Holocene Climate

The Holocene geological epoch starts at the beginning of the current interglacial warm period about 11,700 years ago and continues to the present.  As discussed in previous posts, the current interglacial warm period is the fifth interglacial in the last 500,000 years that was dominated by four intervening glacial periods each lasting about 80,000 to 120,000 years.  See the previous posts for more details.

Several ice core climate analyses covering the Holocene are available from the US NOAA/NCDC Paleoclimatology Program.  Temperature reconstructions from four of these ice core analyses are presented in the graph below in Figure 1.  Two are from Antarctica, EPICA (Jouzel 2007) and Vostok (Petit 2001), and two are from Greenland, GISP2 (Alley 2000) and GRIP (Vinther 2011).  The graph data are presented as anomalies relative to each data-set average for the 1,000 year period from 200 to 1,200 years before the present (2014) as the zero baseline.  Also shown in red on the graph are the annual HadCRUT4 global temperature anomalies estimated from temperature measurements provided by the UK Meteorological Office Hadley Centre which are referenced to a 1961-1990 zero baseline.

Figure 1. Comparison of surface temperature reconstructions for the last 11,000 years based on oxygen isotope ratio analyses of ice cores from Antarctica (EPICA and Vostok) and from Greenland (GISP2 and GRIP) along with the more recent HADCRUT4 global surface temperature anomaly analysis (click to enlarge).

Since these ice core climate reconstructions are likely to be more representative of regional climate rather than global climate, two reconstructions were selected for each hemisphere for comparison.  The southern hemispheric Antarctic reconstructions can be compared to each other and to the northern hemispheric Greenland reconstructions which can also be compared to each other for consistency and variability.  This is a crude attempt to characterize the general global climate, analogous to selecting data from two weather stations in Antarctica and two in Greenland to estimate modern global temperature anomalies and thus is likely to have some large uncertainties for this purpose.

In general, all of the reconstructions show frequent variations in temperature of plus or minus one to two degrees Centigrade(C)  about the reference baseline and show much wider variation than what has been seen for global climate in the most recent 164 years as indicated by the HadCRUT4 data.  Temperature variations tend to be larger over time in polar regions than near the equator, so global average temperature variations are not likely to have been as large.  However, even considering the large uncertainties in the accuracy of the reconstructions and their relevance to global average temperatures, the implication is that climate variations much larger than those seen in the most recent industrial era may have been quite common in the past during the Holocene with no influence from human activities.  As a consequence, trying to separate a relatively small human-induced climate signal from the large natural climate variation noise may not be as easy as some researchers claim.

It is also interesting to note the general divergence of the Greenland versus Antarctic climate reconstructions during the period from about 4,000 to 11,000  years ago with the largest separation about 8,000 years ago when much warmer anomalies are indicated in Greenland than in Antarctica.

Figure 2 shows the most recent 4,000 years in more detail.  This graph uses the same normalized temperature anomaly data as in the previous graphs, except the GISP2 data are from a higher temporal resolution reconstruction based on deuterium and argon isotope analyses (Kobashi 2011).  During this period the ice core climate reconstructions show considerable variation ranging from -3C to +4C.  The temperature anomalies from 2,000 to 4,000 years ago averaged about 0.5C above the reference normal for 200 to 1,200 years ago, which may be comparable to very recent temperatures in the last ten years.

Figure 2. Comparison of surface temperature reconstructions for the last 4,000 years based on oxygen isotope ratio analyses of ice cores from Antarctica (EPICA and Vostok) and from Greenland (GISP2 and GRIP) along with the more recent HADCRUT4 global surface temperature anomaly analysis (click to enlarge).

Looking back at the most recent 2,000 years, Figure 3 below again shows considerable variation in temperatures over relatively short time periods.  This graph presents the same data used in Figure 2.  As mentioned previously, the rapid variations in temperature may be more indicative of local climate variations than of global climate variations since these shorter variations do not correlate very well between data sets.  Uncertainty in the age estimates and variable amounts of smoothing between the data sets may also come into play.

Figure 3. Comparison of surface temperature reconstructions for the last 2,000 years based on oxygen isotope ratio analyses of ice cores from Antarctica (EPICA and Vostok) and from Greenland (GISP2 and GRIP) along with the more recent HADCRUT4 global surface temperature anomaly analysis (click to enlarge).

To better estimate the global temperature average during the last 2,000 years, the four ice core climate reconstructions presented in Figure 3 were interpolated by year and averaged together for each year over the last 2,000 years.  The resulting global climate reconstruction is shown below in Figure 4, with plus and minus one standard deviation as a rough indication of uncertainty.  Coincidentally, the composite ice core global temperature reconstruction matches nicely with the HadCRUT4 temperature where they meet in 1850.  The implication is that the HadCRUT4 reference normal period of 1961-1990 may closely correspond to the ice core reconstruction reference normal for 200 to 1,200 years ago.

Figure 4. Composite of EPICA, Vostok, GISP2, and GRIP ice core surface temperature reconstructions referenced to the 1000 year period ending 200 years ago and compared with the recent HADCRUT4 global surface temperature anomaly analysis (click to enlarge).

For easier interpretation relative to history, Figure 5 shows the same data as Figure 4, but scaled in years AD with increments marked for each century.  Note that year 0 in Figure 5 is actually year -1, since there is no year 0.

Figure 5. Composite of EPICA, Vostok, GISP2, and GRIP ice core surface temperature reconstructions referenced to the 1000 year period ending 200 years ago and compared with the recent HADCRUT4 global surface temperature anomaly analysis, with scale in years AD (click to enlarge).

