I can’t remember if I’ve written about this before, but since I haven’t written a post for a few days and would prefer to address something physicsy (rather than something controversial), I thought I might address the issue of us simply recovering from the Little Ice Age (LIA). There are some people who suggest that some (or all) of the warming we’ve experienced is simply a recovery from the LIA. This has a tendency to then irritate a lot of other people. The reason for this is that it doesn’t really make physical sense. It’s not that you can’t use the word recovery, but you do have to be careful about what you mean.
So, why is it irritating? Well, our climate isn’t some kind of bouncing ball that has an equilibrium that’s defined by it’s own properties. The mean temperature of the planet is essentially determined by 3 factors, the amount of energy we get from the Sun, the amount that we reflect directly back into space (albedo), and the composition of our atmosphere (the greenhouse effect). If one of these things changes (what we’d typically call a change in forcing) we will warm if the changes causes us to get more energy than we’re losing, and it will cause us to cool if we lose more energy then we’re gaining. There are other effects (internal variability) that can cause the surface temperature to change, but if these other effects do not change the solar flux (impossible), our albedo (unlikely), or the composition of our atmosphere (possible in some cases), we will return to equilibrium quickly.
So, the period referred to as the Little Ice Age, was a period where the solar flux was lower than today (we were getting less energy from the Sun) and – I think – was a period of enhanced volcanic activity. Volcanoes release aerosols which act to reflect incoming sunlight, producing a negative forcing and hence producing cooling. So, some of the warming since the LIA has been because the volcanic activity has reduced, and the aerosols have precipitated out (which happens quite quickly). Could this still be occurring today? No, because the “recovery” time after enhanced volcanic activity is quite short. As I point out in this post, even in the extreme case of cooling resulting in us falling a full degree below the equilibrium temperature, it would still only take a few decades to “recover”.
What about responding to the increasing solar forcing? Sure, that’s indeed happening and is incorporated in all the models. What to bear in mind though, is that the solar forcing has dropped in the second half of the 20th century and we’re still warming. Also, we’ve warmed by maybe 1 K since the LIA and solar forcing has increased by no more than 1 Wm-2 (probably less). This gives a sensitivity of at least 1 K/Wm-2 which – multiplying by 3.7 Wm-2 – would imply a climate sensitivity of 3.7 K per doubling of CO2. So, if you want to attribute our warming mainly to the slowly increasing solar forcing since the LIA (which has started dropping since the mid-1900s) you have a bit of a contradiction. How can we be so sensitive to changes in the solar forcing while being completely insensitive to changes in greenhouse gas forcings?
There’s probably more that could be said, but I hope this helps to explain why you may find yourself irritating others if you happen to suggest that we’re simply recovering from the LIA.