Paleo climate

The Most Common Skeptical Arguments About Climate Change (from the Grist articles: How to Talk to a Climate Skeptic – see site for images)


It was warmer during the Holocene Climatic Optimum

Objection: It was warmer during the Holocene Climatic Optimum than it is today — without any human influence.

Answer: Though some temperatures during that period were in the same range as today, they were confined to the northern hemisphere and the summer months.

What’s more, the cause is understood (orbital forcing similar to what controlled the Ice Ages), just as today’s cause is understood (CO2 emissions), and these causes are very different. NOAA has a page on this that contains the following quote:

In summary, the mid-Holocene, roughly 6,000 years ago, was generally warmer than today, but only in summer and only in the northern hemisphere. More over, we clearly know the cause of this natural warming, and know without doubt that this proven “astronomical” climate forcing mechanism cannot be responsible for the warming over the last 100 years.

As an aside, it’s worth noting that even if the Holocene had been as warm as or warmer than today, it would do nothing to undermine the theories and data that indicate today’s warming is rapid and anthropogenic.


The medieval warm period was just as warm as today

Objection: It was just as warm in the Medieval Warm Period (MWP) as it is today. In fact, Greenland was green and they were growing grapes in England!

Answer: There is no good evidence that the MWP was a globally warm period comparable to today. Regionally, there may have been places that exhibited notable warmth — Europe, for example — but all global proxy reconstructions agree it is warmer now, and the temperature is rising faster now, than at any time in the last one or even two thousand years.

Anecdotal evidence of wineries in England and Norse farmers in Greenland do not amount to a global assessment.

On its website, NOAA has a wide selection of proxy studies, accompanied by the data on which they are based. Specifically, they have this to say on the MWP:

The idea of a global or hemispheric “Medieval Warm Period” that was warmer than today, however, has turned out to be incorrect.

With regard to the “grapes used to grow in England” bit, here is some fairly solid evidence that grapes are in fact growing there now, denialist talking points aside. If that is not enough, RealClimate has a remarkably in-depth review of the history of wine in Great Britain, and how reliable it is as a proxy for global temperatures. (Hint: not very.)


The hockey stick is broken

Objection: The Hockey Stick graph — the foundation of global warming theory — has been shown to be scientifically invalid, perhaps even a fraud.

Answer: The first order of business here is to correct the mischaracterization of this single paleoclimate study as the “foundation” of global warming theory.

What’s going on today is understood via study of today’s data and today’s best scientific theories. Reconstructions of past temperatures are about, well, the past. Study of the past can be informative for scientists, but it is not explanatory of the present nor is it predictive of the future. The scientific foundation of global warming theory contains much more than a few tree-rings and the temperature during the Medieval Warm Period.

RealClimate has an interesting article about what it would mean for today’s climate theories if the MWP had indeed been warmer than today.

Now, about that pesky bit of sporting equipment …

The infamous “Hockey Stick” graph was featured prominently in the IPCC TAR Summary for Policymakers. It was important in that it cast serious doubt on the notion both of a global Medieval Warm Period warmer than the 20th century and of a global Little Ice Age, both long-time (cautiously) accepted features of the last 1,000 years of climate history. It seems these periods were regional, not globally synchronized — though the LIA seems to have been more widely experienced.

This caused quite an uproar in the skeptic community, not least because of its visual efficacy. Two Canadians, an economist and a petroleum geologist, took it upon themselves to verify this proxy reconstruction by getting the data and examining the methodology for themselves. They found errors in the description published in Nature of the data used — errors that prevented them from duplicating the study. Mann et al., the hockey stick’s creators, published a correction in Nature, noting where the description did not match what had actually been done. The Canadians, McIntyre and McKitrick, then published a paper purporting to uncover serious methodological flaws and problems with data sets used.

Everything from this point on is hotly disputed and highly technical.

All the claims made by M&M have been rebutted in detail by many other climatologists; M&M insist they are completely in error. All of it fits nicely with the expectations of both sides of the global warming issue, both the conspiracy theorists and the champions of peer review.