The composite is considerably less variable than the individual data sets, but even the remaining variability may still include a fair amount of “noise” rather than accurately depicting global temperature variations.  Nevertheless, the composite does suggest a possible warmer period globally from about 250 AD to about 1000 AD and a cooler period from about 1200 AD to about 1950 AD.  There is a hint of the “Little Ice Age” with the coldest period from about 1630 AD to 1780 AD when the composite temperature is mostly in the range from 0.3C to 1.0C below the reference normal.  The warmest period indicated by the composite was from about 625 AD to 720 AD where the estimated global temperature was about 0.5C to 1.3C above the reference normal.  For comparison, the HadCRUT4 estimated global temperature anomaly has been near 0.5C for 2005-2014 AD.  The peak period from about 640-660 AD may possibly have been as much as 0.5-0.8C warmer than our 2005-2014 AD average and without any human influence.

This analysis reinforces my suspicion that global climate variability from natural causes can easily exceed what has been observed in the current industrial era, making it difficult to confidently estimate the human influence, if any.

Update through 2022

Figure 5 above has been updated to include the HadCRUT5 global surface temperature anomaly estimates through 2022.  Also Figure 6 has been added below to provide a closer view of the current 1,000 year period ending 2100.

Figure 6. Composite of EPICA, Vostok, GISP2, and GRIP ice core surface temperature reconstructions referenced to the 1000 year period ending 200 years ago and compared with the recent HADCRUT4 global surface temperature anomaly analysis, for 1100 to 2100 AD (click to enlarge).

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Three Million Years of Climate Change

Using reconstructions of global temperature based on oxygen isotope ratio analyses of ocean sediment cores and polar glacial ice cores we can look back at the Earth’s climate for about 800,000 years in considerable detail. To go farther back in relatively high temporal detail, we have to rely on the ocean sediment core analyses that provide data back to five million years.

The three previous blog posts looked at the Earth’s climate over the last half million years using these same proxies for global temperature. During that period the Earth’s climate was dominated by five intense glacial periods each lasting about 60,000 to 100,000 years, alternating with much shorter interglacial warm periods lasting about 2,000 to 25,000 years. The Earth is currently in the most recent interglacial period that first reached near modern “normal” temperatures about 12,000 years ago. The glacial-interglacial cycle time was about 80,000 to 120,000 years during this period.

Looking back another half million years to a million years ago, the glacial periods were about the same duration but progressively weaker going backward in time as can be seen in the graph below.

Global temperature reconstructions for the last million years from oxygen isotope ratio analyses of ocean sediment cores (Bintanja), Antarctic ice cores (EPICA and Vostok), and a Greenland ice core (GISP2). Click on graph to enlarge.

The interglacial warm periods were similar in length to the most recent half million years, but were weaker and did not quite reach the modern “normal” temperature. Consequently, the average global temperature was about the same as for the most recent half million years, a little over 5 degrees Centigrade (C) below our modern “normal” temperature. Thus, over the last million years, the Earth has averaged a little over 5C colder than our current modern “normal” temperature. This estimate is based on adjusting the ocean sediment core reconstruction to match the Antarctic ice core reconstructions from EPICA and Vostok. The average is even lower in the unadjusted ocean sediment reconstruction as will be shown later.

Another difference is that the range in temperature during each cycle was only about 8C during the during the earlier half of the last million years, as compared to about 14C during the most recent half of the last million years.

Looking back all the way to three million years ago, the adjusted ocean sediment core reconstruction shows that global temperature progressively dropped from levels much closer to the modern “normal” around three million years ago to the much colder average of the last million years. Thus, the start of our current ice age was about three million years ago. The cycling between cold glacial periods and warm interglacial periods was present throughout the last three million years but each cycle was only about 40,000 years duration prior to one million years ago compared to 100,000 years duration in the most recent million years as can be seen in the graph below.

Global temperature reconstructions for the last 3 million years from oxygen isotope ratio analyses of ocean sediment cores (Bintanja), Antarctic ice cores (EPICA and Vostok), and a Greenland ice core (GISP2). Click on graph to enlarge.

Without adjustment, the ocean sediment core reconstruction shows even larger swings in amplitude of both warming and cooling with each cycle as seen in the graph below. The large difference between the adjusted and unadjusted global temperatures is an indication of the uncertainty involved in making these reconstruction estimates.

Global temperature reconstruction for the last 3 million years based on oxygen isotope ratio analyses of ocean sediment cores (Bintanja). Click on graph to enlarge.

Note that in the unadjusted ocean sediment core reconstruction the global average temperature is estimated to have been over 7C warmer than our modern “normal” for a period of several thousand years as recently as a little over 2.9 million years ago. With temperatures that warm there would have been much less permanent ice than today and sea levels would have been substantially higher.

One of the challenges to understanding the Earth’s climate is to determine what caused this gradual trend into our current ice age. Another is to understand what caused the 40,000 year cold to warm cycle to increase fairly abruptly around a million years ago to about 100,000 years. And yet another challenge is to determine what caused the increase in amplitude of the cycles about half a million years ago.

One of the leading hypotheses as to what started this ice age is that it may have resulted from the connection of North America to South America by the uplifting that created the Isthmus of Panama and blocked ocean currents from passing between the Atlantic and Pacific.

The 40,000 year cycling corresponds well with orbital/rotational mechanics of the Earth that induce changes of solar radiation at the poles. But what caused the shift to 100,000 year cycling and a greater amplification is more difficult to explain. Until our climate models can reproduce these past changes in climate, I have little confidence that they will be able to accurately predict the future climate. In the mean time, extrapolating past climate cycles is probably our best estimate of the future climate changes we can expect, as was attempted in the previous blog post linked below.

Interglacial Comparisons