I came across something a couple of days ago which gave me pause for thought. I made a passing reference to it in a recent blog comment, but this is my first proper attempt to outline my current understanding.
The current state of the science
The image below illustrates the carbon cycle.
(Image source: NASA)
As you can see from the image, there are a number of emitters of carbon dioxide (sources) and a number of absorbers (sinks). It is assumed that until fairly recently (the latter decades of the ninetheenth century) carbon dioxide sources and sinks were more or less in balance.
Human activities have provided a new source of carbon dioxide, through fossil fuel use and other activities. Our activities have also change the CO2 balance through, for example, changes in agricultural methods and other land use changes. And unsurprisingly, this has led to an increase in measured atmospheric CO2. We can see this increase in this graph of atmospheric CO2 concentrations from Mauna Loa, below
(Image source: NOAA)
So: humans start adding more CO2 to the atmosphere than occurs naturally, and the CO2 increases. It wouldn’t seem to take a genius to work out that one thing caused the other. But scientists wanted to be sure, so they looked at various lines of evidence, the most important of which appears to be the ratio between different Carbon isotopes in atmospheric CO2. I’ll let Real Climate explain:
CO2 produced from burning fossil fuels or burning forests has quite a different isotopic composition from CO2 in the atmosphere. This is because plants have a preference for the lighter isotopes (12C vs. 13C); thus they have lower 13C/12C ratios. Since fossil fuels are ultimately derived from ancient plants, plants and fossil fuels all have roughly the same 13C/12C ratio - about 2% lower than that of the atmosphere. As CO2 from these materials is released into, and mixes with, the atmosphere, the average 13C/12C ratio of the atmosphere decreases.
Isotope geochemists have developed time series of variations in the 14C and 13C concentrations of atmospheric CO2. One of the methods used is to measure the 13C/12C in tree rings, and use this to infer those same ratios in atmospheric CO2. This works because during photosynthesis, trees take up carbon from the atmosphere and lay this carbon down as plant organic material in the form of rings, providing a snapshot of the atmospheric composition of that time. If the ratio of 13C/12C in atmospheric CO2 goes up or down, so does the 13C/12C of the tree rings. This isn’t to say that the tree rings have the same isotopic composition as the atmosphere - as noted above, plants have a preference for the lighter isotopes, but as long as that preference doesn’t change much, the tree-ring changes wiil track the atmospheric changes.
(Source: Real Climate).
So that would appear to be that. We have a correlation between increasing CO2 and human emissions, and we have the 12C/13C ratio which appears to show that the extra CO2 is from human sources. End of story.
Well, not quite. Earth’s temperature is, as we know, increasing. The global average temperature has increased by around around 1.5°C since 1900. It is asserted that this is due in large part from the increase in CO2, which is known to be a greenhouse gas.
There is a long geological record of the correlation between atmospheric CO2 concentrations and temperature, for example as provided by the Vostok ice cores, however it is also clear from the ice core data that CO2 concentration lags temperature changs by between 200 and 800 years. (Source: Caillon et al., 2003, Science magazine (208k PDF))
Does this mean that CO2 isn’t responsible for our increasing global temperature? Is it possible that the correlation is the other way around, and that a warming planet is actually causing the increase? Again, lets’s turn to Real Climate for the answer to this question:
Does this prove that CO2 doesn’t cause global warming? The answer is no.
The reason has to do with the fact that the warmings take about 5000 years to be complete. The lag is only 800 years. All that the lag shows is that CO2 did not cause the first 800 years of warming, out of the 5000 year trend. The other 4200 years of warming could in fact have been caused by CO2, as far as we can tell from this ice core data.
The 4200 years of warming make up about 5/6 of the total warming. So CO2 could have caused the last 5/6 of the warming, but could not have caused the first 1/6 of the warming.
(Source: Real Climate).
Game, set and match, to the AGW hypothesis. CO2 causes warming, and the fact that the ice core shows that warming occurs *before* an increase in CO2 is a special case which although it might seem to suggest the correlation is δT -> δCO2, it could be the other way around.
Game over? Not quite
A recent podcast by Murry Salby, Chair of Climate, Macquarie University, entitled Atmospheric Science, Climate Change and Carbon - Some Facts, would seem to cast some doubt on the notion that increases in CO2 are due to human activity.
Summary of the podcast:
Carbon dioxide is emitted by human activities as well as a host of natural processes. The satellite record, in concert with instrumental observations, is now long enough to have collected a population of climate perturbations, wherein the Earth-atmosphere system was disturbed from equilibrium. Introduced naturally, those perturbations reveal that net global emission of CO2 (combined from all sources, human and natural) is controlled by properties of the general circulation - properties internal to the climate system that regulate emission from natural sources. The strong dependence on internal properties indicates that emission of CO2 from natural sources, which accounts for 96 per cent of its overall emission, plays a major role in observed changes of CO2. Independent of human emission, this contribution to atmospheric carbon dioxide is only marginally predictable and not controllable.
Professor Murry Salby holds the Climate Chair at Macquarie University and has had a lengthy career as a world-recognised researcher and academic in the field of Atmospheric Physics. He has held positions at leading research institutions, including the US National Center for Atmospheric Research, Princeton University, and the University of Colorado, with invited professorships at universities in Europe and Asia. At Macquarie University, Professor Salby uses satellite data and supercomputing to explore issues surrounding changes of global climate and climate variability over Australia. Professor Salby is the author of Fundamentals of Atmospheric Physics, and Physics of the Atmosphere and Climate due out in 2011. Professor Salby’s latest research makes a timely and highly-relevant contribution to the current discourse on climate.
(You can listen to the podcast here.)
Sounds pretty dull, doesn’t it? But the key point in the podcast is this: that although the decline in 12C/13C ratios can be explained by human emissions, it can also be explained by other sources which are also rich in 12C. Salby then argues that we do not understand the composition of the natural sources sufficiently well, nor even really the composition of the anthropogenic sources, to be able to rely on the 12C/13C argument.
Does this disprove the hypoothesis that human activity is causing the temperature to rise? No, it doesn’t, but it does take away a key support from this so-called “settled” science.