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AGU, Climate Scientists and the Global Warming Hiatus

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Like 22,000 other earth scientists, I headed to San Francisco last week for the annual American Geophysical Union (AGU) fall meeting, eager for stimulating science learning and discourse. One of the questions at the forefront of my mind (besides where to get the best Indian food) was: What's the latest on the global warming pause?

I know I couldn't have been the only one asking this. So, after filling my schedule with growing number of climate literacy sessions, I searched the program directory for the term "hiatus."("Plateau" and "pause" are also used to describe global temperature trends of the last 15 years, but related terms like "Tibetan Plateau" and "tropopause" made them less likely candidates.) Guess how many results came back? One. In a meeting of literally thousands of presentations, many of them devoted to climate, only one came back that outright addressed the current hiatus in global warming.

I had expected at least a full session of a dozen or so talks or posters dedicated to this subject. That there was no such session gives a hint of the dilemma that climate scientists find themselves faced with.

The lone hiatus talk was given by Dr. Gerard Meehl of NCAR. To say it was well-attended would be an understatement. (See picture.) Although Dr. Meehl is a well-known, reputable scientist in his own right, capable of drawing a crowd (the AGU type of crowd, at least), there was clearly a demand from the science community for more information on this topic.

Through a series of model runs, Meehl showed that a 15-year pause in climate change was fully replicable through internal variability of the climate system alone. In other words, there's randomness in the system that can put climate change on pause and there's nothing we can do about it. The following talk by Dr. Piers Forster of the University of Leeds addressed another possible cause: a relatively weak sun over much of the decade 2000-2010 as well as a series of small, but still significant volcanic eruptions. According to Forster, these changes in forcing (factors that drive climate) could account for half of the hiatus.

What's going on here? Natural variations, the sun, volcanoes or a combination of all three? The list of possible causes for the hiatus, as discussed in Box 9.2 of the recently released IPCC Working Group I (page 26), reads somewhat like scientists grasping at straws:
• Sun
• Volcanoes
• Natural variability
• Heat going into the deep ocean
• Not enough temperature sensors in the Arctic
• Decreased stratospheric water vapor
• Aerosols

From a climate communications perspective, this does not look good. Why aren't scientists doing a better job at talking about their lack of understanding about this problem? And why isn't the media doing a better job covering it? A quick Google search of "global warming hiatus" reveals mainly strident articles either on the one side attempting to expose the "global warming hoax" (no real science there), or on the other side, attempting to debunking the "myth" of a pause at all. (It's not a myth -- according to estimates of global temperatures, temperatures have increased only 0.04 degrees Celsius per decade over this time period, which really isn't statistically significant from zero.)

The climate science community has been put on the defensive here, trying to explain a situation that none of the models predicted. And at the same time, they're doing their darndest to get to the bottom of the whole issue.

So, what's to be done? Maybe it's helpful to recollect what the job of science is at its most basic: to understand and explain the world around us. It's a daunting task. The earth system is incredibly complex and it almost NEVER behaves the way you expect it to. And climate models are still just numbers run through a (really big) computer -- not the same thing as reality. Even as a former scientist myself, it's easy to forget just how darn messy data can be when you only read the write-ups in the media. There is still a lot we have to learn about our climate system, especially over short timescales. As one IPCC modeler said to me at AGU, "We're just really bad at timescales between 3 weeks and 20 years."

Although there's a lot we don't know, that's how the process of science works. We'll get there eventually. Models are fully capable of replicating the temperature trends of the 20th century and this gives us confidence that in the long-term, they can make accurate predictions of average temperatures in the next 100 years. That doesn't mean they can reproduce every bump and dip along the way, though. (Check out this walking the dog analogy for a great illustration.)

Does this mean ACE and all other organizations like us should pack up our bags and go home? No. Why? Because the underlying physics remains the same:

• Burning fossil fuels makes CO2. CO2 emits heat. Heat warms things up.

• Our world has already gotten warmer and will continue to do so in the decades and centuries to come.

• Despite pitfalls in the near-term, the picture of human-caused warming is at its clearest at the 50 to 100-year scale, as a long-term average emerges.

• Climate change poses a serious threat to humanity, one that is ultimately not mitigated by the current hiatus. The proof of this will continue to manifest itself in the coming years and decades.

It's a regrettable twist of fate that the climate system operates on its own timescales with patterns emerging only over multiple decades without regard to human deadlines of electoral or news cycles.

Unfortunately, the physics just doesn't care.

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