Key Points Both natural and human factors change Earth’s climate. Before humans, changes in climate resulted entirely from natural causes such as changes in Earth’s orbit, changes in solar activity, or volcanic eruptions. Earth’s temperature is a balancing act Earth’s temperature depends on the balance between energy entering and leaving the planet’s system . View enlarged image Models that account only for the effects of natural processes are not able to explain the warming over the past century. Changes in the greenhouse effect, which affects the amount of heat retained by Earth’s atmosphere Variations in the sun’s energy reaching Earth Changes in the reflectivity of Earth’s atmosphere and surface These factors have caused Earth’s climate to change many times. The historical record shows that the climate system varies naturally over a wide range of time scales. Recent climate changes, however, cannot be explained by natural causes alone. Radiative Forcing View enlarged image References:
Climate Impacts in the Southwest | Climate ChangeKey Points Increasing temperatures and more frequent and severe droughts will likely worsen existing competition for water resources. Drought, wildfire, changes in species' geographic ranges, invasive species, and pests will likely threaten native Southwest forests and ecosystems. View enlarged image Observed and projected temperature change in the Southwest, compared to a 1960-1979 baseline period. The Southwest is bordered by the Pacific Ocean to the west, the Rocky Mountains to the east, and Mexico to the south. There is a wide range of elevation and climate types across the region. The climate of the Southwest is changing. Warming has already contributed to decreases in spring snowpack and Colorado River flows, which are an important source of water for the region.  Future warming is projected to produce more severe droughts in the region, with further reductions in water supplies. Top of page Impacts on Water Resources View enlarged image Impacts on Recreation Piñon Pine Die-off 
Western U.S. Bark Beetles and Climate Change | Climate Change Resource Center (CCRC)Preparer: Barbara Bentz, FWE, Rocky Mountain Research Station, Western Bark Beetle Research Group (WBBRG) This page is an archive, please see our updated Bark Beetles page. Issue Since 1990, native bark beetles have killed millions of trees across millions of hectares of forest from Alaska to southern California. Bark beetle outbreak dynamics are complex, and a variety of circumstances must coincide and thresholds must be surpassed for an outbreak to occur on a large scale. Likely Changes The recent large-scale dieback of piñon (Pinus edulis Engelm.) and ponderosa pine (P. ponderosa Dougl. ex Laws.) and associated bark beetle outbreaks in the Southwestern United States has been linked to the ”climate change type drought” (e.g., dry and warm) that occurred in this region in the early 2000s. We have database models to describe and project the effect of temperature, but not other climate variables, on life-cycle timing of the mountain pine beetle and spruce beetle (D. rufipennis Kirby).