Marcott et al (2013)

A study by Marcott et al that reconstructed the Earth’s climate over the past 11,300 years using data gathered by various proxies at 73 locations globally. The findings of this study showed that the Earth had been gradually cooling over the past 5,000 years up until about 100 years ago, when it began to warm. The Holocene era, the studied time period, experienced gradual climate changes which allowed for the flora and fauna to adjust. The more recent changes though, are more rapid and may exceed the hottest period during the Holocene period. The recent warming is due to human activity, not natural causes. This is evident especially when examining the fact that because of the Earth’s orientation, the Norther Hemisphere should be experiencing cool summers as part of a cooling trend, which is not occurring. It is projected that the temperature will increased anywhere from 2 degrees to 11.5 degrees Fahrenheit by the end of this century.  

Three key findings of the study include the fact that the 1.3 degree cooling that occurred over the last 5,000 years was reversed in just the past 100 years, and the temperature will continue to rapidly increase. Another key finding is that with the natural changes of the Earth the Northern Hemisphere was projected to experience intense cooling over the next few thousand years but most likely will not because of the increase output levels of carbon dioxide and continued warming. A third finding is that this past century is an anomaly because we have not experienced this level of warming since the most recent ice age over 11,000 years ago.

Figure 1: Temperature Anomaly for previous 2,000 and 11,000 years.

Image Source: http://cdn.antarcticglaciers.org

These two graphs temperature data for the Earth over the Holocene period. The figure on the left shows the same data as the figure on the right, zoomed in to 2,000 years. These graphs illustrate the warming and gradual cooling experienced during the Holocene period with the rapid jump in temperature over the past 100 years. As illustrated in the graph on the right, the current temperature is very close to the warmest time of the Holocene period.

According the Michael Mann of Pennsylvania State University, the main conclusion of this study by Marcott et al is “the rate of warming appears to be unprecedented as far back as the authors are able to go (to the boundary with the last ice age). And the rate of warming appears to have no analog in the past”. The warming that we are experiencing, and the rate at which it is occurring is not by means of nature as it has been in the past. With increased human activity and output of carbon dioxide among other greenhouse gases into the environment, the Earth is going through rapid changes that we have not seen before.

Concerns that Robert Rohde, the chief data analyst behind the Berkeley Earth Surface Temperature project, has are that in the study they relied too heavily on their proxy data which was spread out in time. This can blur or obscure the fluctuations in the data. Because of this he thinks that, “we can’t use the analysis of Marcott et al. to draw any firm conclusions about how unique the rapid changes of the twentieth century are compared to the previous 10,000 years.” Ultimately he recognizes the impact that the findings have in building upon current knowledge as well as assisting in future studies, but he urges the use of caution when comparing their findings and current events.

According to Richard Alley, of Pennsylvania State University, the big-picture messages from this study are, for one, “Our high assessed confidence that the recent warming is mostly human-driven, and that the costs will become large if the warming becomes large, do not primarily rest on how much warmer or colder today is than some particular time in the past, or even on how fast the recent changes are relative to those in the past.” Meaning scientists aren’t conducting these studies and saying that because today is hotter than it was 1,000 years ago means that we’re experiencing global warming; they look at the many factors that go into what climate is and what the climate is and has been around the world. Another takeaway is, “Whether the past was naturally warmer or cooler than recently, and whether the changes were faster or slower than recently, are of great interest to climate scientists in learning how the climate system works, including the strength of feedbacks.” So while we cannot look at 1 day or 1 year for comparison, it is still extremely important and valuable that scientists continue to study the past in attempts to understand how climate works that that we may appropriately prepare for the future.

Roger Pielke, professor at the University of Colorado, addresses in a blog post that the media made some mistakes when covering this study. There has been various coverage of the study that explains the resulting temperature data as fitting into the “hockey stick” analogy. The coverage explains that there has been a sharp increase in temperature in the past 100 years, the blade of the stick. Pielke explains this is wrong. He explains that the 20th century data is “not statistically robust” therefore there can be no “blade” for Marcott et al’s study.