Emil Reed has written a reply to Brian J. Baker, which addresses some of the scientific issues raised. He claims that Brain J. Baker has given "an extremely misleading picture of the state of knowledge about climate change." And that "instead of making a positive contribution to the understanding of climate change Brain J. Baker repeats tired old arguments."

I am writing this article in response to the publication of an article last week by Brian J. Baker. I was dismayed to see this article on In Defence of Marxism, and that it was apparently considered to be a serious contribution to the debate. The perspective given is not that of a socialist, but that of a conspiracy theorist. On top of this, BJB gives an extremely misleading picture of the state of knowledge about climate change. He also makes innumerable factual errors. For these reasons, I believe that publication of the article was a mistake.

To set out my position, I do not believe in the conspiracy that BJB alleges. I believe that the evidence we currently have strongly indicates that anthropogenic global warming is a reality, that the scientists working to understand the situation are honest, and that there has been no attempt to suppress the views of the "global warming skeptics". On the contrary, they have been given plenty of coverage in the media. However, there are many issues which are as yet poorly understood, and the science of climate forecasting is in its infancy.

This is a crucial issue for the Labour Movement to get to grips with. The continued existence of capitalism poses a serious threat to our environment, and on a capitalist basis it is hard to see how there can be any solution to the problem of carbon dioxide emissions. This is not an issue where we can risk creating confusion. At In Defence of Marxism we should be giving a clear lead, not helping to spread confusion. But in what follows I will confine myself to addressing some of the scientific issues raised by BJB. I hope other people will criticise his method and his political views.

This article will cover BJB's portrayal of climate change over the last millennium and the recent trend in global warming. In a future article I hope to address the issue of sunspots, and BJB's fear that we may be headed for another Ice Age.

1. The Medieval Warm Period

Because we only have good instrumental temperature records going back to 1850, the only way we can establish global temperatures before this time is to use so-called "proxies"—these are indirect ways in which we can measure the historical climate. These include looking at tree rings, annual density bands in corals, glacier lengths, sediment layers in marine environments, using boreholes in soil and ice, etc. This science is called "paleoclimatology", and without it we would not be aware of global warming.

The first issue I will look at is that of the "Medieval Warm Period". Incidentally the section of BJB's article that deals with this is copied verbatim from an article by Ross McKitrick entitled "What is the ‘Hockey Stick' Debate About?" (PDF is available here.) The copying starts at "In 1995 David Deming..." and ends a few paragraphs later with "...cold years of the ‘Little Ice Age'." In fact there is one original contribution of BJB—the 150-year study of Deming has become a 1500-year one!

The first IPCC report, from 1990, included a graph which shows a clearly marked period of warmer temperatures between about 1000-1300 AD, followed by a period of lower temperatures. BJB reproduces this graph, although he incorrectly attributes it to the 1995 report. (See here for details.) BJB contends that in the Medieval Warm Period, the temperatures were "higher than those of today." He also claims that the truth of this has been suppressed by certain people who represent a "politicised science" that endeavours to "eradicate history." In fact "Stalin could have learnt lessons from this brigade."

Let us see if there is any evidence for this conspiracy. First, the graph from the 1990 report was taken from a 1975 report of the United States National Research Council, which had based it on a 1966 diagram from a book of Hubert Lamb. (Source.)

But in 1990, the IPCC report's authors did not have any more up-to-date information, and they included the 1975 graph (on page 202), although it was described as a "schematic diagram". According to the IPCC 2007 report:

"Even in the first IPCC assessment (IPCC, 1990), many climatic variations prior to the instrumental record were not that well known or understood. Fifteen years later, understanding is much improved, more quantitative and better integrated with respect to observations and modelling." (p. 438) (Available online here.)

So BJB takes this 1975 graph, based on 1966 data, included in passing in a 1990 report (as a "schematic diagram", not a presentation of data), and claims that this was the IPCC's view of "World Climate History" in 1995. In my opinion he is being extremely misleading to readers.

BJB says that by the time of the Third Assessment Report, the Medieval Warm Period was dismissed as a localised phenomenon. Incidentally BJB says that this report came out in 1998, and was presented by Michael Mann. In fact it came out in 2001 and Mann was one of dozens of authors and did not have a coordinating role. (See list of authors.)

