The University of Arizona

Temperature Changes

By Melanie Lenart | The University of Arizona | September 14, 2008

The Southwest is projected to warm faster than the world as a whole in coming decades, with summer temperatures rising even faster than winter ones. Average annual temperatures in many parts of the region could be 5 to 8 degrees F higher than they were even during the hot quarter century that began in the 1970s.

Figure 1. Using an ensemble of 18 global climate models and the moderate A1B emissions scenario, researchers at the NOAA Earth System Research Laboratory (ESRL) predict warming in the Southwest at the end of this century of approximately 7-8 degrees F for summer (June-August) relative to average temperatures 1971-2000.
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Credit: Jeremy Weiss (Geosciences, The University of Arizona) created this map using data from NOAA ESRL.

In 2007, the Intergovernmental Panel on Climate Change (IPCC) projected that average temperatures could rise a couple of degrees Fahrenheit every quarter century. The panel was comprised of some of the world’s most respected climate scientists, whose work also faced closed scrutiny by 113 governments.1 For their efforts, the scientists shared a Nobel Peace Prize that year with former U.S. Vice President Al Gore, whose movie An Inconvenient Truth helped propel the notion of climate change into mainstream consciousness.

The projected temperature rise is comparable to what much of the Southwest has seen in the past three decades. This page describes:

  • Projections of how temperature is expected to fare in the Southwest
  • Observations of historic climate in the Southwest, focusing on recent decades

Projections

The Intergovernmental Panel on Climate Change (IPCC) projects the world’s average annual temperature could rise by 2 to 12 degrees Fahrenheit during this century, with the “best estimate” falling between 3 and 7 degrees.1

Figure 2. Using an ensemble of 18 global climate models and the moderate A1B emissions scenario, researchers at the NOAA Earth System Research Laboratory (ESRL) predict warming in the Southwest at the end of this century of approximately 5-7 degrees F for winter (December-February) relative to average temperatures 1971-2000.
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Credit: eremy Weiss (Geosciences, The University of Arizona) created this map using data from NOAA ESRL.

Meanwhile, the average annual temperature in the U.S. Southwest is projected to rise by 5 to 8 degrees Fahrenheit by the end of this century, based on the “ensemble” results from numerous climate models used by the IPCC.2 (Figures 1 and 2) What’s more, summer is projected to warm even more than winters in the Southwest.3

Temperature results from Global Climate Models (GCMs) typically correlate by 95 percent or more with observations of actual near-surface temperatures, a 2008 assessment of climate models found.4 This confirms that GCMs do a good job of capturing past temperature changes, which suggests they also perform well when predicting future temperature.

The number of extremely hot days is also projected to rise over the decades. By the end of the century, parts of the Southwest are projected to face heat waves lasting two weeks longer than those occurring in recent decades, according to a study led by Noah Diffenbaugh.5

This study incorporated the results from a GCM into a regional model using dynamical downscaling. The results also suggested a fivefold increase in unusually hot days by the end of the century compared to 1961–1985. In effect, the high temperatures that formerly occurred on only the hottest 5 percent of days could become the norm for a quarter of the year—100 days or more—in much of the Southwest. Similar results held for northern Mexico.

Figure 3. Using a normal distribution, or bell curve, to represent natural variation in temperature from an average, this figure illustrates how climate change will not only affect average temperatures, but also extremes. Since projections are for warmer temperatures, this implies more hot and extreme heat days than what has occurred in the past.
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Credit: Intergovernmental Panel on Climate Change, 2007

This general expectation for more extreme events, in this case hot days and heat waves, fits into the concept that shifting an average even slightly can lead to shifting the extremes on either end of an average curve quite dramatically (Figure 3).

Observations

Observations based on measurements from weather stations indicate the temperature rise projected for the future is on par with the rate of increase much of the Southwest has already registered in recent decades.

Temperatures in Arizona and New Mexico have  been rising, particularly since the mid-1970s. Since 1976, the average annual temperature increased by 2.5 degrees F in Arizona and 1.8 degrees F in New Mexico (Figure 4).

The recent temperature increase is unusual even in the context of records dating back more than 1,000 years that were compiled from tree rings and other natural archives of temperature for the northern hemisphere.

So far, winter temperatures have warmed up even more than summer temperatures in the Southwest, at least when comparing the time frame from 1950 to 2003. This has serious implications for snow cover, an important natural reservoir of water in the West. The shortening of winter also affects the timing of natural cycles such as plant blooming and peak river flows.

Figure 4. Southwest temperatures have been rising. Since 1976, the average annual temperature increased by 2.5 degrees F in Arizona and 1.8 degrees F in New Mexico. Data were averaged from the respective states' climate divisions by Ben Crawford, CLIMAS.
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Credit: Ben Crawford, CLIMAS, The University of Arizona

Throughout the West, the number of days in the frost-free season, which varies by location, has been increasing more rapidly than in the East.4 The expansion of the warm season is a general theme around the world.1

Summer temperatures have also climbed, especially since the mid-1970s. Maximum temperatures regularly reach above 100 degrees F daily for weeks on end in many southwestern cities—and the number of days above 99 degrees appears to be on the rise, too, in some of them.

In cities, some of the recent temperature rise relates to the urban heat island effect. In the Phoenix metropolitan area, for instance, about two-thirds of the recent increase appears to relate to the heat island effect as concrete and pavement replaced grasslands and desert.

 

 

The heat island effect was removed from global assessments of recent temperature rise using statistics.1 Similarly, projections for future warming do not include the heat island effect. As a result, growing cities are likely to warm faster than other parts of the Southwest, especially in terms of nighttime temperatures.

The temperature rise alone has some predictable effects on aridity in the region. For instance, higher temperatures increase evaporation rates because warmer air can hold more moisture. Higher temperatures and a drier landscape increase wildfire hazard and put extra stress on ecosystems. Thus, even without a change in precipitation, the Southwest would be expected to become more arid. But in addition, climate change projections suggest that precipitation rates may drop, further increasing the risk of future drought. This, coupled with the expectation for declining snow cover, concerns policy makers and residents throughout the region.

References

  1. Arblaster, J., et al. 2007. Summary for policymakers. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
  2. Lenart, M., (ed.) et al. 2007. Global warming in the Southwest: projections, observations and impacts. University of Arizona, Climate Assessment for the Southwest, Tucson, Arizona.
  3. Meehl, G.A., et al. 2007. Global climate projections. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change . [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller (eds.)] Cambridge University Press, Cambridge, United Kingdom, and New York, NY, USA.
  4. Bader, D.C., et al. 2008. Climate models: An assessment of strengths and limitations. A Report by the U.S. Climate Change Science Program and the Subcommittee on Global Change Research. Department of Energy, Office of Biological and Environmental Research, Washington, D.C.
  5. Diffenbaugh, N., J. Pal, R. Trapp, and F. Giorgi. 2005. Fine scale processes regulate the response of extreme events to global climate change. Proceedings of the National Academy of Sciences, 102:1574–1578