seen. Based upon data derived from annual tree rings, NOAA has recorded patterns of drought covering most of the U.S. for every year since 1700. Certain species of trees have given evidence over a longer period, in particular Montezuma Cypress and Bristlecone pine trees. The University of Arkansas has produced a 1238-year tree-ring based chronology of weather condition in central Mexico by examining core samples taken from living Montezuma Cypress trees. Sediment core samples taken at the volcanic caldera in Valles Caldera, New Mexico and other locations. The cores from Valles Caldera go back 550,000 years and show evidence of megadroughts that lasted as long as 1,000 years during the mid-Pleistocene Epoch during which summer rains were almost non-existent. Plant and pollen remains found in core samples from the bottom of lakes have been also studied and added to the record. Fossil corals on Palmyra Atoll. Using the relationship between tropical Pacific sea surface temperatures and the oxygen isotope ratio in living corals to convert fossil coral records into sea surface temperatures. This has been used to establish the occurrence and frequency of La Niña conditions. During a 200-year mega drought in the Sierra Nevada that lasted from the 9th to the 12th centuries, trees would grow on newly exposed shoreline at Fallen Leaf Lake, then as the lake grew once again, the trees were preserved under cold water. However, a 2016–2017 expedition by the Undersea Voyager Project found evidence that the ancient trees did not grow there during an ancient drought, but rather slid into the lake during one of the many seismic events that have occurred in the Tahoe Basin since it was formed. The 2000–present southwestern North American megadrought was the driest 22-year period in the region since at least 800. Both 2002 and 2021 were drier than any other years in nearly 300 years and were, respectively, the 11th and 12th driest years between 800 and 2021. References External links Global Drought Information System Current worldwide drought conditions US Drought Monitor Current U.S. drought conditions Persistent drought in North America: a climate modeling and paleoclimate perspective Lamont–Doherty Earth Observatory of Columbia University Droughts Water supply Weather hazards