Global Temperatures for 11,300 Years

On Friday the Journal Science published a peer reviewed paper by Shaun A. Marcott, of the College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Jeremy D. Shakun, of the Department of Earth and Planetary Sciences, Harvard University, and Peter U. Clark and Alan C. Mix also of the College of Earth, Ocean, and Atmospheric Sciences, Oregon State University “A Reconstruction of Regional and Global Temperature forthe Past 11,300 Years.” The authors lead by post-doctoral researcher Shaun Marcott used existing data from 73 records from multiple paleoclimate archives and 22 temperature proxies to create a reconstruction of global temperatures back to the end of the last Ice Age in order to view today’s climate conditions in a larger context during the Holocene period, the time of man. The last ice age is currently believed to be the eighth ice age during the last million years.

Drs. Marcott, Shakun, Clark and Mix’s research shows that over the past 5,000 years, the Earth on average cooled about 1.3 degrees (Fahrenheit) – until the past 100 years, when it warmed about 1.3 degrees (F) bringing the average global temperature back to the previous level. The largest temperature increase changes were in the northern hemisphere, where there are more land masses and greater human populations.

Figure taken from Dr. Marcott et als paper

The scientists did not take any new samples, but used existing core samples to derive age primarily with carbon 14 dating of organic material; and tephrochronology or annual layer counting were used where appropriate. These methods are the current established and accepted scientific procedure. They recalibrated the radiocarbon data of the original studies (some of which were decades old). The basic methology for determining surface temperature in the past depends on extrapolation of information from core samples. In ice cores drawn from glaciers, oxygen isotopes are measured and can be used to infer the temperature at the time when the snow was originally deposited. The isotopic composition of the ice in each layer reflects both the temperature in the region where the water molecules originally evaporated and the temperature of the clouds in which the water vapor molecules condensed to form snowflakes. The temperature can thus be inferred from the data.  

Data sets from cores taken from the sediments at the bottoms of lakes and oceans followed the original authors’ suggested values. Sediment cores can be analyzed for oxygen isotopes, the ratio of magnesium to calcium, and the relative abundance of different microfossil types with known temperature preferences (such as insects) or with a strong temperature correlation like some algae to determine the temperature of the water. This in turn, can be related to the local surface temperature. Beyond 2,000 years there was a higher level of uncertainty in the age of the samples. To account for that uncertainty, the authors used a Monte Carlo simulation. The uncertainty between the age-control points was modeled as a random walk. For the layer-counted ice-core records, they applied a ±2% uncertainty for the Antarctic sites and a ±1% uncertainty for the Greenland site.

According to Peter Clark, “When you just look at one part of the world, the temperature history can be affected by regional climate processes like El Niño or monsoon variations, but when you combine the data from sites all around the world, you can average out those regional anomalies and get a clear sense
of the Earth’s global temperature history.”

We already knew that on a global scale, Earth is warmer today than it was over much of the past 2,000 years,” Dr. Marcott said. The new study, shows that the earth average temperature is still within the high point before the “Little Ice Age,” but as can be seen on the graph below warmer than most of the past 11,300 years. This period of time, the Holocene, spans the entire period of human civilization. “The Earth’s climate is complex and responds to multiple forcings, including CO2 and solar insolation,” Marcott said. “Both of those changed very slowly over the past 11,000 years. But in the last 100 years, the increase in CO2 through increased emissions from human activities has been significant. It is the only variable that can best explain the rapid increase in global temperatures.”

taken from Dr. Marcott et al 's paper