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Banner image: A researcher hauls equipment across the snowy Antarctic听landscape.听(Credit: Bradley Markle)

By analyzing Antarctic ice cores, CU Boulder scientists and an international team of collaborators have revealed the most detailed look yet at the planet鈥檚 recent climatic history, including summer and winter temperatures dating back 11,000 years to the beginning of what is known as the Holocene.

Published today in , the study is the very first seasonal temperature record of its kind, from anywhere in the world.

鈥淭he 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,鈥� said Tyler Jones, lead author on the study, and assistant research professor and fellow at the Institute of Arctic and Alpine Research (滨狈厂罢础础搁).听

The study also validates one aspect of a long-standing theory about Earth鈥檚 climate that has not been previously proven: how seasonal temperatures in polar regions respond to Milankovitch cycles. Serbian scientist Milutin Milankovitch hypothesized a century ago that the collective effects of changes in Earth's position relative to the sun鈥攄ue to slow variations of its orbit and axis鈥攁re a strong driver of Earth's long-term climate, including the start and end of ice ages (prior to any significant human influence on the climate).

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

These more highly detailed data on long-term climate patterns of the past also provide an important baseline for other scientists, who study the impacts of human-caused 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.

鈥淭his research is something that humans can really relate to because we partly experience the world through the changing seasons鈥攄ocumenting how summer and winter temperature varied through time translates to how we understand climate,鈥� said Jones.听

Finer definition amidst diffusion听

Scientists around the world have long studied Earth鈥檚 past climate using ice cores gathered from the poles. These slender, cylindrical columns of ice, drilled from ancient ice sheets (mostly in Antarctica and Greenland), provide valuable long-term data trapped in time about everything from past atmospheric concentrations of greenhouse gases to past temperatures of the air and oceans.听

The West Antarctic Ice Sheet (WAIS) Divide ice core, the longest ice core ever drilled by U.S. researchers, measures 11,171 feet (or over 2 miles) long and 4.8-inches in diameter鈥攃ontaining data from as old as 68,000 years ago. Ice cores like this one are then carefully cut into smaller sections which can be safely transported to and stored or analyzed in ice core labs around the country鈥攍ike the Stable Isotope Lab at CU Boulder.听

For this study, researchers 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 numbers of neutrons) reveal data about past temperatures and atmospheric circulation, including transitions between ice ages and warm periods in Earth鈥檚 past.听

Measuring seasonal changes in our planet鈥檚 history from ice cores is especially difficult, however, due to the fine detail required for their shorter timescales. A process within ice sheets known as diffusion, or natural smoothing, can blur this needed detail.听

These water isotopes tend to not 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 sufficiently solidifying. This process can 鈥渂lur鈥� the data researchers are trying to examine. But by using the high-quality ice cores from the West Antarctic Ice Sheet, extremely high-resolution measurements and advances in ice core analysis from the past 15 years, the team was able to correct for the diffusion present in the data and complete the study.

鈥淓ven 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,鈥� said Jones.听

Studying stable isotopes听

While the study details the history of Earth鈥檚 climate, the work behind it has a history of its own. 听

For more than three decades, researchers at INSTAAR鈥檚 Stable Isotope Lab have been studying a variety of stable isotopes鈥攏onradioactive forms of atoms with unique molecular signatures鈥攆ound everywhere from the inside ice cores and the carbon in permafrost to the air in our atmosphere. Jones joined the lab in 2007 as a master鈥檚 student and has never left.听

鈥淚 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鈥攚hich is extremely rare. In our understanding, no one had ever done this before,鈥� said Jones. 鈥淲e looked at each other and said, 鈥榃ow, this is going to be a really big deal.鈥欌��

It then took almost a decade to figure out the proper way to interpret the data, from ice cores drilled many years before that meeting.听

Bruce Vaughn, co-author and manager of the Stable Isotope Lab, and Bradley Markle, co-author on the study and assistant professor at INSTAAR and the Department of Geology, were there to collect the ice in West Antarctica that was shipped back and analyzed.听

The team鈥檚 next step is to attempt to interpret high-resolution ice cores in other places鈥攕uch as the South Pole and in northeast Greenland, where cores have already been drilled鈥攖o better understand our planet鈥檚 climate variability.听

鈥淗umans 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,鈥� said Jones. 听听

Additional authors on this publication include: Valerie Morris and Bruce Vaughn of the Institute of Arctic and Alpine Research at the 麻豆视频; Bradley Markle, Abigail Hughes, Kevin Rozmiarek and Chloe Brashear at the Institute of Arctic and Alpine Research and Department of Geological Sciences at the 麻豆视频; Kurt M. Cuffey of the University of California Berkeley; William Roberts of Northumbria University; Eric Steig and T. J. Fudge of the 听University of Washington; Max Stevens of the Cryospheric Science Laboratory at NASA Goddard Space Flight Center and Earth System Science Interdisciplinary Center at the University of Maryland; Paul Valdes of the University of Bristol; Michael Sigl at the Physics Institute & Oeschger Centre for Climate Change Research, University of Bern; Joshua Garland of Arizona State University; Bo Vinther at the Niels Bohr Institute; and James White at the University of North Carolina, Chapel Hill.听