When Michael Mann, Raymond Bradley and Malcolm Hughes first presented their findings that suggested the globe’s warming was greater than had been experienced in the past, they never would have predicted that one simple graphic would alter domestic and international politics for decades to come. Mann et al. had unintentionally created the “hockey stick controversy.” In their 1998 paper, Mann et al. were interested in investigating the three following questions: Was the globe warming; If so, was the warming within normal patterns of past warming; Finally, if the globe was warming and the warming was not within the trend of past warming periods, was it caused by humans?
In order to investigate these questions, Mann et al. used two different types of measurement methods. The first type is what I call “controversial measurements.” These are measurements taken using tree rings, corals, ice cores, and other historical records to estimate the earth’s past temperatures. Since these measurements are estimates of unknown past temperatures, the accuracy of the data is questioned, which is why they are deemed “controversial.” Furthermore, those who oppose the claim that the earth is warming reference their dissatisfaction and skepticism of these measurements to support their side of the global warming debate.
The second type used in Mann’s study was what I call “thermometer measurements.” As the name suggests, these measurements are thermometer records of the earth’s temperature. They are more contemporary and considered accurate measurements. These data have been collected since 1902 and continue to be collected and recorded today. By combining the two types of data collected, Mann et al. were able to track the earth’s temperature records back until 1400 (in the data set I used) and in later versions of the paper as early as 1000 AD. This allowed the team of researchers to compare the earth’s temperatures in the past to the earth’s temperature of the present and begin to answer their first question.
Their first question of interest was whether or not the globe was in fact warming. Mann et al. did find evidence that suggested that the earth was warming. Once coming to this conclusion, the team moved onto their second question: was this pattern of warming outside of the normal ebb and flows of global warming and cooling? Mann and his team argued that the data suggested that the contemporary pattern of warming was outside the normal trend. After this conclusion, the group of researchers moved onto their final question, which was: Is global warming caused by humans? After much analysis, Mann et al. determined that it was manmade.
In order to present their findings, the team put together a graphic that illustrated the global warming pattern they believed the data revealed. The graphic they created is what is known today as “The Hockey Stick Graph.” The graph reveals that from 1400 AD until about 1900 the earth’s temperature was relatively consistent. In the graph, this period of time is represented by an essentially flat or steady line and creates the “shaft” of the hockey stick. Around 1900, the earth’s temperature began to rise steadily and continues to increase today. This rise in temperature is represented in the graph by a climbing line and this line creates the blade of the hockey stick.
The graphic alone reveals Mann’s answers to his three questions. It clearly illustrates that the earth’s temperature was and is rising. It also suggests that this increase is greater than the normal heat waves the earth had experienced in the past. This is evident when one compares the increases found in early time periods (the blue line), which were measured using the controversial methods, to the increases found since 1900 (the red line), which are measurements taken using the thermometer method. Once the red line hits the 1990’s, record temperatures were being documented. In no previous period presented on the graph were temperatures of equal magnitude being found. This suggests that the current heating of the earth is some how different from earlier periods. Finally, the graph also suggests that this pattern of abnormal global warming was and continues to be caused by humans. An observer of the graph can see that the earth’s rising temperatures began around 1900. This, of course, is towards the end of the Industrial Revolution, a time period of massive manufacturing, transportation, and mining transformation. More importantly, it was a time period when humans had begun burning fossil fuels. The fact that the peak of the Industrial Revolution coincides with the rise in the earth’s temperatures suggests that the two may be related. If this is the case, then it serves as evidence that global warming is caused by humans.
Despite the initial clarity of the graph, many global warming critics accuse the use of the graphic as misleading. These critics, as previously mentioned, are extremely skeptical of the use of the “controversial measurements” of the Earth’s past temperatures. They argue that the uncertainty in these measurements, when ignored, lead observers of the analysis with a sense that the findings are conclusive and absolute.
In response to the critique that the graph is misleading, however, I argue that Mann et al. were very open and honest with their data. After all, within the hockey stick graph, they reveal the standard errors for the controversial measurements. The gray shaded area behind the blue line is the standard error and any informed reader of the graph would understand that this shaded area represents the level of uncertainty present in the data. In other words, the earth’s “true” past temperature is estimated to be within the shaded area. So, the past temperature may be greater than what the blue line suggests, which would mean that the contemporary warming of the globe is not exceptional. And the Earth’s past temperature may also be less than what the blue line suggests, which would mean the current heating of the earth is more severe than originally believed to be. Had Mann and his fellow researchers excluded the presentation of the standard errors altogether, I would have agreed with their critics that the graph was misleading. However, Mann did present the standard errors and so this criticism is not justified.
In response to the claim that the “controversial measurements” cannot support the conclusion that the globe is in a current state of warming that differs from previous periods of warming because the measurements are inaccurate depictions of past temperatures, I argue that there is a simple way to show how accurate or inaccurate these measurements are. Starting in 1902, there is data collected using both methods (the controversial and thermometer methods). By comparing the measurements taken using both methods, one can see if they produce the same results. I did this very test in my extension and what I found was that both methods measured the earth’s temperature at essentially the same level (look at my second graph under extension). This test also discredits global warming critics’ claim that the data used to measure past temperatures are inaccurate.
