Our global thermometer since 1850

This is the 7th episode in a series recounting the history of measurements and data related to Global Climate Change. If you’re just joining, you can catch up on the previous episodes:

  • Episode 1: Beginnings (or two British scientists’ adventures with leaves and CO2 measurements)
  • Episode 2: First measurement of anthropogenic global warming
  • Episode 3: Our “large scale geophysical experiment” (1940-1960)
  • Episode 4: Dave Keeling persists in a great idea
  • Episode 5: Icy time capsules
  • Episode 6: The “geologic eons of time”

Episode 7

In the last 3 episodes of our history of global climate change evidence, we’ve focused on measurement of Earth’s atmospheric CO2 record, finding in the last episode that it’s now over 40% higher than the entire pre-industrial experience of the human species spanning over 200,000 years. But we have not checked in on global temperature measurements since Episode 3, where the intrepid steam engineer, Guy Callendar (1961) and Landsberg & Mitchell, Jr. (1961) had independently measured what appeared to be a slight but discernible warming between 1880 and the late 1950’s. You may also recall from Episode 3 that the physicist, Gilbert Plass, had used some of the first computers to refine calculations of infrared absorption by CO2, predicting we would observe about a 1 degree temperature increase between the years 1900 and 2000, whereupon we would also begin to observe obvious effects of climate change.

Well, the year 2000 has come and gone and we have thermometers all over the world. Let’s grade Dr. Plass’ work, shall we?

In the 1960’s and 1970’s, others continued to document surface temperature records from collections of meteorological stations, but the data were gathered primarily from stations in the Northern Hemisphere and there wasn’t a standardized method of obtaining a truly global temperature average. During that time, James Hansen, a physicist and astronomer at the NASA Goddard Institute for Space Studies, was studying the planet Venus. Specifically, he was calculating the influence of Venus’ thick atmosphere on its extremely hot surface temperature. (Fun fact: scientists believe Venus’ atmosphere several billion years ago was similar to Earth’s and it had liquid water on its surface, but Venus now has a thick atmosphere and a scorching surface temperature of 864 degrees Fahrenheit due to the occurrence of a runaway greenhouse effect.)

In the late 1970’s, Hansen turned his attention to similar calculations of the effects of Earth’s atmosphere on its surface temperature. As part of this work, he tackled the problem of creating a standardized method for calculating global average temperature trends. The method begins with the recognition that, while absolute temperatures are widely variable from place to place on the Earth, even for locations relatively close to one another, temperature changes of nearby locations tend to be very similar. For example, while the absolute temperatures in New York and Pittsburgh might be quite different on a particular day, if one is having a hotter than average month, the other is likely having a month hotter than average by around the same amount. Thus, global temperature trends are plotted, not as absolute temperatures, but as temperature differences, called “temperature anomalies,” relative to some reference temperature.

The second key element of the method is the Earth’s surface is divided up into a grid formed by squares of equally spaced latitude and longitude lines, such that each square contains a sufficient number of weather stations to obtain an accurate record of historical temperature data. At any given time in history, then, the temperatures of the squares are averaged to get an estimate of the global average temperature. Various statistical methods are used to correct for errors, such as the known artificial urban warming around weather stations in or near cities. The gathering of sufficient, widespread temperature data to apply this method began in the late 1800’s. Hansen’s method was initially published in the peer-reviewed scientific journal, Science, in 1981, and has since been updated as the techniques have continued to improve (1987, 2010).

Similar methods have now been applied independently by four major research groups. They make their data publicly available for download (see links in the caption below). Here are the four readings of the “global thermometer” (orange, pink, red, and purple lines) plotted on top of the global CO2 record (green and blue circles) we saw in Episode 5:

All data publicly available, downloaded and plotted by me. Green and blue circles: atmospheric CO2 concentration from Law Dome ice cores (green) and direct atmospheric sampling (blue) from Scripps (see figure captions in Episode 5 for detailed references). Orange line: Temperature anomaly, 1880-2016, according to U.S. NASA Goddard Institute for Space Studies (public data, reference). Pink line: Temperature anomaly, 1880-2016, according to U.S. NOAA National Climatic Data Center (public data, reference). Red line: Temperature anomaly, 1850-2017, according to U.K. Hadley Centre/Climate Research Unit (public data, reference). Purple line: Temperature anomaly, 1891-2016, according to Japan Meteorological Agency (public data, reference). All temperature anomalies re-scaled by me to be relative to a common reference baseline of the 1891-2010 average temperature.

