GG 140: The Atmosphere, the Ocean, and Environmental Change

Lecture 28.?Global Warming II

https://oyc.yale.edu/geology-and-geophysics/gg-140/lecture-28

The Holocene as a Climatic Reference Period [00:00:00]

We are now in the middle of a section on global warming. And we spent Friday looking at how carbon dioxide levels have changed in the atmosphere.?So we have a pretty good idea of how--what kind of carbon dioxide is being emitted, how much is being emitted every year into the atmosphere. How much of that is being retained by the atmosphere. How those levels are growing. We'll come back to that either at the end of this lecture or on Friday when we talk about projections into the future.?

Today I want to start off by thinking of this recent period--the last 10,000 years called the Holocene by geologists--and investigating what kind of climate change there's been during that period, and whether that would serve as a reference. I doubt that there's any perfect reference for climate change. It's been changing all through geologic history. But it's still worth I think questioning whether or not the Holocene might be a reasonable choice as a reference for looking at more recent climate changes.

And basically, we start the Holocene with that last large set of ice sheets retreating. So the Laurentide ice sheet, the Scandinavian ice sheet retreating, the most rapid rate of retreat was 11,000 years ago. By the time you got to about 8,000 or 9,000 years ago, most of those ice sheets were gone. And we're left with just the big two that we have today--Greenland and Antarctica. And then we're into what's called the Holocene.

So there was this period of time centered on about 1,000 A.D. but lasting for a couple hundred years called the Medieval Warm Period. After that, the Little Ice Age came along. I want to talk briefly today about the Year Without a Summer.?And then the curious change in notation. As the earth began to warm up about 100 years ago when climatologists studied this, there was a kind of change of attitude associated with a change in notation that was used.?

Thousands of years before present, coming out of the last ice age, the heavy curve is the average of a bunch of different glacial proxies. And it's an anomaly. So they've taken this period through here as their reference, and then everything else is either colder than that or warmer than that. It's in degrees C, but not in absolute degree Celsius. It's not based on the normal zero Celsius. It's based instead on taking this period of time as a reference.

So coming out of last ice age, slightly warm period. Only a few tenths of degrees warmer. Than cooling off after that as we get closer to the present day. Different authors will plot this up in different ways to make a different emphasis. And in this case, they show some of the outlier proxies to give you a little stronger sense of the Medieval Warm Period. But in this data set, they showed that variation as a grey background so you get less of a sense of the Holocene Optimum as being an important feature.

Chapter 2: Medieval Warm Period [00:04:47]

So a word about this Medieval Warm Period.?Historians seem to agree that this was a fairly big deal in Europe and in the Americas. And it was a period when the Vikings were able to leave their homeland and travel west across the Atlantic, setting up villages, even farming on the coast of Greenland.

And this is a dwelling left from that period of time. It was abandoned, of course, when we got into the Little Ice Age. The climate was too cool to support those settlements, and the Vikings disappeared from that area.

It was also during that medieval warm period, having established these settlements in Greenland, they were able to then go the rest of the way across the Atlantic and establish brief residences up in Newfoundland. Hence the belief that really the Vikings discovered America, not Columbus. They were there, well, almost 500 years before Columbus came. But again, taking advantage of this warm period of time to make that crossing.

Little Ice Age [00:06:03]

Then came the Little Ice age. 1400 to 1800 roughly. The historians will argue forever about what those dates are. It's impossible to set definite dates.

But a picture like this, a painting by Bruegel from the 1500s gives you a sense that it was a colder period of time in Europe. The Thames froze over. A lot of the rivers in Europe froze over that don't freeze over today. And that was recorded in many different ways, not only in art, but in many of the economic and other records of the time.

Year without a Summer [00:06:43]

Right at the end of that, there was a curious event called the Year Without a Summer. The year was 1816, and this was a big event both in Europe and in the Northeast United States. There were several killing frosts, at least one in each of the summer months--June, July, and August. And so crops that were planted died in the frost, replanted, were frozen again. And so very little successful agriculture was carried out in the Year Without a Summer--1816.

