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The PAGES-2k synthesis

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Guest commentary by Darrell Kaufman (N. Arizona U.)

In a major step forward in proxy data synthesis, the PAst Global Changes (PAGES) 2k Consortium has just published a suite of continental scale reconstructions of temperature for the past two millennia in Nature Geoscience. More information about the study and its implications are available at the FAQ on the PAGES website and the datasets themselves are available at NOAA Paleoclimate.

The main conclusion of the study is that the most coherent feature in nearly all of the regional temperature reconstructions is a long-term cooling trend, which ended late in the 19th century, and which was followed by a warming trend in the 20th C. The 20th century in the reconstructions ranks as the warmest or nearly the warmest century in all regions except Antarctica. During the last 30-year period in the reconstructions (1971-2000 CE), the average reconstructed temperature among all of the regions was likely higher than anytime in at least ~1400 years. Interestingly, temperatures did not fluctuate uniformly among all regions at multi-decadal to centennial scales. For example, there were no globally synchronous multi-decadal warm or cold intervals that define a worldwide Medieval Warm Period or Little Ice Age. Cool 30-year periods between the years 830 and 1910 CE were particularly pronounced during times of weak solar activity and strong tropical volcanic eruptions and especially if both phenomena often occurred simultaneously.
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References

  1. . , "Continental-scale temperature variability during the past two millennia", Nature Geoscience, vol. 6, pp. 339-346, 2013. http://dx.doi.org/10.1038/ngeo1797

Response by Marcott et al.

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Readers will be aware of the paper by Shaun Marcott and colleagues, that they published a couple weeks ago in the journal Science. That paper sought to extend the global temperature record back over the entire Holocene period, i.e. just over 11 kyr back time, something that had not really been attempted before. The paper got a fair amount of media coverage (see e.g. this article by Justin Gillis in the New York Times). Since then, a number of accusations from the usual suspects have been leveled against the authors and their study, and most of it is characteristically misleading. We are pleased to provide the authors’ response, below. Our view is that the results of the paper will stand the test of time, particularly regarding the small global temperature variations in the Holocene. If anything, early Holocene warmth might be overestimated in this study.

Update: Tamino has three excellent posts in which he shows why the Holocene reconstruction is very unlikely to be affected by possible discrepancies in the most recent (20th century) part of the record. The figure showing Holocene changes by latitude is particularly informative.

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Urban Heat Islands and U.S. Temperature Trends

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Guest Commentary by Zeke Hausfather and Matthew Menne (NOAA)

The impact of urban heat islands (UHI) on temperature trends has long been a contentious area, with some studies finding no effect of urbanization on large-scale temperature trend and others finding large effects in certain regions. The issue has reached particular prominence on the blogs, with some claiming that the majority of the warming in the U.S. (or even the world) over the past century can be attributed to urbanization. We therefore set out to undertake a thorough examination of UHI in the Conterminous United States (CONUS), examining multiple ‘urban’ proxies, different methods of analysis, and temperature series with differing degrees of homogenization and urban-specific corrections (e.g. the GISTEMP nightlight method; Hansen et al, 2010). The paper reporting our results has just been published in the Journal of Geophysical Research.
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References

  1. D.E. Parker, "Large-scale warming is not urban", Nature, vol. 432, pp. 290-290, 2004. http://dx.doi.org/10.1038/432290a
  2. X. Yang, Y. Hou, and B. Chen, "Observed surface warming induced by urbanization in east China", Journal of Geophysical Research, vol. 116, 2011. http://dx.doi.org/10.1029/2010JD015452
  3. J. Hansen, R. Ruedy, M. Sato, and K. Lo, "GLOBAL SURFACE TEMPERATURE CHANGE", Reviews of Geophysics, vol. 48, 2010. http://dx.doi.org/10.1029/2010RG000345

On Sensitivity Part II: Constraining Cloud Feedback without Cloud Observations

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Guest Commentary by Karen M. Shell, Oregon State University

Link to Part I.

Clouds are very pesky for climate scientists. Due to their high spatial and temporal variability, as well as the many processes involved in cloud droplet formation, clouds are difficult to model. Furthermore, clouds have competing effects on solar and terrestrial radiation. Increases in clouds increase reflected sunlight (a cooling effect) but also increase the greenhouse effect (a warming effect). The net effect of clouds at a given location depends the kind of clouds (stratus, cumulus etc.), their distribution in the vertical and on which radiative effect dominates.

Not only is it difficult to correctly represent clouds in climate models, but estimating how clouds and their radiative effects will change with global warming (i.e., the cloud feedback) is very difficult. Other physical feedbacks have more obvious links between temperature and the climate variable. For example, we expect and have strong evidence for the increase in water vapor in a warmer climate due to the increased saturation specific humidity, or the reduced reflection of sunlight due to the melting of snow and ice at higher temperature. However, there isn’t a simple thermodynamic relationship between temperature and cloud amount, and the complexities in the radiative impacts of clouds mean that an increase in clouds in one location may result in net heating, but would correspond to a cooling elsewhere. Thus, most of the uncertainty in the response of climate models to increases in CO2 is due to the uncertainty of the cloud feedback.

