Forecasts of future climates for the purpose of projecting possible impacts of climate change on ecosystems are considerably more consistent than one is led to believe by the popular press. Of course, all the coupled Atmosphere-Ocean General Circulation Models (AOGCMs) are in agreement that the planet will warm over the 21st century within a range that at its minimum will be similar to that of the middle-Holocene thermal maximum (ca. 5,000 to - 8,000 years ago) and at its maximum will warm to a level that is comparable to a full glacial-interglacial cycle. The fossil record is replete with evidence of major changes in the distributions, carbon loadings, and presumably changes in hydrology and all other ecological functions at even the minimal projected warming. The climate models are also in agreement that with increasing temperature, more water will evaporate from the oceans and more rain will fall, as the hydrologic cycle is sped up and storms will become more intense, as will droughts.
The models also agree that most of the increased precipitation will fall at the higher latitudes, that are already wet, and that the dry subtropical latitudes will increase in breadth and likely become drier, at least in the winters. The AOGCMs, being of coarse grid resolution, do not simulate summer monsoon rains very well, but those may also increase as they did in the hotter times of the past, again due to the enhanced hydrologic cycle of the subtropical high pressure cells. Thus, the gradient of wet to the north (within the lower 48 states) and drier to the south, will likely increase, making specific forecasts extremely difficult within most of the conterminous United States. However, in addition to the well-known El Niño—La Niña oscillation there is now increasing recognition of the importance of interdecadal climate variability, such as the Pacific Decadal Oscillation (PDO), the North Atlantic Oscillation (NAO) and many others. These interdecadal oscillations appear to be the primary drivers of major drought episodes, such as the 1930s and others. Given the intermediate latitudinal placement of most Western ecosystems along the wet-to-dry gradient, which is becoming increasingly steep, the patterns of interdecadal variability appear to be among the greatest immediate threats. It has now been recognized that much of the recent dieback of interior western ecosystems was initiated by drought stress, induced by increasing temperatures, with longer growing seasons and more evaporative demand during the growing season, as well as multiple dry years. The stressed forests have become magnets for bark beetles and perhaps other insects and diseases, causing even further dieback. Over longer periods, the Great Basin sagebrush ecosystem could be subject to continued invasion of woody plants, especially in the north, in part owing to increased water-use-efficiency from elevated CO2 concentrations; a relaxation of winter frost stress will allow much of the southwestern diversity of all forms of life to invade into the interior West. In the drier parts of the West, carbon densities could increase owing to an earlier growing season, more in line with the spring cyclonic storms, even as there is also more fire, owing to greater fuel loadings. However, the wetter ecosystems of the Northwest could become even wetter and more dense, or drier with more frequent fire, depending on whether the winters become wetter or drier, as different climate models suggest. It is also highly possible that these decade-scale, wet-dry cycles could become the dominant pattern, as has already occurred through two massive wet-dry cycles, beginning with the recent onset of extremely rapid warming in the mid-1970s. These long-wave climate oscillations tend to resonate with the intrinsic inertia of ecosystems, causing regional synchronization of growth episodes followed by catastrophic drought-induced dieback, infestation, and fires. Over the same 30+ years of warming, some vegetation zones have shifted as much as 65 meters upslope, and large latitudinal displacements of species are being observed. Thus, future ecosystem states and dynamics will become increasingly uncertain, and management options must be adapted to manage for change, per se, and to decrease the risk of catastrophic disturbances from rapidly increasing temperatures possibly coupled with high-amplitude, multi-year swings in wet-dry cycles.