Tracking seasonal temperature changes over 11,000 years

Scientists have been drilling and removing ice cores in Antarctica for many years with the aim of identifying characteristics of Earth’s atmosphere and climate dating back hundreds of millennia. The concentration of carbon dioxide in air bubbles trapped in the ice gives clues about levels of this greenhouse gas in the past, and the ratios of different water isotopes present in the ice can reveal how temperatures in the Antarctic have changed. 

Using information derived from Antarctic ice cores, scientists have previously identified that, going back 800,000 years, the Antarctic climate consisted of a succession of long, cold ‘glacial’ periods interspersed roughly every 100,000 years with warm “interglacial” periods (of which the last 11,000 years is the most recent). 

Now, in a new study of ice core data, an international team of scientists led by the University of Colorado Boulder has analyzed temperature changes on a seasonal scale, recording all the winter and summer temperatures going back 11,000 years to the start of the Holocene Period. Published in the journal Nature, this study is the very first seasonal temperature record of its kind, from anywhere in the world.

“The goal of the research team was to push the boundaries of what is possible with past climate interpretations, and for us that meant trying to understand climate at the shortest timescales, in this case seasonally, from summer to winter, year-by-year, for many thousands of years,” said Tyler Jones, lead author on the study, and assistant research professor and fellow at the Institute of Arctic and Alpine Research (INSTAAR).

The researchers used data from the West Antarctic Ice Sheet (WAIS) Divide ice core, which is the longest ice core ever drilled by U.S. researchers. It was removed from a dome of ice in central West Antarctica in 2011 and measures 11,171 feet (3405 meters), which is over 2 miles long. This core contains atmospheric and climate data from as much as 68,000 years ago, although the researchers only used sections dating back 11,000 years. 

For this study, the scientists analyzed a continuous record of water-isotope ratios from the WAIS ice core. The ratios between the concentration of these isotopes (elements with the same number of protons but different atomic masses) reveal information about past temperatures and atmospheric circulation, including transitions between ice ages and warm periods in Earth’s past.

In addition to providing the first data on summer and winter temperature changes in the recent past, the study also validates one aspect of a long-standing theory about Earth’s climate that has not been previously proven. It shows that seasonal temperatures in Polar Regions respond to Milankovitch cycles. 

A century ago, Serbian scientist Milutin Milankovitch hypothesized that cyclical orbital changes in the Earth’s movement around its axis and around the sun, affect how much solar radiation reaches its surface, and can thus drive long-term climate changes and trigger the start and end of glaciation periods (ice ages). 

“I am particularly excited that our result confirms a fundamental prediction of the theory used to explain Earth’s ice-age climate cycles: that the intensity of sunlight controls summertime temperatures in the polar regions, and thus melt of ice, too,” said Kurt Cuffey, a co-author on the study and professor at the University of California Berkeley.

It was not a straightforward matter to measure seasonal changes in Earth’s temperature from ice cores, due to their short timescales. Water isotopes tend not to stay in one place in the upper ice sheet, but instead move around in interconnected pathways (similar to the air pockets in Styrofoam) as they change states between vapor and ice, over decades or centuries, before solidifying. This process, known as diffusion, can “blur” the finely detailed data that the researchers were trying to examine. 

Luckily the high-quality ice cores from the WAIS allowed high-resolution measurements down to seasonal level. This, along with recent advances in ice core analysis during the past 15 years, enabled the team to correct for the diffusion present in the data and obtain the detail they sought.

“Even beyond that, we had to develop new methods entirely to deal with this data, because no one’s ever seen it before. We had to go above and beyond what anyone’s done in the past,” said Jones.

“This research is something that humans can really relate to because we partly experience the world through the changing seasons – documenting how summer and winter temperature varied through time translates to how we understand climate,” he added.

These finely detailed data on long-term climate patterns of the past also provide an important baseline for other scientists, who study the impacts of greenhouse gas emissions on our present and future climate. By knowing which planetary cycles occur naturally and why, researchers can better identify the human influence on climate change and its impacts on global temperatures. Interestingly, they are confident that the current period of warming is not related to any Milankovitch cycles, but is instead the result of human-produced greenhouse gases in the atmosphere.

This particular study has a long history of its own. For more than three decades, researchers at INSTAAR’s Stable Isotope Lab at CU Boulder have been studying a variety of stable isotopes – non-radioactive forms of atoms with unique molecular signatures – found everywhere from inside ice cores and permafrost to the air in our atmosphere. Jones joined the lab in 2007 as a master’s student and has never left.

“I have this distinct memory of walking into my advisor, Jim White’s office in about 2013, and showing him that we would be able to pull out summer and winter values in this record for the last 11,000 years – which is extremely rare. In our understanding, no one had ever done this before,” said Jones. “We looked at each other and said, ‘Wow, this is going to be a really big deal.’”

It then took almost a decade to work out how to cope with problems such as diffusion, and how to interpret the data from ice cores drilled many years earlier. 

The team’s next step is to attempt to interpret high-resolution ice cores in other places – such as the South Pole and in northeast Greenland, where cores have already been drilled –to understand our planet’s climate variability better.

“Humans have a fundamental curiosity about how the world works and what has happened in the past, because that can also inform our understanding of what could happen in the future,” said Jones.  

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By Alison Bosman, Earth.com Staff Writer

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