We think we know the reason for that. We were just coming out of the Little Ice Age. Remember, this year could even have been included in our definition of the Little Ice Age because the curve is so wiggly, so it's hard to know where to draw the end. So this is right at the end of the Little Ice Age. But there was an event that caused that particular episode. And that was Mount Tambora in Indonesia, with a huge volcanic explosion, putting dust into the atmosphere.

A number of writers at the time noted the red sunsets, the very unusually red sunsets from all that scattering material in the atmosphere. And as far as we know, that was the largest volcanic explosion since 180 A.D. So it'd been more than 1,000 years since there'd been such a large explosive volcanic event on the earth, as far as we know.?

What it does is put dust into the stratosphere, which increases the albedo of the planet. And if you remember, the formula we derived for the temperature of the planet--that formula that had the fourth root of S times one minus the albedo--when the albedo is larger, the planet cools. So that formula probably doesn't give us a quantitative way to understand it, but at least it shows qualitatively that an increase in reflectivity, caused by a layer of dust in the stratosphere, would cool the planet. The dust probably only lasts three or four years, and then the climate would quickly return to normal after that.

But then immediately after the Year Without a Summer, migration of farmers to the West began to occur. Not only because of this event, but that's when new lands are becoming open to settlement in the Midwest, in the Ohio Valley, for example, where the soils were better than in New England. Because remember, the glaciers had scraped all the topsoil off of New England, but not in the Midwest. There were large areas where the glaciers did not reach, and therefore they had deep, high quality top soils.?

So the Holocene had many small regional or temporary climate events. But generally, global temperature--and as we saw earlier, sea level--varied quite little during that 10,000 years. So in that sense, it's probably a pretty good reference.?But a lot was going on in terms of human history. Civilizations were rising and falling during those 10,000 years, so we saw a lot of human change in the way they lived and structured their societies in this relatively constant period of time. Perhaps it is the best reference for later climate change, but I think it's quite open to argument. I won't make the point too strongly there.

Recent Amelioration [00:13:55]

And you're quite familiar with this kind of curve from the data sets you've been plotting up for the lab. And starting in 1880, this shows a temperature anomaly. So again, a mean value has been separated out to make data points earlier than 1940 appear negative, and ones after that appear positive.?And it's curious because we seem to have a roughly flat period of climate. Then a rapid increase to the 1940s. Then either flat or maybe even a slight decrease. And then a really rapid increase since then.

And now very often you'll find in the global warming skeptic literature the point made--and it's a good one, but I'll address it--that the carbon dioxide rise has been much smoother than that. You get the annual cycle due to uptake by forests and oceans, but generally the CO2?rise has been much more smooth than this. And so if the two are causally related, why do the curves resemble each other so poorly? They're both rising, but they have different structures.

And the total amount of increase, if you go from, say, minus 0.3, let's say, up to plus 0.5 or 0.6, that total increase has been about 0.8--0.7, 0.8, or 0.9 degrees Celsius. Something less than one degree Celsius over that period of time.

And as you know from the data sets, when you combine the land and the ocean, you wash out the fact there's a pretty big difference between the land record and the ocean record. The land record is much noisier and there's more overall increase. The ocean record is much smoother--less year to year variation--and less trend with time. So be aware that you're hiding a lot of information when you sum together these things. There's extra information to be gained by looking at the continents and the oceans separately.

The terminology I want to point you to here--I came across this in the climatology literature, reading books from the 1940s and 50s where they were commenting on this rise, but they refer to it as quote, "the recent amelioration of climate." They were aware of the Little Ice Age. That was already in the literature. They were considering this warming to be a beneficial thing because it was taking us back to a more pleasant climate coming out of the Little Ice Age. It wasn't till we got to about 1970 and 1980 that this new term global warming was coined. And now, of course, that's what's used to describe a curve like this.

And there's a different implication, though, isn't there in those two terms? Global warming suggests that this is something that's maybe a little unusual--not just returning to a more normal state, but actually taking us into a new kind of climate zone we hadn't seen before.

