The Last 100 Years in Perspective
Levi32, 5:43 PM GMT on February 17, 2011
Although I generally try to stick to the tropics in this blog, it is a fact that other things like AGW (Anthropogenic Global Warming) are discussed in the off-season by many on a daily basis, and often by this site's very founder, Dr. Masters. No matter which side you are on, it is a very relevant meteorological (and political?) issue in today's society that is shaping lots of things in the world as time goes on, and it is worth discussing once in a while. In search for the truth, which is what science is, I often feel the urge to dig down deep into the data myself and see what I can find.
Recently I decided to stop scrutinizing the 20th century for a moment and zoom out a lot farther for a bigger perspective. The data that we possess before 1850 is almost all in the form of paleoclimatological data sets, temperature reconstructions based off of other clues in nature before a widespread weather observation network existed. The longest records that we currently have are those of ice cores from glaciers or polar ice caps, some extending back for many thousands of years. These ice cores are used to estimate temperature using oxygen isotope data, and are some of the better proxies that we utilize. All paleoclimatological data sets must be considered with scrutiny, as they all have problems, but they do capture the general trends and patterns.
I decided to plot three of the best ice cores that we have and examine their data together: the Vostok core from Antarctica, the Greenland core from the GISP2 project, and the Penny Icecap core from Baffin Island, Canada. All ice core data used in this post was obtained from the National Climatic Data Center, here. Below I have plotted the last 12000 years of these three cores together, showing the end of the last ice age between 10000 and 9000 B.C., and the current holocene interglacial period (a warm period in between ice ages).
Figure 1. The last 12000 years of temperature data from the Vostok, Greenland, and Penny ice cores. Anomalies were calculated using the 9000BC-1900AD mean.
At first glance this graph doesn't tell us a whole lot. It is common knowledge that the Earth goes through ice ages and interglacials due to orbital variations around the sun. However, a great way to compare the last 100 years of global warming to history is by looking at the variation in the data set. Based on the NOAA and HadCRUT3 temperature records, the average global warming between 1910 and 2010 has been about 0.65C. The warming during the last century has often been described by climate scientists as "unprecedented" and "very concerning." It would be helpful to examine century-scale temperature swings in the past and compare them to the temperature changes we are seeing now. I took the 100-year temperature change of both the Greenland and Penny data sets (Vostok has too coarse of a time interval between measurements), using the Penny 25-year means and the Greenland 20-year means provided by their respective records. This will give us a good indicator of climate variance during the period of record. I then overlaid a static y-value representing the global warming that occurred between 1910 and 2010 that was mentioned above in order to compare.
Figure 2. Average 100-year temperature change in degrees Celsius for the Greenland and Penny ice cores. The global warming between 1910 and 2010 according to the NOAA and HadCRUT3 data sets is shown by the red line running across the graph.
Notice the massive variation at the far left side of the graph where the Earth was still coming out of the last ice age. This is something ice cores have revealed to us about climate variation, that it can be rather wild and erratic during cold periods, but tamer with less variance during warm periods. Nonetheless, what jumps out about the graph is how the global warming of 0.65C during the last 100 years does not appear to be very significant against the average variance observed during the current interglacial. In fact, it is well-within the range of variation that often exceeds 1C on century timescales.
Before drawing any conclusions, it would be wise to consider the fact that we are using two locations rather far north in the northern hemisphere as a rough proxy for global temperature patterns. Although the general patterns of global temperature will indeed be depicted just fine by these cores, the temperature variance may not be, given the polar location of the drilling sites. Thus, it would be prudent to compare not just the global change over the last 100 years, but the local change at the ice core locations as well.
Using NOAA and HadCRUT3 data from the Koninklijk, Netherlands Meteorological Institute Climate Explorer, I derived the temperature difference between the 1890-1910 period and the 1990-2010 period for the Greenland area (65N-80N, 305E-340E) to be about 1.15C of warming, and for the Baffin Island area (61N-70N, 270E-298E), about 0.43C of warming. The ice cores themselves show significantly less 100-year warming as recently as 1980 and 1992, but we will honor our current global temperature data sets for this comparison. Taking these values, I tacked them onto the end of their respective ice core data sets and overlaid them on the same graph as before, both in red so that they stand out.
Figure 3. Same graph as figure 2, but with 1910-2010 warming from the local drilling areas tacked onto the end of the graph.
Again, as with the global change, the local change does not appear to deviate from the normal noisy variation during the interglacial period. This is an interesting find given the statements by scientists that the current warming is at an unprecedented rate compared to historical records.
There are two big things that can be taken away from this ice core data. One is that, according to the cores, we have certainly been warmer at multiple times during the last 10000 years than we currently are today (see figure 1). Second, and most importantly, the temperature swing during the last 100 years is in fact not very significant compared to the average 100-year variation during the current interglacial period. According to these ice core records, the current global warming amounts to nothing more than just one of the hundreds of tiny spikes making up the temperature record of the last ten millennia.
We shall see what happens!
Updated: 8:57 PM GMT on February 17, 2011
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