Global warming deniers tell us that “the climate has always changed, and it always will.” The deniers assert this truth as grounds for complacency and inaction. They imply that the climate scientists who are warning us about the dangers and causes of global warming don’t know this fact and that the researchers are alarmists. Often, the deniers follow their truth with a falsehood: that the scientists are uncertain as to the cause of the present warming. Once the scientists have figured out the causes, then we can deal with problems we know are real they say.
As part of your suggestion that we devote part of our blog to fact-checking and idea checking of matters of current interest about which we have useful perspectives, I’m posting this ninth in my series on global warming. I’ll put this issue of historical climate change, warming and cooling in perspective. I’ll show how changes in the world’s climate during the period of settled agricultural, civilized existence compare to changes in the geologic past. My conclusion, and yours and of our readers to this, ought to be one of alarm. Indeed, when deniers accuse someone as an “alarmist” the accusation carries the connotation that the alarmist is unnecessarily fearful and likely exaggerating the danger. In the case of warming of the Earth caused by human’s burning of fossil fuels, alarm is appropriate.
Here is a reconstruction of the Global Mean Temperature throughout the Phanerozoic Era; that is for the past 550 million years, about 1/8th of Earth’s history. Click the chart to view a larger version.
Professor Christopher R. Scotese shared this remarkable data with me.
The horizontal axis is millions of years, with the distant past to the left. The vertical axis is in Celsius and runs from 10 C (50 F) to 28 C (~82 F). For context, from this data, 2016 was 14.5 C (59 F). The acronyms are: PETM Paleocene-Eocene Thermal Maximum (55.8 Ma, million years ago), EECO Early Eocene Climatic Optimum (54-46 Ma), MECO Mid-Eocene Climatic Optimum (42 Ma), EOT Eocene-Oligocene Transition (40-33 Ma), MMCO Mid-Miocene Climatic Optimum (15-13 Ma), LGM Last Glacial Maximum (21,000 years ago), PAW Post-Anthropogenic Warming (+5000 – 10,000 years in future). This last is a prediction, of course.
This temperature estimate did not arise from thermometers, of course, except for the one for 2016. The record is a proxy. In this case, the professor and his colleagues studied rock layers around the world from two hundred years of field geology. From the known conditions that lead to various sedimentary rocks and from changes in those rocks’ locations over millions of years, they construct the general climate conditions in latitudinal zones. As with all proxies, these results are approximate. Professor Scotese and his colleagues work hard to compare their results with those of other researchers using different methods. You can easily see that the Global Mean Temperature relates to the geologic era and to the character of the climate and of the plants and animal communities. Notice that sharp drop at about 65 Ma. That’s the record of the meteor strike and sudden freeze that wiped out the dinosaurs and many other animals.
The Global Mean Annual Temperature, GMAC, at the Last Glacial Maximum, LGM, the most intense part of the most recent Ice Age, a temperature minimum, was 12.4 C (~54 F), only 2 C or 4 or 5 F cooler than today. During pre-industrial times, which I happen to know extend from about 10,000 BP, before the present day, until about 1850, the GMAC was about 13.8 C (~56 F). In other words, using the GMAC, a single number, to suggest the Earth’s climate shows that about 2 or 3 C (~5 F) is sufficient to take us from New York City and Boston, Canada and the Northern US, and most of Europe under a mile of ice to today’s conditions. Dr. Scotese supposes that humans will insist on burning all our available fossil fuels during the next few centuries, and that this will lead to a Post-Anthropogenic Warming temperature of about 20 C (68 F)! As adding CO2 to the atmosphere and the seas will cease when the fuel is gone, the CO2 will gradually over thousands of years leave the atmosphere and the climate will return to “normal.”
There you have it. It is true that the climate has always changed, and it always will change.
Here’s the most recent 11,000 years.
This data is from a NOAA climate research group that the new administration plans to wipe out. Except for the years since 1880 on the far right, this is proxy data. Typically, this data comes from ice cores in Greenland and Antarctica. The scientists calculated the average temperature from 1961 to 1990, computed the average, and subtracted this value from their data. Thus, we are seeing the history of differences from that average. The depth of the most recent ice age was about 20,000 years ago when the temperature would have been -4 or -5 C on this chart. At that time, North America into the northern states of the US were covered with a vast ice sheet. New York and Boston were under a mile of ice. So was most of Europe. Beginning about 14,000 or 12,000 years ago the climate warmed, driven by natural forces such as tiny changes in the Earth’s orbit. During the past 11,000 years, the Earth’s surface temperature has varied by about half a degree C, plus and minus. That is, mostly less than one tenth of the difference between an Ice Age and today’s climate that we see as temperate.
