Time scale presents one of the big challenges when discussing the science around climate change. Climate change can take place in the short time frame, say decades; in the medium time frame, say hundreds or thousands of years, and in the very long time frame, say tens of thousands or hundreds of thousands or even millions of years.
Some critics of human-caused global warming raise the objection that Earth’s climate has always changed and that, even if global temperatures are rising, that’s probably just a result of natural cycles rather than greenhouse gas emissions. Look at the Medieval Warm Period! Look at the Little Ice Age! And wasn’t the Earth pretty hot when the dinosaurs were around?
So the thinking person wonders: How has Earth’s temperature changed throughout the ages? And how do past temperatures compare with what we experience today?
The Short Time Frame
Much very specific information about the Earth’s surface temperature has come forward during what might be called the “Instrumental Period” starting around 1850 (see “Uncertainty estimates in regional and global observed temperature changes: a new dataset from 1850,” P. Brohan, J. J. Kennedy, I. Harris, S. F. B. Tett & P. D. Jones, Journal of Geophysical Research, Atmospheres, June 24, 2006). Although meteorological stations were taking temperature measurements before then, the 1850-2007 dataset, called HadCRUT3, provides a more or less global set of instrumental data measuring surface temperature. A dataset from NASA called GISTEMP provides instrumental measurements going back to about 1880.
The Medium Time Frame
On the geologic time scale, we live in the period scientists refer to as the Holocene, which began at the end of the Pleistocene after the last glacial period, supposedly about 12,000 years ago.
To reconstruct global surface temperatures for the last thousand years or so, researchers have combined proxy evidence from such sources as tree rings, corals, sediments, cave deposits, ice cores, glaciers, and even documentary sources.
The Very Long Time Frame
The geologic time scale goes back 4.6 billion years, and Earth’s climate has naturally changed greatly throughout that period. The Holocene and Pleistocene I mentioned before are a relatively short part of the longer Cenozoic era, estimated to have begun about 66.4 million years ago. Mainstream science scholarship says that this was the period during which large mammals and humans appeared.
Climate change during the Cenozoic has been reconstructed by studying ocean sediment records. For about the last 800,000 years, global temperature has been determined from ice cores.
So How Have Global Temperatures Changed in the Past?
Scientists are certain that during Earth’s 4.6 billion years, global climate has changed significantly, many times over. Yet, life as we know it on Earth can only exist within certain temperature limits. Scientists think some “extremophiles” might be able to live at very high or very low temperatures. But by far most of the life that exists now and has ever existed on the planet has required a relatively moderate temperature to survive. While many people would love to find life on other planets, there’s no evidence that life exists on Mars (other than some controversial squiggles in the dirt), where the average surface temperature is -55 °C (-67°F), or on Venus, where the average surface temperature is 460°C (860°F).
I’m including here an interesting timeline showing the estimated changes in Earth’s temperature during the Paleozoic (dated 570-245 million years before present), Mesozoic (245-66.4 million years before present), and Cenozoic (66.4 million years before present up to now) eras. If you want to see it in more detail, click here for the original.
As you can probably tell, this timeline is a mashup of several sources. The zero point on this chart’s “y” axis would be the baseline global mean surface temperature in the early part of the 20th century before the rapid warming of the past several decades, which would be about 14ºC (57ºF).
You can see that by and large Earth’s temperatures were 2-6 degrees Celsius warmer than modern times during the Paleozoic and Mesozoic. During the ice ages of the Pliocene and Pleistocene, temperatures got to be maybe 2-3 degrees C cooler than modern times.
But think about that: The difference between a world in which Canada was covered by two miles of ice and a world in which Earth had no ice cover and dinosaurs lived in Siberia and Alaska seems to be, if I figure correctly, only about 10ºC or 18ºF. A relatively small warming or cooling can correlate with a tremendous difference in climatic conditions.
During the Mesozoic era (dated about 245 to 66.4 million years ago), when dinosaurs lived, the Earth was supposed to have been much warmer than today. The Mesozoic era is made up of the Triassic, Jurassic, and Cretaceous periods. According to Lee R. Kump and colleagues at Pennsylvania State University (The Earth System, Lee R. Kump, James F. Kasting, and Robert G. Crane, 2010), fossil evidence indicates that
[T]he Mid-Cretaceous climate was on the order of 2 to 6ºC (3.6-11º F) warmer at the equator and 20 to 60ºC (36-110ºF) warmer at the poles.
