With the rise in global temperatures stalled for about a decade now, scientists are trying to figure out what exactly is behind the so-called hiatus. And is it linked to the mid-20th century pause in rising temps? (From NOAA)
The current pause in global temperature rise may be a repeat of another pause in the mid-20th century.
First of all take note: despite headlines to the contrary, global warming has not stopped. Heat continues to build in the oceans. But the last decade has been quite a surprise when it comes to atmospheric temperatures. For all intents and purposes we have seen a "hiatus" in the warming trend in the atmosphere. This is not to say that temperatures are not elevated -- they are; the average temperature over the past decade is higher than any other 10-year average on record.
It's also relevant to note that the rise in global temperatures has never been smooth and steady. Nor would we expect it to be. Myriad factors in addition to greenhouse gases influence global temperatures, each pushing and pulling in different directions. And so, while carbon dioxide (CO2) continues its slow and steady climb upward, global temperatures move in fits and starts, showing rapid warming over some periods and slow warming over others; and in some cases the warming going on hiatus for a while. (See graphic above).
So a pause in warming even as CO2 continues to rise is not at all unusual. But the current pause, now approaching 10-15 years in duration, is beginning to look a bit unusual. With one exception, discussed below, it has lasted longer than any other pause we've seen since the late 1800s.
Why Is It Happening?
The fact is we don't really know why the global temperature climb is on a hiatus. The consensus is that it probably has something to do with extra heat going into the ocean driven by so-called "internal variability." Here's how the latest report by the Intergovernmental Panel on Climate Change (IPCC) sums it up [pdf]:
"The observed reduction in surface warming trend over the period 1998-2012 as compared to the period 1951-2012, is due in roughly equal measure to a reduced trend in radiative forcing and a cooling contribution from internal variability, which includes a possible redistribution of heat within the ocean (medium confidence)."
Of course such an explanation begs the question: what internal variability? What is going on inside the climate system that is causing global temperatures to respond in this way? One interesting explanation that has surfaced in recent months starts by positing that the current hiatus is linked to an earlier hiatus that also lasted for a more than a decade.
An Earlier Hiatus
This earlier hiatus ran through the middle of the 20th century. See "Hiatus 1" in the graphic above. At the time we knew far less about the climate, and scientists were even more flummoxed about what was going on then than we are now. For example, there was some concern -- but no consensus -- about global cooling.
Eventually, the hiatus ended and the warming trend renewed (at an even faster pace). But scientists continued to puzzle over the cause of the hiatus. By the late 1990s and early 2000s, with our growing understanding of the role of aerosols in the climate, the general consensus grew that the stalled warming in the mid-20th century could be laid at the feet of air pollution. Emissions, primarily from the burning of coal, increase the atmospheric burden of suspended particulates (or aerosols) that cool and that, therefore, can counteract the effect of greenhouse gases like CO2. The hiatus had arrived in the late 1940s when the rapid growth of the then coal-dependent economies of the United States and Europe began to spew huge amounts of pollution into the atmosphere. When these countries began to clean up their air-pollution act (e.g., the Clean Air Act of 1970 in the United States), emissions decreased, the cooling effect shrank, greenhouse warming took over, and the rise in global temperatures began again in earnest.
Line plot of global mean land-ocean temperature index, 1880 to present, with the base period 1951-1980. The dotted black line is the annual mean and the solid red line is the five-year mean. The green bars show uncertainty estimates. [This is an update of Fig. 1A in Hansen et al. (2006).] (Source: Goddard Institute for Space Studies)
So that was then and this is now, and we are faced with trying to figure the cause of another long-running hiatus. Scientists have advanced a plethora of ideas. Most of these treat the current hiatus as a phenomenon essentially unrelated to previous hiatuses. But what if the mid-20th century hiatus and our current hiatus are not unrelated? What if they are caused by the same thing?
Another possible explanation posits that the mid-20th century and current hiatus periods share a common climatic trigger: the internal variability in the climate system known as the Atlantic Multidecadal Oscillation or AMO.
