Friday, May 3, 2013

Southwest Climate Change Assessment Report

The complete Southwest Climate Change Assessment Report was released earlier this week.  At you'll find a full color PDF of the entire report and links to specific chapters and fact sheets.  With 20 chapters, it is the most comprehensive assessment of climate change for our region to date.  Here are what I see as some of the key findings, with emphasis on climate, weather, and water changes of interest to readers of the Wasatch Weather Weenies.

Present Weather and Climate: Evolving Conditions (Chapter 5, Hoerling et al. 2013)

  • The decade 2001–2010 was the warmest and fourth driest in the Southwest of all decades from 1901 to 2010 (high confidence)
  • Average annual temperature increased 1.6ºF (+/- 0.5ºF) over the Southwest during 1901–2010, while annual precipitation experienced little change (high confidence)
  • Fewer cold waves and more heat waves occurred over the Southwest during 2001–2010 compared to their average occurrences in the twentieth century (high confidence)
  • The frequency of extreme daily precipitation events over the Southwest during 2001–2010 showed little change compared to the twentieth-century average (medium-high confidence)
  • Streamflow and snowmelt in many snowmelt-fed streams of the Southwest trended towards earlier arrivals from 1950–1999, and climate science as attributed up to 60% of these trends to the influence of increasing greenhouse gas concentrations in the atmosphere (high confidence)
  • Streamflow and snowmelt in many of those same streams continued these earlier arrivals during 2001–2010, likely in response to warm temperatures (high confidence)
  • The period since 1950 has been warmer in the Southwest than any comparable period in at least 600 years, based on paleoclimate reconstructions of past temperatures (medium-high confidence)
  • The most severe ans sustained droughts during 1901–2010 were exceeded in severity and duration by several drought events in the preceding 2000 years, based on paleoclimatic reconstructions of past droughts (high confidence)
Future Climate: Projected Average (Chapter 6, Cayan et al. 2013)
  • Temperatures at the earth's surface in the Southwest will rise substantially (by more than 3ºF [1.7ºC] over recent historical averages) over the twenty-first century from 2001–2100 (high confidence)
  • Climate variations of temperature and precipitation over short periods (year-to-year and decade-to-decade) will continue to be a prominent feature of the Southwest climate (high confidence)
  • There will be lower precipitation in the southern portion of the Southwest region and little change or increasing precipitation in the northern portion (medium-low confidence)
  • There will be a reduction of Southwest mountain snowpack during February through may from 2001 through 2100, mostly because of the effects of warmer temperature (high confidence)
  • Substantial parts of the Southwest region will experience reductions in runoff and streamflow from the middle to the end of the twenty-first century (medium-high confidence)
Future Climate: Projected Extremes (Chapter 7, Gershunov et al. 2013)
  • Heat waves, as defined relative to current climate, are projected to increase in frequency, intensity, duration, and spatial extent (high confidence)
  • Wintertime cold snaps are projected to diminish in their frequency, but not necessarily their intensity, into the late twenty-first century (medium-high confidence)
  • Enhanced precipitation extremes are generally expected due to greater moisture availability in a warming atmosphere, even if average precipitation declines (medium-low confidence)
  • Snowmelt-driven spring and summertime floods are expected to diminish in both frequency and intensity (high confidence)
  • Drought, as expressed in Colorado River flow, is projected to become more frequent, more intense, and longer lasting, resulting in water deficits not seen during the instrumental record (high confidence)
Water: Impacts, Risks, and Adaptation (Chapter 10, Udall et al. 2013)
  • A large portion of the Southwest is expected to experience reductions in streamflow and other water stresses in the twenty-first century (high confidence)
I encourage you to dig into some of these chapters for more information.  Educators and others interested in using media from the assessment can access electronic versions of all of the figures at  


  1. I don't believe they covered this topic in the agricultural section (since the publication is essentially focused on climate), but another important aspect is the direct effects of increased CO2 on plant growth (for example, apart from any changes to climate. For example, crop yield in some cases has been shown to be much higher with less water use when CO2 levels are higher. Even though this is not climate related, the impacts could be similar or greater than those due to climate. It is an interesting topic.

  2. Not so fast! CO2 levels continue to rise and are at historic levels but the earths temperature has been steady since the late 1990's. It was easy to point to CO2 in the 80's and most of the 90's when the earths temperature was rising along with CO2 but that is just no longer the case. Go to for many temperature and CO2 charts and you will clearly see there is NO link! Climate change is real and the earth will continue to warm and yes COOL in the future, future plans need to take into account warming and cooling.