Last week, a group of climate alarmists headed by Shaun Marcott, a geologist at Oregon State, released a study that purported to resurrect the infamous hockey stick. Relying on an assortment of data sources, Marcott and his colleagues tried to reconstruct global temperatures over the last 11,000 years, since the end of the last Ice Age. While acknowledging that the Earth has often been warmer than it is today, they claimed that the Earth is now the warmest it has been “in at least 4,000 years,” and that the rate of warming over the last 100 years is unprecedented. Voila! The hockey stick returns. Naturally, the study was acclaimed uncritically in the press; this NPR story titled “Past Century’s Global Temperature Change Is Fastest On Record” offers a good summary of Marcott’s research.
There are two fundamental problems with Marcott’s claims. First, thermometers have been in existence for only a very small portion of the last 11,000 years. It is hard enough–some say impossible–to calculate even the current average temperature of the Earth. To compare 100 years of contemporary records against reconstructions of thousands of years based on proxies like tree rings and ice cores is, at best, matching an apple against millenia of oranges. Second, the Marcott reconstruction is contradicted by just about everything we know about the temperature history of the last 11,000 years.
Anthony Watts is in the midst of a three-part analysis of Marcott’s study. Part 2 addresses the inconsistency between Marcott’s reconstruction and the climate record as revealed by our most reliable data sources. Here are some excerpts:
Both the Greenland GISP2 temperature curve (Figure 1B) and the oxygen isotope curve (Figure 1C) clearly show that except for the Little Ice Age and Dark Ages Cool Period, temperatures for all of the past 4,000 years have been warmer than the end of the ice core (1950 AD). The Medieval Warm Period was 1.1° C warmer than the top of the core (1950) and at least four other warm periods of equal magnitude occurred in the past 4,000 years; four other warm periods were ~1.3°C warmer; two other warm period were 1.8-2.0°C warmer; and one warm period was 2.8°C warmer. At least a dozen periods more than 1°C warmer than 1950 occurred, clearly contradicting the Marcott et al. conclusions.
The top of the GISP2 ice core is 1950 AD, so we need to look at more recent temperatures in Greenland in order to get to the “present temperature,” i.e., has the temperature in Greenland risen since 1950? Figure 2 shows 1880 to 2004 temperatures in Greenland (Chylek et al., 2004, 2006). Temperatures in 2004 were slightly lower than in 1950, so temperatures at the top of the Greenland ice core are not significantly different than those “at present.”
Next, Watts addresses Marcott’s assertion that “Global temperature…has risen from near the coldest to the warmest levels of the Holocene within the past century. A heat spike like this has never happened before, at least not in the last 11,300 years.”
Let us test this conclusion against real-time data. First, their statement that “Global temperature…has risen from near the coldest to the warmest levels of the Holocene within the past century” is not true. The coldest part of the Little Ice Age occurred about 400 years ago, during the Maunder Minimum, so right off the bat, their conclusion is flawed. They appear to be unaware of the cyclic nature of temperature change and use the low point of the 1880-1915 cool period as their starting point for assessing the rate of warming over the “past century,” rather than 1913-2013. Comparing the depth of cooling in a cool period with a warm period peak is comparing apples and oranges. It distorts the real rate, which should be measured from cool peak to cool peak or warm peak to warm peak. The 1880-1915 cool period was followed by the 1915-1945 warm period, the 1945-1977 cool period, and the 1978-1998 warm period (Figure 4). The rate of warming from 1913 to 2013 is about 0.7°C per century (which is about the same as the warming rate over the past 400 years as we have been thawing out of the Little Ice, long before atmospheric CO2 began to rise significantly).
So let’s compare this rate (0.7°C per century) to rates of temperature increase in the past 11,300 years. Figure 5 shows rates of temperature change in the Greenland GSP2 ice core from the end of the last Ice Age through the Holocene (Figure 4A). Figure 4B shows some of the higher rates of temperature change in Figure 4A. The highest rates occurred at the transition from the Ice Age to Holocene when warming rates in Greenland were 20 to 24°F per century and the huge continental ice sheets that covered large areas of North America and Eur-Asia melted dramatically. As shown in Figure 4B, the rate for the past century (0.7°C) is puny indeed compared to late Ice Age/early Holocene rates.
Holocene rates of warming and cooling were not as profound as those at the end of the last Ice Age, but were nonetheless greater than or equal to recent warming rates. Marcott et al. contend that “If any period in time had a sustained temperature change similar to what we have today we would have certainly seen that in our record.” As shown in Figure 4A, we do indeed have a record of warming rates far in excess of those in the past century.