LOS ANGELES (MCT) — First the good news: In the last 11,300 years, humans have endured a planet warmer than today's, even as they set about building their earliest civilizations.
Now the bad news: That will no longer be true 87 years from now, according to scientists who have conducted a comprehensive analysis of the planet's climate history since the world's ice sheets began their most recent retreat from North America and Europe.
New research into the Earth's ancient climate is providing a clearer, more detailed view of how the planet's average surface temperature fluctuated over the period known as the Holocene epoch, which continues through to the present day. It's the time in which humans truly began making their mark on the planet, abandoning their hunting and gathering traditions and adopting a settled, agricultural lifestyle.
In a study being published in Friday's edition of the journal Science, researchers used eight indirect temperature indicators — such as pollen and shells from marine organisms — to chart long-term global warming and cooling trends. The research team concluded that temperatures in the last decade have not exceeded the Holocene's steamiest periods from thousands of years ago. However, if current warming trends hold, those records will be broken by the end of the century.
"By the year 2100, we will be beyond anything human society has ever experienced," said study leader Shaun Marcott, a post-doctoral researcher at Oregon State University's College of Earth, Ocean and Atmospheric Sciences.
According to NASA, the global average temperature for 2012 was 58.3 degrees Fahrenheit, making it the ninth-hottest year in recorded history.
However, there is no single, agreed-upon method of calculating these temperatures, so scientists tend to discuss climate change by highlighting deviations from a specific reference point.
While a 1-degree Fahrenheit increase sounds small, it represents an enormous amount of heat energy. For instance, a 10-degree drop would plunge the world into another period of major glaciation, while every 1.8-degree increase would gradually equate to a roughly 65-foot rise in sea level due to melting polar ice, according to NASA climatologist James Hansen.
Previous efforts to measure past climate conditions have relied heavily on measurements of tree ring thickness. At high latitudes, tree growth is controlled mostly by temperature, so thick rings suggest warm years. But trees don't live longer than several thousand years, so those efforts have focused on shorter periods of time — just 1,500 to 2,000 years.
These earlier studies have also featured the now-famous "hockey stick" graphs, in which average temperatures fluctuate in a long band that vaguely resembles the shaft of a hockey stick before rising sharply in the last 100 years, like the hockey stick's blade. Such diagrams have generated virulent criticism from those who refute the idea that man-made greenhouse gases are heating the climate.
Marcott and colleagues from Harvard University said that by estimating temperature fluctuations for the entire Holocene, they hoped to provide a new perspective on the debate.
The researchers collected data from 73 sites across the globe on land and beneath the sea. They included ice cores from Greenland, stalagmites in Borneo and fossilized pollen in Scandinavia.
Some of the data came from the shells of long-dead aquatic microbes that were buried 50 feet or more below the ocean floor. The chemical makeup of the shells gives scientists clues about the water temperature at the time the creatures existed — shells formed in warm water will have a greater percentage of stable oxygen isotopes, while shells formed in cold water will have a lower ratio.
Similarly, sediment core samples taken from the bottom of a pond, lake or ocean will contain fossilized pollen grains, which have very distinctive shapes when viewed under a microscope. By examining the variety of species and their abundance, scientists can gauge the area climate.
Yet another temperature indicator used in the study were trans fats produced by a specific type of algae. The chemical bonds of these fat molecules, called alkenones, change according to water temperature. When the algae die, the alkenones sink to the bottom of sea or lake bed, where they are mostly preserved.
What the researchers found was a climate that warmed and cooled gradually over a period of millennia, then experienced a sudden, unprecedented rise in temperatures — similar to earlier hockey stick graphs.