What Do We Really Know About Climate Change?

A Guest Post by Basil Copeland


Like many of Anthony’s readers here on WUWT, I’ve been riveted by all the revelations and ongoing discussion and analysis of the CRUtape Letters™ (with appropriate props to WUWT’s “ctm”). It might be hard to imagine that anyone could add to what has already been said, but I am going to try. It might also come as a surprise, to those who reckon me for a skeptic, that I do not think that anything was revealed that suggests that the global temperature data set maintained by CRU was irreparably damaged by these revelations. We’ve known all along that the data may be biased by poor siting issues, handling of station dropout, or inadequate treatment of UHI effects. But nothing was revealed that suggests that the global temperature data sets are completely bogus, or unreliable.

I will return to the figure at the top of this post below, but I want to introduce another figure to illustrate the previous assertion:


This figure plots smoothed seasonal differences (year to year differences in monthly anomalies) for the four major global temperature data sets: HadCRUT, GISS, UAH and RSS. With the exception of the starting months of the satellite era (UAH and RSS), and to a lesser degree the starting months of GISS, there is remarkable agreement between the four data sets – where they overlap – especially with respect to the cyclical pattern of natural climate variation. This coherence gives me confidence that while there may be problems with the land-sea data sets, they accurately reflect the general course of natural climate variation over the period for which we have instrumental data. While we need to continue to insist upon open access to the data and methods used to chronicle global and regional climate variation, and refine the process to remove the biases which may be present from trying to make the data fit the narrative of CO2 induced global warming, it would be wrong to conclude that the “CRUtape Letters” prove that global warming does not exist. That has never really been the issue. The issue has been the extent of warming (have the data been distorted in a way that would overstate the degree of warming?), the extent to which it is the result of natural climate variation (as opposed to human influences), and the extent to which it owes to human influences other than the burning of fossil fuels (such as land use/land cover changes, urban heat islands, etc.). And flowing from this, the issue has been whether we really know enough to justify the kind of massive government programs said to be necessary to forestall climate catastrophe.

Figure 2 plots the composite smooth against the backdrop of the monthly seasonal differences of the four global temperature data sets:

Many readers may recognize the familiar episodes of warming and cooling associated with ENSO and volcanic activity in the preceding figure. With a little more smoothing, we get a pattern like that depicted in Figure 3, which other readers may notice looks a lot like the cycles that Anthony and I have attributed to lunar and solar influences (they are the same):

In either case, the thing to note is that over time climate goes through repetitive episodes of warming and cooling. You have to look closely on Figures 2 and 3 – it is much clearer in Figure 1 – but episodes of warming exist when the smooth is above zero, and cooling episodes exist when the smooth is below zero. Remember, by design, the smooth is not a plot of the temperature itself, but of the trend in the temperature, i.e. the year to year change in monthly temperatures. The intent is to demonstrate and delineate the range of natural climate variation in global temperatures. It shows, in effect, the trend in the trend – up and down over time, with natural regularity, while perhaps also trending generally upward over time.

Which brings us to Figure 1. Here we are focusing in on the last 30 years, and a forecast to 2050 derived by a simple linear regression through the (composite) smooth of Figure 3. (Standard errors have been adjusted for serial correlation.) There has been an upward trend in the global temperature trend, and when this is projected out to 2050, the average is 0.114°C per decade ± 0.440°C per decade. Yes, you read that right: ± 0.440°C per decade. Broad enough to include both the worst imaginations of the IPCC and the CRU crowd, as well as negative growth rates, i.e. global cooling. Because if the truth be told, natural climate variation is so – well, variable – that no one can say with any kind of certainty what the future holds with respect to climate change. Be skeptical of any statistical claims to the contrary.

I think we can say, however, with reasonable certainty, that earth’s climate will remain variable, and that this will frustrate the effort to blame climate change on CO2 induced AGW. Noted on the image at the top of this post is a quote from Kevin Trenberth from the CRUtape Letters™: “The fact is that we cannot account for the lack of warmth at the moment, and it is a travesty that we can’t.” Trenberth betrays a subtle bias here – he cannot acknowledge the recent period of global cooling. It is, rather, “a lack of warmth.” But he is right that it is a “travesty” that we cannot fully account for the ebb and flow of earth’s energy balance, and ultimately, climate change. I think Trenberth just sees it as a lack of monitoring methods or devices. But I think there still remains a considerable lack of knowledge, or understanding, about the mechanics of natural climate variation. If you look carefully at Figure 1, you will notice that there seem to be upper and lower limits to the range of natural climate variability. On the scale depicted in Figure 1 (the scale is different with other degrees of smoothing), when warming reaches a limit of approximately 0.08-0.10°C per year, the warming slows down, and eventually a period of cooling takes place, always with the space of just a few years. Homeostasis, anyone? While phenomenon like ENSO are the effect of this regularity in natural climate variation, they are not the cause of it.

In my opinion, what is the real travesty of the global warming ideology is the hijacking of climate science in the service of a research agenda that has prevented science from investigating the full range of natural climate variation, because that would be an inconvenient truth. We see this, quite clearly, in the CRUtape Letters™ where the Medieval Warm Period is just “putative,” and a rather inconvenient truth that needs to be suppressed. Or the “1940’s blip” that implies that global temperatures increased just as rapidly in the early part of the 20th Century, as they did at the end of the 20th Century, an inconvenient truth at odds with the narrative preferred by the IPCC.

It is a truism that “climate varies on all time scales.” With respect to the variability demonstrated here, I’m convinced that someday it will be acknowledged that variability on this scale is dominated by lunar and solar influences. On longer scales, such as the ebb and flow from the Medieval Warm Period, through the Little Ice Age, and now into the “Modern Warm Period,” I do not think climate science yet has any real understanding of the underlying causes of such climate change. If we are, as seems possible, on the verge of a Dalton or Maunder type minimum in solar activity, we may eventually have an answer to whether solar activity can account for centennial scale changes in earth’s climate. And I do think it is reasonable to conclude, at the margin, that human activity has had some influence. It is hard to imagine population growing from one to six billion over the past one and a half centuries without some effect. Most likely, the effect is on local and regional scales, but this might add up to a discernible impact on global temperature. But until all of the forces that determine the full range of natural climate variability are understood better than they are now, there is no scientific justification for the massive overhaul of economic and government structures being promoted under the guise of climate change, or global warming.