I’ve noticed that Judith Curry is discussing the early twentieth century warming. The idea is that there was a period of warming during the early twentieth century that was similar to the warming we’ve experienced since the second half of the twentieth century. Tamino has already pointed out that the warming since the mid-1900s far exceeds the early twentieth century warming, so I won’t say much more about that.
The suggestion, though, is essentially that this early period of warming confounds our understanding of anthropogenically-driven warming. In other words, if it warmed this much during a period when we weren’t emitting much, how can we be confident that most of the modern warming is anthropogenic? As Judith highlights, there is a recent paper that estimates that about half of the early twentieth century warming was externally forced (anthropogenic and natural) and about half was due to internal variability. So, internal variability could have contributed 0.1-0.2K of warming during the early 20th century. This is entirely consistent with what we might expect over a period of a few decades.
So, does this mean that internal variability could be responsible for a reasonably large fraction of the overall warming we’ve experience to date? Well, no. Internal variability can produce quite substantial warming on short timescales, but it’s very difficult for this to persist for a long time. By itself, internal variability just moves energy around. For there to be long-term warming requires some kind of response (for example, changes in clouds or water vapour) that prevents any enhancement in surface temperature from simply being radiated into space (which would happen in months without this response). However, since the system is long-term stable, these responses are – on average – smaller in magnitude than the Planck response (how much an increase in surface temperature changes how much energy we lose to space). Therefore, on long, multi-decade timescales, any internally-driven warming should decay back to the equilibrium set by the external factors (i.e., the Sun and the planetary greenhouse effect). It’s therefore very difficult for it to provide substantial warming on multi-decade/century timescales.
There’s also a paradox. For internal variability to contribute substantially to our long-term warming, there needs to be some kind of radiative response. As I said above, this would be changes in clouds, or changes in water vapour. However, these also respond to externally-driven warming, such as due to changes in atmospheric CO2. As I try to explain in this post, if internal variability has produced more than half of our observed warming, then somehow the system is responding to internally-driven warming, but not to externally-driven warming. This doesn’t make any sense, as the climate system can’t know if some temperature change was internally-driven, rather than externally-driven. Therefore, arguing that internal variability could have contributed substantially to our observed warming introduces a paradox that noone who suggests this has ever addressed.
Basically, the early twentieth century warming doesn’t really challenge our overall understanding of anthropogenically-driven warming. It seems likely that internal variability contributed to some of this early warming, but that doesn’t mean that it has contributed to a substantial fraction of our overall warming. It may have had some impact (~0.1K) but that can work in either direction. In fact, the overall analysis suggests that it may have produced some cooling since 1950, so that anthropogenic influences – by themselves – would have produced slightly more warming than we’ve actually experienced. Those who promote the role of internal variability typically seem, for some reason, to ignore this possibility.
And then there’s the energy imbalance – post I wrote about the significance of the planetary energy imbalance.
95%, attribution, and all that – post I wrote trying to show how it’s very difficult to construct a physical plausible scenario in which most of the observed warming is not anthropogenic.
IPCC attribution statements redux: A response to Judith Curry – Realclimate post responding to Judith Curry’s suggestion that more than half of the observed warming could be internally-driven.
The feedback paradox – post I wrote about the paradox of feedbacks operating only when the warming is internally-driven.
Predictable and unpredictable behaviour – Realclimate post that also discusses the feedback paradox.