Open Mind

Tales from the Thermometer

November 8, 2006 · 5 Comments

Global warming is the “hottest” environmental issue of the day, quite possibly of all time. Yet it’s increasingly clear that most people, even those who are passionate about the issue (on both sides), aren’t very well-informed about what earth’s temperature is doing, what it has done in the past, and what it’s likely to do in the future. There’s quite a gap between what most people know about the subject, and what people need to know.

I’d like to discuss the history of earth’s temperature according to thermometers.

Worldwide, there are two organizations that are (in my opinion) foremost as “keepers of the thermometer.” That’s not to say there aren’t many excellent record-keepers and researchers, but these two seem to be leading the field. Both collect temperature measurements every day, every night, every season of every year ad infinitum, from thousands upon thousands of locations over land and sea. They’ve collected past observations and checked and double-checked them so that errors can be corrected when possible and discarded when not. They determine the changes over time, and the differences between different regions of the earth. And both of them make their data freely available from the web. One is the Hadley Center/Climate Research Unit (HADCRU) associated with the University of East Anglia in the U.K. The other is the Goddard Institute for Space Studies (GISS), part of NASA.

Temperature Anomaly

What climate researchers are mostly interested in is temperature change. Also, temperature differences can generally be measured with much greater precision and reliability than absolute temperature. So, what is usually studied is not temperature itself, but temperature anomaly.

Temperature anomaly is just the difference between the temperature, and what it used to be back in some “reference period.” If it’s hotter now than during the reference period, the temperature anomaly is positive; if it’s colder now, the anomaly is negative. For HADCRU, the reference period is 1961 through 1990, for GISS it’s 1951 through 1980. Global estimates are computed by determining the geographic distribution of temperature anomaly worldwide, and averaging that to get the best estimate of global temperature anomaly. This is the data that will tell us whether or not earth’s temperature has changed, or is changing.

Historical Thermometer Readings

Until the latter part of the 19th century, we didn’t have enough temperature measurements from enough places to do the job. The HADCRU time series runs from 1856 to the present, while the GISS data go from 1880 to today. Let’s look at the HADCRU data, starting with a graph very similar to the first one of its kind I saw: global average temperature anomaly, every year from 1860 to 2000, plotted in figure 1. Each red bar represents the difference between the year’s temperature and the average from 1961 through 1990. The coldest year recorded is 1862, at -0.52; the hottest is 1998, at +0.58.

Figure 1. Global average yearly temperature anomaly (HADCRU).

I’ll also show the same exact information, plus a little more, in a different form. figure 2 shows the global average yearly temperature anomaly, starting way back in 1856 and going all the way up to 2005, as a scatter plot/line graph. Each little blue square plots the average yearly temperature and I’ve connected them with a line.

Figure 2. Yearly average global temperature anomaly (HADCRU).

You may have heard that the hottest year on record is 2005, not 1998. That conclusion is based, not on data from HADCRU, but on data from GISS. figure 3 shows the annual average temperature anomaly according to GISS (recall that for the GISS data, the “reference period” is 1951 through 1980 rather than 1961 through 1990). There are small differences between the two reports, but if you compare figures 2 and 3, you can see that the overall behavior is the same. I have chosen to focus on the HADCRU data because it starts earlier (1856) than the GISS data (1880).

Figure 3. Yearly average global temperature anomaly (GISS).

We can see first of all that there are lots of fluctuations from year to year. But superimposed on these year-to-year changes are some more persistent trends. In particular, there are two episodes in time when temperature overall seems to have increased. From about 1915 to about 1945, and again from about 1975 to the present, there’s a steady rise in yearly average temperature superimposed on the year-to-year fluctuations.

These graphs (the forms most people have seen) are actually slowed-down versions of real temperature change. In real life, temperature changes from moment to moment. Within a single day, it can easily swing by 20 degrees or more. But by taking averages over 1-year periods, we have “slowed down” the changes, eliminating those that happen on timescales much shorter than a year. We can go into greater time detail by looking at global average temperature, averaged not over every year, but over each month, as in figure 4. Each point represents the difference between the given month’s temperature, and the average for that month from 1961 to 1990. The (relatively) coldest month is Oct. 1862, at -0.93, while the hottest is Feb. 1998 at +0.82.

Figure 4. Monthly average global temperature anomaly (HADCRU).

The monthly averages show even more up-and-down fluctuations than the yearly averages. Each month is unique, with its own set of influences that are nearly impossible to predict. This leads to short-term, month-to-month variation that is, essentially, unpredictable but has a limited range of variation. Such variations happen on very rapid timescales. I’ll call this very fast natural variations; it’s sometimes also called scatter.

