How Can We Know How Much Global Warming is Responsible for Extreme Weather? – #365papers – 2017 – 115

#365papers for April 25, 2017

Diffenbaugh, Singh, Mankin, Horton, Swain, Touma, Charland, Liu, Haugen, Tsiang, and Rajaratnam, 2017, Quantifying the influence of global warming on unprecedented extreme climate events: Proceedings of the National Academy of Science, doi: 10.1073/pnas.1618082114

What’s it about?

Global warming is an important issue in the modern world. Many people are attributing extreme weather events – hurricanes, droughts, flooding, etc. – to global climate change. This paper is an attempt to assess and quantify how much global warming was an influence for such events. Continue reading

Global Warming; Shrinking Mammals – #365papers – 2017 – 76

#365papers for March 17, 2017

D’Ambrosia, Clyde, Fricke, Gringerich, Abels, 2017, Repetitive mammalian dwarfing during ancient greenhouse warming events: Science Advances, v. 3, e1601430.

What’s it about?

Rapid global warming in Earth’s past had occurred more than once. The most commonly studied episode occurred 55 million years ago, at the boundary between the Paleocene and Eocene epochs (Paleocene Eocene Thermal Maximum, PETM). Several other episodes have happened, including the ETM2 and H2 episodes which are discussed in this paper.

Dwarfing of mammalian species has been documented at the PETM. This paper shows dwarfing of mammals also occurred at the ETM2. Continue reading

What Can A Clam Teach Us About Climate Change?

One of the many projects I work on involves the study of climate change in the fossil record. I’ve put a bit of it on-line here. What I’ve published thus far deals mostly with interpreting general climatic and environmental factors using bulk geochemistry (all isotopes) from rocks and the fossil contained therein. That is to say, I take a big rock or fossil and grind it (or part of it) down into a single sample. I analyze that and call that a ‘average’ for that entire rock layer.

It turns out that clams (and mollusks in general) do a good job of recording environmental signals not just in bulk, but on a fine scale, such that we can see yearly, monthly, even daily records of weather. Continue reading

The Paleocene-Eocene Thermal Maximum – Is Modern Global Warming Anything to Worry About?

There are lots of names for it, some good, some bad: Climate Change, Global Warming, Climate-gate, The Climate Hoax. Unless you’ve had your head in the sand, you’ve heard at least one of these things. You know that there is a lot of talk about how every year seems to be warmer than the last – “the warmest on record” – and that there have been a lot of wacky weather phenomena of late, including Hurricane Sandy, heat waves in Australia and Europe, massive wildfires in the western United States. Some reports are pretty alarmist, while others claim that these are merely coincident anomalies that we only know of due to more complete modern measuring techniques and records. Some say that the Earth is warming at an alarming rate and we need to prepare for a “The Day After Tomorrow” type scenario, while others say that we have no need to worry and that it’s all hype. And really, how bad can one or two degrees of temperature increase be?

If you’ve read other posts of mine, you probably know where I stand on this. For this post, my views on the legitimacy of modern global warming are irrelevant. What I want to address here is not whether warming is occurring, but what would happen if those noisy scientists are right and we are heading toward a warmer Earth? What could the possible outcomes of a few degrees of warming be? There are models, of course, all mathematical and computerized, that show where things will get wetter or drier and such, but let’s think about something more real.

What if the hype is correct and we are warming? What will happen if we do nothing to mitigate it?

The fossil record provides an opportunity to look at past climate changes and see what effects these changes had on the animals that were alive during that time. The fossil record shows that there have been multiple episodes of global warmth in Earth’s history, much warmer than is projected as a possible outcome of today’s warming. But being warm and warming rapidly are two different things. Gradual warming occurs slowly enough that organisms can adapt. But modern warming is occurring within a single to just a few generations of animals, much too quickly for adaptation to occur. What happens then?

Does the fossil record capture any past episodes of rapid global warming? If so, what happened?

The short answer is ‘yes,’ and it was bad news for many animal groups.

The specific example I will use is the Paleocene-Eocene Thermal Maximum (PETM). This is an episode of global warming that occurred about 55 million years ago (about 10 million years after the dinosaurs went extinct). The entire PETM lasted 150,000 to 200,000 years, with the warming occurring over the span of about 10,000 years. Depending on what you read, the warming was between 5-9° Celsius (9-16° F). Compare that with modern projections of warming of 4° Celsius (or more) in a few hundred years. Warming rates are much faster today than they were at the PETM, and rates at the PETM were much, much faster than most other rates of climate change recorded in the rock record.

