A paper just published in Science Magazine helps explain variation we see in the long term Carbon-pollution caused upward trend Earth’s surface temperatures. The research also, and rather ominously, suggests that a recent slowdown in that trend is likely to reverse direction in the near future, causing the Earth’s surface temperature to rise dramatically.
The graph shown above represents the ongoing warming of the Earth’s surface owing to the increased atmospheric concentration of human generated greenhouse gas pollution, mainly CO2. But, have a look at the following graph of changes in concentration of CO2 in the Earth’s Atmosphere:
As you can see, the increase in CO2 is very steady, while the changes in Earth’s surface temperature is very squiggly. Why? In particular, the Earth’s surface temperatures seem to undergo a series of rapid increases or decreases, and now and then, seem to squiggle up and down along a slowly ascending plateau, as has been happening recently. Climate science deniers have taken this recent slowing in the increase of temperature as a signal that the link between CO2 concentrations and global surface temperatures is a hoax. But real climate scientists focus instead on actually explaining, rather than making up stories about, this variation.
There are several different factors that may cause the shorter term squiggles that we see superimposed on the longer term warming trend. The sun’s energy varies over decades, and this contributes a small amount to the variation. Aerosols (dust), either from human activities or volcanic activity, can produce a cooling effect, and this effect varies across time. If you look at the graph of temperatures, you’ll see a strong downward trend associated with the vast eruption of Mount Pinatubo in 1991, for example. A third source of variation in the upward march of the Earth’s temperature is not really a source of cooling or heating at all, but rather, a shift in where the heat goes. The graph on the top of this post is of “surface temperature,” which is a combination of land-based thermometers at roughly head-height, located at weather stations around the world, and sea surface temperatures. But well over 90% of the heat added to the Earth’s system by the human-caused greenhouse effect actually ends up in the ocean. A small percentage of variation in how much heat goes into, or comes out of, the ocean can cause a large variation in the “surface temperature.” You can think of the surface temperature measurements as a relatively small tail attached to a rather large dog, where the dog is the ocean and the tail is the land based thermometers and the sea surface. (I’ve developed this analogy here.)
That the behavior of the ocean is important can be understood by noting that while surface temperature increase has slowed in recent years, the temperature in the top couple of kilometers of the world’s oceans has continued to increase apace. You can also look at the relationship between the squiggle of the surface temperature curve and El Niño and La Niña events. The former are periods of time when the Pacific ocean is sending heat out into the atmosphere, and the latter are periods of time when the Pacific is sucking more heat in. The following graphic from Skeptical Science illustrates this nicely.
“ENSO” refers to the El Niño-La Niña cycling. The top line, in red, represents the change over time in surface temperature just during El Niño periods, while the blue line, along the bottom, represents change over time in surface temperature just using La Niña years. As you can see, many of the ups and downs in the long term surface temperature trend seem to represent ENSO variation.
Now, to the recently published study. The paper is “Atlantic and Pacific multidecadal oscillations and Northern Hemisphere temperatures” by Byron Steinmann, Michael Mann, and Sonya Miller, and is published in tomorrow’s Science. (Yes, I have a time machine.) From the abstract:
The recent slowdown in global warming has brought into question the reliability of climate model projections of future temperature change and has led to a vigorous debate over whether this slowdown is the result of naturally occurring, internal variability or forcing external to Earth’s climate system. To address these issues, we applied a semi-empirical approach that combines climate observations and model simulations to estimate Atlantic- and Pacific-based internal multidecadal variability (termed “AMO” and “PMO,” respectively). Using this method, the AMO and PMO are found to explain a large proportion of internal variability in Northern Hemisphere mean temperatures. Competition between a modest positive peak in the AMO and a substantially negative-trending PMO are seen to produce a slowdown or “false pause” in warming of the past decade.
The research (also reviewed here by Chris Mooney) combines observational data (temperature records and the indices for the AMO and PMO) with sophisticated modeling techniques to parse out the contributions of the Pacific and Atlantic oceans, the big dogs of climate change (the Pacific being the much bigger dogs) on surface temperature variability. Essentially, they are trying to determine how much of the squiggling, specially the recent slowing down of temperature increase, is accounted for by “internal variability” as opposed to “forcings.” The former includes the interactions of the surface and the ocean. “Forced” variation is, according to Michael Mann, means “… governed by drivers, be they human (increased greenhouse gas concentrations, sulphate pollutants) or natural (volcanoes, solar output changes). The internal variability is what’s left, it is the purely natural oscillations in the system that have no particular cause, just as weather variations on daily timescales have no particular cause, they just happen.”
One of the findings of this paper, important in climate research but perhaps a bit esoteric, is that the Pacific and Atlantic have mostly independent effects as sources of internal variation. This is not really new, but confirmed by this work. More exactly, treating them as independent provided good results.
But the most important finding is summarized in the following figure, taken from Figure 3 and also reproduced in a writeup by author Mann at Real Climate:
This shows the AMO, PMO, and the derived (combining the two) NMO values over time. Assume that the highest and lowest values are close to the maximum and minimum that these measures normally reach. Note that there is something of a periodicity in these values. That there would be makes sense. These values represent the way in which the oceans interact with the air, and we know that although there is not perfect periodicity (regularity) in that relationship, historically, every year the ocean is in a phase of removing heat from the atmosphere there is an increased chance of a reversal in that relationship. Now, step back from the contentious issue of climate change for a moment, and imagine that these are values of a blue chip stock you are thinking of investing in. Remember the cardinal rule of getting rich on the stock market: Buy low, sell high! Now, decide if you want to put your hard earned money ito the AMO or the PMO. Clearly, the PMO is at a minimum. Buy now because it is going to go up soon!
Remembering that the PMO was found to be a much bigger source of internal variability than the AMO, and that it is a major player in determining surface temperatures, this can only mean one thing. Things are going to heat up soon. Study author Michael Mann told me, “The PMO appears to be very close to a turning point, based on the historical pattern. So we don’t expect it to continue to plunge downward. We expect a turning point soon.” In his summary of the work in Real Climate, Mann notes that “the most worrying implication of our study [is] that the “false pause” may simply have been a cause for false complacency, when it comes to averting dangerous climate change”
We just had the warmest calendar year on record. Last month, January 2015, was probably the second warmest January on record. Using a 12 month moving average (like in the graph at the top of this post), the last 12 months were the warmest 12 months on record. I hear rumors that February, the month we are in, is relatively warm. We have been seeing signs of the Pacific belching out more heat lately, with El Niño threatening. This could all be a very short term trend, as we expect to happen frequently with the general upward march of surface temperatures owing to greenhouse gas pollution. But this latest paper indicates that it might not be; it could be the beginning of a longer upward trend. Whatever effects of surface warming you might be concerned with — increased storms, drought, more rapid melting of glacial ice, killer heat waves — expect more over the next decade than we have over the last decade. And we had quite a bit of that over the last decade.