There’s been a good deal of talk about a “pause” in global warming. The Intergovernmental Panel on Climate Change IPCC) report that was just released comments on it. One friend of mine, who should know better, even talked about “decoupling temperature from CO2.” Is this possible pause real or just an artifact of the multiple complex processes that make up the climate system. A recent article in National Geographic Daily News addresses this directly:
Although climate models have been predicting increasing average global temperatures over the next century or so, the past decade has not shown as much warming as most scientists had expected. The year 2012 was no warmer than 2002 . . . The IPCC draft report acknowledges a “global warming hiatus,” according to media reports.
“Governments are demanding a clear explanation of what are the possible causes of this factor,” Arthur Petersen, chief scientist at the Netherlands Environmental Assessment Agency and part of the Dutch delegation that is reviewing the IPCC report, told BBC News.
And, of course, global warming skeptics are themselves quite warmed by this supposed pause.
To start with, such apparent pauses are not new. Look at the process of climate change over the past 130 years as shown in NASA’s Global Land-Ocean Temperature chart at right. You will notice a long plateau from the 1940s to about 1980, as well as several shorter pauses in the 1980s and 90s shown by the downturns in the red (average temperature) line. Looked at over the long term, however, they are all variations along a rising trajectory.
So, even in terms of history, the current “pause” is not so remarkable.
Moreover, there are some specific, well-known phenomena that have probably contributed to the current leveling. For example, some scientists hypothesize that the warming energy may be being absorbed by the deep oceans.
Out of the Pacific
Another element that partly explains what’s happening involves a process in the Pacific Ocean — the El Niño Southern Oscillation. I learned about El Niño processes from Tom Wysmuller, a colleague in the Sierra Club. It works like this: In some years, Pacific waters are very warm, pushing energy in the form of heat into the air and raising the air pressure in thes Eastern Pacific. This tends to push up temperatures in the western hemisphere and result in rain, especially in South America. La Niña processes are the opposite: Pacific waters are cooler and air pressure is lower, which lowers temperatures.
So, when we look at the past 15 years, roughly from 1998, El Niño and La Niña have strongly influenced temperature over this period. 1998 was an El Niño year (shown in red in the graph, left).
By raising worldwide temperature for that year, subsequent years look flat in comparison. In fact, this is exactly why skeptics like to compare current temperatures to 1998: If 1998 had not been an El Niño year, we would be hearing less about how there was no temperature increase since 1998. Using 1998 as the starting point is actually intellectually dishonest.
The following two years, 1999 and 2000, were La Niña years, shown in blue, lowering worldwide temperature. Interestingly, the La Niña years of 2008, 2011 and 2012, by themselves, show an upward trend in world temperature. This upward trend in La Niña temperature is even more notable when compared with 1999 and 2000. Moreover, just because they are normally cooler years than they would otherwise have been, this suggests that the upward trend in temperature over time is especially clear.
In fact, this post shows that the average temperature of each decade since 1970, including the 2001–2010 decade, has been warmer than the prior decade. If temperatures have actually stopped rising, how can this be explained?
More to Learn
This does not resolve all questions about what has been going on with temperature in the past decade. If we start with 2001, temperature does not show much of a trend. Several hypotheses have been put forward for this. One, mentioned above, is that heat is being absorbed by the deep ocean. In any case, whatever the reason, knowing that “pauses” are not unusual should reassure us that our focus on global warming remains warranted.
In fact, this is how science works: Models are never perfect. Anomalies are pointers to those imperfections and then scientists work on the anomalies. At some point, scientists will very likely nail down what’s behind the current “pause” and it will become just another part of the global warming story.