Journalist Alexander Cockburn once again questions global warming, asking “Global Warming: Farce, Fraud, or Both?” in a syndicated column that appeared on Christmas day in my local newspaper. I usually agree with Cockburn about politics, but I think he’s seriously mistaken about global warming.1
Cockburn puts forward four arguments:
(1) Researcher Michael Mann produced the famous “hockey stick” graph, which Cockburn thinks is erroneous because it omitted the Medieval Warm Period (800-1300 AD), when it may have been warmer than the 20th century.
(2) In an email, climate researcher Kevin Treberth of the National Center for Atmospheric Research (NCAR) remarked: “The fact is that we can’t account for the lack of warming at the moment and it is a travesty that we can’t.” Cockburn takes this as evidence of a cover-up.
(3) Cockburn thinks that the greenhouse effect violates the Second Law of Thermodynamics.
(4) Cockburn thinks that the surface temperature of the Earth has decreased in the last eight years, proving that global warming has stopped.
Let’s take up these assertions in order.
(1) Michael Mann published the first attempt to reconstruct the global temperature of the Earth over last 1000 years. In their 1998 paper, Mann and co-authors Raymond Bradley and Malcolm Hughes reconstructed the temperature record over the past 1000 years and concluded that the warming in the second half of 20th century is unprecedented in the last six centuries, and perhaps in the last 1000 years.2
Critics charged that the 1998 paper had some subtle flaws in the ways the authors handled statistics. The Mann study was investigated by a special panel of the National Research Council, whose 2006 report concluded that there were shortcomings in the statistics, but that the effect of these shortcomings was small. So the critics were entitled to their quibble, but it made very little difference to the conclusion. In the last decade, at least a dozen other research teams (that did not include Michael Mann) have investigated the same questions, using different statistical methods, and found similar results to those of Mann et al. So Michael Mann and his co-workers got the big idea right.
One common objection to Michael Mann’s work is that the Medieval Warm Period (800-1300 AD) does not appear in the “hockey stick” graph. At one time, researchers wondered if the Medieval Warm Period was a truly global phenomenon or only a regional phenomenon. The current thinking among researchers is that the Medieval Warm Period was a regional (European and nearby areas) effect, not a global effect. As a regional phenomenon, the Medieval Warm Period doesn’t test our understanding of global climate change and is thus only of mild interest. There is no consensus about the temperature during the Medieval period, because the uncertainties are so large.3
Those who believe that the Medieval Warm Period is very important are making the assumption that there is only one factor determining the climate. If you make that assumption, and if the sole factor is burning of fossil fuels, then our understanding of global warming would be challenged, because of course massive burning of fossil fuels did not happen in medieval times. Some six decades ago, many scientists in fact assumed that only one factor influences climate, although they couldn’t agree on what factor, and global climate change was thus not understood. Today we know better: a number of factors influence global climate, including the intensity of the sun, aerosols, the greenhouse effect, Milankovich cycles (changes in the Earth’s orbital motion), volcanic eruptions, and other factors.
(2) Kevin Trenberth, researcher at NCAR, said in an email: “The fact is that we can’t account for the lack of warming at the moment and it is a travesty that we can’t.” Trenberth did not say (or intend to say) that he doubted the manmade character of recent global warming.
Climate scientists understand the long-term upward trend in the surface temperature of the earth: it’s because of global warming, due to the enhanced greenhouse effect arising from increased carbon dioxide in the atmosphere.
What climate scientists do not fully understand are the short-term fluctuations above and below the long-term trend. As part of those fluctuations, energy is transferred between different parts of the earth’s climate system: glaciers, polar ice, the deep ocean, etc. Trenberth asks why the January 2008 temperature was unusually low. “Was it because a lot of heat went into melting Arctic sea ice or parts of Greenland and Antarctica, and other glaciers?” Currently we just can’t say.
To reiterate: the oft-cited email does not mean Trenberth is doubting the reality of global warming or the manmade cause of global warming. Those concern the long-term trend. Instead, Trenberth was bemoaning the lack of accurate measurements of the energy flows that accompany the short-term fluctuations (“natural variability”).
The email by Kevin Trenberth doesn’t say anything that is not also said in a published article by Trenberth, which is available online.4
(3) Cockburn is impressed by a scientific argument, claiming that the greenhouse effect violates the Second Law of Thermodynamics. He relies on a publication by Gerlich and Tscheuschner (GT), “Falsification of the Atmospheric CO2 Greenhouse Effects within the Frame of Physics.”5 However, the greenhouse effect can be easily demonstrated in the laboratory. The BBC broadcast a tabletop demonstration of the greenhouse effect, which can be found at the BBC website (at <news.bbc.co.uk/2/hi/science/nature/8394168.stm>). The video, a little over 2 minutes long, is well worth watching. Physics is an experimental science, and if theory disagrees with experiment, the theory must be flawed.
