During the Pliocene epoch (5.3 to 2.6 million years ago) climate became cooler and drier, and seasonal, similar to modern climates. The global average temperature in the mid-Pliocene was 2â€“3 Â°C higher than today, global sea level 80 feet higher and the northern hemisphere ice sheet was ephemeral before the onset of extensive glaciation over Greenland that occurred in the late Pliocene around 3 million years ago. Scientists are looking at what climate conditions were like 3.3 to 3 million years ago, during a geologic period known as the Pliocene, and they are confident in the accuracy of their data. The Pliocene is the most recent period of sustained global warmth similar to what is projected for the 21st century. Climate during this time period offers one of the closest analogs to estimate future climate conditions.
The USGS is leading research to reconstruct Pliocene ocean temperatures primarily using fossils contained in sediments from that time period.
"Confidence in data, as discussed in this paper, refers to the overall quality of our Pliocene temperature estimates," said USGS scientist Marci Robinson. "For each temperature estimate, we looked at factors such as the abundance of fossils, the number of samples analyzed, fossil preservation, and the techniques used for analysis."
Scientists from around the world are using the Pliocene reconstructions to compare climate model simulations from fourteen general circulation models. This is an international effort with models developed by the United States, Japan, France, United Kingdom, China, Germany and Norway.
In this study, published in the journal Nature Climate Change, an initial comparison was made between four existing models. Conclusions showed that the models are close in agreement with each other and USGS data, except in the North Atlantic where modeled temperatures differ slightly from the Pliocene data and from each other.
"The processes that impact North Atlantic climate are complex, and we have analyzed many sites in the area," said Dowsett. "Based on this study, we have a high degree of confidence in our North Atlantic data, and we will wait to see how the rest of the models compare and plan future research to better understand the complexities."
Processes that influence North Atlantic Ocean temperatures include ocean circulation, the shape and characteristics of the seafloor, and concentrations of atmospheric CO2 and other trace gases. The Earth's orbit is another factor to consider because it affects the amount of sunlight and, therefore, heat that reaches the Earth's surface.
This Pliocene climate data reconstruction will be useful in predicting which climate model will best represent the future. Carbon dioxide concentration during the mid Pliocene has been estimated at around 400 ppmv.
Several mechanisms have been proposed to explain global cooling the end of the Pliocene and the onset of extensive northern hemisphere glaciation. They are:
Panama seaway closure due to salinity contrast between the Atlantic and the Pacific Oceans.
Collapse of permanent El NiÃ±o
Uplift of the Rocky mountains and Greenland west coast
Research on the mid-Pliocene constitutes the most comprehensive global reconstruction for any past warm period.
For further information: http://www.usgs.gov/newsroom/article.asp?ID=3147&from=rss&utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+UsgsNewsroom+%28USGS+Newsroom%29&utm_content=Google+Reader
Photo: University of Nebraska Lincoln