There are also changes that have some persistence, but still happen rapidly. Such changes last way longer than a single month or two but not more than a few years. This is fast natural variation of global temperature, and it too has a limited range of variation. Sometimes we can figure out exactly why a particular fast natural variation has occured.

Finally, there are slower, even more persistent changes. For example, the temperature is higher, on average, today than it was in the past; we noted the two periods in the thermometer record, from about 1915 to 1940 and again from about 1975 to the present, when in addition to scatter and fast natural variations, temperature also showed a steady rise. Such persistent changes are slow variations; we can also call them trends.

Slow and Fast Changes

It’s natural to want to separate the long-term (slow) changes from the short-term (fast) changes from the very-short-term (very fast) changes, in fact we want to know the changes on all timescales. We’ve already seen one way to do this; by averaging over every year, we eliminated changes whose duration is much less than a year. Essentially we got rid of the very fast changes, giving us a better look at the fast changes (and the slow changes too).

We can carry this technique further, by averaging over even longer timescales. If, instead of 1-year averages, we take 10-year averages, then we’ll eliminate not only the very fast variations, but the fast variations as well, leaving only the long-term changes: the trends. Plotting the slow 10-year averages in red, and the fast 1-year averages in blue, and the very fast 1-month averages as black dots, we get figure 5. Magnifying the temperature axis for a better view gives us figure 6.

Figure 5. Monthly temperature anomaly (black dots) with 1-year averages (blue) and 10-year averages (red).

Figure 6. Monthly temperature anomaly (black dots) with
1-year averages (blue) and 10-year averages (red).

Again it’s clear that global temperature shows two episodes of significant increase: from about 1915 to about 1940, and from about 1975 to the present. I’ve also identified two short-term events, labelled “Mt. Pinatubo” and “el Nino.”

The explosion of the Mt. Pinatubo volcano injected massive quantities of particles into the upper atmosphere (where they could disperse around the globe). These “aerosols” tend to block sunlight from getting to earth’s surface, and have a net cooling effect on earth’s climate. The Mt. Pinatubo explosion caused a minor cooling of the globe for a few years afterward.

El Nino is a spreading out of the pool of warm water in the west Pacific ocean. When it spreads out, it’s exposed to more air, releasing more of its heat into the atmosphere and warming the climate. 1998 was the strongest el Nino ever recorded, making it the 2nd-warmest year in the GISS data set (2005 is the warmest), and the warmest year in the HADCRU data.

Another way to smooth the data is to fit a mathemematical curve, but tune the curve so that it “smooths out” fluctuations faster than the limit we’re interested in. Figure 7 shows two smooth-fit curves to the data, one tuned for 10-year changes (red) and another for 1-year changes (blue). Figure 8 shows the same data, but zoomed in on the temperature axis. The smallest details differ from the graph of averages, but the results are exactly the same: fast natural variations like el Nino and Mt. Pinatubo, and two episodes of warming, from about 1915 to 1940, and about 1975 to the present.

Figure 7. HADCRU temperatures with 10-year smooth (red) and
1-year smooth (blue).

Figure 8. HADCRU temperatures with 10-year smooth (red) and
1-year smooth (blue).

In fact we can mathematically estimate the time periods and their rates of change for both warming episodes as well as both non-warming episodes:

I: From 1880 to about 1916, temperature declined ever-so-slightly, changing by -0.03deg.C over the 36-year period. This gives an estimated rate of -0.001 +/- 0.004deg.C/yr. Note that the error range of the estimated rate is four times larger than the value itself, so we can’t be sure whether the -0.03deg.C net cooling is part of a long-term trend, or just due to those fast natural variations.

II: From 1916 to 1940 temperature increased by +0.31deg.C over 24 yr, for a rate of 0.013 +/- 0.004deg.C/yr.

III: From 1940 to 1975 it declined by a net -0.06deg.C, at an average rate -0.002 +/- 0.004deg.C/yr (again, error range larger than value).

IV: From 1975 to today it has increased +0.57deg.C over 31 yr at a rate of 0.018 +/- 0.004deg.C/yr.

Is the current rate of warming unusual? Has it been more or less in the more distant past? Thermometers alone can’t answer this question; for anything even close to a global estimate, we can only go back a little more than a century. But there are some isolated places with longer temperature records than that. The longest of all, and the one which has been subjected to the most scrutiny, is the Central England Temperature.

Central England Temperature (CET) has been reconstructed from thermometer measurements from 1659 to the present. The earliest data, from 1659 to 1670, are so much less precise than later data that monthly average temperature is only recorded to the nearest degree Celsius. From 1671 to late 1722, temperature is estimated to the nearest half a degree, except for a brief episode from 1699 to 1706 when data are more precise, to a tenth of a degree. Since late 1722 CET has been recorded as average monthly temperature to the nearest tenth of a degree.

