There’s a new paper doing the rounds of the blogosphere, called Varying planetary heat sink led to global-warming slowdown and acceleration. Judith Curry has a post about it. The basic result of the paper is that the “missing heat” is going into the deep parts of the Atlantic because a salinity variation is allowing warm water to sink rapidly. That sounds plausible, but I don’t really know enough to judge. The reason it’s generated some interest seems to be because it suggests that the “hiatus” will last another decade or so, and because it suggests (although this appears to only be in the press release) that only half the warming between 1970-2000 was anthropogenic (or due to global warming).
The paper concludes with
The next El Niño, when it occurs in a year or so, may temporarily interrupt the hiatus, but, because the planetary heat sinks in the Atlantic and the Southern Oceans remain intact, the hiatus should continue on a decadal time scale. When the internal variability that is responsible for the current hiatus switches sign, as it inevitably will, another episode of accelerated global warming should ensue.
Many seem to ignore the very end of that sentence and focus only on the part that says the hiatus should continue on a decadal time scale. Now, the paper seems to have gone through the historical records (including the Central England record) and found some kind of 60-70 year pattern of warm and cool phases, and is then arguing that we’re in some kind of cool phase now and that it will last another decade or so, since it has existed for about 15 years already.
One issue I have is that it is very likely that none of the cool phases in the past were coincident with the planet being significantly out of energy balance – as we are now. Even though we are in this supposed cool phase, we are still warming at something like 0.1 degrees per decade (which is something else the paper rather failed to point out). It’s possible that we could sustain this slowdown for another decade or so, but if we continue increasing our emissions, that would imply that we could remain in a cool phase with slow surface warming even when the energy imbalance is > 1 Wm-2. I find that somewhat implausible, but I may well turn out to be wrong.
Although I find the possibility that the “hiatus” could continue for more than another decade, or longer, a little unlikely, there is a claim in the press release that I find rather strange. It says
Rapid warming in the last two and a half decades of the 20th century, they proposed in an earlier study, was roughly half due to global warming and half to the natural Atlantic Ocean cycle that kept more heat near the surface.
When people say things like this, it makes me think that they don’t really understand anthropogenic global warming (AGW). AGW is simply the fact that we are increasing atmospheric greenhouse gas concentrations (GHGs) which then act to trap outgoing radiation, producing an energy imbalance, and increasing the energy in the climate system. There isn’t really some special anthropogenic mechanism that simply heats the surface. The surface warms because some of this extra energy heats the surface, increasing the surface temperature, which then reduces the energy imbalance.
On the other hand, its very likely that variability means that sometimes the surface will warm faster than at other times. Therefore one could define the anthropogenic (global warming) contribution as the mean long-term trend, and the natural contribution as being variations from this mean. If we consider the period 1970-2000, the Skeptical Science Trend Calculator suggests that we were warming at 0.16 degrees per decade. If half of this is global warming and half is natural, that would suggest that the global warming contribution was 0.08 degrees per decade. If so, that would suggest that over about a 60 year period, the trend should be 0.08 degrees per decade. However, this would imply that the trend since 2000 would have to be about 0 degrees per decade, which it isn’t. In fact, if you go back to the Skeptical Science Trend Calculator, the trend from 1970-2014 is also about 0.16 degrees per decade. So, if half the warming between 1970 and 2000 was natural, and we’ve been in a cooling phase since 2000, how can the 1970-2014 trend be the same as the 1970-2000 trend. It doesn’t really make sense.
So, as far as I can tell, the press release has a claim that isn’t in the paper and that doesn’t really make much sense, and the conclusion about the hiatus continuing for another decade or so is really just based on identifying some kind of 60-70 year cycle in the historical records – none of which contain periods really comparable to what we’re going through today. If I remember correctly, there was a massive outcry when the press release for the Marcott et al. Holocene temperature reconstruction paper rather overplayed the significance of part of their reconstruction. I wonder if the same people will be similarly shocked by the, apparently, unsupported claims in this press release? Don’t bother answering that question. I suspect we all know the answer. And, to be honest, I don’t really care. I do wish people wouldn’t overplay the significance of their papers in press releases, but they do and it’s really a systemic problem, rather than just a few individuals.