The warming associated with the PETM is particularly interesting for two reasons. 1) It’s thought that the warming was due to an increase in carbon dioxide in the Earth’s atmosphere, much like today’s warming. 2) Mammals were around then, and the dominant large-bodied animals living on land. We can look at the record of change in mammals at the PETM as an analogue for what might happen if modern global warming is ‘true.’

So, what happened?

Changes in vertebrate taxa at Polecat Bench, Bighorn Basin, Wyoming. © Philip Gingerich, 2006

The chart above shows a lot of things. It was published in 2003 in Geological Society of America Special Paper 369. It is available here, from Philip Gingerich’s personal web page focusing on his research on the PETM. I suggest reading the entire paper to get the full context, but for the sake of this post focus only on the columns on the right hand side. There are two columns labeled ‘stable isotopes,’ and a series columns (some highlighted in green and others in red) that represent the stratigraphic ranges of specific vertebrate groups. The heavy red line marks the Paleocene-Eocene boundary, and the box in the stable isotope column encloses the isotopic evidence of the PETM – a negative spike in carbon isotopes and a positive spike in oxygen isotopes. It is the positive spike in oxygen that provides the evidence of warming. The negative spike in carbon provides information about the source of the warming (carbon dioxide in the atmosphere). The details of how the isotopes provide such information is a topic for a different blog post.

Focus now on the highlighted vertebrate groups. In green are the Plesiadapidae. Plesaidapids are a group of mammals thought be closely related to modern primates. They go extinct at the Paleocene-Eocene boundary. Modern primates, highlighted in red, appear after the Paleocene-Eocene boundary. It’s possible, then, the the PETM, was responsible for the extinction of the the plesiadapids and appearance of modern primates. Perhaps one evolved into the other, we are not sure at this point, but the loss of one and appearance of the other coincides with the PETM.

You also see, highlighted in red, the first appearance of the groups Perissodactyla and Artiodactyla. These are all the modern hoofed mammals. (Perissodactyla includes horses, rhinos, and tapirs. Everything else is in the Artiodactyla.) It is the appearance of the first perissodactyl, Sifrippus (also called Hyracotherium or Eohippus) that defines the beginning of the Wasatchian North American Land Mammal “age” which is thought to be coincident with the Paleocene-Eocene boundary. Prior to the PETM, there were no true hoofed mammals, though it’s though that the ancestors to perissodactyls and artiodactyls could be found in a group of mammals loosely called the condylarths. Condylarths dwindled after the PETM, to be replaced by the recognizable, modern groups of mammals.

Thus it’s possible that rapid global warming at the Paleocene-Eocene boundary resulted in the rapid evolution of mammalian species, resulting in the loss of many groups that had previously been dominant, and their replacement with new groups. This is a big change. This is not an example of just a few species going extinct. We’re talking about entire orders of mammals here, including the Order Primates, of which we are a member.

Now consider again that warming at the PETM took place over several thousands of years. Modern global warming is occurring over several hundreds of years. If warming at the PETM forever altered mammalian history, what would modern global warming do? Perhaps we should think about this before we say that there’s no need to be concerned.

***UPDATE***

This post has been translated into Spanish by Jorge Moreno-Bernal, a student at the University of Nebraska-Lincoln.  See the translation here. How cool is that?

Orbital Cycles and Climate

One of the things that comes up when someone talks about climate change is the apparent cyclicity of climatic changes. The Earth has been through several rounds of ice ages and warming in recent millennia, how is this new episode of this warming not just part of that? Well, let’s look at the cycles.

Temperature change over the last 400,000 years. Notice the approximately 100,000 year cycle. Modern conditions are on the right end of the graph.

What we see here is a repeating 100 thousand-year cycle of glaciations and warming. We’re in a warm spot, having just come out of an ice age about 10,000 years ago. If we look at the pattern for the last three deglaciations, we see sudden, rapid warming, followed by cooling into another ice age. We’ve already warmed, and have been warm for a while, so we should be cooling down now. That’s why, back in the 1970’s, people were being warned about the coming ice age.

According to the glacial cycles, that’s where we should be heading. Things should be getting cooler. And they were up until about 50 years ago. Then we started seeing increases in annual temperatures. When looking at this graphically, we get what has been referred to as the “Hockey Stick.” You can read more about where the Hockey Stick comes from here.

The “Hockey Stick” showing recent rapid warming. Northern Hemisphere only. Modern conditions are on the right end of the graph.

