BOE: Rates of Change in Antarctic Ice Core Temperatures
Another line of argument for a catastrophic impact of climate change is in the rate of change of global temperatures and/or ocean acidification. In this post, I’m going to poke a bit at the rates of change in global temperature via ice core data. This step – from globally to particular Antarctic locations – is questionable and I’ll look closer to see if this is justified in a latter post. With that said, what do the ice cores hold ….?
First up, as seen in the chart above, is the venerable 420K Vostock Ice Core data provided by the NOAA Paleoclimate data store. I have simply taken the time series and calculated change in temperature (dT)(degC) for every discrete time step in the series (dt)(years). Dividing dT by dt (dT/dt) provides the average rate of change over that time step. I have multiplied by 100 years to get the changes in temperature per century. I have overlain a simple exponential decay curve on the data.
The Vostock chart certainly supports the idea that changes in temperature greater than about 1C per century are very unusual and greater than 1.5C per century are down right rare. But lets take a look at a second chart, below, for the EPICA Dome C 800K ice cores.
The EPICA data contains much greater frequency of high rates of warming (>1C/century). One reason for this is the finer resolution of the EPICA data in which 36% of the time steps are less than 50 years, as opposed to the Vostock data in which only 7.5% of the time steps are less than 50 years. And shorter time spans show a much greater range of warming/cooling trends. The scatter plot below shows the discrete warming/cooling changes plotted against the length of the time span for that step for both time steps.
One take away is that over the Quaternary, sharp changes in Antarctic temperatures only occur over periods of several decades or less. Centuries long warming tends to occur at a rate of about 1C/century or less. 11% of the data with a time step between 50 and 100 years showed warming greater than 2C/century, and 44% of the data with time steps less than 50 years show this much warming/cooling. But 0% of the data with a time step greater than 100 years showed warming that great.** If the Antarctic data can be extrapolated to the world, and if the warming predicted by modeling of ~2C/century is sustained for periods longer than century, AGW will force a truly unusual rate of warming.
But there could be problem. The above statistics are only meaningful if the time steps in the core data are roughly random. If an analysis method was used where sampling resolution increased as deuterium changes increased, then the frequency data is forced and the statistics quoted above meaningless. A quick look Jouzel 2007 and the SOM found no indication of the sampling methodology. Perhaps an alternate method would be to extrapolate the data along a time series and then resample at different time scales.
Trying this idea out, I took the EPICA Dome C data and fitted it to a time line with a time step of one year via a linear approximation function (approx). Then I sampled the data at different time steps of 1, 10,100,1000 years. Sure enough, the resampled data has a much lower percentage of high value trends. Trends of 2C/century or greater occurred only 4.3, 3.8, 0.1, and 0 per cent of the time when averaged over 1, 10, 100, and 1000 year time steps. (I am guessing that the original sampling method deliberately sampled at higher resolution when it detected greater changes – but this is only a guess. It could be that changes associated with high rates of change in temperature caused physical changes in the samples which are reflected in this data.)
Having scoped the data space a bit, time to zoom on time scales of interest in AGW. What follows is for time steps of 60, 120, 240, and 480 years. For these time periods, warming greater than 2C/century occurs 1.1% in 60 year time steps and drops off very quickly. So a global warming trend this high sustained for over a century would be very unusual (if Antarctic and global trends are comparable).
What about a lower trend of 1C/century? Even this lower trend is pretty unusual when sustained over time, occurring in about 6% of the 60 year cuts but in less than 1% of the 240 year trends and 0% of the 480 year trends.
Bottom line: If AGW causes a sustained warming (centuries) similar to that seen over the last decade or projected in IPCC AR4 models, that would be a highly unusual climate change and it seems likely that the rate of change would be of more consequence to natural systems than the magnitude of the rise itself.
Petit, J.R., et al., 2001,
Vostok Ice Core Data for 420,000 Years,
IGBP PAGES/World Data Center for Paleoclimatology
Data Contribution Series #2001-076.
NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
Jouzel, J., et al. 2007.
EPICA Dome C Ice Core 800KYr Deuterium Data and Temperature Estimates.
IGBP PAGES/World Data Center for Paleoclimatology
Data Contribution Series # 2007-091.
NOAA/NCDC Paleoclimatology Program, Boulder CO, USA.