Compared to the big changes in the long-term chart covering half a billion years, the Earth’s average surface temperature has changed little for the past 10,000 years. You can clearly see that there has been a long and gradual decline from about 6000 years ago until recently. I like to ask my students what was happening in the world 10,000 years ago. The answer is that at that time, our ancestors began to settle down in agricultural communities, towns, and cities. That is 10,000 years is the entire span of human civilization. During this period humans have lived in stable climatic conditions. The world now has more than 7 billion people living in a civilization based upon agriculture, the cultivation of certain plants under certain conditions. But look at the end of the chart, in 100 years the temperature has jumped about a degree C. Is this significant?
Most of what we consider modern civilization fits within the last 11,000 years—a period of remarkable climatic stability in which people have been able to continuously inhabit in the same regions for millennia. The United Nations Educational, Scientific, and Cultural Organization describes Tell es-Sultan (ancient Jericho) as “the oldest town on earth.” Photo courtesy Seetheholyland.net (some rights reserved).
The aerial photograph shows Jordan, in the West Bank. You can see evidence of human occupation both ancient and modern. This place has been the continuous site of human settled life beginning as long ago as 9000 BCE. Note UNESCO’s description of this place as “the oldest town on earth” in the caption. Note the climate scientists’ remark that those 11,000 years have been “a period of remarkable climatic stability.”
Here’s an example of the, or a, “hockey stick” graph. The famous, or notorious, Michael Mann hockey stick graph from 20 years ago has been the subject of some scientific and even political attacks. I’ll just summarize this by saying that scientifically Mann’s work has survived the scientific criticisms and is generally consistent with many important studies that use different proxies and methods. As these controversies are not the subject of this post, I’ll just show one of Mann’s early versions, which I will take as typical of all of the others.
This shows a proxy reconstruction of the Northern Hemisphere’s surface temperature from 1000 CE until almost 2000 CE relative to the average from 1961 until 1990. The scientists call this type of difference an “anomaly.” The vertical scale is in Celsius from -1 C to +1 C, but most of the action takes place between -0.5 C and +0.5 C. The gray area indicates Mann’s belief of his error bars. Look carefully at the graphed curves and the legend. The first two data sets go back to 1000 CE, the third one to 1600, and the fourth to 1400. These four proxy records end around 1980. Finally, beginning at 1880 you can see the instrumental, that is measured by thermometers, temperature record.
Scholars call the several centuries from around 800 or 900 until about 1200 or 1300 the Medieval Warm Period, and the centuries from the end of the Warm Period until about 1850 the Little Ice Age. If you squint, you might be able to see evidence for these periods in these graphs. Significant for this blog, however, is the fact that you can barely see them. Around the year 1000 Norse seamen discovered Greenland, now an ice-covered island. Why do you suppose they called it Greenland? As the climate there cooled the intrepid and tough Norse settlers were driven away. That is, climatic temperature changes much smaller than those at the modern end of this graph transformed a region suitable for agriculture into an island covered with ice a mile thick. There are many other examples of major social and economic changes from historical records in Europe of this period.
Looking at these data, since the end of the Ice Age, you can see that the most recent peak in the past was about 7 or 8,000 years ago. The climate entered a long slow cooling trend, until, that is, about 1850. From that time there has be a rapid, more rapid than anything in any of these records, increase in the temperature. The hockey stick data above ends around 2000, and 2016 would be somewhat off the top of this chart. Indeed, the most recent three years have each been a global high temperature record.
Although I haven’t discussed the causes of this sudden warming here, knowledgeable scientists know that it is mainly the result of humans burning fossil fuels. That will be a topic for a subsequent post. This post, I see, justifies alarm. Modern climate changes are smaller than those known to us from the geologic record, but are much faster than previous changes known to us when we have sufficient resolution. Relative to the large climate swing of the geologic past, the climate during the entire span of settled agricultural civilized existence has been stable. The Earth’s population today is more than a thousand times larger than it was during most of this 10,000-year period of agriculture. What reasons do the deniers have for their belief that these recent changes will have little effect on human life?