Taking raw temperature data from the Global Historical Climatology Network (GHCN-Monthly) data base, we can plot average temperatures taken from ground stations over the previous three centuries. I took all the data from all weather stations in all countries and averaged the temperature values per month. Plotting each month separately you get the following graphs.
Then to get a better idea of what's going on overall I calculated the average global temperature per year, and with minimum amd maximum values.
Assuming that the original data is good, and you can find descriptions of how it was collected on the GHCN site, the results seem pretty clear. Over the last three centuries there has been a steady rise in average global temperatures. However, no recent exponential trend is evident. Instead this looks more or less like a linear increase, with the slope beginning before the industrial revolution gets into high gear in the 19th century. The overall average graph (blue line) in the above image shows this best, and I can hold a ruler up to the screen and get a pretty good fit.
The good, the bad and the inconvenient
So the bad news is that after even a cursory examination of the data we can clearly see that global warming is a real phenomena rather than merely a PR stunt or something else which was invented by Al Gore. However, the good news is that this doesn't appear to be significantly related to human activities - despite our inflated sense of self importance as a species. If the warming were primarily due to industrial activity I would expect to see a significant deviation upwards, approximately half way along the graph as factories multiply and begin to belch out smog, steam engines are built, the internal combustion engine is invented and eventually jumbo jets cruise through the atmosphere.
Where you can see alarming deviations though is if you take a small window along the graph, corresponding to a few decades. From the perspective of an individual human life span this seems like a long time, but in the scheme of things it's really not. On smaller scales random deviations begin to look significant, and you could perhaps persuade yourself that an exponential trend was beginning - rather like seeing faces in clouds or trying to find small features within noisy camera images. Also if you're selective about which data you choose then all kinds of theories are potentially supportable. So if I only look at the average minimum temperatures, or if I only look at the average temperature for November I might be able to persuade myself that some sort of anthropogenic foul play was going on.
So which camp would this put me in? Probably I'm a non-anthropogenic or maybe a slightly-anthropogenic warmer. Humans may have had some effect, but it's just not highly noticeable within the data. From this I'd predict that all the proposed carbon dioxide capture, footprint and trading schemes will have no noticeable impact upon the overall temperature trend. This still means that rising sea levels and changing weather patterns could be a significant problem though, and we should take whatever precautions are considered necessary to mitigate against these risks.
Approximate linearity
A line can be fitted fairly well to the average annual global temperatures. There's more variation in the past, but I expect that this can be explained due to less accurate thermometers and calibration methods. It may also be possible to fit a shallow curve.
The more imaginative you get, the more complicated lines or curves could be fitted to the graph, but usually according to the principle of Occam's razor the simplest explanation tends to be the best one. So with a bit of creativity we could maybe fit a nearly horizontal line to the left hand side, and a steeper slope to the right hand side. Or a shallow parabola or circular curve might be fitted. Or using a sliding windowed average the graph could be turned into a complicated looking series of oscillations.
In the interests of full disclosure, so that you can see that I havn't introduced my own fudge factors, the C# source code used to produce these graphs can be downloaded here. The program loads the data from a file called v2.mean and saves a few CSV files, which can then be visualized using the chart tool within OpenOffice spreadsheet.
8 comments:
The IPCC report says:
"Most of the observed increase in global average
temperatures since the mid-20th century is very
likely due to the observed increase in anthropogenic
greenhouse gas concentrations."
This seems rather questionable.
The question is how do they know that humans are likely to be the cause? As far as I can tell this seems to be merely an assumption.
The temperature data tends to suggest that if there is an anthropogenic component it's weak compared to the overall warming trend, which begins long prior to the mid 20th century. There might be other evidence suggesting a strong anthropogenic link, but I'm not aware of it.
Also, looking at temperatures only for the last few decades it's very easy to "see faces in clouds", since there is a high degree of variability/noise. It's only when you put it into the context of the last three centuries that you can see the trend.
IPCC policy summary says: "The observed patterns of warming, including greater
warming over land than over the ocean, and their changes over time, are only simulated by models that include anthropogenic forcing."
The idea that "humans did some of it" seems quite believable and defensible to me. However, the idea that "humans did more than 50% of it" puts the IPCC's neck on the chopping block. The scientific support for that seems far from obvious. It seems like a central claim - and I would not be remotely surprised by it being overturned.
Ok, so the anthrophogenic evidence comes from simulation. Simulations are only as good as the assumptions built into them though, and it's possible to get all kinds of results out of simulations - pretty much any result you like in fact.
The question then arises as to what simulations the IPCC report is referring to, and whether its results are independently verifiable. Relying solely upon simulation as evidence would seem to be problematic. We do rely upon simulations in many different endeavors ranging from designing engines to predicting the positions of planets, but a living system like the Earth's biosphere is significantly more complicated.
Probably humans are having some effect on the atmosphere, but I don't think it's possible to ascribe a percentage warming value to this without more information.
From their technical report:
"Estimates of Northern Hemisphere temperatures over the last one to two millennia, based on natural ‘thermometers’ such as tree rings that vary in width or density as temperatures change, and historical weather records, provide additional evidence that the 20th-century warming cannot be explained by only natural internal variability and natural external forcing factors. Confidence in these estimates is increased because prior to the industrial era, much of the variation they show in Northern Hemisphere average temperatures can be explained by episodic cooling caused by large volcanic eruptions and by changes in the Sun’s output. The remaining variation is generally consistent with the variability simulated by climate models in the absence of natural and human-induced external factors. While there is uncertainty in the estimates of past temperatures, they show that it is likely that the second half of the 20th century was the warmest 50-year period in the last 1300 years. The estimated climate variability caused by natural factors is small compared to the strong 20th-century warming."
The list of models used is here:
cccma_cgcm3_1_t47 cccma_cgcm3_1_t63 cnrm_cm3 gfdl_cm2_0 gfdl_cm2_1 giss_aom giss_model_e_h giss_model_e_r iap_fgoals1_0_g inmcm3_0 ipsl_cm4 mri_cgcm2_3_2a ncar_ccsm3_0 miroc3_2_hires miroc3_2_medres miub_echo_g ncar_pcm1 ukmo_hadcm3 ukmo_hadgem1
Good luck to anyone wanting to check their working.
So to understand the strong anthropogenic case you would need to get into the details of particular climate models.
http://en.wikipedia.org/wiki/Climate_model
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