The Atlantic Multidecadal Oscillation
The National Oceanic and Atmospheric Administration defines the AMO like so:
"an ongoing series of long-duration changes in the sea surface temperature [SST] of the North Atlantic Ocean, with cool and warm phases that may last for 20-40 years at a time and a difference of about 1°F between extremes. These changes are natural and have been occurring for at least the last 1,000 years."
Basically we're talking about an oscillatory type of behavior where for a while the sea surface temperatures in the Atlantic are relatively warm and then they're relatively cool.
Why does the Atlantic oscillate like that? I don't think we really know although it’s generally believed to be related to variations in the deep ocean circulation.
How does the AMO relate to the hiatus? Well, just prior to the extended cool period in the 1960s and '70s, the AMO began to cool (that is, go from a warm phase to a cool phase); and, interestingly enough, the current lull in warming also seems to correspond to the onset of a cooling period in the AMO.
Coincidence? No, wrote Ka-Kit Tung and Jiansong Zhou of the University of Washington in the Proceedings of the National Academy of Science back in January. Using temperature reconstructions going back more than 300 years, the authors were able to show that decadal temperature oscillations like those seen with the AMO in the 20th century predate anthropogenic warming going back some 300 years.
The AMO’s Push and Pull
The authors conclude that anthropogenic forcing is alive and well in the climate system and exerts a “remarkably steady” warming trend on global temperatures. That steady trend, however, is being obscured by the oscillatory overlay from the AMO. Tung and Zhou write: "The presence of multidecadal internal variability (i.e., the AMO) superimposed on the secular trend (from greenhouse gases) gives the appearance of accelerated warming and cooling episodes at roughly regular intervals."
Let me translate by way of a hypothetical real-world scenario. Imagine you are running a race. For 20 minutes the wind is in your face, and then for 20 minutes it's on your back. You keep running at the same pace but to the folks in the stands cheering you on, you appear to be running slower for 20 minutes and then faster. In a strong wind you may even appear to have stopped running all together, but you haven't. Similarly, if Tung and Zhou are correct, it may appear that warming has stopped, but it hasn't really. And like you running with the wind at your back, once the AMO shifts back to its other phase, the warming will accelerate, reflecting not only anthropogenic forcing but also the added push from the AMO itself.
Enter the North Atlantic Oscillation
A new wrinkle to this storyline was added last week by Jianping Li of the Chinese Academy of Sciences, Beijing, and co-authors in the journal Geophysical Research Letters. That new wrinkle is the North Atlantic Oscillation (NAO) -- a phenomenon similar to the El Nino Southern Oscillation involving a shifting of high and low pressure systems in the Atlantic (instead of the Pacific) Ocean. These authors argue that the NAO-induced turbulence triggers a 15-20 year process that leads to a switch between the warming and cooling phases of the AMO. And the NAO can therefore be used as a predictor of the future state of the AMO and, if AMO cooling is tied to a dampening of climate change, it can also be used to infer the length of a given hiatus. And on that basis they predict that the current hiatus will be with us through 2027.
That's quite a prediction -- I admire scientists willing to go out on a limb -- and wouldn't it be fortunate if that prediction proved true. But before you get too carried away, you should keep a few things in mind. In the first place, both Tung and Zhou's and Li et al.'s works are largely based on statistical manipulation of temperature records to show correlations -- there's not a whole lot of explanation by way of physical mechanisms. And don't forget that correlations establish a potential connection between phenomenon, not a cause and effect. Moreover, there are a number of scientists who find potential flaws in Tung and Zhou's analysis -- see here and here.
Still, I find a certain elegance in the Tung and Zhou AMO-hiatus concept. And if true, it would give us a bit more time to get our low-carbon economy in gear. But, let's just not forget what a hiatus is -- it's a temporary pause. If we are lucky enough to get a multi-decadal pause in warming, we should use it to prepare because the warming will be back.
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