Measuring Long-Term Climate Change In North America During The Common Era

Recent floods and droughts have resulted in billions of dollars of damages and loss of life. The occurrence of these extreme weather events has varied in frequency and magnitude over past years and decades.

Identifying regions that have been susceptible to drought or wetter-than-average conditions in the past can help us understand the mechanisms behind these changes and more accurately predict which regions might be vulnerable to similar conditions in the future, which is essential for disaster relief, mitigation planning, and resource management.

Climate is defined as the long-term behavior in the trends and patterns of temperatures, rainfall, evaporation, winds, and other conditions occurring over decades, centuries, millennia, and longer timescales. Natural climate changes in the past have resulted from several factors, such as varying amounts of radiative energy entering the earth’s atmosphere and internally-driven changes to ocean and atmosphere circulation. However, a lot of uncertainty remains about natural climate variability at centennial (100-year) and longer timescales, in part because instrumental and observational climate data does not extend far enough back in time to capture patterns and trends for more than one or two centuries.

Without thermometers and rain gauges stationed across the landscape thousands of years ago, how do we figure out the long-term history of climate? Scientists utilize an array of observations in the geologic record to reconstruct environmental conditions in the past. Decades of research has shown that there have been major alterations to vegetation on the landscape, water levels in lakes, water drip rates in caves, and other environmental archives. By using the relationships between climate and the processes driving change in these environmental systems today, along with statistical and theoretical models, we are able to determine the degree to which these shifts relate to local changes in climate, such as temperature and rainfall. Measurable physical, chemical, and/or biological properties of geologic samples, such as pollen abundances and mineralogy of sediment cores and isotopic values of speleothems, that change in response to changing climate parameters are known as paleoclimate proxies.

In 2018, the U.S. Geological Survey published a study that compiled paleoclimate proxy data from across the North American continent to determine whether there have been broad regional or continent-scale patterns of change during the past 2000 years. This time period is referred to as the Common Era (CE), where the year 0 CE is the same as 0 AD.

What we found was that large areas of North America tended to become dry for centuries at a time, while other areas tended to become wetter than average. Not only were these shifts persistent for hundreds of years, but there were also several fluctuations between wet, dry, and average conditions (i.e. hydroclimate) over the course of the last 2000 years in any given region. These findings are important because they imply that natural variations in hydroclimate at the centennial timescale have been a common trait of North American landscape history. These findings beg the questions: why did these wet and dry patterns emerge, and what can we expect moving forward?

In this study, we compared the newly-compiled data with similar datasets that reconstructed average North American and Northern Hemisphere temperatures for the past several centuries. We found that a greater portion of localities in our study became drier during warmer centuries, particularly between 50 BCE (years before the Common Era) and 450 CE and between 800 and 1100 CE. However, numerous sites became wetter than average during these same time intervals and regions that were dry during one of the warm intervals were not necessarily dry during the second and vice versa.

Interestingly, significant hydroclimate pattern transitions occurred out of sync with reconstructed mean temperature changes. This means that, while temperature might play a role in when and where century-scale shifts toward drier conditions occur, other factors are clearly involved. More research that incorporates additional paleoclimate proxy data and computer model simulations are needed in order to better understand the mechanisms that drove century-scale hydroclimate changes in the past, which is crucial information for predicting future trends and patterns of drought.

These findings are described in the article entitled A North American Hydroclimate Synthesis (NAHS) of the Common Era, recently published in the journal Global and Planetary Change. This work was conducted by Jessica R. Rodysill, Lesleigh Anderson, Thomas M. Cronin, Miriam C Jones, Robert S Thompson, David B Wahl, Debra A Willard, Jason A Addison, Jay R Alder, Katherine H Anderson, Lysanna Anderson, John A Barron, Christopher E Bernhardt, Steven W Hostetler, Natalie M Kehrwald, Nicole S Khan, Julie N Richey, Scott W Starratt, Laura E Strickland, Michael R Toomey,  Claire C Treat, and G. Lynn Wingard from the U.S. Geological Survey.