Climate Change Impacts on the Sonoran Desert

In the words of Melanie Lenart, "Global warming will impact different regions and sectors in different ways, creating many losers and even a few winners around the world. Unfortunately, it looks like the Southwest will be on the losing side." [32] A few of the major consequences of climate change in the Sonoran include the following:
• Exacerbation of drought
• Warmer temperatures
• Earlier spring
• Loss of species not able to survive the new climate
• Losing forests, homes, and other resources to wildfires


First, we discuss the climate factors impacting the Sonoran and then we will discuss each of these consequences in further depth.

Abiotic Stresses Affecting Sonoran Ecosystems

According to the IPCC 2007 Report, deserts of North America will become warmer and drier at faster rates than most other regions. [7] This and many other abiotic stresses will affect many different levels of the Sonoran Ecosystems. The picture below depicts some specific abiotic stresses facing primary producers of all ecosystems.

Overall, the primary factors that will be affected by climate change and be most influential on communities in the Sonoran Desert are: 1) water availability and 2)temperature. 
We describe and analyze the importance of both of these below.


1. Water Availability
• Low and variable precipitation:
Precipitation in the Sonoran Desert is expected to decrease in annual amounts although increase in degree of variability throughout the year. [6]  Animals depend on water to regulate their body temperature, transport nutrients throughout their body, and remove waste products.[7] Plants also require water for nutrient transport, in addition to photosynthesis and maintenance of their structure.[7] In the Sonoran desert, water availability is already a large issue for organisms, as the bimodal precipitation trend exposes the area to extended periods with minimal or no rainfall separated by short bursts of heavy precipitation. This large variation in intra-annual precipitation is accompanied by variations interannually.


• High evaporative demand:
The warm temperatures of the region cause plants to rely on evapotranspiration for cooling. Furthermore, soil loses water readily in these hot temperatures, which will be an issue for a region  already characterized by dry soil.




2. Temperature
• Maximum temperatures:
These directly limit growth and survival of organisms while indirectly increasing water loss by organisms, augmenting the problem of low water availability. [31]


• Minimum temperatures:
The freezes that occur in the winters in the Sonoran can pose great challenges for organisms adapted to the warm temperatures throughout much of the year. These freezes can cause deaths of some species, and limit distributions and productivities of many more. [31] However, these freezes are actually of great importance, as they especially limit distributions and fitness of invasive species and thus can be of advantage for competing, native species.


Overall, it has been found that cold season minimum temperatures and warm season precipitation have the largest effects on Sonoran Ecosystems. [4]








Impacts on Sonoran Desert Ecosystems


Exacerbation of drought:
Higher temperatures increases water loss from soil by increasing evaporation from the soil. This limits water availability to plants, thus limiting carbon uptake by pants, causing cessation of  growth. [2]


 Furthermore, increased variability in cool-season precipitation will place additional strain on water resources during dry periods that may not be accounted for by wet periods.
Rather, flash floods will become more common as soil dries out and is unable to absorb as much water during spells of heavy precipitation. [6] Lastly, rivers and streams will continue to exhibit increasingly lower flows and water levels. The Colorado Rivers annual flow is predicted to drop by 50% just by the mid twenty-first century. [32]


Warmer temperatures:
The mean annual temperatures of 2010-2011 were over 2 degrees Fahrenheit of those of the 1971-2010 average.  [32] Increasing temperatures inhibit photosynthesis of plants, causing a decrease in desert vegetative growth and threatening all types of organisms' survivals. [2]


More importantly, as the Sonoran Desert experiences warmer temperatures, it has had the largest increase in number of frost-free days across all of North America. [CLIMAS] This has many impacts on freeze-intolerant species, allowing them to proliferate, while also having indirect consequences on competitors of freeze-intolerant species. This yields not only regional vegetation changes towards warm-season plants but biome shifts upward in elevation where species may have been intolerable before. [1] In conclusion, as freeze events become more rare, range shifts and shifts in species abundances cause and will continue to cause restructuring of the Sonoran ecosystems.


Earlier Spring:
Warmer temperatures and drier winter conditions have and will promote earlier arrival of spring phenological events for many species.  [32] This can disrupt life cycles between paired species in cases of phenological mismatch, when shifts in phenology differ between trophic levels. This too can lead to restructuring of ecosystems as species abundances change in either bottom-up or top-down cascades. However, species which don't adjust their phenology are at risk of exposure to unfavorable, potentially lethal conditions, as later spring days become warmer and drier.


Furthermore, as growing season ends in springtime as winter precipitation ends, drier winter-spring conditions will truncate the growing season of many species. [2] This will reduce food availability not only for humans but also for other primary consumers.


Wildfires:
In addition to advancing timing of spring events, earlier termination of cool-season precipitation has caused a 1-4 week advancement in the onset of the fire season in the Sonoran Desert. [6] This exposes the region to an increased period of "extreme fire danger" and doubled risk of wildfire. [6]


Increased frequency of wildfires is also a drought, which causes drier soil in addition to insect invasions which can trigger large wildfires. [32] These wildfires destroy desert landscapes, species' habitats, and human homes. More detrimental, however, are the widespread deaths of organisms and risk of irreversible extinction of many populations.


For current information on drought specifically impacting Arizona, visit: http://azdroughtwatch.org/faces/xhtml/index.xhtml