Tag Archives: paleo climate

Holocene Climate

The Holocene geological epoch starts at the beginning of the current interglacial period about 11,700 years ago and continues to the present.  As discussed in previous posts, the current interglacial 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 2020

Figure 5 above has been updated to include the HadCRUT4.6 global surface temperature anomaly estimates through 2020.  Also Figure 6 is being 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).

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.

Climate Reconstructions

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.

Climate Reconstructions

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.

Climate Reconstruction

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

 

Interglacial Comparisons

Most people don’t realize that the Earth is still in a long term ice age that started about three million years ago and has had many alternating cold glacial periods interspersed with warmer interglacial periods.  We are currently in an interglacial period where global temperatures have been near our modern “normal” for about 12,000 years now.  In addition to our current interglacial period, there have been four previous interglacial periods in the last 500,000 years.  Each one has been spaced about 100,000 years apart and lasted about 2,000 to 25,000 years with temperatures at or above our current modern “normal”.  These observations are based on the EPICA Antarctic ice core climate reconstruction using oxygen isotope ratios as a proxie for global temperature change.

The graph below shows the current and last four interglacial periods plotted together, normalized to the year where the estimated global temperature first reached the level of our modern “normal” climate. The approximate year where each interglacial episode first reached the modern “normal” temperature is shown in the legend.  Notice that all four previous interglacials had global temperatures reaching 2 to 4 degrees Centigrade higher than our current modern “normal” without any help from humans, based on this reconstruction.

Interglacial Period Comparison

This graph compares the current interglacial period with the previous four interglacials using the EPICA ice core climate reconstruction. Each interglacial period has been normalized to the time the global temperature departure first reached the current “normal” temperature. Click on graph to enlarge.

I find it amazing how abruptly and similarly each glacial period ended in about 5,000 years to start each following interglacial warm period.  In contrast, the duration of the interglacials has been much more variable. The most recent previous interglacial period that started about 130,000 years ago lasted about 14,000 years at temperatures at or above our current modern “normal”. The second previous interglacial lasted about as long as our current interglacial while the third previous was by far the shortest, lasting about 2,000 years, but arguably had somewhat of a double peak. However, the latter secondary peak did not quite reach the warmth of our current modern “normal”. The fourth previous interglacial which started about 418,000 years ago was by far the longest at about 25,000 years.

As repeatable as the glacial cycles have been over the last 500,000 years, I see little reason not to expect more of the same in the future. Using this interglacial comparison as a climate persistence forecast, we might expect about a 75% chance that the global average temperature will begin to drop dramatically sometime within the next few thousand years and about a 25% chance of staying warm for another 10,000 years or so … at most. Perhaps we need all the anthropogenic warming we can muster to stall or prevent the next glacial period?

Our understanding of what causes these glacial cycles, which are relatively recent on a geological scale, is still very limited although there are plenty of hypotheses. Our current climate models cannot predict them and therefore to me are somewhat useless. Until we can create climate models that can accurately track past glacial and interglacial periods I will not be too impressed and I certainly don’t believe our infant and untested climate models should be used to shape policy regarding “climate change”.

Update 2016 November

Below is a link to an interesting analysis of the causes of glacial cycles, along with conclusions made by the author, which seem reasonable to me.  The author hypothesizes that evidence suggests that the current interglacial period is likely to be only average in length and therefore should be ending soon, most likely sometime within the next two thousand years.

Nature Unbound I: The Glacial Cycle

Conclusions

1) Obliquity is the main factor driving the glacial-interglacial cycle. Precession, eccentricity and 65°N summer insolation play a secondary role. There is no 100 kyr cycle. Milankovitch Theory is incorrect.

2) The current pacing of interglacial periods is the consequence of the Earth being in a very cold state that prevents almost half of obliquity cycles from successfully emerging from glacial conditions. The rate for the past million years has been 72.7 kyr/interglacial, or 1.8 obliquity cycles between interglacials. This can be generally described as one interglacial every two obliquity cycles except when close to the 413 kyr eccentricity peaks, when interglacials take place at every obliquity cycle.

3) Glacial terminations require, in addition to rising obliquity, the existence of very strong feedback factors manifested as very low glacial maximum temperatures. High northern summer insolation at the second half of the rising obliquity period is a positive factor, and if high enough during eccentricity peaks can drive the termination process.

4) CO2 can only produce a minor effect in glacial terminations since the measured change in concentration (roughly a third of a doubling which represents half of the warming effect of a doubling) is too small to account for any important contribution to the large observed temperature changes.

5) Since the precession cycle has bottomed and the obliquity cycle is half way down we should expect the next glacial inception to take place within the next two millennia.

Matching Ice Core and Ocean Sediment Core Climate Proxies

In my previous blog entry I showed the large discrepancies in the magnitude of reconstructed paleo climate estimates between the ice core and ocean sediment proxies based on oxygen isotope ratio analysis. I made two sets of adjustments to the proxy data to bring them into better alignment – a high grouping and a low grouping. The high grouping shows higher glacial period temperatures and the low grouping lower glacial period temperatures.

In the high grouping I multiplied the Greenland GISP 2 ice core analysis by 0.5 and the Bintanja ocean sediment analysis by 0.6 to reduce their offsets from our modern “normal” climate reference and thus bring them into better alignment with the Vostok and EPICA ice core analyses from Antarctica, as shown below (click to enlarge).

Climate Reconstructions Adjusted to Match

A comparison of adjusted climate reconstructions from a composite of many ocean sediment cores (Bintanja) versus three different ice core reconstructions (Greenland GISP2 and Vostok and EPICA from Antarctica). The ice core reconstructions have been adjusted by the factor indicated to better match the ocean sediment reconstruction.

For the low grouping I increased the offsets from the modern “normal” climate reference by multiplying the Vostok and EPICA ice core analyses by 1.6 to bring them into better alignment with the Bintanja ocean sediment analysis. In this case I also multiplied the GISP 2 analysis by 0.9 to bring it into better alignment with the Bintanja analysis. The result is shown below (click to enlarge).

Climate Reconstructions Adjusted to Match

A comparison of adjusted climate reconstructions from a composite of many ocean sediment cores (Bintanja) versus three different ice core reconstructions (Greenland GISP2 and Vostok and EPICA from Antarctica). The composite ocean core reconstruction and Greenland GISP2 ice core reconstruction have been adjusted by the factor indicated to better match the Antarctica Vostok and EPICA ice core reconstructions.

Notice that even though the patterns are very similar between the two adjusted groupings, the temperature scale is much different.  The high grouping shows the lowest glacial period global temperatures about 8 to 10 degrees Centigrade (C) below our modern “normal” while the low grouping shows global temperatures about 12C to 18C below.  Likewise, the high grouping shows maximum interglacial warm period temperatures as much as 1C to 4C above our modern “normal” while the low grouping shows peak interglacial temperatures 2C to 8C above our modern “normal”.

I find the high grouping to be more aesthetically appealing because it doesn’t look quite as noisy, but I’m not sure which of these two groupings might be more accurate. The differences between them reenforce the uncertainty of the magnitude of past global climate variations. The timing of major events is remarkably similar between these proxies, although that might result from similar methods in relating the proxie measurements to a time scale for the reconstruction.

I my next post I will make a paleo climate persistence forecast based on the EPICA ice core analyses by comparing our current interglacial period with the previous four interglacials.

Interglacial Comparisons