Okay, not quite, but bear with me. This post is really just a response to those who seem to be suggesting that half of the warming between 1970 and 2000 could be natural, rather than anthropogenic. See my earlier post or Nic Lewis on Bishop-Hill. I’m going, here, to assume that those who suggest that half the warming in the last few decades of the 20th century could be natural, mean that it was due to some process internal to the climate system (as it can’t have been the Sun and can’t have been volcanoes). It’s this that I would argue doesn’t make sense and that, therefore, in our current situation, there is essentially no such thing as natural warming : it is all anthropogenic. I’ll try to explain why – some may disagree, but this could be an interesting discussion nonetheless.
So, here’s my reasoning. What we’ve done through burning fossil fuels over the last 100 – 200 years is to add so much CO2 to our atmosphere that we’ve produced a radiative forcing of about 2 Wm-2. A consequence of this is that it has taken our planet out of energy balance, so that we are receiving more energy than we’re losing. We are therefore gaining energy, some of which will heat the surface and raise the surface temperature. The increase in surface temperature increases the amount of outgoing energy and reduces the energy imbalance, and this rise in temperature will continue until we return to energy equilibrium.
Therefore, the fact that we are warming is entirely a consequence of our actions. If it wasn’t for us burning fossil fuels and increasing the atmospheric CO2 concentrations, we would not be warming (or, at least, not by much). So, that we are warmer today than we were in the past is entirely a consequence of anthropogenic influences and anyone who claims that half of the recent warming could be natural, should go and learn – or brush up on – their basic physics.
Okay, so the above is clearly quite a strong statement and there are some caveats. The Sun can clearly play a role, but it’s relatively small. Volcanoes can too, but they tend to produce cooling. Additionally, natural variability can also play in role in our warming, but what it does is change the rate at which we warm, but can’t really warm in its own right. In other words, this variability means that sometimes we will warm faster than at others. This doesn’t change, however, that the reason that we’re warming is entirely anthropogenic. Given this, however, one could define the long-term trend as the anthropogenic component, and variations about this trend as the influence of natural variations.
So, let’s then consider the possibility that natural variability doubled the rate at which we warmed during the period 1970 – 2000 : i.e., that half of the warming was natural, and not anthropogenic. If you go to the Skeptical Science Trend Calculator you’ll discover that the trend for this period was about 0.16 degrees per decade. If half is natural, the anthropogenic warming trend is then about 0.08 degrees per decade. If you then consider the period 1970 – 2014, the trend is still 0.16 degrees per decade. We’ve, therefore, now had 44 years with a trend of 0.16 degrees per decade, so if the long-term trend is actually 0.08 degrees per decade, we will very soon need to have a multi-decade period (almost 50 years) with no warming.
However, if we continue to increase our emissions, we’ll continue to move the planet further and further out of energy balance. So, those who are arguing that half of the warming between 1970 and 2000 was natural are essentially suggesting that we will have about 50 years of no warming, despite the planet moving further and further from energy balance. I think that’s physically implausible, even impossible. However – and here’s a challenge – if anyone can give a physically plausible explanation for how half the warming between 1970 and 2000 was natural, they’re welcome to try.
I’ve actually been thinking about a post like this all day, but it was a comment from Fred Moolten that motivated me to go ahead and write this. Fred says,
confusion on the part of the authors between the capacity of natural variability to affect the rate of warming from anthropogenic forcing and the role of natural variability as a source of warming in its own right.
I think this is the crucial point. There is a difference between natural variability being able to influence the rate at which we warm (it can) and its ability to warm in its own right (it can’t, or not by much). If people could understand this distinction, I think it would resolve much of the confusion.