Actually, there was no conspiracy to cover up the Medieval Warm Period. What happened was that studies found that the warm temperatures of this period were simply not as pronounced as Lamb had suggested, and were not experienced by the whole world. In fact a 2006 report by the US Board on Atmospheric Sciences and Climate (BASC) says:

"Large-scale surface temperature reconstructions yield a generally consistent picture of temperature trends during the preceding millennium, including relatively warm conditions centered around A.D. 1000 (identified by some as the ‘Medieval Warm Period') and a relatively cold period (or ‘Little Ice Age') centered around 1700." (p. 2) (Available online here.)

The 2007 IPCC report says:

"The evidence currently available indicates that the northern hemisphere temperatures during medieval times (950-1100) were indeed warm in a 2000-year context...However, the evidence is not sufficient to support a conclusion that hemispheric mean temperatures were as warm, or the extent of warm regions as expansive, as those in the 20th century as a whole, during any period in medieval times." (p. 469)

Indeed the report concludes:

"Average Northern Hemisphere temperatures during the second half of the 20th century were very likely higher than during any other 50-year period in the last 500 years and likely the highest in at least the past 1300 years." (p. 9)

Now BJB is welcome to believe that the report says this because some nefarious people have decided to rewrite history. But the fact is that this is what the studies done since the early 1990s have shown. The IPCC 2007 report has a temperature chart (p. 467) that presents the results of eleven studies of worldwide temperature since 700 AD. (See here and a similar chart here.) None of them show the medieval warm period that "dwarfed 20th century changes" as BJB claims.

BJB says that Huang et al (1998) is the source for his figure 4. In fact he should have given the source as McKitrick (2005). McKitrick based this diagram on data from Huang et al (1997), not 1998 (Original paper here.) This diagram is actually a zoomed-in section of a larger graph which presents data going back 20,000 years. However, scientists believe that temperature reconstructions from borehole data are only accurate to 500 years. According to the 2006 BASC report:

"The time resolution and length of borehole-based surface temperature reconstructions are severely limited by the physics of the heat transfer process...The time resolution of the reconstruction thus decreases backward in time. For rock and permafrost boreholes, this resolution is a few decades at the start of the 20th century and a few centuries at 1500. Borehole temperatures thus only reveal long-term temperature averages and trends prior to the period of instrumental records; they tell us nothing about decadal variations or specific years, except for the most recent ones. For rock and permafrost boreholes, the thermal smearing is strong enough to prevent recovery of clear temperature signals prior to about 1500." (p. 80)

In fact in 1998 the same authors published another paper where they restrict themselves to looking at the last 500 years. (Available here.) They conclude that "in the 20th century, the average surface temperature of Earth has increased by about 0.5 degrees C and that the 20th century has been the warmest of the past five centuries." They have not referenced their 1997 paper again, and it is not referenced by the IPCC reports.

However, this 1997 paper is enough for BJB to conclude in Part 4 of his article: "Recent temperature profiles, over the past 2000 years have exhibited cooler and warmer periods than are experienced today. Evidence from ground borehole measurements rather than tree data is proof." I think that our readers deserve better than this.

2. The hockey stick

BJB's figure 5 is from the 2001 IPCC report (although it is originally from Mann et al (1999).) It is the now-famous "hockey stick" graph of temperature reconstruction over the last 1000 years. (Original here.) BJB tells us that "two Canadian statistical researchers McIntyre and McKitrick, investigated how the datasets [used in the temperature reconstruction] were arrived at and they found several procedural, data splicing and mathematical errors. When these were corrected the hockey stick vanished."

BJB then goes on to copy verbatim another paragraph from McKitrick (2005). The copying begins at "In Mann's program..." and continues until "...the variance of these series" at which point there is a full stop (period) in the original, but in our version BJB adds on a bit of his own. He also changes "like flat static" into "statically flat".

BJB concludes: "In other words the famous hockey stick was a product of the computer program used to analyse the data and nothing more."

