From time to time, there is discussion about whether the recent warming trend is due just to chance. We have heard arguments that so-called ‘random walk‘ can produce similar hikes in temperature (any reason why the global mean temperature should behave like the displacement of a molecule in Brownian motion?). The latest in this category of discussions was provided by Cohn and Lins (2005), who in essence pitch statistics against physics. They observe that tests for trends are sensitive to the expectations, or the choice of the null-hypothesis .
Greenhouse gases
Natural Variability and Climate Sensitivity
One of the central tasks of climate science is to predict the sensitivity of climate to changes in carbon dioxide concentration. The answer determines in large measure how serious the consequences of global warming will be. One common measure of climate sensitivity is the amount by which global mean surface temperature would change once the system has settled into a new equilibrium following a doubling of the pre-industrial CO2 concentration. A vast array of thought has been brought to bear on this problem, beginning with Arrhenius’ simple energy balance calculation, continuing through Manabe’s one-dimensional radiative-convective models in the 1960’s, and culminating in today’s comprehensive atmosphere-ocean general circulation models. The current crop of models studied by the IPCC range from an equilibrium sensitivity of about 1.5°C at the low end to about 5°C at the high end. Differences in cloud feedbacks remain the principal source of uncertainty. There is no guarantee that the high end represents the worst case, or that the low end represents the most optimistic case. While there is at present no compelling reason to doubt the models’ handling of water vapor feedback, it is not out of the question that some unanticipated behavior of the hydrological cycle could make the warming somewhat milder — or on the other hand, much, much worse. Thus, the question naturally arises as to whether one can use information from past climates to check which models have the most correct climate sensitivity.
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Methane hydrates and global warming
There is an enormous amount of methane (CH4) on earth frozen into a type of ice called methane hydrate. Hydrates can form with almost any gas and consist of a ‘cage’ of water molecules surrounding the gas. (The term ‘clathrate’ more generally describes solids consisting of gases are trapped within any kind of cage while hydrate is the specific term for when the cage is made of water molecules). There are CO2 hydrates on Mars, while on Earth most of the hydrates are filled with methane. Most of these are in sediments of the ocean, but some are associated with permafrost soils.
Methane hydrates would seem intuitively to be the most precarious of things. Methane hydrate melts if it gets too warm, and it floats in water. Methane is a powerful greenhouse gas, and it degrades to CO2, another greenhouse gas which accumulates in the atmosphere just as fossil fuel CO2 does. And there is a lot of it, possibly more than the traditional fossil fuel deposits. Conceivably, climate changes could affect these deposits. So what do we know of the disaster-movie potential of the methane hydrates?
Debate over the Early Anthropogenic Hypothesis
There have been a few mentions of the ‘early anthropocene’ hypothesis recently (cf. the EPICA CO2 results, and Strange Bedfellows). We therefore welcome Bill Ruddiman to RealClimate to present his viewpoint and hopefully stimulate further discussion – gavin.
[Addendum: For a non-technical backgrounder on the ‘early anthropocene’ hypothesis and its significance in the context of anthropogenic climate change, see Bill Ruddiman’s article “How Did Humans First Alter Global Climate?” from the March 2005 issue of “Scientific American” (first two paragraphs available for free; full article must be purchased). -mike]
Guest posting from Bill Ruddiman, University of Virginia
The hypothesis (Ruddiman, 2003) that early agriculture caused large enough emissions of greenhouse gases millennia ago to offset a natural climatic cooling remains controversial. The centerpiece of the hypothesis was a comparison of the increases of CO2 and CH4 values in Vostok ice during the current (Holocene) interglaciation versus the (natural) drops during similar portions of the three previous interglaciations. [Read more…] about Debate over the Early Anthropogenic Hypothesis
Greenhouse gases help seasonal predictions
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) The conventional wisdom in meteorology has been that certain factors such as the complete oceanic state and the exact concentrations of greenhouse gases are of minor importance for a normal weather forecast. Moreover, whereas sea surface temperatures (SST) are important, the deep sea temperatures are believed to have little impact for predictions for the next few days. The reason is that the ocean reacts slowly to changes in the atmosphere (has much higher inertia and much higher heat capacity). Hence, the most important information needed for such a weather forecast is the atmospheric initial conditions, a description of what the atmosphere and the SST look like when the weather model starts computing the weather evolution.