But beyond noting that the GT theory is refuted by experiment, it is worthwhile examining where GT went wrong. They claim that greenhouse gases in cold upper atmosphere cannot possibly transfer heat to the warmer earth, without violating the Second Law of Thermodynamics.
Let’s be clear about what Second Law of Thermodynamics does and does not say. Suppose that you have two objects at two different temperatures, and suppose that light (visible or infrared) from either object can reach the other object. There will be a flow of heat from the hot object to the cold object and a smaller flow of heat from the cold object to the hot object. There are thus heat flows in both directions: from hot to cold and from cold to hot.
The Second Law says that the flow of heat from hot to cold is greater than the flow of heat from cold to hot. Hence the net flow of heat is from the hot object to the cold object. Note that the existence of a smaller flow of heat from the cold object to the hot object does not refute the Second Law.
At this point, we return to Cockburn’s argument (from GT). Heat flows from the warm earth to the cold atmosphere and also from the cold atmosphere to the warm earth. (Heat also flows from the cold atmosphere to outer space, which is even colder.) The flow of heat from the earth to the atmosphere is greater than the flow of heat from the atmosphere to the earth, so the net flow of heat is from the earth to the atmosphere.
But there is also a (smaller) flow from the atmosphere to the earth. This smaller flow keeps the earth warmer than it would be if there were no greenhouse gases in the atmosphere. This is what the greenhouse effect is all about. On this point, Cockburn has been misled by GT, who have advanced degrees in physics but have made a serious mistake in thermodynamics.
Readers with a background in physics and calculus can read a comprehensive refutation of the GT paper by Arthur P. Smith, “Proof of the Atmospheric Greenhouse Effect.” Smith’s article begins: “The results presented here are not new.” Indeed, they are over a century old and found in standard textbooks. Smith has presented the subject in great detail in order to answer objections raised by GT to the treatment found in standard textbooks.
The greenhouse effect has been known for over a century. The greenhouse effect is quite a big effect: the Earth’s surface is about 59 F warmer than it would be in the absence of greenhouse gases in the atmosphere. The greenhouse effect was entirely natural until the industrial revolution. In the last two centuries, the burning of fossil fuels has added a manmade contribution to the greenhouse effect. It is surprising that the GT paper survived peer review, which is a quality-control policy that makes it harder to publish erroneous papers. Harder, but evidently not impossible.
(4) Finally, Cockburn claims the average temperature has decreased significantly in the last eight years or so. It is important to stress the difference between weather and climate. Weather is short-term behavior, while climate is long-term behavior. Climate is weather, averaged of a period of several decades. The Earth is getting warmer at about 0.2 degrees C per decade, in agreement with models. Any trend over a short time (e.g., only eight years) is weather and not climate. Short-term decreases or increases in temperature are not important for climate; what matters is the long-term trend. The latest data is that the July 2009 sea-surface temperature is the highest sea-surface temperature ever recorded.
Modern climate science explains a great deal of information that would be difficult or impossible to explain in any other way. By way of analogy, consider the central role that evolution plays in biological sciences. Evolution explains a great deal of information that would be difficult or impossible to explain in any other way. Biologists argue about the details of evolution, but not about whether or not evolution is happening at all. Similarly, climate scientists debate the amount and pace of global warming, but very few climate scientists think that global warming is not happening at all.
Of course there still are people who don’t believe in evolution, but (with incredibly rare exceptions) they’re not biologists. The case for modern anthropogenic climate change has become stronger in recent decades, as the science has improved and as the Earth has continued to warm. I predict that, within a decade or two, those who believe that global warming isn’t happening at all will be where the creationists are now.
1 This essay is an update of my 2008 article: John W. Farley, “The Scientific Case for Modern Anthropogenic Global Warming,” Monthly Review, July-August 2008.
2 Michael E. Mann, Raymond S. Bradley, Malcolm K. Hughes, “Global-scale Temperature Patterns and Climate Forcing over the Past Six Centuries,” Nature 392 (1998): 779–787.
3 “Surface Temperature Reconstructions for the Last 2,000 Years,” National Academy Press (2006).
4 Kevin Trenberth, “An Imperative for Climate Change Planning: Tracking Earth’s Global Energy,” Current Opinion in Environmental Sustainability 1 (2009): 19-27.
5 Gerhard Gerlich and Ralf D. Tscheuschner, “Falsification of the Atmospheric CO2 Greenhouse Effects within the Frame of Physics,” International Journal of Modern Physics B 23 (2009): 275-364.
John W. Farley is a professor in the Department of Physics and Astronomy of the University of Nevada, Las Vegas. The opinions expressed are his own. His list of links related to global warming (including rebuttals to frequently raised skeptical arguments) can be found at <www.physics.unlv.edu/~farley/links/Global_Warming.html>.