Basically, the question is whether the climate has changed sufficiently to push some ecosystems beyond resilience into active adaptation, ie change in vegetation type. Variation has increased, but many stop short of saying that the climate has actually changed. I have included a number of papers to peruse, none of which makes a complete argument in favor of climate over biological factors. Obviously both occur, and grazing and fire-suppression have exacerbated climate-driven changes. The hard question is to what extent each factor has "driven" the observed changes.
The old school of thought on climate change (which is still prevalent among scientists today) argues that climate change is happening but isn't here yet. In other words, that we are still within natural ranges of variability. However, my recent research has convinced me that the changes that are being predicted are already here, all around us.
Large forest die-offs have occurred in old-growth Amazonian and Indonesian rainforest, in addition to other undisturbed forests around the world (Allen 2010) and in the Western US (van Mantgem 2009). Here in the Southwest, Pinon die-off has been linked to an increase in temperature that increased the severity of an otherwise normal drought (Breshears 2005). Dr. Gutzler here at UNM has created a map showing what a 3 degree increase in temperature would do if precipitation variability remains unchanged (see below; Gutzler 2010). The trend is, indeed, toward unprecedented warming (Kaufman 2009). This kind of increased ET could also explain the increase in stand-replacing Ponderosa forest fires, which have occurred without 20th century overgrazing and fire supression (Meyer and Pierce 2003). Indeed, Dr. Archer at University of Arizona, an authority on shrub invasion of grasslands, also points out situations where differing grazing and fire regimes fail to modify climate-driven shrub encroachment (Fensham 2005). Dr Fredrickson (Fredrickson 2006) has shown that climate change can explain loss of Black grama grasslands on the Jornada in the absence of grazing pressure.
In a particularly interesting rebuttal to a long-held 'sacred-cow', Dr. Meyer at UNM recently used stratigraphy to show that beaver have not significantly affected hydrologic processes during the Holocene, but instead, that climate determined beaver abundance (Meyer 2008). I could include other papers, but I particularly like several of these. I intentionally saved papers on historic arroyo cutting for another time, but could have cited Leopold, Shumm, and many others to argue for climate-driven processes.
Bill McKibben has written a new book with essential the same argument: "Most accounts terrifically underplay what’s actually going on already. " Tim Flannery's book The Weather Makers makes a similar argument, even though it was published in 2006: "the first victims of climate change have already died".