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Climate change in New Mexico

Reconstruction of annual rainfall from El Malpais National Monument, New Mexico, based on tree-ring analysis showing long-term (more than 100 years) climatic trends. (Adapted from Grissino-Mayer 1997).

Climate is of course inextricably linked to patterns of land cover and land-use, particularly in a semi-arid region of such variable topography as the Colorado Plateau. The biotic communities we see today are the products of species evolution and migration over time on a constantly shifting landscape driven by changes in climate at a variety of temporal and spacial scales.

Variablility in temperature, humidity and precipitation affects biotic productivity and diversity both regionally and locally. Shifts from one climatic regime to a new pattern can be abrupt. For example, evidence from Greenland ice cores suggest that a 1300-year cold spell from 11,200–10,000 years ago ended over a period of perhaps only a few years.

Dendrochronology, the study of tree growth rings, has been especially useful in understanding both long and short-term environmental variation in the Southwest. Dr. Thomas Swetnam and his colleagues at the Tree Ring Laboratory in Tucson, Arizona have been able to determine that the region is strongly influenced by a three to five-year Southern Oscillation in the Pacific ocean-atmosphere system (SO); El Niño years bring increased annual precipitation (but less rain in summer) and La Niña years bring the opposite. Short-term fluctuations such as the SO influence floods, drought cycles, fire frequencies, insect population outbreaks and pulses of tree reproduction.

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Clouds and snow in Bryce Canyon National Park, Utah. Photo © 1999 Ray Wheeler

Combined data from geology, paleobotany, and dendrochronology studies from many sites on the Colorado Plateau permit an approximate reconstruction of its long-term climate history. Fossil pollen and plant macrofossil studies by paleoecologist Scott Anderson have recorded dramatic vegetation changes over time at Potato Lake, a high elevation site (7,500 feet) on the Mogollon Rim. In the mid-Wisconsin Period (35,000–21,000 B.P.) the area was dominated by mixed-conifer species; in the late-Wisconsin (21,000–10,400 B.P.) the forest was nearly pure Engelmann spruce; and for the past 10,000 years newly arrived ponderosa pine has dominated the area's forests.

By examining the sediments in the exposed banks of deeply cut arroyos at Chaco Canyon, palynologist Stephen Hall has determined that before 8,000 B.P. a relatively cold and moderately wet climate prevailed; the canyons contained a mixed-conifer forest and the mesa tops a cold desert steppe. During the Altithermal Period (8,000–4,000 B.P.) pinyon and juniper migrated into the area and replaced the mixed-conifer forests; warm desert grasses replaced the sagebrush of the cold desert steppe. The cause of these vegetation changes is thought to have been the arrival under generally warmer conditions of a monsoonal circulation with warm wet summers. A cooler and dryer Neoglacial Period lasted from 5,000 to 2,000 B.P.

The climate of the past 2,000 years includes several notable global events including the Medieval Warm Period from 1000 to 1350 A.D. and the Little Ice Age from about 1450 to 1850 A.D. Both are implicated in the mystery of the Anasazi collapse at the close of the thirteenth century. In the Southwest, higher average summer temperature and precipitation persisted from 950–1130 A.D. and prolonged summer droughts occurred from 1130–1180 A.D.

Northeastern Arizona Precipitation

Northeastern Arizona precipitation history. Graphic courtesy USGS.

In modern times the decade of the 1950s was notable for the severity of drought conditions throughout the Southwest, although Henri Grissino-Mayer's tree ring data at Malpais National Monument in northwestern New Mexico are evidence that the past two centuries have been the wettest period of the past 1,500 years in that region. Grissino-Mayer's data also suggest that the past 20 years have been the wettest of all, with rainfall 23 percent above the long-term New Mexico average. The implication of this and other long-term climate data is that the farms, cities and irrigation networks of the modern Southwest were built on rivers swollen by unusually wet weather. Vegetation in such critical watersheds as the Rio Grande and Colorado River basins is apt to change in the near future.

Global warming

(a) Near-global annual-mean surface air temperature change, based on meteorological station network, (b) global land-ocean surface temperature index, which combines sea surface temperature measurements for ocean areas with surface air temperature measurements at meterological stations. Source: NASA Goddard Institute for Space Studies

The future impacts of climate on the Colorado Plateau are complicated by current and projected global warming and intensified land use. Although global temperature fluctuates considerably from year to year due to chaotic variability of the atmosphere and ocean, there has been a long-term global warming trend underway since the early 1960s. Researchers at the NASA Goddard Institute for Space Studies who analyze data collected from several thousand meteorological stations around the world report that 1998 was the warmest year on record. Global surface temperatures in 1999 fell back from the 1998 record setting high level, with 1999 being approximately the sixth warmest year in the record. The ranking of years is approximate because of incomplete global coverage of measurement stations and small errors in the measurements.


Click here for a description of modern climatic conditions on the Colorado Plateau.
Click here for a description of the causes of long-term cycles of climate change in the Southwest.


Fire-Southern Oscillation Relations in the Southwestern United States. A close linkage between fire and climate could diminish the importance of local processes in the long-term dynamics of fire-prone ecosystems. The structure and diversity of communities regulated by fire may have nonequilibrial properties associated with variations in global climate. Successful prediction of vegetation change hinges on a better understanding of climatically driven disturbance regimes and the relative contributions of regional versus local processes to community dynamics. Adapted from a journal article by Thomas W. Swetnam and Julio L. Betancourt.

Packrat Midden Research in the Grand Canyon. On the Colorado Plateau the ice age (Pleistocene) vegetation of the Grand Canyon has been determined through the analysis of plant fossils preserved in caves and fossil packrat middens.  Large changes occurred as the most recent ice age ended and the Holocene era began. Adapted by Kenneth L. Cole from his journal article.

Paleobotany and Paleoclimate of the Southern Colorado Plateau. The biota of the Colorado Plateau during the middle (50,000-27,500 B.P.) and late (27,500-14,000 B.P.) Wisconsin time periods was dramatically different from that seen today. Differences were primarily a result of major climate changes associated with the last major glacial period. This site examines the environment of the southern plateau during this time. Adapted by R. Scott Anderson from his journal article.


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