What would be the consequences for the estimations of future climate change if the reconstructions of the climate of the past few millennia were wrong? Since estimations of future climate change are presently solely based on model simulations, they would not need to be modified. However, past reconstructions do have a subtle and, for many perhaps surprising, implications in the understanding of global climate, and in this sense they also project into the future.
Projection of future climate change are basely almost exclusively on the results of simulations with climate models. Climate models do not include or use any information derived from proxy data. They are not calibrated, or their skill is scored, against past climate reconstructions. The consequence of this simple fact is that revisions of the reconstructions of past climates due to the new data or to the application of new statistical reconstruction methods do not require any revision of future climate change projections. In this sense the world of climate models and the world of proxy-based reconstructions are truly independent. Actually, one of active avenue of research is to try to build bridges between both worlds, for instance by assessing the skill of climate models by comparing simulations of past climate with proxy-based reconstructions (e.g. Hegerl et al, Nature 440, 1029). But this still work in progress. In other words, if the hockey is wrong, nothing changes in the climate projected for 2100. The projections are as correct or incorrect as if the hockey-stick were right.
However, reconstructions of past climates, in particular the climate of the past millennium, do interact in a subtle way with the understanding of the basic functioning of the Earths climate.
The key word for this link is the much vaunted concept of climate sensitivity and its connection to the amplitude and phase of past climate variations. Climate variations can basically be classified as externally forced and internally generated. The externally forced variations are due to variations in external agents that may affect climate, .i.e. solar variations, concentrations of greenhouse gases, volcanic eruptions, anthropogenic land-use changes, etc. Roughly speaking the amplitude of external forced variations of surface temperature are proportional to the amplitude of the variations of the external climate forcing, the proportionality constant being the climate sensitivity. It is plausible, although by no means proven, that the climate sensitivity is independent of the nature of the external forcing (advocates of the solar influence on climate would rather defend, rightly or wrongly, the concept of forcing-dependent sensitivity). The timing of the externally-forced variations must be also related to the variations in the external forcing, although perhaps they are not necessarily simultaneous. For instance, if solar irradiance reaches a maximum in some particular decade, the global mean temperature would also reach a maximum after some lag, provided that all other external forcings remaining constant.
By contrast, internal climate variations are not related to the external forcing. Their amplitude and timing is random, and their spatial structure is determined by the physical processes that define the climate system. Known examples of internal variability which vary with time scales of a few years are the North Atlantic Oscillation, ENSO, etc . Also, the slowdown of the global temperatures observed in the past decade may be the result of natural internal variations superimposed to an underlaying rising trend However, internal variations with typical timescales of several decades or even longer may well exist, although they are not really so well characterized as the high-frequency internal variations. One example of these slow internally generated climate variations is the Atlantic Multidecadal Oscillation. Multidedadal and centennial internal variations may display other unknown spatial structures.
Internal variations are not caused by external forcings, but there could be a subtle connection between them and the climate sensitivity. This connection is based on the fluctuation-dissipation theorem. This is not the place to explain this theorem and its applicability to the climate system, which is also debated. But the meaning of this theorem can be intuitively illustrated. If the climate system is a stiff system. i.e. it exerts a large resistance to external influences (low sensitivity), then the very same processes that are responsible for this stiffness will tend to quickly wipe out any random fluctuations that may be internally generated. On the other hand, if the system is soft and it does not try to resist the effects of the external forcings (high sensitivity), any random fluctuations will tend to persist for longer times.
Thus, we have therefore different 'scenarios' to describe past temperature variations:
-a very small climate variability, with temperature variations more or less following the external forcing. This implies a low-climate sensitivity or very small variations in the external forcing in the past. This is the hockey-stick scenario. If the amplitude of past solar variations had not been that small, the hockey-stick scenario would imply low-climate sensitivity, and therefore also small climate changes in the future. This explains why the recent estimations of past solar variations pointing to very small amplitude were warmly and rapidly welcomed in some quarters.
-a large past variability, as for instance hinted by other temperature reconstructions (Moberg et al. Esper et al.). The larger variability would require larger climate sensitivity or larger variations in the external forcing. Although one would think that indications of a larger climate sensitivity could find an easier way into the IPCC quarters, it raises another problem: the role of solar forcing in the temperature trend observed in the last two centuries or so would have been also larger, and therefore the influence of greenhouse gases should should have been smaller to accommodate the observed the temperature increase.
An intriguing scenario, which I am not sure it would be possible at all, is to have large internal climate variations with small climate sensitivity. For this scenario to be correct, the temperature variations should be uncorrelated with the external forcing. In essence, this would be one of the alternative 'theories' to explain the 20th century warming. For this theory to be true the Medieval Warm Period could have been a prominent feature but the solar irradiance should not be very high, otherwise a warm MWP could be easily explained by the standard paradigm of climate sensitivity. I must admit that I have several problems with the idea of an important role of past internal variations. Does the fluctuation-dissipation theorem applied to an open system require that high-sensitivity goes hand-in-hand with high amplitude internal variations as well, or only with long-lived internal variations? Perhaps some readers, with more knowledge than me, may have some comments to the applicability of the fluctuation-dissipation theorem in this context.
An interesting question is related to our pseudo-proxies studies (von Storch et el 04). Given that the main conclusion of that study was that past climate variations had been underestimated in the proxy-based reconstructions, and that they were probably externally forced (at least this is what climate simulations indicate), these results hinted at a high climate sensitivity. Why then were Mann, Jones and Rahmstorf so fiercely opposed to this and other similar studies?