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Instrumental Record

Historical climatology in Greenland

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Gavin Schmidt & Michael Mann

Extending the instrumental record of climate beyond the late 19th Century when many of the national weather centers were first started is an important, difficult and undervalued task. It often is more akin to historical detective work than to climatology and can involve long searches in dusty archives, the ability to read archaic scripts and handwriting, and even Latin translations (for instance, when going through the archives of the Paris Observatory) (sounds like a recent bestseller, only less lucrative, no?).
[Read more…] about Historical climatology in Greenland

NOAA: Hurricane forecasts

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Guest commentary from Thomas Crowley

NOAA has issued its annual forecast for the hurricane season, along with its now-standard explanation that there is a natural cycle of multidecadal (40-60 year) length in the North Atlantic circulation (often referred to as the “Atlantic Multidecadal Oscillation”–see Figure), that is varying the frequency of Atlantic tropical cyclones, and that the present high level of activity is due to a concurrent positive peak in this oscillation. [Read more…] about NOAA: Hurricane forecasts

On a Weakening of the Walker Circulation

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by Ray Pierrehumbert and Rasmus Benestad

Second article of our 3-part series on atmospheric circulation and global warming

In Part I we outlined some general features of the tropical circulation, and discussed ways in which increases in anthropogenic greenhouse gases might affect El Niño. Now we take up the question of how global warming might affect the quasi-steady east-west overturning circulation known as the Walker Circulation. The Walker circulation affects convection and precipitation patterns, the easterly Trade Winds, oceanic upwelling and ocean biological productivity; hence, changes in this circulation can have far-reaching consequences. It also provides the background state against which El Niño events take place, and so changes in the Walker circulation should form an intrinsic part of thinking about how global warming will affect El Niño. In a paper that recently appeared in Nature, Vecchi, Soden, Wittenberg, Held, Leetmaa and Harrison present intriguing new results which suggest that there has already been a weakening of the Walker circulation in the past century, and that the observed changes are consistent with those expected as a response to increases in anthropogenic greenhouse gases. The discussion in Vecchi et al. also raises some very interesting issues regarding the way the hydrological cycle might change in a warming world.

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Significant Warming of the Antarctic Winter Troposphere

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The “iconic” Antarctic temperature trends are the large warming seen on the Antarctic Peninsula, which has had various repercussions including the collapse of several ice shelves (some documented in a previous post). Elsewhere, though, the pattern of surface warming is more complex – trends are smaller, and while more are positive than negative they are generally not significant – see this map. Contrary to what you might have heard, this is in general agreement with model predictions.

But meanwhile, there is a record for the upper atmosphere derived from radiosondes, which we have been working on – finding old datasets and digitising them to fill in gaps. What this shows is that around East Antarctica there is a general warming of the troposphere, greatest at around mid-height (at 600 hPa) at 0.7 ºC/decade over the last 30+ years.

In itself, this is an interesting observation. The obvious question is, what does it mean? Is this natural variability; is it a response to global warming, or to changes in ozone; or something else? Ozone is unlikely, because this is winter (which conveniently means that the radiosonde temperature corrections, often a source of potential trouble, are not a problem). Two ways of trying to interpret the record are to see what GCMs run for the same period show; or to look at the re-analyses (essentially, the archived outputs from the weather-prediction models). The latter, of course, incorporate many of the radiosonde observations that we are using, and so don’t count as independant. Despite this, the ECMWF re-analyses show *greater* trends than we see in the observations; and a maximum trend over West Antarctica (which has no radisonde stations to allow us to verify this). A climate model (HadCM3, with an ensemble of four members) shows similar patterns to the observations, but this time too little warming; and a good deal of variation between the ensemble members. So neither of these is helps much with the interpretation.

So we are currently left with an open question; hopefully, this will stimulate us and other researchers to explain it in the future.

[Quick addendum: the paper itself is available via this.]

Meteors, Nuclear Tests and Global Warming

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What is one to make of a recent press release and submitted preprint blaming global warming on the Tunguska meteor event in 1908? Well, although it is not unknown for impact events to affect climate (the K/T boundary event springs to mind) there are a number of hurdles for any such theory to overcome before it moves into the mainstream from the wilder shores of unsubstantiated speculation.

