Quantitative reconstructions of past climates are important to fully understand our rapidly changing climate and to accurately predict its trajectory. Investigating the patterns of natural temperature changes over space and time helps us understand and quantify the processes that cause climate to change, which is important as we prepare for the full range of future climate changes due to both human and natural causes. For this project, we assembled major new quality-controlled databases of previously published paleo (preindustrial) climate records, and we used them to generate important new understandings of global climate evolution over the present interglacial period (Holocene, past 12,000 years). One of the databases focuses on western North America and the adjacent Pacific Ocean. It includes data that attest to changes in both temperature and hydroclimate from 209 study sites. The other focuses on just temperature changes, but with global coverage at 679 study sites. The two data compilations were used to:

(1) Reconstruct the global surface temperature. We developed and applied five different statistical methods to the Temperature 12k database to estimate a realistic range of global temperature and the uncertainties over the past 12,000 years. The results show that the warmest 200-year-long interval took place around 6500 years ago when the global average temperature was about 0.7 ?C warmer than the 19th century average. This compares with global warming of about 1.0?C over the past 150 years (see figure).

(2) Trace the evolution of the equator-to-pole temperature gradient across the Northern Hemisphere. The data show that the weak latitudinal temperature gradient of the early- to middle-Holocene, which was caused by orbital cycles, coincided with a substantial decrease in mid-latitude net precipitation. This is a robust feature of both the paleoclimate data and simulations by climate models. It implies that the recent decrease in the temperature gradient associated with global warming and Arctic amplification may lead to future mid-latitude drying.

This project was part of our larger on-going effort to develop community-endorsed benchmark datasets of preindustrial climate variability and change. Our data products integrate records from both marine and terrestrial archives, such as lake deposits, marine sediments, peat and glacier ice, where ecological, geochemical and biophysical evidence has been used to infer past climate changes. The databases include a large variety of metadata to facilitate subsampling, analysis, and intelligent reuse. All of the data have been formatted for the Linked PaleoData (LiPD) structure. Use of LiPD for standardizing the reporting of preindustrial proxy climate data and metadata is growing as the availability of analytical tools built for this format continues to grow. The big-picture findings that we have drawn from the data compilations for this project demonstrate the power of well-curated databases, and we expect that these publicly accessible collections will be used in future research that address a variety of climate-science questions. As such, these data compilations help promote a culture of data stewardship that aims to reduce the loss and increase the use of valuable proxy climate data.

Results from this project were disseminated through peer reviewed journals, public presentations, social media, and press releases. The award supported two university faculty members in their early career.

This Project Outcomes Report for the General Public is displayed verbatim as submitted by the Principal Investigator (PI) for this award. Any opinions, findings, and conclusions or recommendations expressed in this Report are those of the PI and do not necessarily reflect the views of the National Science Foundation; NSF has not approved or endorsed its content.

Last Modified: 11/30/2020
Modified by: Darrell S Kaufman