On the recently published Marcott et al. study of global temperatures over the Holocene period, or past 11,300 years: It is safe and justifiable to say that our current rate of warming is probably unprecedented and certainly extreme when compared to Holocene temperature trends. Based on every plausible IPCC emissions scenario, temperatures by the end of the century are projected to be higher than any recorded in the entire Holocene.
The following is a guest post by Climate Nexus:
Setting the Temperature Record Straight: The Last 11,300 Years Explained
Authors Marcott et al. recently published a study of global temperatures over the Holocene period, or past 11,300 years. The study reconstructed past temperatures using data from sources such as ice cores and ocean bottom cores that are known to record changes in temperature, allowing them to be used as proxies in the absence of a thermometer record. The study found that current temperatures measured by thermometers are warmer than a large majority (~75%) of the temperatures of the last 11,300 years. In interviews, the authors explained that the warming trend over the past century is more rapid than temperature changes that took place over the past 11,300 years. The study conclusions and author interviews received harsh criticism from some.
Roger Pielke Jr. and several other bloggers have denounced what they see as a misleading presentation of the study. Pielke posted that the study “does not have the ability to reproduce 20th century temperatures in a manner that is ‘statistically robust.’” Bloggers Steve McIntyre and Ross McKitrick criticized the study further, focusing on specific ways in which they believe the 20th century part of the proxy temperature record is unreliable, and claimed that data-smoothing effects in the combining of temperature records would mask detection of any large warming spikes in the distant past. All three bloggers accused the study authors of purposely concealing aspects of the study while giving alarmist interviews to the media.
In a mostly uncritical account of the debate, the Washington Post reprinted these criticisms.
The scientists compared their record of past temperatures (derived from proxy sources and known as paleoclimate temperatures) to the modern instrumental record as well as to future climate projections to draw their conclusions.
The authors of the paper recognized the statistical issues Pielke and others raised, and accounted for them in advance. They acknowledged in their paper and reiterated in their FAQ that the 20th century portion of the proxy record is not strong enough to base conclusions on, because they include less data for this period. The authors also acknowledged that their data have an average resolution of about 120 years due to geologic sampling techniques, and their practice of combining the data from many sources means that a temperature change lasting less than 300 years may not be recorded reliably.
It is reasonable of the authors to expect that, as scientists, they will rightly be asked about what their work means to humans today. Answering that question means finding the relationship between paleotemperatures and current temperatures. So, to establish that relationship the authors brought in the reliable data of the 20th century from thermometers. This thermometer-based temperature record has been reviewed and validated by a multitude of different studies, and it has been independently found to be consistent with high-resolution proxy records.
By using the paleotemperature data and the historical record, the authors were able to confidently compare the ranges of Holocene and recent temperature changes. They made corrections for the smoothing effects of the geologic record, thus avoiding the problem of comparing high resolution short-term changes from the past century with lower-resolution proxy data.
But what about their more contested statements in the media, comparing the rates of temperature change? These statements reflect the information offered by the averaged proxy trends reported in the study, as well as the larger body of science on this topic, which address the criticisms on rate of change when taken together.
Technically it is possible that wild temperature spikes occurred in the past, yet returned to normal levels so quickly that the averaging techniques rendered them insignificant. However it is also likely that a spike of this magnitude and speed would leave at least a moderate bump even in the smoothed temperature record (see blogger Tamino’s analysis). Furthermore, there is no clear natural mechanism for such an abrupt shift in temperatures, particularly a shift that leaves no trace in any of the independent temperature proxies or in the ice-core greenhouse gas records.
The increase in temperatures during the last century is so striking that it would take a stunning series of coincidences for a similar up-spike in the last 11,300 years to leave no trace in the reconstruction. But there's no trace to be found. The paleotemperature data include known proxy records such as ice cores that have plenty of time resolution to show such an event. But there’s no trace there either. The available high-resolution ice cores from the northern and southern hemispheres show no evidence for coordinated global events of a duration and magnitude similar to 20th century warming.
Finally, the Marcott et al. study notes that based on a range of greenhouse gas emissions scenarios, climate models project that global average temperatures will exceed even the highest recorded Holocene temperatures by the end of this century.
Together these factors make it safe and justifiable to say that our current rate of warming is probably unprecedented and certainly extreme when compared to Holocene temperature trends.
- The Holocene reconstruction is unique in that it looks back 11,300 years and has a resolution of 300 years. The past 2000 years are much more extensively studied, and many high-resolution reconstructions have been made of this period. The Holocene cooling trend in Marcott et al.’s data agreed closely (given its resolution and inherent smoothing) with these higher-resolution studies (See page 1199 of the paper for a direct graphic comparison).
- Blogger and statistician Tamino created a simulated Holocene temperature record with three large warming spikes similar to the one the world is currently experiencing. He then smoothed the record using Marcott’s technique, and found that the spikes were reduced but still quite noticeable.
- Based on every plausible IPCC emissions scenario, temperatures by the end of the century are projected to be higher than any recorded in the entire Holocene. A dramatic graph produced by Climate Progress superimposes one recent temperature estimate for the year 2100 on the Holocene record, giving the appearance of past temperatures hitting a vertical wall.
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Also see this compelling post by Jon Koomey at Stanford: If we don’t change our direction, we’ll end up where we’re headed (re-posted at Climate Progress)
At Skeptical Science: Real Skepticism About the New Marcott 'Hockey Stick'
At Ars Technica: Climate science once again finds itself fighting with hockey sticks