Due to differences between the chosen data sources, gridding methods, and error correction methods used by the four independent groups (for details, see references in the caption above), the four temperature records are not identical. They show remarkable agreement, however. They generally have peaks and valleys in the same places, and their basic conclusions are all the same – the world is about 1.1 degrees Celsius warmer now than it was in pre-industrial times. Check out the video below, where the NASA and NOAA gridded data have been used to show how different parts of the globe have changed in temperature.

Video credit: NASA Goddard Space Flight Center (link to web page). Video using a color coding of NASA and NOAA gridded global temperature anomaly data to show how the Earth’s temperature has changed since 1880.

There is no obvious evidence of a “Chinese hoax” here. Instead, these appear to be the serious, well considered and extensively peer-reviewed conclusions of four independently funded and well-respected scientific groups (a British group, a Japanese group, and 2 U.S. groups – one of which, NASA, has brought us other generally well-regarded scientific achievements such as the moon landings).

In their 1981 paper, James Hansen and his coworkers calculated the temperature increase, relative to the global temperature around 1975, at which we would have a greater than 98% statistical confidence that global warming is “real” (not just a result of random temperature variations). That would be when the temperature rose above the light grey range in this graph, about 0.2 degrees C higher than the 1975 temperature, which the NASA scientists predicted would occur in the 1990’s.

Figure 7 from Hansen, et al. (1981). Calculation of the temperature change, relative to the temperature in the late 1970’s, at which our statistical confidence that global warming had exceeded previous natural variation would reach >85% confidence (represented by the dark grey range) and >98% (light grey range).

A look at the temperature data above shows that had indeed occurred by the 1990’s. Now, we are a full 0.8-1.0 degrees C above the 1975 temperature, and there can really be no doubt.

Strikingly, the temperature graphs above have almost exactly the same shape as the CO2 graph! But, if we’ve been paying attention to our history of evidence, this should not be a surprise. Rather, it should be a confirmation of our expectations. Sure, the Earth’s climate is a highly complex system, and there have been real questions about things like the role of the deep oceans, as we saw in Episode 3. But those questions were settled by around 1960, by which time Dave Keeling had also begun direct measurements of the atmospheric CO2 concentration. Once we see CO2 going up, we expect warming with mathematical certainty. Based on physics known since the early 1800’s, CO2 absorbs infrared radiation reflected from the Earth’s surface, generating heat. It’s as simple as that. At the end of the day, the basic physics driving global warming are far simpler than those at work every moment inside your smart phone.

Anyone denying the reality of global warming would have to not only explain why at least four formidable groups of well-respected scientists, not evidently influenced by Chinese hoaxters, don’t know how to process data from thermometers. They would also need to explain how the undeniable increase in atmospheric CO2 through the combustion of fossil fuels has somehow not resulted in warming, when anyone with a basic laboratory infrared instrument can verify the infrared absorption of CO2. In fact, did you notice in Episode 4 how the weekly atmospheric CO2 concentration at Mauna Loa is measured? By the infrared absorption of collected air samples! So, every time we measure the CO2 concentration of the atmosphere, by the method precise enough to reveal the seasonal respiration of plants, we verify the very physical phenomenon that drives global warming!

OK, so it’s time to grade Dr. Gilbert Plass’ 1956 prediction of around 1 degree Celsius of warming between 1900 and 2000, and readily observed effects of global climate change, due to infrared adsorption by increased atmospheric CO2. The verdict?

  • Actual warming between 1900 and 2000? Around 0.8 degrees C. Not bad. Maybe an A-. But pretty impressive given that Dr. Plass was using the world’s very first computers and considering only the effects of infrared absorption by CO2.
  • Readily observable effects of global climate change? Absolutely.

To be continued…

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