## Friday, 1 November 2013

### Atmospheric warming hiatus: The peculiar debate about the 2% of the 2%

Dana Nuccitelli recently wrote an article for the Guardian and the introduction read: "The slowed warming is limited to surface temperatures, two percent of overall global warming, and is only temporary". As I have been arguing before, how minute the recent deviation of the predicted warming is, my first response was, good that someone finally computed how small.

However, Dana Nuccitelli followed the line of argumentation of Wotts and argued that the atmosphere is just a small part of the climate system and that you do see the warming continue in the rest, mainly in the oceans. He thus rightly sees focusing on the surface temperatures only as a form of cherry picking. More on that below.

## The atmospheric warming hiatus is a minor deviation

There is another two percent. Just look at the graph below of the global mean temperature since increases in greenhouse gasses became important.

The anomalies of the global mean temperature of the Global Historical Climate Network dataset versions 3 (GHCNv3) of NOAA. The anomalies are computed of the temperature by subtracting the mean temperature from 1880 to 1899.

The temperature increase we have seen since the beginning of 1900 is about 31 degree years (the sum of the anomalies over all years). You can easily compute that this is about right because the triangle below the temperature curve, with a horizontal base of about 100 years and a vertical size (temperature increase) of about 0.8°C: 0.5*100*0.8=40 degree years; the large green triangle in the figure below. For the modest aims of this post 31 and 40 degree years are both fine values.

The hiatus, the temperature deviation the climate ostriches are getting crazy about, has lasted at best 15 years and has a size of about 0.1°C. Thus using the same triangular approach we can compute that this is 0.5*15*0.1=0.75 degree years; this is the small blue triangle in the figure below.

The atmospheric warming hiatus is thus only 100% * 0.75 / 31 = 2.4% of the total warming since 1900. This is naturally just a coarse estimate of the order of magnitude of the effect, almost any value below 5% would be achievable with other reasonable assumptions. I admit having tried a few combinations before getting the nice matching value for the title.

## Warming of the climate system

That is not all, as indicated in the introduction the atmospheric warming itself is just a small part of the warming of the climate system. Thus let's look at Dana Nuccitelli's article before putting it all together. I especially like the introduction:
Many popular climate myths share the trait of vagueness. For example, consider the argument that climate has changed naturally in the past. Well of course it has, but what does that tell us? It's akin to telling a fire investigator that fires have always happened naturally in the past. That would doubtless earn you a puzzled look from the investigator. Is the implication that because they have occurred naturally in the past, humans can't cause fires or climate change
But for this post, the important piece of information is:
One key piece of information that's usually omitted when discussing this subject is that the overall warming of the entire climate system has continued rapidly over the past 15 years, even faster than the 15 years before that. The speed bump [hiatus, vv] only applies to surface temperatures, which only represent about 2 percent of the overall warming of the global climate.
As the figure below from The Guardian shows, most of the energy goes to the heating of the oceans, but also the cryosphere (mainly melting of glaciers, sea ice and ice sheets) and continents take up some heat.

Here one also has to consider that the warming in the last 15 years is minute, likely again a few percent of the total warming since 1900. Observing such small changes is challenging. Fortunately, ocean observation have become a lot more comprehensive lately, which is illustrated by the decrease in the error bars in the figure below. See for example the recent study by Balmaseda et al. (2013), discussed by RealClimate.

Energy accumulation within distinct components of Earth’s climate system from 1971 to 2010. From the 2013 IPCC report.

## Putting it all together

Combining both arguments, you can see that the discussion about the hiatus is a discussion about 2% or 2%, which is 0.0004. Taking into account the uncertainty in the estimates, this means that the discussion is less than a thousandths of the warming.

That is a number that is certainly interesting for some scientists, but I guess you have to be a climate ostrich to think that this is worth at least almost 6 Million webpages. And now one more. You would almost get the impression that this minor deviation would refute the theory that greenhouse gases warm the climate system, or would be on the verge of doing so.

## To refute AWG you need a temperature drop, not just a pause

The consensus opinion in climatology is that the global mean temperature has been increasing since the industrial revolution and that humans are the main cause of this. Even if the atmospheric surface temperature stays at the current high level for several more decades, that statement remains true. The temperature will have to have an unexplained drop towards previous levels in the coming decades before one would conclude that additional CO2 and other greenhouse gasses does not cause warming.

Depending on what the causes of the pause are, it may influence our projections. Let’s see what the science will tell us the coming years. We have some first indications, see the literature below, but finding the cause of such a minor fluctuation will be difficult.

As someone working on the quality of station data, I am naturally wondering whether the increased used of ventilated automatic weather stations could explain part of an explanation of the pause. It would be a large project to find out, unfortunately.

The projections are not part of any consensus. There is much uncertainty there, especially about whether humans will get their act together. And uncertainty can go both ways, I may have to add because many climate ostriches seem the think that uncertainty means nothing will happen. I am sure we will see some surprises, such as the current unexpected rapid decline of the Arctic ice cap or consequences we simply did not think of.