The rebuttals have been objected to and the objections denied and the denials rejected. The specific issues are highly technical and require considerable time and energy to fully understand. Steve McIntyre has a website devoted to his continued probe of this study and Michael Mann is a contributor to RealClimate, which consumes considerable web space refuting his attacks.

In short, M&M raise many specific and technical objections, and climate scientists seem pretty unified in denying the charges. To my knowledge, the worst indictment from the climate science community came from a study led by Hans Von Storch that concluded M&M was right about a particular criticism of methodology, but that correcting it did not change the study results.

If you want to evaluate the issue for yourself, and do it fairly, you must read the copious material at the sites mentioned above. You must also be prepared to dig into dendrochronology and statistical analysis.

Where does that leave the rest of us — you know, the ones with lives?

I confess immediately that the technical issues are over my head. I don’t know PCA from R^2 from a hole in the ground. But the most critical point to remember, if you are concerned about this for its impact on the validity of AGW theory, is that the fight is over a single study, published eight years ago, focused on paleoclimate. It verges on historical minutia. If you feel the study may be tainted, simply discard it.

The fact is, there are dozens of other temperature reconstructions. They tend to show more variability than the original hockey stick (their sticks are not as straight), but they all support the general conclusions the IPCC TAR presented in 2001: late 20th century warming is anomalous in the last one or two thousand years, and the 1990s were likely warmer than any other time in that period.

Here’s a superimposition of numerous global, hemispheric, and regional temperature reconstructions for the last 2,000 years, together with an average. References can be found at the bottom of this Global Warming Art link. Regional variations are of course greater than global, so don’t be surprised by how wavy some of the lines are.

(Disclosure: one of the reconstructions used in that page is by the same team that did the infamous hockey stick — but it is not the same study. To the best of my knowledge, M&M have claimed no problems with that one, though they have expressed some concerns that span the entire field of dendrochronology).

Does the 20th century stand out?

Recently the National Academy of Science in the U.S. did a report on the hockey stick study and found it “plausible,” though more uncertain the farther back in time it went. But then, true to form for this debate, another report commissioned by another Senate committee came out right afterwards and condemned it. Sigh.

I have read as much about this controversy as I ever intend to, and come to the firm conviction that I don’t have the technical background and/or time required to make a scientific judgment on the issue one way or another. I suspect 95% of the people arguing about this have chosen their position ideologically and won’t be able to explain the merits of the various arguments.

So while in my mind MBH are in no way guilty of fraud or incompetence (many of the accusations do go that far), judgment of their research must be approached in reverse: given reason to doubt, I will reject it until it is proven to me that the criticisms are invalid. I can’t decide for myself until I devote the required time to both the statistical background and the technical details of M&M vs MBH98. That isn’t going to happen!

So where does that leave me and (I suspect) most of you?

Well, it leaves me with dozens of other proxy reconstructions, some by the same team or involving some of its members, some by completely different people, some using tree rings, some using corals, some using stalagmites, some using borehole measurements — all supporting the same general conclusion. That general conclusion is what’s important to me, not whether or not one Bristlecone pine was or was not included correctly in a single eight-year-old study.

The general conclusion is:

Although each of the temperature reconstructions are different (due to differing calibration methods and data used), they all show some similar patterns of temperature change over the last several centuries. Most striking is the fact that each record reveals that the 20th century is the warmest of the entire record, and that warming was most dramatic after 1920.

End of story.

To conclude where I started: study of the past can be informative for scientists, but it is not explanatory of the present, nor is it predictive of the future. The science of global warming is rooted in what we know about today.

Now, can we all get off of the hockey rink and back into the lab?


Vineland was full of grapes

Objection: Newfoundland was so warm in the Medieval Warm Period that when the Vikings landed they called it Vineland and brought boatloads of grapes back to Europe.

Answer: Once again: you can’t draw conclusions about global climate from an anecdote about a single region, or even a few regions. You need detailed analysis of proxy climate indicators from around the world. These proxy reconstructions have shown that the Medieval Warm Period (around the time the Vikings are said to have discovered North America) was not as pronounced or as warm as today’s warmth. From NOAA’s paleoclimate website comes these quotes:

What records that do exist show that there was no multi-century periods when global or hemispheric temperatures were the same or warmer than in the 20th century. In summary, it appears that the 20th century, and in particular the late 20th century, is likely the warmest the Earth has seen in at least 1200 years.

