Last Saturday I took the kids up to the Mesa Lab to see the “Climate Discovery” exhibit at the National Center for Atmospheric Research (NCAR). NCAR has a lot of graphs and other illustrations about climate change. I asked the tour guide how the climatologists sort out which is the cause and which is the effect, between carbon dioxide and temperature?
The guide brought out Caspar Ammann and Carrie Morrill, both of whom I knew from past presentations (they both have PhDs). This was a public conversation at a public event, so I can report it here. Caspar pointed out the correlated graphs of CO2 and temperature proxies, taken from the Vostok ice core (~420,000 years). He remarked that it is difficult to sort out cause and effect from the ice cores alone. As the air bubbles become trapped in the ice during compression over the first 100 years or so, some CO2 migrates by diffusion between the annual layers. The effect is that the annual CO2 and temperature signals are not as precise over very short time scales, and the lead/lag relationship between the peaks can be obscured. I didn't pursue this line of inquiry further because I plan to investigate the ice core data myself as a project for Climatology class during the upcoming fall semester.
However, Carrie told me that the increase in CO2 displayed by the Keeling Curve can indeed be attributed to human burning of fossils fuels, and here's how: The air is getting older. Carbon-14 has a half-life of 5,730 years, and it can be used to date objects up to about 50,000 years old. Carbon-14 decays into Nitrogen-14 through beta decay:
http://en.wikipedia.org/wiki/Carbon-14
Objects older than 50k years have only the N-14 isotope. By measuring the ratio of Carbon isotopes in organic material, one can determine how many years have passed since that organism was last exposed to the air. We can measure the age of all this carbon dioxide that's building up in the atmosphere. The following article at RealClimate.org states:
How do we know that recent CO2 increases are due to human activities?
http://www.realclimate.org/index.php?p=87
“Sequences of annual tree rings going back thousands of years have now been analyzed for their 13C/12C ratios. Because the age of each ring is precisely known** we can make a graph of the atmospheric 13C/12C ratio vs. time. What is found is at no time in the last 10,000 years are the 13C/12C ratios in the atmosphere as low as they are today. Furthermore, the 13C/12C ratios begin to decline dramatically just as the CO2 starts to increase -- around 1850 AD. This is exactly what we expect if the increased CO2 is in fact due to fossil fuel burning.”
I spent some time looking for a graph of Carbon isotope ratio vs. time that would show the “dramatic” change at 1850, but couldn't find one. If you have a link, please post it below.
If temperature rise were currently forcing CO2 rise “naturally”, we would expect newer CO2 to get flushed from the earth's surface. But it's old CO2 that's getting flushed instead, and the most obvious cause is human burning of fossil fuels.
The astute reader may wonder, How do we know that the older CO2 isn't merely coming out of the Arctic peat bogs? The bogs will flush more CO2 as the Arctic climate gets warmer, and peat bogs are really old.
Carl's answer: The trend of older atmospheric CO2 has been going on since about 1850, which is the start of the Industrial Revolution:
http://www.ipcc.ch/present/graphics/2001syr/large/02.01.jpg
Global temperature rise didn't really get going in earnest until about 1975. We've only been flushing out the peat bogs for about 30 years, not 150 years:
Several comments:
What's clear is that temperature and CO2 concentration is correlated. One may cause the other, or a third something may cause both. But for the sake of argument, if CO2 caused past changes in temperature, what caused the change in CO2? Why can't the same cause be occuring now -- and I'm sorry 10,000 years just isn't a long enough time frame to look at Carbon isotope ratios when the an ice age cycle is on the order of 100,000 years. We don't have isotope measurements covering this same period from a prior ice age.
The industrial revolution began long before 1850 - James Watt improved the steam engine in 1770. Any particular date for the start is inherently arbitrary, and what is of interest is in a sense unknowable - human production of CO2, although I'm sure there are proxies such as human population and industrial production. I could just as well state that the temperature increase coincided with the end of "the little ice age" in 1850. To be fair, my eye puts the start of increased CO2 concentrations in the graph between 1750 and 1800.
What am I to make of the decrease in temperature from 1940 to 1980 in the graph in light of ever increasing CO2 concentrations?
I'm not saying that the earth isn't warming from human released CO2, I'm just saying that I don't think such a claim is currently supported by the data.
Radiocarbon dating is relevant to the 2006 theory of global warming because isotopes indicate that the extra Carbon Dioxide is coming from fossils fuels, not from the “fresh” biosphere. The duration of Carbon-14 (10k years) is useful as a measuring tool for human activities, not the glacial cycle of 100k years.
There are lots of factors that influence the earth's climate, across all different timescales. To name just a few:
The 11-year sunspot cycle.
The El Nino/Southern Oscillation (ENSO).
The North Atlantic Oscillation (NAO).
The Maunder Minimum (of sunspots).
Volcanic eruptions.
The three Milankovitch orbital cycles (100 kyrs, 41 kyrs, and ~21 kyrs):
Milankovitch Cycles in Paleoclimate
http://deschutes.gso.uri.edu/~rutherfo/milankovitch.html
I believe that much (most?) of the paleoclimate variation is temperature forcing of CO2, not carbon dioxide forcing of temperature. The sun just gets brighter or dimmer! Or the earth moves away or toward the sun in Milankovitch Cycles, producing the same effect. In these cases the temperature changes first and causes corresponding changes in CO2. As noted earlier, CO2 can amplify temperature changes.
But CO2 changes can also be the forcing factor. Here are some factors that could possibly cause changes in the atmospheric content of carbon dioxide:
Volcanic outgassing.
Humans burning fossils fuels, starting in the 1800s on a large scale.
Humans dying off in large numbers from Black Death, allowing the European forests to grow back again. (Go to http://en.wikipedia.org/wiki/Black_Death and search for "Thomas van Hoof".)
Australian Aborigines burning off the Australian continent?
I don't know why the temperature decreased from 1940 to 1980. I'll ask my professor when Climo class begins. Remember that the temperature curve is a lot of signals piled on top of each other. I have seen some model runs that duplicate the 20th-century curve shape, so the answer is out there somewhere (probably buried in somebody's Fortran code).
To summarize, here's why I think CO2 forcing is the cause of the current temperature increase:
1. Atmospheric carbon is getting older, pointing to fossils fuels.
2. The Keeling Curve increase leads the temperature curve increase.
3. The observed temperature rise is not enough to account for the measured increase in CO2 from the oceans.