A great post, with tons of neat graphs.
To Tell the Truth: Will the Real Global Average Temperature Trend Please Rise? Part I
A guest post by Basil Copeland
Everybody talks about the weather, but rarely has a scientific debate engaged the public as have concerns about climate change and and anthropogenic global warming. It is a scientific issue or debate that everyone can have an “informed” opinion about just by going outside, or by thinking about how climate has changed in their lifetime. If they cannot understand the physics of GCM’s (global climate models) they can read a thermometer and opine whether it is getting colder or warmer “than it used to be.” Few scientific issues or debates are as reducible to an everyday metric — a thermometer reading — as the debate over global warming.
The experts merely fan the fires when they issue press releases about how this year or that is the warmest since whenever, or that the earth’s temperature is rising at X degrees per decade and is likely to continue to rise Y to Z degrees for the rest of the century. The truth is that taking the earth’s temperature is no easy task. Some would argue that it is not even possible to speak of a global temperature as such, e.g. that climate is regional, not global. Others, such as the host of this blog, have drawn attention to serious questions about the accuracy of the station records on which estimates of global average temperatures are frequently based.
Then there are the stat geeks, like myself, who understand how hard it is to accurately or meaningfully measure the “average” of anything! It begs reciting the old saw about a statistician being someone who can stand around with one foot in a bucket of boiling water, and the other foot in a bucket of ice water, and say that “on the average” they feel fine.
…However you look at the data, since 2001 the “trend” in all four metrics has been either flat, or negative. There has been no “global warming” since 2001, and if anything, there has been “global cooling.” But is it “statistically significant?” I imagine that one could fit some simple trend lines through the data in Figure 3 and show that the trend is negative. I would also imagine that given the variability in the data, the trends might not be “statistically significant.” But since statistical significance is often measured by reference to zero, that would be just another way of saying that there has been no statistically significant warming since 2001.
But that may not be the most insightful way to look at the data, or frame the issue. Prior to 2001 we have a much longer series of data in which there has likely been a positive trend, or “global warming.” What can we say, if anything, about how the period since 2001 compares to the period before it? Rather than test whether the trends since 2001 are significantly different than zero, why not test whether the trends since 2001 are significantly different than the trends in the 23 years that proceeded 2002?
