This chapter summarized the scope of what is known and not known about climate in the Southwestern United States. There is now more evidence and more agreement among climate scientists about the physical climate and related impacts in the Southwest compared with that represented in the 2009 National Climate Assessment (Karl, Melillo, and Peterson 2009). However, there remain uncertainties about the climate system, the complexities within climate models, the related impacts to the biophysical environment, and the use of climate information on decision making.

Uncertainty is introduced in each step of the climate planning-an-response process--in the scenarios used to drive the climate models, the information used to construct the models, and the interpretation and use of the model' data for planning and decision making (Figure 19.1).

There are server key challenge, drawn from recommendations of the authors of this report, that contribute to these uncertainties in the Southwest:

- There is a dearth of climate observations at high elevations and on the lands of Native nations.

- There is limited understanding of the influence of climate change on natural variability (e.g. El Niño-Southern Oscillations, Pacific Decadal Oscillation), extreme events (droughts, floods), and the marine layer align coastal California.

- Climate models, downscaling, and resulting projection of the physical climate are imperfect. Representing the influence of the diverse topography of the Southwest on regional climate is a particular challenge.

- The impacts of climate change on key components of the natural ecosystems (including species and terrestrial ecosystems) are ill-defined.

- The adaptive capacity of decision-making entities and legal systems to handle climate impacts is unclear. This creates a challenge for identifying vulnerabilities to climate in the Southwest.

- Regulation, legislation, and political and social responses too climate all play important roles in our ability to adapt to climate impacts and mitigate greenhouse gas (GHG) emissions.

- Climate change is one of multiple stresses affecting the physical, biological, social, and economic systems of the Southwest, with population growth (and its related resource consumption, pollution, and land-sue changes) being particularly important.