I cannot go into the details of the objections as they are technical in nature. They concern a statistical process called principal component analysis. However, I will point out that other scientists have addressed their criticisms, and there is no evidence of any cover-up whatsoever. The 2007 IPCC report explains how McIntyre and McKitrick (2003) claimed that they could not replicate the results of Mann et al (1998), and how Wahl and Ammann (2007) showed that this was because of a misunderstanding. It also says that McIntyre and McKitrick (2005) raised further objections concerning the inclusion of tree ring data, but that Wahl and Ammann (2006) showed that the effect is insignificant. In total five papers by McIntyre and McKitrick are cited—hardly evidence of a cover-up.

The truth is, that while the criticisms by McIntyre and McKitrick may have some foundation, the results of Mann et al (1998) are in line with those of more recent studies. So there is not a great deal to be gained by arguing about details of Mann et al (1998). In addition Rutherford et al (2005) (Available here. And see commentary here.) showed that the hockey stick is still evident without using principal component analysis. A 2006 report of the National Academy of Sciences says:

"The basic conclusion of Mann et al. (1998, 1999) was that the late 20th century warmth in the Northern Hemisphere was unprecedented during at least the last 1000 years. This conclusion has subsequently been supported by an array of evidence that includes both additional large-scale surface temperature reconstructions and pronounced changes in a variety of local proxy indicators, such as melting on ice caps and the retreat of glaciers around the world." (p.3) (Available online here.)

Although McIntyre is not convinced by the new studies and chooses to believe that "there is a distinct possibility that researchers have either purposefully or subconsciously selected series with the hockey stick shape." (Quoted in New Scientist. Available here.)

3. Has there been any global warming after 1998?

In Part 2 BJB makes the claim that the "oceans and satellite records show no net warming since 1998" while Fred Weston in the introduction quotes the idea that ground records "show no warming whatever since 1998."

Zeke Hausfather from the Yale Forum on Climate Change and the Media comments:

"There are two ground-based global temperature datasets: NASA's GISSTemp and the Met Office Hadley Center's HadCRU...Both GISSTemp and HadCRU show a strong positive warming trend of roughly 0.18 degrees C per decade over the past 32 years. GISSTemp shows that seven of the 10 warmest years on record have occurred in the past decade, and HadCRU shows nine of the ten warmest years in the same period. GISSTemp identifies 2005 as the warmest year on record, with 1998 and 2007 tied for second. HadCRU has 1998 as the warmest year, followed by 2005 and 2003, with 2007 placing sixth." (Source.)

Anyone who doubts the warming trend can consult the data for themselves. (See GISSTemp and HadCRU.) The websites of both agencies have graphs which are extremely informative. Presumably BJB's research did not extend to consulting these.

1998 was a hot year because of a strong El Niño effect. However, when looking at measurements of a natural phenomenon we must look for trends in the data, not isolated spikes or "noise". The blogger tamino comments:

"Noise is the "random" part of the data. It may be measurement error, or a random physical process, or even a deterministic physical process which mimics a random process. But it's there...The right approach is to look for the trends, not the wiggles, and to apply statistical significance testing to determine whether they're real changes in the system or just accidental fluctuations." (Source.)

So what is the trend? According to a recent paper by Fawcett and Jones (2007):

"There is very little justification for asserting that global warming has gone away over the past ten years, not least because the linear trend in globally-averaged annual mean temperatures (the standard yardstick) over the period 1998-2007 remains upward. While 1998 was the world's warmest year in the surface-based instrumental record up to that point in time, 2005 was equally warm and in some data sets surpassed 1998. A substantial contribution to the record warmth of 1998 came from the very strong El Niño of 1997/98 and, when the annual data are adjusted for this short-term effect (to take out El Niño's warming influence), the warming trend is even more obvious." (Source.)

The claim that global warming stopped in 1998 appears to have originated in a 2006 article in the Daily Telegraph by Bob Carter. (Available online here.) So instead of making a positive contribution to the understanding of climate change BJB repeats tired old arguments.

In the introduction a reference is made to the claim that apparently satellite measurements show no global warming since 1978. Well they certainly could not have showed any before 1978, as this was the first year of operation! This is perhaps an attempt to home in on a weak spot because for a long time it was not understood how to correctly interpret the data from the satellites. However, scientists now believe they can, and the data is in agreement with ground-based sources. (See here.)

[To be continued at a later date]