Writing in the journal Science, James Zachos (University of California) and colleagues say that during the Cenozoic (“Trends, Rhythms, and Aberrations in Global Climate 65 Ma to Present,” James Zachos, Mark Pagani, Lisa Sloan, Ellen Thomas, Katharina Billups, Science, April 27, 2001),
Earth’s climate has undergone a significant and complex evolution, the finer details of which are now coming to light through investigations of deep-sea sediment cores. This evolution includes gradual trends of warming and cooling driven by tectonic processes on time scales of 105 to 107 years, rhythmic or periodic cycles driven by orbital processes with 104- to 106-year cyclicity, and rare rapid aberrant shifts and extreme climate transients with durations of 103 to 105 years.
Daniel Sigman of Princeton University and Edward Boyle of MIT write in an article for the journal Nature (“Glacial/Interglacial Variations in Atmospheric Carbon Dioxide,” Oct. 19, 2000) that during the past 2 million years, Earth’s climate has fluctuated between glacial periods and periods of warming linked partly to variations in Earth’s orbit:
The past two million years have been characterized by large cyclic variations in climate and glaciation. During cold ‘ice age’ periods, large continental ice sheets cover much of the polar Northern Hemisphere. During intervening warm periods, or ‘interglacials,’ Northern Hemisphere glaciation wanes drastically. The ultimate pacing of these glacial cycles is statistically linked to cyclic changes in the orbital parameters of the Earth, with characteristic frequencies of roughly 100,41 and 23 kyr [thousands of years] … These orbitally driven variations in the seasonal and spatial distribution of solar radiation incident on the Earth’s surface, known as the ‘Milankovitch cycles’ after their discoverer, are thought to be the fundamental drivers of glacial/interglacial oscillations.
The temperature changes associated with the Milankovich cycles are demonstrated by analysis of ice cores collected at the research station at Vostok, Antarctica. The ice cores provide a record of atmospheric composition going back in time. Temperature is determined by measuring the deuterium content of the ice at different levels. The following chart shows carbon dioxide content, dust concentration, and Antarctic temperatures going back about 400,000 years.
Antarctic temperature variation is shown on the top graph in blue. Low points are glacial periods, high points are interglacials.
What about the last 1,000 years?
The following chart from a report by the U.S. National Academy of Sciences (NAS) (“Surface Temperature Reconstructions for the Last 2,000 Years,” 2006) shows estimates of global average surface temperatures during about the past 1,100 years.
NAS says that:
Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the “Medieval Warm Period”) and a relatively cold period (or “Little Ice Age”) centered around 1700. The existence of a Little Ice Age from roughly 1500 to 1850 is supported by a wide variety of evidence including ice cores, tree rings, borehole temperatures, glacier length records, and historical documents. Evidence for regional warmth during medieval times can be found in a diverse but more limited set of records including ice cores, tree rings, marine sediments, and historical sources from Europe and Asia, but the exact timing and duration of warm periods may have varied from region to region, and the magnitude and geographic extent of the warmth are uncertain.
NAS stresses that the further back in time the estimates go, the less certain they are:
The main reason that our confidence in large-scale surface temperature reconstructions is lower before A.D. 1600 and especially before A.D. 900 is the relative scarcity of precisely dated proxy evidence. Other factors limiting our confidence in surface temperature reconstructions include: the relatively short length of the instrumental record (which is used to calibrate and validate the reconstructions); the fact that all proxies are influenced by a variety of climate variables; the possibility that the relationship between proxy data and local surface temperatures may have varied over time; the lack of agreement as to which methods are most appropriate for calibrating and validating large-scale reconstructions and for selecting the proxy data to include; and the difficulties associated with constructing a global or hemispheric mean temperature estimate using data from a limited number of sites and with varying chronological precision. All of these considerations introduce uncertainties that are difficult to quantify.
So, it’s really true that Earth’s climate, along with its global surface temperature, has been changing for a long time. The NAS chart shows temperature rising since the end of the Little Ice Age around 1850. The following chart from Berkeley Earth Surface Temperature shows that temperature is still climbing, and has been doing so even more rapidly since the middle of the 20th century. (See my previous article about Berkeley Earth’s study of global temperatures: “So, Have They Figured Out That Global Warming Is Real?“)
Nearly all climate scientists assert that the recent rise of global surface temperatures is in part human-caused (anthropogenic), specifically through emissions of carbon dioxide and other greenhouse gases, principally through burning of fossil fuels. Skeptics raise various objections, including the assertion that temperatures could be rising for natural reasons independent of CO2 emissions or other human causes.
So why do climate scientists focus on CO2? After all, isn’t CO2 beneficial — in fact, a necessary ingredient for photosynthesis? How could very small concentrations of this beneficial gas cause global temperatures to rise?
I plan to investigate these questions around CO2 in an upcoming article. In the meantime, please take a look at some other articles I’ve written in the past examining the climate-change controversy:
As always, please feel free to use the comment space below to express your wise ruminations, thoughtful observations, well-considered commentary, or lunatic ravings.