Program scientists also updated the National Climate Change Viewer (NCCV), a premier web application for visualizing climate projects across the contiguous United States. The updated tool incorporates the latest CMIP6 climate change models and integrates new guidance on model summarization and weighting. 

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Coastal wetlands have the ability to keep pace with moderate amounts of sea level rise by trapping sediments and sequestering carbon, raising their surface elevation. However, there has been ongoing scientific debate about how much sea level rise coastal wetlands can handle. A new study published in Nature by USGS scientists and their academic partners from around the world, examines how with rising temperatures, sea level rise rates will increase faster, and this will impact the resilience of coastal wetlands. The study determined that at rates above 7mm/year, which are predicted with 2 degrees of warming above pre-industrial levels, most coastal wetlands will reach a tipping point that exceeds their capacity to adjust to these rising sea levels and very likely drown. This research has important implications for management and coastal communities around the world who rely on wetlands as natural protection against storms, erosion, and flooding.

Saintilan, N., Horton, B., Törnqvist, T.E., Ashe, E.L., Khan, N.S., Schuerch, M., Perry, C., Kopp, R.E., Garner, G.G., Murray, N., Rogers, K., Albert, S., Kelleway, J., Shaw, T.A., Woodroffe, C.D., Lovelock, C.E., Goddard, M.M., Hutley, L.B., Kovalenko, K., Feher, L., and Guntenspergen, G., 2023, Widespread retreat of coastal habitat is likely at warming levels above 1.5 °C. Nature, v. 621, p. 112–119, https://doi.org/10.1038/s41586-023-06448-z

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Fire is a major disturbance across the western United States. Annual area burned has increased along with temperatures during recent decades leading to record-breaking fire seasons during warm, dry years. Anticipating how wildfire patterns may respond to projected climate changes is a top priority because wildfires have many societal (human-health and economic) and ecological consequences. USGS scientists identified thresholds in annual aridity that distinguish years with limited, moderate, and extensive area burned. The scientists used this observation to develop a new model that simulates annual area burned under a range of potential future conditions. The results indicated that the frequency of extreme fire years and annual area burned will continue to increase under most scenarios in the coming decades, and under all scenarios after 2060.  Importantly, the model is very adaptable and can readily incorporate new data or be applied at broader scales making it useful for many different managers.

Henne, P.D., and Hawbaker, T., 2023, An aridity threshold model of fire sizes and annual area burned in extensively forested ecoregions of the western USA. Ecological Modelling, v. 477, https://doi.org/10.1016/j.ecolmodel.2023.110277

USGS scientists and their partners published two separate papers this past year that contribute to our understanding of human migration into North America and the climate conditions that may have impacted their survival.

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The first new study published in PNAS explores how humans could have gotten to North America earlier than previous estimates when ice sheets blocked the interior continental route. It explores historic environmental conditions, including presence of glaciers, ocean sea ice, strength of ocean currents, and climate conditions, and suggests that a coastal route is a likely option for how humans got to North America, pointing to places where scientists could look for evidence of human migration some twenty thousand years ago. The results suggest that 24,500 to 22,000 and 16,400-14,800 years ago were the most environmentally favorable windows of time for a coastal migration route to have been available (when interior routes were blocked). The study suggests that humans may have taken advantage of winter sea ice that connected islands and coastal refugia since there would have been rich food supply at marine ice edges and traveling on coastal ice could have been easier than trying to take a boat or cross on land over glaciers. These paleoenvironmental insights could help focus archaeological reconnaissance efforts to find evidence for human occupation around the North Pacific Rim along paleoshorelines and now-submerged islands.

Praetorius, S.K., Alder, J.R., Condron, A., Mix, A., Walczak, M., Caissie, B.E., and Erlandson, J., 2023, Ice and ocean constraints on early human migrations into North America along the Pacific Coast. Proceedings of the National Academy of Sciences, v. 120, no. 7, https://doi.org/10.1073/pnas.2208738120

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The second new study builds on USGS research from September 2021. At that time, USGS researchers and an international team of scientists announced that ancient human footprints discovered in White Sands National Park, NM were between 21,000 and 23,000 years old. This discovery pushed the known date of human presence in North America back by thousands of years and implied that early inhabitants and megafauna co-existed for several millennia before the terminal Pleistocene extinction event. In a follow-up study this year, published in Science, researchers used two new independent approaches to date the footprints, both of which resulted in the same age range as the original estimate. Now that three separate lines of evidence point to the same approximate age, it is highly unlikely that they are all incorrect or biased and, taken together, provide strong support for the 21,000 to 23,000-year age range for the footprints. Their continued studies at White Sands focus on the environmental conditions that allowed people to thrive in southern New Mexico during the Last Glacial Maximum.

