Articles | Volume 14, issue 5
Clim. Past, 14, 665–686, 2018

Special issue: Climate of the past 2000 years: regional and trans-regional...

Clim. Past, 14, 665–686, 2018

Research article 28 May 2018

Research article | 28 May 2018

Placing the Common Era in a Holocene context: millennial to centennial patterns and trends in the hydroclimate of North America over the past 2000 years

Bryan N. Shuman1, Cody Routson2, Nicholas McKay2, Sherilyn Fritz3, Darrell Kaufman2, Matthew E. Kirby4, Connor Nolan5, Gregory T. Pederson6, and Jeannine-Marie St-Jacques7 Bryan N. Shuman et al.
  • 1Roy J. Shlemon Center for Quaternary Studies, Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, 82071, USA
  • 2School of Earth Sciences & Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona, 86011, USA
  • 3Department of Earth and Atmospheric Sciences, University of Nebraska, Lincoln, Nebraska, 68588, USA
  • 4Department of Geological Sciences, California State University, Fullerton, Fullerton, California, 92834, USA
  • 5Department of Geosciences, University of Arizona, Arizona, 85721, USA
  • 6Northern Rocky Mountain Science Center, US Geological Survey, Bozeman, Montana, 59715, USA
  • 7Department of Geography, Planning and Environment, Concordia University, Montreal, Quebec, H3G 1M8, Canada

Abstract. A synthesis of 93 hydrologic records from across North and Central America, and adjacent tropical and Arctic islands, reveals centennial to millennial trends in the regional hydroclimates of the Common Era (CE; past 2000 years). The hydrological records derive from materials stored in lakes, bogs, caves, and ice from extant glaciers, which have the continuity through time to preserve low-frequency ( > 100 year) climate signals that may extend deeper into the Holocene. The most common pattern, represented in 46 (49 %) of the records, indicates that the centuries before 1000 CE were drier than the centuries since that time. Principal component analysis indicates that millennial-scale trends represent the dominant pattern of variance in the southwestern US, northeastern US, mid-continent, Pacific Northwest, Arctic, and tropics, although not all records within a region show the same direction of change. The Pacific Northwest and the southernmost tier of the tropical sites tended to dry toward present, as many other areas became wetter than before. In 22 records (24 %), the Medieval Climate Anomaly period (800–1300 CE) was drier than the Little Ice Age (1400–1900 CE), but in many cases the difference was part of the longer millennial-scale trend, and, in 25 records (27 %), the Medieval Climate Anomaly period represented a pluvial (wet) phase. Where quantitative records permitted a comparison, we found that centennial-scale fluctuations over the Common Era represented changes of 3–7 % in the modern interannual range of variability in precipitation, but the accumulation of these long-term trends over the entirety of the Holocene caused recent centuries to be significantly wetter, on average, than most of the past 11 000 years.

Short summary
A synthesis of 93 published records reveals that moisture availability increased over large portions of North America over the past 2000 years, the Common Era (CE). In many records, the second millennium CE tended to be wetter than the first millennium CE. The long-term changes formed the background for annual to multi-decade variations, such as "mega-droughts", and also provide a context for amplified rates of hydrologic change today.