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650,000 years of greenhouse gas concentrations
) () The latest results from the EPICA core in Antarctica have just been published this week in Science (Siegenthaler et al. and Spahni et al.). This ice core extended the record of Antarctic climate back to maybe 800,000 years, and the first 650,000 years of ice have now been analysed for greenhouse gas concentrations saved in tiny bubbles. The records for CO2, CH4 and N2O both confirm the Vostok records that have been available for a few years now, and extend them over another 4 glacial-interglacial cycles. This is a landmark result and a strong testament to the almost heroic efforts in the field to bring back these samples from over 3km deep in the Antarctica ice. So what do these new data tell us, and where might they lead? [Read more…] about 650,000 years of greenhouse gas concentrations
Busy Week for Water Vapor
It’s been a busy week for water vapor, and I have two recent papers to discuss. The first is the paper "Anthropogenic greenhouse forcing and strong water vapor feedback increase temperature in Europe" by Rolf Philipona et al. (GRL, 2005, subscription required for full text), which has attracted a certain amount of media attention. The overall goal of the paper is to understand, from a physical standpoint, why European temperatures have been increasing three times faster than the Northern Hemisphere average. It focuses on the changes between 1995 and 2002, over which time good surface radiation budget observations are available. The paper reports some results on the role of large scale circulation changes (which they conclude are minor) but I’ll concentrate on the results relating to water vapor.
What is a first-order climate forcing?
Roger Pielke Sr. (Colorado State) has a blog (Climate Science) that gives his personal perspective on climate change issues. In it, he has made clear that he feels that apart from greenhouse gases, other climate forcings (the changes that affect the energy balance of the planet) are being neglected in the scientific discussion. Specifically, he feels that many of these other forcings have sufficient ‘first-order’ effects to prevent a clear attribution of recent climate change to greenhouse gases.
In general, I heartily agree – other forcings are important, even essential, for understanding observed climate variability and, as a community, we are only just starting to get to grips with some of the more complicated effects. Obviously, though, not all forcings are of the same magnitude (either globally or regionally) and so it is useful to separate the ‘first-order’ forcings from those that are relatively minor. But what exactly is ‘first-order’ and what is not? [Read more…] about What is a first-order climate forcing?
Climate sensitivity and aerosol forcings
In a new review paper in Nature this week, Andreae, Jones and Cox expand on the idea that uncertainty in climate sensitivity is directly related to uncertainty in present day aerosol forcing (see also this New Scientist commentary). This was discussed here a couple of months back in the Global Dimming and the climateprediction.net posts, and so it is worth revisiting the question in the light of their analysis.
The Acid Ocean – the Other Problem with CO2 Emission
The Royal Society has just issued a summary report on the effects of CO2 on the pH chemistry of seawater and aquatic organisms and ecosystems. In addition to its pivotal role in the atmosphere in the regulation of global climate, CO2 and its sister chemical species, HCO3– and CO32- comprise the carbonate buffer system which regulates the pH of seawater. The new report can be found here. Acidifying the ocean is particularly detrimental to organisms that secrete shell material made of CaCO3, such as coral reefs and a type of phytoplankton called coccolithophorids [Kleypas et al., 1999]. The ocean pH change will persist for thousands of years. Because the fossil fuel CO2 rise is faster than natural CO2 increases in the past, the ocean will be acidified to a much greater extent than has occurred naturally in at least the past 800,000 years [Caldeira and Wicket, 2003].
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