Pigati, J.S., Springer, K.B., Honke, J.S., Wahl, D., Champagne, M.R., Zimmerman, S.R.H., Gray, H.J., Santucci, V.L., Odess, D., Bustos, D., and Bennett, M.R., 2023, Independent age estimates resolve the controversy of ancient human footprints at White Sands. Science, v. 382, no. 6666, https://doi.org/10.1126/science.adh5007

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Wetlands naturally produce methane in their o2-free soils. As the international community works to reduce greenhouse gases to achieve climate-mitigation targets, understanding these natural emissions and any future changes are key to meeting targets. A new study published in Science Advances by USGS researchers determined that methane emissions from wetlands in the prairie pothole region (North America’s largest wetland complex) are likely to increase by 2-3 times by 2100 due to warming temperatures. This means that natural sources of greenhouse gases are going to increase in the future, possibly canceling out mitigation actions to cut human emissions. Therefore, efforts to decrease greenhouse gases in the atmosphere need to jointly account for human emissions as well as natural emissions in order to meet climate-mitigation targets.

Bansal, S., Post van der Burg, M., Fern, R., Jones, J., Lo, R., McKenna, O.P., Tangen, B., Zhang, Z., and Gleason, R.A., 2023, Large increases in methane emissions expected from North America’s largest wetland complex. Science Advances, v. 9, no. 9, https://doi.org/10.1126/sciadv.ade1112

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Arctic sea ice plays an important role in helping to keep the Arctic cool and moderating global climate. Recently, Arctic sea ice has been rapidly declining and models predict seasonally ice-free conditions by mid-century. Arctic sea ice loss has consequences not only for global climate, but also ocean circulation, Arctic wildlife and ecosystems, and Indigenous communities. To better understand sea ice change, scientists look to the past, specifically back to the Last Interglacial (LIG) (~129-115 thousand years ago) when summer Arctic temperatures were ~4-5^oC warmer than today. However, there is still some debate and uncertainty around the extent and seasonality of sea ice during that time. In a recent paper published in Nature Geoscience, USGS researchers and their colleagues at Stockholm University used data from microfossils in sediment cores to fill data gaps and demonstrate seasonally ice-free conditions in the Arctic during the LIG. Using data from across the central Arctic Ocean, they show that sea-ice extent was substantially reduced, and summers were probably ice free. The results reinforce that the LIG is a prime analogue for studying seasonally ice-free Arctic Ocean conditions and impacts. 

Vermassen, F., O’Regan, M., de Boer, A., Schenk, F., Razmjooei, M., West, G., Cronin, T.M., Jakobsson, M., and Coxall, H.K., 2023, A seasonally ice-free Arctic Ocean during the Last Interglacial. Nature Geoscience, v. 16, p. 723–729, https://www.nature.com/articles/s41561-023-01227-x.

Shen, Z., Zhou, W., Li, J. et al. A frequent ice-free Arctic is likely to occur before the mid-21st century. npj Clim Atmos Sci, v. 6, no.103 (2023). https://doi.org/10.1038/s41612-023-00431-1

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The prevalence of microplastics throughout the environment is quickly becoming a topic of concern for society. They are considered a contaminant of concern and have been found in every environmental compartment across the globe. Yet, the way they enter, move through, and where they eventually end up in the environment is not fully understood. In order to better understand how microplastics move and accumulate in river systems, USGS researchers and colleagues compared microplastic abundance between two nearby river systems, one in a highly urbanized watershed and the other in a less urbanized watershed, and analyzed microplastic concentrations from the upstream to downstream. They found that the degree of urbanization does affect microplastic patterns along a river with the urbanized watershed containing higher levels. They also found that river diversions (removing water for societal use) removed microplastic particles from the river reducing loads downstream. In cases where water was diverted for drinking purposes, many of the microplastics may get removed since water treatment plants are able to move the vast majority of microplastics, but when water is diverted for agriculture, the microplastics are dispersed to the wider terrestrial environment and the particles could eventually wash back into rivers. The implications of this work point to the need to consider the way that microplastics are redistributed back into a watershed in large scale models that look to quantify plastic fate and transport to the oceans. It may be that more microplastics are being distributed into terrestrial ecosystems than the ocean then previously thought. Water management practices can have a significant impact on microplastic fate and transport.

Kukkola, A., Runkel, R.L., Schneidewind, U., Murphy, S.F., Kelleher, L., Smith, G.S., Nel, H.A., Lynch, I., and Krause, S., 2023, Prevailing impacts of river management on microplastic transport in contrasting US streams: Rethinking global microplastic flux estimations. Water Research, v. 240, https://doi.org/10.1016/J.WATRES.2023.120112