What causes these glacial cycles? What is this 100,000 year periodicity? This pattern is caused by Milancovitch Cycles, changes in the intensity of the sun that hits the Earth due to properties of the Earth’s orbit and rotation about its own axis. There are three (or four) parts to Milancovitch Cycles.

The first of these is an approximately 21,000 year cycle called precession. This is where the Earth’s rotation axis wobbles, much like how a top wobbles as it spins. This changes the position in the Earth’s orbit at which the equinoxes take place.

Obliquity is a 41,000 year cycle in which the tilt of the Earth’s axis varies from 21.5° to 24.5° from perfectly vertical relative to the plane of the Earth’s orbit around the sun. With greater tilt, the difference between the seasons becomes greater.

The shape of the Earth’s orbit around the sun shifts from being closer to circular to being more oval. This shift is called eccentricity and varies on scales of 100,000 and 400,000 years.

Milankovitch Cycles

Each of these (precession, obliquity, and eccentricity) have an effect on the amount of sun (insolation) that hits the Earth and therefore Earth’s climate. The term for this is solar forcing. We can take the individual impacts on solar forcing for each of these and add them up to summarize solar forcing at any given time. We can then compare this, and the individual forcings, to the pattern of glaciations. What we see is an approximately 100,000 year cycle of glaciations, which coincides with minima (or low insolation) in the 100,000 year eccentricity cycle.

Solar forcings due to Milankovitch Cycles and their relationship to temperature changes over the last million years. In this chart, modern conditions are on the left hand of the plot.

As we are approaching a minimum in the eccentricity cycle, we might expect to be heading into an ice age – though it might be a few thousand years off. What we are seeing instead is rapid warming. Perhaps we should be concerned.

Beware of Movies! Climate Change

The Beware of Movies! series is meant to point out some of the scientific inaccuracies of popular movies, specifically in points related to the geological sciences.

This post will point out the major inaccuracies portrayed in movies about climate change, and how it would affect the Earth.

Climate change is a sensitive topic. It’s become politically charged. It’s now taboo to talk about it in polite company. I’m not here to incite riots. I have my opinions that, though I won’t state them explicitly, they’ll probably be obvious. My objective here is to talk about how we understand climate change, how we can infer that it is happening. I want to demystify all the numbers and data points and graphics that we’re bombarded with every day. Continue reading

Decoding the Hockey Stick – Part 1

There is a lot of discussion about climate change these days. It’s quite a polarizing topic, actually. It’s astounding to me to see how science – or a scientific result – is suddenly a taboo topic in polite company, just like politics and religion. It upsets me. Why are we not interested in the science? Why can’t I talk about it?

Well, I talk about it anyway, at least to those who are interested in listening. If people want to argue, then I usually shut down. It’s not that I don’t feel that such discussion isn’t worthwhile, and with some people I will try to be engaged, but honestly, most of the arguments stem from a fundamental misunderstanding of how science works and how scientists look at data. It’s frustrating, and it’s not something easily explained. There’s also a certain amount of mistrust of science, which I find disturbing.

Rather than trying to explain the entire body of climate science, perhaps I’ll take a moment to talk about one aspect of the climate debate. One thing that some argue is completely bunk.

The Hockey Stick.

Most everyone has heard of this. I’m not talking about the game of Hockey, here. I’m talking about the Hockey Stick. Considered by some to be the smoking gun proving global warming and by others as manipulated data. The gist of it is that if we can look at average annual temperatures over the last several hundred years, we see that there’s some fluctuations around an average, but that the last few decades have been getting warmer and warmer, more so than at any other time on record. This, then, is the rapid global warming that everybody is arguing about (but that you don’t talk about with your family at Christmas).

Well, where does this come from? When you see images of the Hockey Stick, you see time along the bottom and you expect to see temperature on the vertical axis, so that when the lines go up, you’re looking at warmer temperatures. What the vertical axis shows, however, is what’s called the “temperature anomaly” (although it is, at least, labeled in degrees). What the heck is that?

The temperature anomaly is the difference between any given year’s average annual temperature and the average of all the annual temperatures over a specified period of time (sometimes from 1951-1980, sometimes from 1902-1980, sometimes something else, always defined). During that span of time, temperatures were relatively constant. The decision to use this period of time as a baseline by which to compare everything else was arbitrary. (Or I assume, so. I wasn’t there when this decision was made!) The fact is, they just needed a ‘zero’ point against which to compare everything else. Presumably, records during that period of time were precise and accurate enough for the researchers to be confident in them.