What I am saying is nothing new. I only hope that maybe this way of formulating the question and its quantification is interesting for some. Already the National Geographic wrote:
"Gavin Schmidt, a climate scientist with NASA's Goddard Institute for Space Studies, told National Geographic that focus on a global warming pause over the past 15 years is a "misplaced" distraction that misses the big picture. He said, "The IPCC and the issue of climate change is not about the weather next year or the next five years; it's about the long-term climate change that we are engendering."

## Related information

Global warming has not stopped by Ari Jokimäki at AGW Observer
Ari Jokimäki presents the latest research papers on the warming since 1998 and its variability.
About the Lack of Warming… by John Nielsen-Gammon at the Climate Abyss.
John Nielsen-Gammon created a beautiful plot showing the relationship between global mean surface temperature and El Nino. By giving El Nino and La Nina years a different color and symbol you can see the influence of SOI, without having to "fudge" the data.
Tropical ocean key to global warming ‘hiatus’ by Jeff Tollefson at Nature magazin.
An editorial article in Nature on the Kosaka and Xie paper, which studied the influence of El Nino on the temperature, which could largely explain the atmospheric warming hiatus. By taking into account historical forcings (greenhouse gases, sun, etc.) and El Nino they are able to reproduce the global surface temperature with a of correlation 0.97 (!) since 1970. Impressive!
Learning From the Hiatus
Another great, but technical, post by John Nielsen-Gammon on the Kosaka and Xie paper. The post includes a discussion of the mistakes in the first quick post of Judith Curry at Climate Etc. about the paper.
Does "Global Warming Pause" Debate Miss Big Picture? by Brian Clark Howard at the National Geographic.
A well written longer piece on the surface warming hiatus. Gotta love the subtle hint: "Yet some public scepticism has persisted, especially in the United States."
Does the global warming 'pause' mean what you think it means? by Dana Nuccitelli at The Guardian.
The original article on the 2 percent.
Climate Science: Connecting the Hiatuses by Bill Chameides at The Scientific American
Discusses possible causes for the current hiatus on the basis of previous ones.

## Scientific articles on the "hiatus" in atmospheric warming*

Foster, G. and S. Rahmstorf, 2011: Global temperature evolution 1979–2010. Environ. Res. Lett., 6, no. 044022, doi: 10.1088/1748-9326/6/4/044022.

Fyfe, J.C., N.P. Gillett, F.W. Zwiers, 2013: Overestimated global warming over the past 20 years. Nature Climate Change, 3, pp. 767–769, doi: 10.1038/nclimate1972.

Guemas, V., F.J. Doblas-Reyes, I. Andreu-Burillo, et al., 2013: Retrospective prediction of the global warming slowdown in the past decade. Nature Climate Change, 3, no. 7, pp. 649-653, doi: 10.1038/NCLIMATE1863.

Hunt, B.G., 2011: The role of natural climatic variation in perturbing the observed global mean temperature trend. Climate dynamics, 36, no. 3-4, pp. 509-521, doi: 10.1007/s00382-010-0799-x.

Kaufmann, R.K., H. Kauppi, M.L. Mann, et al., 2011: Reconciling anthropogenic climate change with observed temperature 1998-2008. PNAS, 108, no. 29, pp. 11790-11793, doi: 10.1073/pnas.1102467108.

Kosaka, Y., S.-P. Xie, 2013: Recent global-warming hiatus tied to equatorial Pacific surface cooling. Nature, 501, no. 7467, pp. 403-, doi: 10.1038/nature12534.

Lean, J.L., D.H. Rind, 2009: How will Earth’s surface temperature change in future decades?
Geophysical Research Letters, 36, no. 15, doi: 10.1029/2009GL038932.

Meehl, G.A., J.M. Arblaster, J.T. Fasullo, et al., 2011: Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods. Nature Climate Change, 1, no. 7, pp. 360-364, doi: 10.1038/NCLIMATE1229.

Meehl, G.A., A. Hu, J.M. Arblaster, et al., 2013: Externally Forced and Internally Generated Decadal Climate Variability Associated with the Interdecadal Pacific Oscillation. J. Climate, 26, no. 18, pp. 7298-7310, doi: 10.1175/JCLI-D-12-00548.1.

Meehl, G.A., H. Teng, 2012: Case studies for initialized decadal hindcasts and predictions for the Pacific region. Geophys. Res. Lett., 39, art. no. L22705, doi: 10.1029/2012GL053423.

Santer, B.D. , C. Mears, C. Doutriaux, P. Caldwell, P.J. Gleckler, T.M.L. Wigley, S. Solomon, N.P. Gillett, D. Ivanova, T.R. Karl, J.R. Lanzante, G.A. Meehl, P.A. Stott, K.E. Taylor, P.W. Thorne, M.F. Wehner, F.J. Wentz, 2011: Separating signal and noise in atmospheric temperature changes: The importance of timescale. Journal of Geophysical Research: Atmospheres (1984–2012), 116, no. D22, doi: 10.1029/2011JD016263.

