Thursday, 16 February 2012

A 4000-Year History of Greenland Surface Temperature

Reconstructed Greenland snow surface temperatures for the past 4000 years as adapted from Figure 1 of Kobashi et al. (2011). The blue line and blue band represent the reconstructed Greenland temperature and 1σ error, respectively. The green line represents a 100-year moving average of the blue line. The black and red lines indicate the Summit and AWS decadal average temperatures, respectively, as calculated by others.
From CO2 Science:

Reference
Kobashi, T., Kawamura, K., Severinghaus, J.P., Barnola, J.-M., Nakaegawa, T., Vinther, B.M., Johnsen, S.J. and Box, J.E. 2011. High variability of Greenland surface temperature over the past 4000 years estimated from trapped air in an ice core. Geophysical Research Letters 38: 10.1029/2011GL049444. 



Background
The authors write that, supposedly, "Greenland recently incurred record high temperatures and ice loss by melting, adding to concerns that anthropogenic warming is impacting the Greenland ice sheet and in turn accelerating global sea-level rise." However, they state that "it remains imprecisely known for Greenland how much warming is caused by increasing atmospheric greenhouse gases versus natural variability."


What was done
In rigorously exploring this question of recent warmth attribution, Kobashi et al. reconstructed "Greenland surface snow temperature variability over the past 4000 years at the GISP2 site (near the Summit of the Greenland ice sheet; hereafter referred to as Greenland temperature) with a new method that utilizes argon and nitrogen isotopic ratios from occluded air bubbles," as described in detail by Kobashi et al. (2008a,b).

What was learned
The eight researchers report that "the temperature record starts with a colder period in 'the Bronze Age Cold Epoch'," which they say was followed by "a warm period in 'the Bronze Age Optimum'," which was followed by a 1000-year cooling that began "during 'the Iron/Roman Age Optimum'," which was followed by "the Dark Ages," which was followed by "the Medieval Warm Period," which was followed by "the Little Ice Age" - which they describe as "the coldest period of the past 4000 years" - which was followed, last of all, by "the recent warming." For comparative purposes, they also note that "the current decadal average surface temperature at the summit is as warm as in the 1930s-1940s, and there was another similarly warm period in the 1140s (Medieval Warm Period)," indicating that "the present decade is not outside the envelope of variability of the last 1000 years." In fact, they say that "excluding the last millennium," there were fully "72 decades warmer than the present one, in which mean temperatures were 1.0 to 1.5°C warmer," and that during two centennial intervals, average temperatures "were nearly 1.0°C warmer than the present decade" (see the authors Figure 1, adapted below).

What it means
Since the Greenland summit's decadal warmth of the first ten years of the 21st century was exceeded fully six dozen times over the prior four millennia, it is clear that it was in no way unusual, unnatural or unprecedented; and, therefore, it is clear that none of Greenland's recent warming need have been caused by increasing greenhouse gases. Indeed, it is far more likely that its recent warmth is nothing more than the next expected phase of the natural oscillation of climate that has produced several-hundred-year periods of alternating warmth and cold over the past four thousand years.

Corals Dying from Weather-Induced Heating and Cooling ... But Surviving Climate-Induced Heating and Cooling

Image: Wikipedia
From CO2 Science:
n a paper published in Global Change Biology, Kemp et al. (2011) write that "considerable attention has been given to worldwide coral reef decline over the last several years with major emphasis placed on the negative effects of increased seawater temperatures," which typically lead to coral bleaching and subsequent death; but they also note that "imposed low-temperature stress can cause coral bleaching by inducing responses similar to elevated-temperature, including reduction in Symbiodinium cell density and chlorophyll a content, as well as photoinhibition," citing the work of Steen and Muscatine (1987), Saxby et al. (2003),  g and Fine (2004) and Hoegh-Guldberg et al. (2005). And they go on to demonstrate this latter fact via an analysis of coral responses to two closely-spaced cold fronts that caused sudden and severe seawater cooling in February and March of 2010 in the upper Florida (USA) Keys that led to "a mass die-off of reef-building corals," thereby convincingly illustrating that both unusually warm and unusually cold temperatures, such as are caused by fluctuations in weather conditions, are equally adept at killing corals and their algal symbionts.

Over the long term, however, when either warmer or cooler conditions are the result of much slower changes in climate, such need not be the case. Consider, for example, the Little Ice Age (LIA). In a study of the Atlantic Warm Pool (AWP) - which is defined by the >28.5°C isotherm and develops annually in the northern Caribbean during early summer (June) and expands into the Gulf of Mexico and western tropical North Atlantic through the late summer (July-October) - Richey et al. (2009) found that "geochemical proxy records from corals, sclerosponges and foraminifera in the region encompassed by the AWP show a large (2-3°C) cooling during the LIA," citing, in this regard, the work of Winter et al. (2000), Watanabe et al. (2001), Nyberg et al. (2002), Haase-Schramm et al. (2003), Black et al. (2007) and Kilbourne et al. (2008). And in reporting the results of a study of a large brain coral that lived throughout the 17th century on the shallow seafloor off the island of Bermuda, Cohen and Madin (2007) say that although seawater temperatures at that time and location were about 1.5°C colder than it is there today, "the coral grew faster than the corals there now."

Other studies have shown earth's corals to be able to cope with climate-induced warmings as well as coolings. In a study of patch reefs of the Florida Keys, for example, Greenstein et al. (1998) found that Acropora cervicornis corals exhibited "long-term persistence" during both "Pleistocene and Holocene time," the former of which periods exhibited climatic changes of large magnitude, some with significantly greater warmth than currently prevails on earth; and these climate changes had almost no effect on this long-term dominant of Caribbean coral reefs. Hence, there is good reason to not be too concerned about long-term changes in climate possibly harming earth's corals. They apparently have the ability to handle whatever nature may throw at them in this regard.

Sherwood, Keith and Craig Idso