 

PRECISION and ACCURACY: These are two terms that are sometimes confused for one another, but in science have very specific meanings. Accuracy is getting the right answer. It’s hitting the bullseye. In the case of temperature, it reflects how correct the temperature reading on any given thermometer is. Precision describes how well the same answer can be found. If you shoot ten arrows at a target, precision is about how close together those ten arrows are. In science, it’s about putting the same thermometer in the same freezer on different days and getting the same temperature reading, or perhaps putting ten seemingly identical thermometers in the freezer at once and seeing how similar all the readings are. Precision is shown on graphs (like the Hockey Stick) with error bars or confidence envelopes.

What’s important is to realize that something can be precise and not accurate and vice versa. I can shoot ten arrows at a target and they can all clump to the upper right of the bullseye, which I was aiming for. That’s precise, but not accurate. Or I can shoot ten arrows and have them spread out, surrounding the bullseye. In this case, they’re accurate, but not precise.

Precision and accuracy is a big deal in science, and particularly in climate science. Both of these are called into question when the legitimacy of the interpretation of the Hockey Stick is discussed.

 

In order to calculate a temperature anomaly, of course, one must first come up with a value for average annual temperature. For more recent years, this comes from instrumental records, aka, thermometers. One of the difficulties faced, however, is how to calculate a global average annual temperature, especially when temperatures vary all over the world, from day to day and season to season. And really, how can you compare annual temperatures in the arctic with annual temperatures on the equator? And then, throw on top of that precision issues with the thermometers themselves. Geez! How do you handle all those data?

Well, it’s complicated. The first thing you have to do is normalize everything. Normalizing means to set everything up onto the same scale so that they can be compared easily. This is where the temperature anomaly comes in. By using an average of a particular set of years and then showing all your annual weather data relative to that, it becomes possible to compare Arctic temperatures with equatorial temperatures. In the Arctic, a temperature anomaly of 1 degree might mean a change from -5 to -4 degrees, whereas on the Equator, it’s a change from 72 to 73 degrees. By normalizing using the temperature anomaly, we can easily see that the temperature went up one degree in both places.

The normalized anomalies can be averaged for specific regions (to help even out the differences between regions that have tons of thermometers and regions that don’t), and then for the whole world to get at a global temperature change. That’s what we’re really interested in.

When you calculate all these averages, you can also calculate the variation of the values. For example, in the Arctic, the anomaly could be 2 degrees, whereas on the Equator it could be 1 degree. You can calculate the average (1.5) but also calculate some statistics to represent the variation. This is where error bars come in. Your average is 1.5, but the range is from 1 to 2 degrees, so you draw a little bar on the graph representing that. (This example is not real, of course. Standard deviation or standard error would be used in a real scientific study, but you get the idea.) The error bars can also be extended (or shortened) depending upon the known precision of the thermometer used.

What you wind up with is a lovely graph of squiggly lines representing the global temperature anomaly over time. A positive anomaly means warmer temperatures than in times past. A negative anomaly means colder temperatures. The Hockey Stick shows warmer temperatures than in the past, and things seem to be getting warmer.

One of the problems with the typical image of the Hockey Stick, when it’s flashed up in the news is that it almost always lacks the error bars. The error bars are important. When looking at instrumental records (thermometers), for which we have data going back into the late 19th century, we can see that the error bars get smaller and smaller over time. This is due to improvements in the technology of temperature measurement. But the errors are still there.

Average global temperature anomalies.

Error bars give you a possible range within which the actual ‘real’ measurement might be. That is to say, that even though there’s a point on the plot, it might not be in exactly the right spot. The error bars give you a measure of how inaccurate the data point might be. It’s possible for data points to show a nice complex pattern, but to have error bars so big, that the pattern might not be real.

I like to think of error bars as bumpers. Imagine that you put a string into the plot between the error bars and pull it tight. If it can make a flat line between the error bars, then the data don’t show any pattern. If you pull the string tight and it still has bends and peaks in it, then those features probably represent true variations.

In the case of the Hockey Stick, the upturn of the temperature anomalies in the last few decades is pretty compelling. With error bars, the increase in temperature anomalies might be a little smaller, but it is still there.

Average global temperature from instrumental records. Colored lines show different possible rates of warming.

But what does this mean? We see an increase in the temperature anomaly over the last few decades, but really, this plot doesn’t look so much like the Hockey Stick you’ve seen elsewhere. The full-blown Hockey Stick goes back about 600 years, but we didn’t have thermometers way back then. How can we measure mean annual global temperatures from that far back.

Alas, that’s a topic for another post.