Susan Solomon, Karen H. Rosenlof, Robert W. Portmann, John S. Daniel, Sean M. Davis, Todd J. Sanford, Gian-Kasper Plattner, 2010: Contributions of Stratospheric Water Vapor to Decadal Changes in the Rate of Global Warming. Science, 327, no. 5970, pp. 1219-1223, doi: 10.1126/science.1182488.

Toth, L.T., R.B. Aronson, S.V. Vollmer, et al., 2012: ENSO Drove 2500-Year Collapse of Eastern Pacific Coral Reefs. Science, 337, no. 6090, pp. 81-84, doi: 10.1126/science.1221168.

Trenberth, K.E., 2009: An imperative for climate change planning: tracking Earth’s global energy. Current Opinion in Environmental Sustainability, 1, no. 1, pp. 19–27, doi: 10.1016/j.cosust.2009.06.001.

Watanabe, M., Y. Kamae, M. Yoshimori, et al., 2013: Strengthening of ocean heat uptake efficiency associated with the recent climate hiatus. Geophys. Res. Lett., 40, no. 12, pp. 3175-3179, doi: 10.1002/grl.50541.

* Extended list using the list of AGW observer.

## References

Balmaseda, M.A., K.E. Trenberth, and E. Källén, 2013: Distinctive climate signals in reanalysis of global ocean heat content. Geophysical Research letters, 40, 1–6, doi: 10.1002/grl.50382.

Daneel Olivaw said...

I get the idea but wouldn't an indefinitely long period of absolutely no warming represent a triangle with exactly zero area and thus be 0%?

Victor Venema said...

For the coming or the last decades, an average temperature increase of about 0.15°C per decade is projected (IPCC AR5, SPM, page 15).

Thus, if the observed temperature trend would be zero, the deviation between observation and projection would increase by about 0.15°C per decade.

Thus, if the temperature had been flat for the last 15 years (1.5 decades), you could compute a deviation of 1.5*0.15=0.225°C.

With that number the small blue triangle be about 5% of the green triangle. However, 1998 was an extreme outlier, if you just make the period one year longer or shorter, you do see a trend and the deviation would be much less. I would say 0.1°C is a fair estimate. However you compute it, the deviation is minute.

Maybe I should not have drawn the base of the blue triangle horizontally, but that makes it easier to understand the computation of the area.

Daneel Olivaw said...

Thanks! I didn't realize you used proyections as the height of the triangle.

Rachel Martin said...

Great post, Victor and very interesting way of looking at the "hiatus". Contrarians make a huge deal out of it but this is partly because the word "hiatus" appears in the scientific literature. I think in many cases though it's a case of not understanding what they're reading. My uncle Bob's have pointed out examples of climate scientists who use this word as an argument that climate change might not be happening after all.

Victor Venema said...

I think that the term hiatus probably stems from the climate ostriches. I found a 2008 post at WUWT on it.

On the Web of Science I could find 10 articles on the topic, starting in 2011. They are now added to the post. I might have missed some, but these scientific articles seems to be from a later date. (If someone knows an article I have missed, I am happy to add it.)

As the term global warming hiatus is not wrong, I see no reason why science should have invented a more appropriate word.

My uncle Bob's have pointed out examples of climate scientists who use this word as an argument that climate change might not be happening after all.

That would be a very weird argument. Did your uncle Bob's give references? Still scientists are also humans. Individual scientists can make trivially wrong statements. Some of them even signed the petition of Cornwall Alliance.

Rachel Martin said...

You say that "the term global warming hiatus is not wrong" but it does imply that global warming has stopped don't you think? When really it is just that the rise in surface temperature has slowed rather than completely stopped while the ice is still melting and the water still heating up. I really don't like the word hiatus when it is used to describe global warming. I didn't realise that climate scientists had stolen the word from fake skeptics. I don't think that was the smartest thing to do.

Victor Venema said...

I had intended to write "atmospheric warming hiatus", not "global warming hiatus".

Still even in that case you have a point, that there is no evidence that the atmospheric warming has actually stopped. The uncertainty over such a climatologically short period is so large that we cannot distinguish between the expected temperature increase of 0.15 degrees per decade and no warming.

Maybe these colleagues just did not think about the right word that much, like I did, they might not spend as much time on the front as you do.

Or maybe they like the press attention that it gives, which is good for your self-esteem, career and the funding of your field. Anthony Watts likes to claim that my job is to proof that the IPCC is right. It is the opposite. You have to do something new and interesting to get published. If you can show that there is a major problem in your field, that means additional funding.

If you can proof any of the denier memes, that makes a great article. The limitation is that scientists cannot sprout nonsense like WUWT and Co. It would not get published in a good journal and it would hurt your reputation. The proof does need to be solid. Choosing a controversial, but not clearly wrong, therm like hiatus is a good compromise.

Wotts Up With That Blog said...

Victor, very interesting post. I'd never thought of it in this way.