So how cool is it then that the recent paper by Fasullo and Trenberth estimates the net climate sensitivity without getting into the details of the cloud feedback then? Quite cool.
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References

  1. J.T. Fasullo, and K.E. Trenberth, "A Less Cloudy Future: The Role of Subtropical Subsidence in Climate Sensitivity", Science, vol. 338, pp. 792-794, 2012. http://dx.doi.org/10.1126/science.1227465

AGU time again…

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This week is Fall AGU, the biggest climate-related conference around. Not everything is related to climate – there is a lot of other geophysics and astrophysics, but it is generally the place to go if you want to see and be seen (and incidentally, be crushed, be excited, be friendly and be frustrated that you can’t be in three places at once).

If you are not going, you should check out the improvements in the ‘Virtual meeting‘, which will offer live streaming of some big sessions, and for those and many additional sessions there is video-on-demand (VOD) after the fact. Many posters will also be available via ePoster. The twitter hashtag is #AGU12.

And if you are going to be there, here is a limited selection of sessions that will discuss issues that often come up here (more details in the scientific program).

Monday, Dec 03:

PP11F. The Climate of the Common Era I + II
8:00 AM – 12:30 AM; 2010 (Moscone West) (including Kevin Anchukatis, Philip Brohan, Eric, and many others).

12:30 PM – 01:30 PM: San Francisco Marriott Marquis – Salon 10 Brown Bag Lunch Workshop with Michael Gerrard on “Legal Duties to Preserve and Disclose Scientific Data and Personal Communications” (note that anyone who wants a private one-on-one session with a lawyer related to these or related issues, can email lawyer(at)climatesciencedefensefund.org to set up a meeting).

PA13B. Countering Denial and Manufactured Doubt of 21st Century Science
1:40 PM – 3:40 PM; 302 (Moscone South)

6:30 PM – 8:30PM; Open Mike Night hosted by Richard Alley. Jillian’s, 175 4th Street, Suite 1070.

Tuesday, Dec 04:

PA21B. Communication of Science Through Art: A Raison d’Etre for Interdisciplinary Collaboration
8:00 AM – 10:00 AM; 104 (Moscone South) (VOD)

GC22B. Communicating Climate Science—Seeking the Best of Old and New Paradigms
10:20 AM – 12:20 PM; 3014 (Moscone West) (including Mike, Richard Alley, Dan Kahan, Richard Somerville and Naomi Oreskes)

PA23B. PA23B. Facebook, Twitter, Blogs: Science Communication Gone Social—The Social Media 101
1:40 PM – 3:40 PM; 302 (Moscone South) (Including Mike, Michael Tobis, Peter Sinclair, and Zeke Hausfather)

Bloggers Forum: Science on the Web:
5:00 pm – 6:00 pm: 3000 (Moscone West)

Wednesday, Dec 05:

Session Title: A32D. New Atmospheric Sciences Fellows Presentations I + II
8:00 AM – 12:20PM; 3002 (Moscone West) (including Tony DelGenio, Mike, Ron Stouffer, Dave Neelin… ) (VOD)

PP31D. Continental Archives of Past Climate and Seismic Events II
8:00 AM – 10:00 AM; 2006 (Moscone West) (including Ray B. discussing this)

PP32A. Emiliani Lecture
10:20 AM – 11:20 AM; 103 (Moscone South)
“No future without a past” or “History will teach us nothing”? (Invited) Richard E. Zeebe (VOD)

12:30 PM – 1:30 PM; Brown Bag lunch with Pete Fontaine “An inside look at the Michael Mann case”. 226 (Moscone South)

GC33F. Construing Uncertainty in Climate Science
2:40 PM – 3:40 PM; 3003 (Moscone West) (including Naomi Oreskes, Gerard Roe)

GC44B. Links Between Rapid Arctic Change and Midlatitude Weather Patterns
5:00 PM – 6:00 PM; 3001 (Moscone West) (including Steve Vavrus, Judah Cohen)

Thursday, Dec 06:

GC43I. Tyndall History of Global Environmental Change Lecture:
2:40 PM – 3:40 PM; 2022-2024 (Moscone West): “Successful Predictions” (Invited), Raymond Pierrehumbert (VOD)

U44A. Dissolving Boundaries Between Scientists, Media, and the Public
4:00 PM – 6:00 PM; 102 (Moscone South) (Including Eric as speaker and panelist).

Friday, Dec 07:

C54B. The Ice Core Record of Carbon Cycle History and Processes
4:00 PM – 6:00 PM; 3007 (Moscone West)

Feel free to advertise other sessions/talks that might be of interest in the comments. Hopefully we’ll get some reports of interesting sessions to share with you (and if anyone wants to send us anything, we’ll post it up that evening).