In the endless discussion around global warming the often obscure ways of measuring the past temperature of the earth—necessary to determine if there has been a long extended changing trend towards warming—get lost; and examining these methods may help explain the hesitancy of conservative thinkers to jump into a huge policy hole with tremendous costs for a very long time, without perhaps some more study.
This article looks at those obscure ways.
An excerpt.
“How do we know anything about the Earth’s past climate? Discussions about climate change—its extent, its causes, and what to do about it—often hinge on what we know about our planet’s temperature history. Climate scientists and policymakers routinely talk about the Earth’s “global mean temperature” and compare today’s temperature to a record dating back hundreds of thousands of years. But where does that record come from? And what does it even mean for a single figure to represent the temperature of our entire planet, with its regional diversity and dynamic atmosphere? Scientists have devised ingenious techniques to peer into our planet’s past temperature record, but the picture they give us is a blurry one…
“Perhaps the dominant proxy used to understand past climate is tree rings—a practice called dendroclimatology. By measuring the widths and densities of a tree’s rings, scientists can tell roughly how favorable or unfavorable to growth were the conditions of that tree’s environment in past growing seasons. Temperature is one of the important factors determining how well a tree can grow, so in many cases there is a correlation between the width or density of a specific ring and the local temperature during the growing season corresponding to that ring…
“Of course, there are several sources of proxy data other than tree rings used to reconstruct the Earth’s past temperature—like samples of ice taken from glaciers, which give scientists data reaching much further back in time than the tree rings. Glaciers accumulate when previous snowfalls are crushed into ice from the weight of more recent snowfalls above. Seasonal cycles of temperature and precipitation lead to discernable annual striations. Researchers can drill down from the surface of the glacier to obtain a core sample—a long record of these bands….
“Temperature information from coral data is inferred in a way similar to the technique used with ice cores: relying on the ratio of oxygen-18 to oxygen-16. This can provide sea-surface temperatures to within about 0.3 degrees Celsius over its data range, although that range is limited to a few centuries. Also, the ratio of oxygen isotopes is affected not just by temperature but also by salinity, causing additional uncertainty. Given these uncertainties, coral data are mostly useful as a confirmatory tool.
“Researchers have also found it surprisingly useful to employ historical and cultural events to place bounds on past temperatures in various regions. These can consist of detailed records of produce, newspaper articles about local events (such as the famous account of the annual frost festival held atop the frozen Thames in London), and even landscape paintings showing the extent of glacial advancement. While all of these techniques are helpful in placing bounds on possible temperature values, obviously they are all vastly imprecise and only available for the last few centuries.
“Perhaps the oddest technique used by scientists to determine the Earth’s past temperature is that of “thermal boreholes.” Essentially, a thermometer is placed into a narrow hole in the ground to measure temperature as a function of depth. The resulting signature can be used to reconstruct estimates of the surface temperatures of the past at a resolution of multiple decades. In its report on temperature reconstructions, the National Academy of Sciences explained this technique by comparing it to a metal spoon placed in a cup of hot tea. A spoon has high thermal conductivity, so heat from the tea would quickly travel its length, heating it from end to end. But one can imagine an object that conducts heat much more slowly—an object for which it could take an hour for the heat to move from the end submerged in the tea to the tip of the handle. Continuing with this analogy, the temperature at the surface of the Earth is like the temperature of the tea and the slowly conducting spoon is like the Earth. But the temperature at the surface of the Earth is not constant. This is akin to changing the temperature of the tea over time. When our specially-crafted spoon is initially placed in this hot tea, the submerged end will heat up first and the heat will begin to slowly travel the length of the spoon. But if we then cool the tea, the submerged end will take on this new temperature, which will then follow the earlier heat signal down the length of the spoon. The act of measuring the temperature at various points along the spoon is like that of measuring the temperature at various depths in the borehole. However, these adjacent hot and cold regions will mix, giving scientists only an extremely low-resolution glimpse of past surface temperatures at a few select sites for, at best, the past few hundred years. Boreholes can place very broad bounds on recent local temperatures, but other techniques are much more useful…
“As both the National Academy of Sciences and the U.N.’s Intergovernmental Panel on Climate Change have stated, the proxy techniques discussed here are sufficient to show with high confidence that there has been warming in the last century that is anomalous relative to what would have been expected based upon the natural variations of the geologically recent past—and human greenhouse-gas emissions are at least partly to blame. That said, the uncertainties of these techniques make them grossly insufficient to provide the basis for some of the more extreme claims that have been made. We have reason to be skeptical of both those who design elaborate hypotheses to explain away global warming and those who would have us panic.”
