Or: Journalists should report what climate science actually “says”, rather than what they mistakenly “believe” it to say – Part II
In Part I we looked at some issues relating to climate science that the Houston Chronicle’s “SciGuy” Eric Berger was mistaken about and had blamed “climate scientists” for. And while pointing out that it isn’t particularly fair for Mr. Berger to blame climate scientists for his misunderstandings, it would also be unfair to say that his confusion was his fault alone.
Fred Pearce wrote a recent column for New Scientist claiming climate modeler Mojib Latif predicted that up to two decades of cooling were coming: “We could be about to enter one or even two decades of cooler temperatures, according to one of the world’s top climate modellers.” Pearce’s claim was promptly picked up by the denialosphere and has been cited by “skeptics” as well as those who believe climate science is undergoing some sort of shake up, like Mr. Berger. Pearce’s story is greatly misleading both in terms of what Latif actually said and the role climate scientists believe natural variability plays in the climate system.
First a bit of background: Pearce’s story was written about a recent climate summit: the World Climate Conference-3. Part of the summit was dedicated to Advancing Climate Prediction Science; Latif’s presentation was concerned with decadal-scale climate predictions- concerning not only their potential value and viability but also the significant challenges that remain before we can make useful ones.
On interannual (more than a single year) and decadal (tens of years) scales, natural variability swamps the long term anthropogenic warming trend. That is to say that variations in naturally occurring aspects of the climate system have more of an impact on the ultimate value of, say, global average temperature over a span of 10 or so years than man-made global warming does. For example, changes in ENSO are one of the largest sources of natural variability and thus influence on global average temperatures in the climate system on interannual scales. In 1998, a very strong El Niño boosted the global average temperature much higher than the overall trend, while in 2008, a persistent La Niña in cahoots with a solar minimum ensured that temperature was in the top 10 (#9 for NASA, #10 for Met Hadley) hottest years on record, but not a record breaker.
While this might be surprising for some readers, let’s be clear: This is not “new” information. This does not represent a “shake up” of the climate science community’s understanding of the system, or a blow to “settled science”. This is acknowledged in the IPCC’s most recent Assessment Report (AR4 WG1 8.3 and 9.4) as well as in the relevant primary literature. For example, the AR4 Synthesis Report states:
On scales [smaller than 50 years], natural climate variability is relatively larger [than human influences], making it harder to distinguish changes expected due to external [e.g.man-made] forcings.
Latif begins the section of his presentation misrepresented by Pearce by confirming that the media incorrectly believes that global warming is monotonic- something that we know the warming is decidedly not; something not claimed by “climate science” or “climate scientists”. Significant natural variability is superimposed on the long term man-made warming trend. Although the press might expect for us to set a new temperature record every year, the existence of natural variability means that we could in theory wait a long time (~17 years) before setting a new temperature record. Latif imagines ‘what if’:
It may well happen that you enter a decade, or maybe even two- you know- when the temperature cools- alright- relative to the present level- alright?
And then- you know- I know what’s going to happen -you know? I will get- you know- millions of phone calls- you know:
“Eh, what’s going on? So, is global warming disappearing?” You know? “Have you lied on [sic] us?”
So- you kn0w- and therefore this is the reason why we need to address this decadal prediction issue.
[ed. note: "entering... two [decades]” depending on the usage can take as little as 11 years, “enter[ing]” a decade” as little as one]
This was not an explicit prediction by Latif- it was a hypothetical scenario that is a real, if not necessarily likely, possibility. Latif is saying that because people don’t understand that global warming isn’t supposed to be monotonic, and that there could be periods where temperatures pause or even dip below the present, the media and/or public will incorrectly believe that global warming has stopped/was wrong/etc. even though such “pauses” in warming are decidedly not contrary to our understanding of the climate system and how we anticipate it will respond to emissions driven warming.
Of course this is like cat nip to the denialists and their fellow travelers like Roger Pielke Jr. It feeds into the caricature, enabled by sloppy journalism, that nearly everything can happen because of global warming [often phrased, "Global warming, is there anything it can't do?" Sometimes with 'global warming' stricken out and replaced with 'climate change'].
Latif goes on to describe a number of phenomena that have an overall trend but are dominated on the interannual and even decadal scales by natural variability: Sahel rainfall, Atlantic tropical cyclones, regional sea levels. Again, none of this is new, none of it was presented as new. This represents no paradigm shift within climate science.
Latif then switches gears to model initialization. When the IPCC offers projections of global temperature change into the next 100 years, these are not predictions- as previously discussed. And dealing with interannual or decadal predictions instead of looking at the changes to temperature trends 100 years out is a difference between an initial value problem and a boundary value (or in Latif’s words, a “boundary force”) problem. Uncertainties about emissions scenarios (how much carbon we decide to burn) and model biases are the dominant areas of uncertainty for end-of-century projections of changes of how temperature will trend.
However, on much shorter scales, such as interannual or decadal scales, can you guess what the largest source of uncertainty becomes? Yep, that’s right, natural variability. Prediction on such short timescales then becomes at least partially an initial value problem. Latif rightly understands that such short term predictions depend on accurate understanding and modeling of initialization factors like variance in the North Atlantic Oscillation. You might remember when a team he was part of made some waves in predicting a temporary pause in warming/global cooling in their attempt to initialize a climate model to make a deliberate prediction (rather than say an end-of-century projection) of temperature for the next few decades. Suffice it to say that not everyone has found the basis of their prediction (of no immediate warming) particularly compelling.
Latif’s warning, garbled though it became regarding the reality and difficulty in predicting natural variability, deserves to be acknowledged. It’s exceedingly difficult for me to see, however, how or why the presentation was subsequently spun in the manner that it was, or why science journalists like Mr. Berger would accept said spin so uncritically.
Pearce’s article gives the false impression that there is a “new” or “growing” dissent from the broad strokes consensus on climate change. This couldn’t be further from the truth. I appreciate Pearce’s concern (that the existence of natural variability can embolden denialists), but it sounds like this concern has caused him to unnecessarily and inaccurately frame Latif’s presentation as a challenge to the scientific consensus on climate change. Natural variability is of course real. It can and will overwhelm man-made warming on shorter timescales. That journalists are beginning to pay attention to this simple fact is not a reflection of a sea change in our understanding of climate science.
Latif’s presentation and audio [LATE UPDATE: The audio has moved, it's now here under "Advancing Climate Prediction Science"; the presentation is available here] are available for anyone to examine. We can look at Mr. Berger and others’ claims about hurricanes/tropical cyclones and anthropogenic warming in a Part III if there is interest.
[Fixed some spelling errors and reworded the penultimate paragraph for clarity]