A reader writes:
Your reader with the climatologist friend is simply wrong about the correlation of tree-rings to temperature. Hot/wet does not necessarily correspond to greater growth, and cold/dry to lesser growth. Most trees have an optimum temperature to which they have adapted. This means that as temperatures rise, growth increases, until it reaches the optimal temperature. Past that point, tree growth actually slows. The reverse is also the case. Cooling can increase growth, if the temperature is falling from a higher than optimal state. Similar patterns in relation to precipitation can occur as well.
And there's many more things that affect tree growth than temperature and precipitation. There's sunlight availability, meaning cloudiness, and local conditions of other trees being taller or shorter than the trees being studied. Trees that start in the shade will increase growth when older trees start to die and expose it to more sunlight. This can give a false signal of rising temperatures when nothing of the kind is the case. There's also nutrient supply, disease, etc.
In short, tree-rings are very poor proxies for temperature, and they do not correlate well with many other proxies over long periods of time. The problem is that these scientists like to cherry pick the proxies that give them the answers they want, and they ignore the rest. This is even true within the tree-ring studies themselves. They have been caught using individual trees which support their theory, and throwing out tree rings, even within the same study, that do not. This is an issue not just skeptics but the mainstream of science has been pointing out for years. This is why even the IPCC has stated that tree rings are not reliable for temperature reconstructions going back more than 2-3 centuries, when they can no longer be correlated with instrumental measurements. The Climategate scandal is simply bringing wider attention to the kinds of shortcuts that CRU has been engaged in for years, and making it clear that their reconstructions of past temperatures are reliable beyond 200-300 years ago.
This matters because one of the crucial questions about the current warming is whether it is natural and within the bounds of normal climate variation, or whether it is so unusual as to force us to rationally conclude it could only be the result of rising greenhouse gas levels. To that end, they have tried to recreate past temperatures such as to wash out any past variations, and to make the recent warming stand out as unprecedented. Skeptics who have reviewed their computer modeling have shown that even putting pure random noise data into their models will produce a hockey-stick graph 99% of the time. It's hard to pretend that this is unbiased science at work. No temperature proxies can be considered completely accurate, but tree-rings are near the bottom of the list, and making them the centerpiece of the temperature record is simply scientific malpractice.
For just one example of an alternative temperature proxy, look at this attached graph of Greenland's temperatures, taken from ice cores going back hundreds of thousands of years, based on the ratio of the isotopes of oxygen in the air trapped in the ice (which is also used to measure past CO2 levels), which is considered a much more reliable proxy for temperature than tree-rings. As you can see, Greenland's temperature has been higher than it is at present for most of the last 10,000 years. Previous warming periods, such as the Medeival warm period, the Roman warm period, and others previous to that, have been warmer than our own by as much as 3C.
There are many temperature proxies that demonstrate significant temperature changes around the world over the last 10,000 years which suggest that our current warming is not out of the ordinary at all. CRU and the AGW crowd has tended to downplay and ignore this data, and use various tricks and political actions to discount it, because it weakens their case dramatically. And this is even aside from the charge that they've manipulated the current temperature data in order to make the current warming seem even more dramatic than it actually is. Overall, the more attention one gives to the AGW theory, the more fragile it begins to seem. And this doesn't even bring up the question of the reliability of the climate models used to suggest catastrophic temperature future temperature increases due to man-made greenhouse gases are examined.
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