As for the specific anecdote that Vineland was a warm land where grapes grew wild: as with the Greenland story, Vineland’s name was most likely a kind of marketing ploy. This 1988 article by Robert McGhee for Canadian Geographic had this to say:

There has been much argument over the location of Vinland, with scholars and local enthusiasts placing it anywhere between Labrador and Florida, and even in the Great Lakes or the Mississippi Valley. The geographical descriptions in the Norse sagas are too vague to allow certain placement on a modern map, but there is growing consensus that they best fit Newfoundland and Labrador (formerly Newfoundland). The main problem with a Newfoundland and Labrador (formerly Newfoundland) site is the absence of wild grapes. Still, there is a strong suspicion that what Leif found were only berries, and that he followed the practice of his father in “giving a land a good name so that men would want to go there”.

(That comes from this page, which has lots of good resources on the Viking expansion.)

So, grapes growing in Newfoundland: paleoclimate’s smoking gun, or medieval marketing ploy? You decide.


We are just recovering from the LIA

Objection: Today’s warming is just a recovery from the Little Ice Age.

Answer: This argument relies on an implicit assumption that there is a particular climatic baseline to which the earth inexorably returns — and thus that a period of globally lower temperatures will inevitably be followed by a rise in temperatures. What is the scientific basis for that assumption?

There is no evidence of such a baseline. The climate is influenced by many factors, which change or remain stable in their own ways. The current understanding of the Little Ice Age is that it was likely the result of a decrease in solar irradiance combined with an increase in volcanic activity, blocking additional sunlight. The LIA was also not particularly well synchronized globally, affecting different regions at different times. Scientists are aware of no century-scale pattern in solar output or volcanic activity, so there’s no reason to expect a reversal of those changes. As it happens, solar output did increase somewhat in the early 20th century, which did contribute to warming at that time. However, that’s not behind current warming.

Another problem with appealing to a natural recovery from the LIA is that temperature has now risen to levels higher than the assumed baseline climate. So even if some recovery were to be expected, why have we now exceeded it? This argument has problems similar to the more general “it is part of a natural cycle” argument.


Global warming is nothing new!

Objection: Global warming has been going on for the last 20,000 years.

Answer: It is true that 20,000 years ago the temperature was some 8 to 10° C colder than it is today. But to draw a line from that point to today and say, “look, 20K years of global warming!” is dubious and arbitrary at best.

If you have look at this graph of temperature, starting at a point when we were finishing the climb out of deep glaciation, you can clearly see that rapid warming ceased around 10,000 years ago (rapid relative to natural fluctuations, but not compared to the warming today, which is an order of magnitude faster). After a final little lift 8,000 years ago, temperature trended downward for the entire period of the Holocene. So the post-industrial revolution warming is the reversal of a many-thousand-year trend.

A closer look at today’s trend, within the context of the last 1,000 and 2,000 years, makes it even clearer that today’s trend is striking — opposite to what one would expect without anthropogenic interference.

If you really want to play the “global warming started X years ago” game, you should talk about how we’re reversing a 5-million-year cooling trend — or go crazy and track global temperatures right back to the origins of the planet! Not that there’d be much point …


Climate is always changing

Objection: Climate has always changed. Why are we worried now, and why does it have to be humans’ fault?

Answer: Yes, climate has varied in the past, for many different reasons, some better understood than others. Present-day climate change is well understood, and different. Noting that something happened before without humans does not demonstrate that humans are not causing it today.

For example, we see in ice core records from Antarctica and Greenland that the world cycled in and out of glacial periods over 120Kyr cycles. That climate cycle’s timing is fairly well understood to be caused by changes in the orbit of the earth, though the mechanism behind the response has not been conclusively established. These orbital cycles are regular and predictable and they are definitely not the cause of today’s warming. The other important difference between the glacial-interglacial cycles and today is the rapidity of the current change. The rate of warming is on the order of 10 times faster today than in the ice cores.

