It is often claimed that we can attribute the recent temperature rise to greenhouse gases because models with natural climate forcings do not show the temperature rise, whereas those with greenhouse gases included do. Via Roger Pielke Sr comes an important comment in the Bulletin of the American Meteorological Society on this reasoning:

That global twentieth-century temperature variations can be explained by using a simple model merely points to a certain consistency between this model or climate model simulations and observations. Furthermore, the fact that the late-twentieth-century warming is unexplained by two factors (solar variations and aerosols) and can be explained by including a third factor (greenhouse gases) does not prove that greenhouse gases are the cause; it just points to a missing process in this model. In fact, this whole line of reasoning does not prove the existence of global warming; it is merely consistent with it. As an example, it is still debated whether or not land surface temperature changes during the twentieth century are affected by anthropogenic non–greenhouse gas processes and whether or not these processes affect surface temperatures on a global scale (Christy et al. 2006; Kalnay et al.2006; de Laat and Maurellis 2006).

As`the IPCC itself acknowledges, our level of scientific understanding of a whole swathe of climatic processes is low. Moreover, as the National Research Council acknowledged in 2005 (referred to by Prof. Pielke in the comments), the use of global average radiative forcing is an inadequate metric to reflect influences on the climate system. Until we devote a lot more research to these questions, the finding of the NRC in 2001 remains true:

“Because of the large and still uncertain level of natural variability inherent in the climate record and the uncertainties in the time histories of the various forcing agents (and particularly aerosols), a causal linkage between the buildup of greenhouse gases in the atmosphere and the observed climate changes during the twentieth century cannot be unequivocally established.”

This is not denial, merely a recognition of the limits of current knowledge.