Working with a continuous record of Arctic climate reaching back 3.6 million years, researchers have documented a period when the region was significantly warmer and wetter than it is today and when the atmosphere's inventory of carbon dioxide was comparable to today's levels.
The period the team has analyzed covers the first 1.4 million years of the record, when the region's climate shifted from warm and wet to conditions that signaled the start of ice ages.
This period is of interest in part because the warmth persisted despite periodic shifts in Earth's orbit that reduced the intensity of sunlight reaching the region.
Temperatures were high enough – about 14 degrees warmer than today in the warmest month of the summer – to suggest that the climate system is more sensitive to small changes in greenhouse-gas concentrations than the sensitivity estimates included in some climate models.
If that's the case, as other paleoclimate studies have indicated, the models may be underestimating the amount of warming likely to result from increasing atmospheric CO2 concentrations, the scientists say.
The period also is of interest because it holds clues about the factors that drove climate from prolonged warmth into a cycle of ice ages – factors that will help researchers understand the role natural variability plays in the region's climate and where climatic “tipping points” may lie.
The work “identifies for the first time a long, continuous story of that history from the Arctic and what I call the Arctic borderlands,” says Julie Brigham-Grette, a geologist at the University of Massachusetts at Amherst who focuses on the Arctic's ancient climate, referring to adjacent regions. Dr. Brigham-Grette is the lead author of a formal report of the results, which are being published in Friday's issue of the journal Science.
The evidence is captured in a 1,034-foot core sample the team drew from the bottom of Lake El'gygytgyn, known informally as Lake E. It formed 3.6 billion years ago after a meteor punched a crater in Russia's northeastern Arctic. The crater filled to form a lake 7 miles across and some 560 feet deep. The area around the lake managed to remain ice free during the ebb and flow of continental ice sheets during the Pleistocene ice ages, allowing sediment layers – and the pollen and other climate indicators they contain – to build up uninterrupted for the last 3.56 million years.
The team published initial results from the Lake E core last year, focusing on the Pleistocene. This latest effort focuses on the climate from the earlier, mid Pliocene through the onset of the glacial cycles.
“There are sites all over the Arctic that have little pieces of information” about the climate at different dates during this time, Brigham-Grette says of other places where teams have taken sediment samples from lakes and ponds. “All these little pieces of information tell us that the Arctic had tremendous forest cover in the past.”
Between 3.56 and 3.4 million years ago, the region would have looked quite exotic compared with today's Arctic, she says. Pollen and other indicators point to a region cloaked in Douglas fir and hemlock all the way to the shores of the Arctic Ocean, which the team posits was ice free in the summer.
Today, “you've got to go pretty far south and west in Russia to get those kinds of forest types,” she says. The vegetation and other indicators speak to summer temperatures in the Middle Pliocene at the site that reached the high 50s to low 60s Fahrenheit, roughly 14 degrees warmer than today. The region saw precipitation amounts of perhaps 23 inches a year, compared with 15 inches today.
The warmth and moisture, along with the vegetation present, are consistent with atmospheric CO2 levels of about 400 parts per million, a level the atmosphere is nudging today – a result of the buildup of CO2 from burning fossil fuels.
Indeed, modeling studies the team performed shows that it takes the warming effect of that much CO2 to sustain the forests over thousands of years through warm and cold phases of Earth's orbit around the sun.
“Our data is supporting the notion that carbon dioxide in the Pliocene must have been similar to what it is today,” she says.
In effect, a Pliocene-like climate may be the Arctic's future as well as it's past.
Between 3.26 million and 2.2 million years ago, the transition to the ice ages began. The climate was still warm, but this time span included cool excursions that approach those of recent glacial periods, the team says. The forest began to change its composition as tundra and other cold-weather plants took over. This gave the surface a lighter hue, reflecting more sunlight into space than the darker firs did. This tended to reinforce cooling.
The task now is to move from making observations about the changes to teasing out in some detail the factors driving them, Brigham-Grette says.
With such a long climate record from Lake E, researchers are in a better position to compare notes with colleagues studying climate change at the bottom of the world over the same period. Sediment records taken from the sea floor below the Ross Ice Shelf, for instance, reach back some 14 million years.
The record in Antarctica shows that at several times during the period Brigham-Grette's team covers in its latest results, the entire West Antarctic Ice Sheet vanished and reappeared several times.
“We've got these extremely warm situations in the Arctic and we can see some parallels in the ice-sheet sizes in Antarctica,” she says. “What were the forcing mechanisms that drove that warmth and what's the connection between the Arctic and Antarctica?”
The answers could yield valuable insights into how the Earth system works and how it might respond to today's warming trend.