I’ve been involved in a discussion on another blog (which I won’t highlight) about there being a period of warming in the early 20th century that seems comparable to the warming we’ve experienced since about 1980. This is a somewhat standard “skeptic” talking point that is meant to suggest that a period of warming in the early 20th century, that is comparable to a similar period in the late 20th century, somehow challenges the fundamentals of anthropogenic global warming (AGW). Well, it doesn’t and you can read this Skeptical Science post that discusses this.
I’ll make a few general comments. If you consider, for example, a 12 month running average of GISTEMP then selecting the period 1910-1940 might seem a bit of a cherry-pick, as it happens to go from a particularly low point, to a particularly high point. Karsten Haustein has also suggested that there might be a data issue with some of the early-1940s data. So, some of this apparent earlier warming could be somewhat exaggerated.
If we put this to one side though, then there does indeed appear to be a period, starting around 1910 and ending sometime in the 1940s, during which the surface warmed at a rate a bit higher than 0.1oC per decade. However, if you consider this paper by Matthew Palmer and Doug McNeall, then it’s quite possible for internal variability to drive trends of around 0.1oC/decade for a period of a few decades. So, that such a period exists, isn’t necessarily all that surprising. Over longer timescales, however, we would expect internally-driven trends to tend to 0.
Furthermore, if you consider the GISS Forcings, then the net change in external forcing (mainly anthropogenic and solar) over the period 1910-1940 is about half that of the change over a similar period starting around 1980. The earlier warming is maybe about 2/3 that of the later period. So, a large part of this earlier warming could simply have been externally forced (mostly solar and anthropogenic), although maybe not all of it. The point, though, is that there are perfectly plausible explanations for this earlier period of warming, and it doesn’t somehow provide some kind of major challenge to our understanding of AGW (that it has warmed before, doesn’t mean we’re not responsible for the warming now).
However, I think this is all largely beside the point. The fundamentals of AGW are very simply that we pumping CO2 into the atmosphere and causing the atmospheric concentrations to rise. This increased atmospheric CO2 reduces the outgoing longwavelength flux and pushes the system out of energy balance; we will be gaining more energy than we’re losing. This extra energy will be distributed throughout the climate system (atmosphere, oceans, cryosphere) and some of it will warm the surface, causing surface temperatures to rise. In fact – given the increase in atmospheric CO2 – the only way to regain energy balance is for surface temperatures to rise.
Of course, our climate is very complicated and there are various circulations patterns/cycles in the atmosphere and in the oceans. This means that there will periods when this extra energy is distributed in such a way as to cause the surface to warm faster/slower than at other times. This doesn’t, however, mean that some of the warming is not anthropogenic; until we regain energy balance, it’s essentially all anthropogenic.
Of course, one could regard the variability about the long-term trend as a consequence of internally-driven cycles. I’m also certainly not arguing against trying to understand how internally-driven cycles influence how the surface warms. I’m simply pointing out that while we still have a planetary energy imbalance (a consequence of rising atmospheric CO2 due to our emissions) the surface will – on average – continue to warm, and all of this warming will – until we regain energy balance – essentially be anthropogenic. Arguing about the causes of short periods when we warmed faster/slower than expected does not somehow mean that the overall warming is not being driven by our emission of greenhouse gases into the atmosphere. That’s my view, at least. Feel free to disagree in the comments.