Figure 9 shows the Central England Temperature Anomaly time series from 1659 to the present. The “reference period” is the entire time span, so hotter-than-average months are positive anomalies and colder-than average are negative. There are a lot of black dots, and lots of variation. Again, it would be nice to isolate the long-term changes. I’ll plot 20-year averages in blue, and a wavelet fit (which can be tuned for slow changes as well as fast) in red, and magnify the temperature axis to make the changes easier to see in figure 10.

Figure 9. Central England temperature anomaly.

Figure 10. Central England temperature, with 20-year averages
(blue) and a 20year smooth (red).

The present 20-year period (from 1990 to the present, actually) has the highest average temperature anomaly by far, and the change since the previous 20-year average (1970 to 1989) is the largest change in the entire CET record. If we stick to the more reliable part of the data, from 1723 to the present, we see that the 20-year average never got more than 0.23 degrees hotter or colder than zero until 1940. The most recent value is 1.01. The recent rapid rise doesn’t look like what’s happened before; the last four twenty-year periods are the four hottest of all.

But this is only indicative, it is far from conclusive, because it’s only one small part of the globe. To estimate global temperature before the age of thermometry, we have to estimate temperature from other data. These are called proxy data, and come in many forms: tree rings, microfossils, borehole temperature profiles, coral reefs, ice cores, and many more. The best approach is to take all the available temperature proxy data and combine it with the best statistical tools. Then we can hope to see the temperature history of the earth over the last few thousand years. And then, maybe we’ll know whether or not current warming is unusual. But that is the subject of a future post.

Categories: Global Warming

5 responses so far ↓

  • mike // January 21, 2008 at 9:58 pm

    Good data. This was very informative. I do agree that the arugment for both sides of the global warming issue are ill-informed. We, as humans, cannot say for certain that global warming is taking place. We do not have enough historical data to make a accurate conclusion. And I think your article proves it.

    Now one can point to the vast increase in avg. temp. over the past 80 years. But 80 years in the history of the world, I don’t think that even comes out to half a percent.

    But all of this is really a moot point anyway. When mother Earth is tired of us, she will get rid of us. Its that simple. Unless of course a gaint rock comes hurling at us nearing speeds 40k+ mph, slamming into crust, then plowing into the mantle, knocking earth of her axis, ultimately killing the rest of us who survived the explosion, fallout, and lack of sunlight. But I guess thats for a different article.

  • None // January 22, 2008 at 5:40 am

    “But 80 years in the history of the world, I don’t think that even comes out to half a percent.”

    O rly?

  • dhogaza // January 22, 2008 at 9:55 am

    I do agree that the arugment for both sides of the global warming issue are ill-informed. We, as humans, cannot say for certain that global warming is taking place. We do not have enough historical data to make a accurate conclusion. And I think your article proves it.

    I see nothing in the article or the graphs presented that supports this argument.

    Tamino is making clear why we need to look at data other than direct thermometer measurements to be certain. Not proving that the AGW “argument” is “ill-informed”.

    And he certainly hasn’t proved that “we can’t say for certain that global warming is taking place”.

    If Tamino agrees with your assessment, I’m sure he’ll correct me …

  • Tony R // February 24, 2008 at 7:22 am

    I’ve been very passionate about this (and so has my wife). Many people wish to prove their point by picking and pointing to certain data in favor of their positions. My wife and I have been very strong supporters of the global temperature increases. Admittedly, some of the things we’ve seen and read have been strongly slanted, but the point remains, as is born out by the early data shown in graphs above, earth is warming - for whatever reason. We are loosing polar ice caps, Greenland is loosing its ice sheets at an alarming rate, and the Himalayas are loosing their glaciers.

    For whatever reason it is happening, IT IS HAPPENING. Never mind the historical thermal records, it is happening.

    While big oil business would downplay this and environmental extremists would exaggerate it, the big question is “what can we do about it?”

    I’m grateful to have found someone who has taken a more level approach to the subject and given me clarity on the issues, I’ll not take the extreme alarmist point I once held, but I can not let this go as some would call it - hog wash.

    Please keep up the good work. You’ve clarified the problem. Now, let’s find solutions, and do something about it.

    Thanks again.

    Tony (and wife)

  • Hank Roberts // February 24, 2008 at 8:05 pm

    > O rly?

    Dang, coffee on my keyboard _again_ …

  • Like gas stations in rural Texas after 10 pm, comments are closed.