Firstly, one would anticipate that immediate effects of the impact on climate would be strongest near the time of the impact (allowing for some inertia in the system) and decay away subsequently. Secondly, the timescales for any mechanism associated with the impact (in this case disruption of the atmopsheric water vapour) would need to be in line with the change one hopes to explain. And thirdly, one has to show that this explanation is better than the alternatives. Unfortunately, none of of these requirements are met by this hypothesis. [Read more…] about Meteors, Nuclear Tests and Global Warming

The Greenland Ice

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by Rasmus Benestad, Eric Steig and Gavin Schmidt

In a recent paper in Science, Eric Rignot and Pannir Kanagaratnam present new satellite observations of the speed of glaciers of Greenland, and find that they are sliding towards the sea almost twice as fast as previously thought. Additionally, between 1996 and 2005, they detected a widespread glacier acceleration and consequently an increased rate of ice discharge from the Greenland ice sheet. However, previous papers have recently noted an increase in snow accumulation in the interior (i.e. Johannessen et al., 2005), so how do these different measurements fit into the larger picture of Greenland’s net mass balance?
[Read more…] about The Greenland Ice

Cloudy outlook for albedo?

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Guest Commentary by George Tselioudis (NASA GISS)

In the past few years several attempts have been made to assess changes in the Earth’s planetary albedo, and claims of global dimming and more recently brightening have been debated in journal articles and blogs alike. In a recent article entitled “Can the Earth’s Albedo and Surface Temperatures Increase Together,” that appeared in EOS, Enric Palle and co-authors use recently released cloud data from the International Satellite Cloud Climatology Project (ISCCP) to explain how it is possible for the Earth to be warming even as it’s albedo is increasing. The need for an explanation arises from the author’s claim that the earth’s albedo has increased since the year 2000, an increase that was not followed by a decrease in surface temperature. They base this claim on Earthshine data (a measurement of the glow of the dark side of the moon that they use to deduce the earth’s reflectance) and on an albedo proxy derived from ISCCP parameters after they are regressed with two years of overlapping, but not global, earthshine observations. Subsequently they claim that the rising reflectance of the Earth has not led to a reversal of global warming because the difference between low and middle-plus-high ISCCP clouds has increased in the last four years. This they say implies that as the low-level, cooling clouds have decreased during the most recent years, the high-level, warming clouds have increased even more negating any potential cloud-induced cooling.

There are several issues connected to the use of earthshine data to calculate the earth’s albedo that have been discussed in peer-reviewed publications and that I will not discuss in this posting. I will say a few things, however, about the selective use of ISCCP data in this article to construct qualitative arguments that do not stand up to detailed quantitative analysis . [Read more…] about Cloudy outlook for albedo?

A New Take on an Old Millennium

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The subject of reconstructions of temperature variations of the past millennium has been discussed many times before on this site (see e.g. here, here, here, and here). Despite the apparent controversy, the basic conclusion–that the global and hemispheric-scale warmth of the past few decades appears anomalous in a very long-term context–has stood up remarkably well in many independent studies (see Figure 1).

Temperatures over the Past 1000 Years
Figure 1. Reconstructions of Northern Hemisphere temperatures for the last 1000 years (various colored curves) compared with instrumental record (black curve).
 [source: Wikipedia] (click to enlarge)

This is not to say that all estimates agree in their details. Indeed, there is a fair scatter among the various published estimates. [Read more…] about A New Take on an Old Millennium

2005 temperatures

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Translations: (Français)

Due to a historical quirk (of unknown origin), the World Meterological Organisation releases its summary for each year based on the Dec to Nov ‘meteorlogical year’ means (rather than the more usual calendar year). Anyway, the WMO summary is now available, as is the NASA GISS analysis and the CRU summary. The point upon which all the analyses agree is that 2005 was exceptionally warm and that it continues the long term mean warming trend. All show record warmth in the Northern Hemisphere since 1860, while GISS gives 2005 as the warmest year globally as well (CRU/WMO have it second after 1998). As the summaries indicate, the differences in ranking are on the order of a few hundredths of a degree (smaller than the accuracy of the analysis) and so a definitive ranking is not possible. Differences in how the separate analyses deal with missing data are responsible for most of the apparent variations. Note too that the convention for the base periods for the anomalies differ between the analyses (1961-1990 for CRU/WMO, 1951-1980 for GISS), but this does not affect the rankings.

Update 7pm: The GISS analysis curiously appears to have gone off line….
Update 8am 16 Dec: The GISS summation is still not back up, but the raw data and new figures do seem to be available http://data.giss.nasa.gov/gistemp . Note that as pointed in comment #5, the WMO/CRU/Hadley Centre analysis is for Jan-Nov, and not for the met. year as stated above (though the GISS analysis is). Don’t ask us why!
Final Update 11pm 16 Dec: The GISS analysis is back!

(traduit par T. de Garidel)

More satellite stuff

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The November 17th issue of Science has an interesting exchange of letters between Christy and Spencer; Mears and Wentz; and Sherwood and Lanzante (ref here; subs required for substance). The context of this discussion is the tropospheric temperature record; see Et tu LT and The tropical lapse rate quandary for two RC posts that discuss the issue, and in particular three papers in the August 11th issue of Science.

[Read more…] about More satellite stuff