Such rapid warming on a global scale is quite rare in the geological record, and while it may not be entirely unprecedented, there is strong evidence that whenever such a change has happened, whatever the cause, it was a catastrophic event for the biosphere.


Historically, CO2 never caused temperature change

Objection: In the geological record, it is clear that CO2 does not trigger climate changes. Why should it be any different now?

Answer: Given the fact that human industrialization is unique in the history of planet earth, do we really need historical precedent for CO2-triggered climate change before we accept what we observe today? Surely it is not far-fetched that unprecedented consequences would follow from unprecedented events.

But putting this crucial point aside, history does indeed provide some relevant insights and dire warnings.

During the glacial/interglacial cycles, temperatures and CO2 concentrations showed remarkable correlation. Closer examination reveals that CO2 does not lead the temperature changes, but lags by many centuries. Even so, the full extent of the warming can not be explained without the effects of CO2. Though these cycles do not demonstrate that greenhouse gas initiated warming, they do lend credence to the importance of CO2 and CH4 in setting the planetary thermostat.

There are also events in geological history when sharp rises in temperature were initiated and driven by large spikes in greenhouse gases — not unlike the fossil-fuel-emissions spike today. The Paleocene Eocene Thermal Maximum is such a case. Roughly 55 million years ago, ocean pH levels dropped drastically and global temperatures rapidly rose over 5oC. The resolution of available proxy records indicates that this occurred in a period of time no longer than 5K years; it’s not possible to know if it happened even faster. The likely cause was massive releases of methane from the ocean floors, perhaps due to some smaller warming or changes in sea level. It took over 100K years for the ocean, atmosphere, and temperatures to return to their previous state. The result was a mass extinction event that took millions of years to recover from.

We can also look at the formation of the Deccan Traps. In this case, a massive and sustained volcanic action altered atmospheric chemistry and caused a drastic climate change, one that lead to the extinction of the dinosaurs. And Snowball Earth theories involve the build-up of greenhouse gases as the mechanism by which the earth eventually escaped its frozen state.

In short, it is simply untrue that history lacks precedent for greenhouse-gas-driven warming. The precedents are there, as are the dire warnings.


If we can’t understand the past, how can we understand the present?

Objection: Climate science can’t even fully explain why the climate did what it did in the past. How can they claim to know what is going on today?

Answer: There are two requirements for understanding what happened at a particular point of climate change in geological history. One is an internally consistent theory based on physical principles; the other is sufficient data to determine the physical properties involved.

It is extremely hard, in some cases impossible, to gather sufficient data about every aspect of the climate system for periods of time or events in the distant past — and especially to do so at a temporal resolution adequate for a full and nuanced explanation. The record in the ice cores is extraordinarily rich in variety and detail, but it only goes back as far as the age of the ice sheets — even less, as there is melting from the bottom even as there is accumulation at the top. Past that time, about a million years ago in the Antarctic, records must come from ocean sediment, rock layers, fossils, and other imaginative sources. These are harder to decipher and are much coarser in temporal resolution. The spatial extent of samples is often far from sufficient to get global information.

In contrast, today we are closely monitoring everything we can think of. We are much more able to generate quality reconstructions of the recent past for those factors we did not think of until now. We know how the sun is behaving. We know when and how hard the volcanoes are erupting. We know the atmospheric levels of ozone, CO2, CH4, NO2, etc. to a high degree of precision, and on a month to month basis, across the globe. We know where the continents are, how the oceans are flowing and the size of the ice sheets.

Consequently, our understanding of what is going on today is leaps and bounds ahead of what we possess with regard to any point in the past. It is no surprise at all that we can speak with much greater confidence about today than we can about yesterday. The real test the past can provide is new and exciting data that can’t be explained by current theories — every scientist’s Holy Grail. This happens frequently in the finer details of climate theory, but the basics only become more and more certain the more and more data we recover.

So far, although we are far from that elusive Perfect Understanding ™ for the reasons described above, there is also no known climate change in the earth’s past that provides substantial contradiction of the theories that underpin anthropogenic global warming.