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The mystery of the offset chronologies: Tree rings and the volcanic record of the 1st millennium

Filed under: — group @ 19 February 2015

Guest commentary by Jonny McAneney

Volcanism can have an important impact on climate. When a large volcano erupts it can inject vast amounts of dust and sulphur compounds into the stratosphere, where they alter the radiation balance. While the suspended dust can temporarily block sunlight, the dominant effect in volcanic forcing is the sulphur, which combines with water to form sulphuric acid droplets. These stratospheric aerosols dramatically change the reflectivity, and absorption profile of the upper atmosphere, causing the stratosphere to heat, and the surface to cool; resulting in climatic changes on hemispheric and global scales.

Interrogating tree rings and ice cores

Annually-resolved ice core and tree-ring chronologies provide opportunities for understanding past volcanic forcing and the consequent climatic effects and impacts on human populations. It is common knowledge that you can tell the age of a tree by counting its rings, but it is also interesting to note that the size and physiology of each ring provides information on growing conditions when the ring formed. By constructing long tree ring chronologies, using suitable species of trees, it is possible to reconstruct a precisely-dated annual record of climatic conditions.

Ice cores can provide a similar annual record of the chemical and isotopic composition of the atmosphere, in particular volcanic markers such as layers of volcanic acid and tephra. However, ice cores can suffer from ambiguous layers that introduce errors into the dating of these layers of volcanic acid. To short-circuit this, attempts have been made to identify know historical eruptions within the ice records, such as Öraefajökull (1362) and Vesuvius (AD 79). This can become difficult since the ice chronologies can only be checked by finding and definitively identifying tephra (volcanic glass shards) that can be attributed to these key eruptions; sulphate peaks in the ice are not volcano specific.

Thus, it is fundamentally important to have chronological agreement between historical, tree-ring and ice core chronologies: The ice cores record the magnitude and frequency of volcanic eruptions, with the trees recording the climatic response, and historical records evidencing human responses to these events.

But they don’t quite line up…
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It never rains but it pause

There has been a veritable deluge of new papers this month related to recent trends in surface temperature. There are analyses of the CMIP5 ensemble, new model runs, analyses of complementary observational data, attempts at reconciliation all the way to commentaries on how the topic has been covered in the media and on twitter. We will attempt to bring the highlights together here. As background, it is worth reading our previous discussions, along with pieces by Simon Donner and Tamino to help put in context what is being discussed here.

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The global temperature jigsaw

Since 1998 the global temperature has risen more slowly than before. Given the many explanations for colder temperatures discussed in the media and scientific literature (La Niña, heat uptake of the oceans, arctic data gap, etc.) one could jokingly ask why no new ice age is here yet. This fails to recognize, however, that the various ingredients are small and not simply additive. Here is a small overview and attempt to explain how the different pieces of the puzzle fit together.

AR5_temp_obs

Figure 1 The global near-surface temperatures (annual values at the top, decadal means at the bottom) in the three standard data sets HadCRUT4 (black), NOAA (orange) and NASA GISS (light blue). Graph: IPCC 2013. More »

2012 Updates to model-observation comparisons

Time for the 2012 updates!

As has become a habit (2009, 2010, 2011), here is a brief overview and update of some of the most discussed model/observation comparisons, updated to include 2012. I include comparisons of surface temperatures, sea ice and ocean heat content to the CMIP3 and Hansen et al (1988) simulations.
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On sensitivity: Part I

Filed under: — gavin @ 3 January 2013

Climate sensitivity is a perennial topic here, so the multiple new papers and discussions around the issue, each with different perspectives, are worth discussing. Since this can be a complicated topic, I’ll focus in this post on the credible work being published. There’ll be a second part from Karen Shell, and in a follow-on post I’ll comment on some of the recent games being played in and around the Wall Street Journal op-ed pages.

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