2. This is the greenhouse effect. CO2 lets energy in, doesn't let as much get out.
CO2 warming explains how the relatively weak forcing from Milankovitch cycles can bring the planet out of an ice age. It begins with the high southern latitudes (eg - Antarctica) warming and releasing CO2 from the oceans. The CO2 mixes through the atmosphere, amplifying and spreading the warming to northern latitudes (Cuffey 2001). This is why warming in the southern hemisphere precedes warming in the northern hemisphere (Caillon 2003). This is confirmed by marine cores that show tropical temperatures lag southern warming by ~1000 years (Stott 2007).
Climate sensitivity - how CO2 amplifies temperature increase
Climate sensitivity is defined as how much global temperature increase if we doubled CO2. Studies of past CO2 and temperature records have helped quantify exactly how sensitive our climate is to changes in CO2.
Temperature and various forcings (including CO2) over the past few centuries shows a climate sensitivity between 1.5 to 6.2°C (Hegerl 2006). One study combines the results from various paleontological studies to narrow climate sensitivity to around 2.5 to 3.5°C (Annan 2006). Basically, multiple studies covering many different periods of earth's history confirm that when CO2 is doubled, global temperatures go up around 3°C.
So what does the CO2 lag tell us? The behaviour of CO2 in the past is well understood by climatologists and confirms the amplifying effect of CO2 in the atmosphere. Also, sharp temperature rises in the past indicate how sensitive climate is to change. Our past history shows how our climate is prone to "tipping points" where warming can lead to positive feedbacks sparking a runaway warming effect.
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Related ArgumentsClimate sensitivity is low
Further readingThat CO2 lags and amplifies temperature was actually predicted in 1990 in a paper The ice-core record: climate sensitivity and future greenhouse warming by Claude Lorius (co-authored by James Hansen):
"Changes in the CO2 and CH4 content have played a significant part in the glacial-interglacial climate changes by amplifying, together with the growth and decay of the Northern Hemisphere ice sheets, the relatively weak orbital forcing"
The paper also notes that orbital changes are one initial cause for ice ages. This was published over a decade before ice core records were accurate enough to confirm a CO2 lag (thanks to John Mashey for the tip).
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