Of course, my simple test does not come anywhere near closing this debate. In fact, the continuous research and repeated findings that corroborate with Mann’s initial findings over the last 20 years have not put the controversy to rest. Despite this continuous research and repeated findings, global warming critics continue to find any mistake, or misuse of the data to argue that “climate change” is nothing but a hoax. These critics’ claims are not without ground, especially in light of the recent IPCC report mistake, which accidentally claimed that the Himalayan Glaciers would disappear in 2035 as opposed to 2350. Incidents like these give fuel to critic’s fire. Furthermore, critics use these few mistakes to completely discount the thousands of valid studies that do support Mann’s initial findings.
Currently, the debate between the two sides is extremely political at both the domestic and international level and the fighting between the two sides only seems to get more “heated.” Of course, hot tempers may simply be a side effect to climate change.
#Loading Data
load("/Users/carrielevan/Downloads/hockey.rdata")
#Creating Line for Thermometer Measurements (Contemporary Measurements)
plot(hockey$date, hockey$raw, type="l", col="red", ylim=c(-1.1, 0.8), ylab="Departures in temperature (C)\ from the 1961 to 1990 average", xlab="Year", main="Northern Hemisphere")
#Creating SE Shaded Region
polygon(x=c(hockey$date, rev(hockey$date)), y=c(hockey$sigpos1, rev(hockey$sigpos2)), col="gray83", border=FALSE)
#Creating Tree Rings, Corals, Ice Cores, and Historical Records Line
lines(hockey$date, hockey$recon, col="dodgerblue3")
#Creating Trend Line
meanRAW<-mean(hockey$raw)
abline(h=meanRAW)
#Creating Lowess Line
lowess<-lowess(hockey$date, hockey$recon, f=0.04, iter=3)
lines(lowess)
Extension of Mann's Analysis
Many of Mann's critics argue that his use of data gathered from tree rings, corals, ice cores, and historical records are inaccurate and so exaggerate the claim that the globe is warming. In order to further investigate this criticism, I will compare the "controversial measurements" (temperature measurements taken from tree rings, corals, etc.) with the contemporary measurements taken with thermometers.
Starting in 1902, there is data available for both the "controversial measurements" and the "thermometer measurements." Since there is overlap, I am able to compare the two measurements and through this comparison, I will be able to show whether or not the two methods for measuring the globe's temperature differ greatly or not at all.
In the first graph, I compare the two measurements over the entire span of time available in the data (1400-1995). One can see that around 1900, the "thermometer measurements" (the red line) and the "controversial measurements" (the blue line) almost perfectly match. This is true until about 1980, when the two lines diverge and appear to be moving in the opposite directions. Of course, this is a startling finding, because Mann claims that in the 90's the greatest global temperature increases occurred. The finding, however, that the thermometer measurements and the controversial measurements no longer match after 1980 seems to injure the validity of Mann's conclusions.
In order to further investigate this pattern, I isolated the time period between 1900 and 1995. In the second graph, one can observe how the two measurements follow each other almost exactly. Furthermore, the supposed differentiation between thermometer and controversial measurements found in the first graph, in the second graph, is found to be simply a lack of "controversial measurements" taken. In fact, between 1980 and 1995 no "controversial measurements" are available. So, upon closer inspection, one can see that the criticism that these "controversial measurements" are inaccurate measurements of past temperatures does not seem to hold, at least for the time period between 1902 and 1980. Both thermometer and controversial measurements of temperature produce essentially the same data.
R Code for Extension
#Comparison Between two Temperature Measurement
#Erasing the Zero Values
recon1<-hockey$recon
hockey$recon1<-recon1
fix(hockey)
hockey$recon1[hockey$recon1==0]<-NA
#Creating the Lowess Lines
lowessRAW1<-lowess(hockey$date, raw1, f=0.04, iter=3)
lowessRECON1<-lowess(hockey$date, recon1, f=0.04, iter=3)
#Creating Plot 1
plot(lowessRAW1, type="l", col="red", ylim=c(-0.5, 0.8), xlim=c(1400, 2000), main="Comparison of Thermometer Measurements\n and Controversial Measurements\n (lowess smoother)", xlab="Year", ylab="Difference in Temperature (C) from 1400-1995")
lines(lowessRECON1, type="l", col="blue")
abline(h=meanRAW)
legend("bottomright", c("Thermometer", "Controversial"), col=c("red", "blue"), lty=1:2)
######Now to Isolate effects between 1902 and 1995
#Creating a Subset of the data for only 1902 and Later
raw2<-subset(hockey$raw, hockey$date>1902)
date2<-subset(hockey$date, hockey$date>1902)
recon2<-subset(hockey$recon, hockey$date>1902)
#Creating Plot 2
plot(date2, raw2, type="l", col="red", main="Closer Look: Comparison of Thermometer Measurements\n and Controversial Measurements", xlab="Year", ylab="Difference in Temperature (C) from 1902 to 1995")
lines(date2, recon2, type="l", col="blue")
legend("bottomright", c("Thermometer", "Controversial"), col=c("red", "blue"), lty=1:2)
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