And the word global in the phrase global warming is a little problematic because I think it tends to overstate the uniformity of the warming. If you look at the global temperature trends 1979 to 2005, for example, which covers basically this part here--so here it is at the surface of the earth, and here it is estimates for the troposphere. There's much more warming on the continents than over the ocean. There's much more warming in the northern hemisphere than the southern hemisphere. There's more warming in the high latitudes than in low latitudes.?So one danger in this term global warming is that it makes you think that the warming is somehow uniform over the globe, and that's not the case.?

When you hear this word global warming, that there's a lot of inhomogeneity. The land warms more than the ocean, the northern hemisphere more than the southern hemisphere. It's a little more uniform when you look up in the troposphere, in the air in the first 10 or 20 kilometers of the atmosphere. But still there's some variation there. And even some regions that have cooled over that period of time.

Factors that Influenced the Climate of the 20th?Century [00:19:45]

These two large volcanic explosions in our own era. Forget about Tambora back in 1815. In the 20th century we had these two large ones. El Chichon in Mexico, April, 1982. There's a picture of it. And Pinatubo in the Philippines in 1991. That material, some of it carried up into the stratosphere. The winds quickly carried it around the globe--only takes a few days to get it distributed around the globe--and then those dust particles stayed up there for a couple of years, and?did cause a temporary cooling in the climate connected with those two things.

Here is from that same Mauna Loa Observatory on the big island of Hawaii where they were measuring CO2. They also measure atmospheric transmission. You have a telescope looks right at the sun and measures the intensity of the radiation coming from the sun. And when that dips, that's usually because some dust has come between you and the sun. And notice these dips connected with both El Chichon and Pinatubo. So we know that had an influence on atmospheric transmissivity, transmittance, and probably on albedo, as well, the fraction of sunlight that was reflected back to space.

The other big thing that was going on along with industrialization was sulfur dioxide emissions into the earth's atmosphere, mostly from coal fired power plants. Coal typically has quite a bit of sulfur in it. And when you burn coal with sulfur, you produce SO2. And the SO2?will often, after it's been emitted, it will react further chemically to form small?sulfate particles. And those particles are a primary source of air pollution, a primary cause of lack of visibility in the atmosphere, and they reflect sunlight back to space just like those volcanic explosion plumes would.

So that peaked in about--well, there are various sources here. This is organized by continent. And North America is the maroon color here. East and Central Asia I think is the yellow. But anyway, we all were synchronous.?And it peaked in about 1880--1980 or 1990 and then slowly begin to decrease. Not because coal was being used less, but because by that point, in the US first and then in Europe, we had passed the Clean Air Act. And the Clean Air Act didn't immediately shut down coal power plants--by no means did it to do that--but it required that they put scrubbers on the smokestacks to remove the sulfate from getting emitted into the atmosphere. And we see that as this beginning of the decline in?SO2?emissions into the atmosphere.

So here's one of the--it's not a great way, but it's a pretty good way to look at the influences of these different inputs into the atmosphere. So these are model runs. These are computer simulations of climate done by a number of different laboratories. Every one of these curves is a different global circulation model. One of these is at the National Center for Atmospheric Research. The Department of Energy has one. NOAA has one. The Germans have one. There's one in the UK. So many GCMs are in this business of trying to model climate and how it changes with these different inputs.

The time scale is 1900 to about 2010 or so. Temperature anomaly is plotted. Again, some period of time has been used as a reference, and that reference temperature has been subtracted. The different models are shown in yellow, all the different curves scribbled together. The average of all the models is shown in red. And the observations, similar to what I just showed you back here, that is basically, with that characteristic peak in around 1940, is shown in the black.

So how well do the models do in predicting the climate of the 20th century? They don't get that peak. They do, however, get the drops in temperature connected with Agung, El Chichon, and Pinatubo. But that comes through because the modelers are putting in those inputs into the model, those increases in dust. They also get the general upward trend. And they get this flat period here before it begins to rise rapidly. So the models do pretty well by putting in the CO2?forcings, the sulfate aerosol, and the volcanic eruptions.

Here's the same set of models, but they've put in natural forcings only.?That means the volcanic eruptions are put in, but the sulfate and the CO2?is left out. Those are both anthropogenic. So the model continues to capture the drops connected with volcanic input, because that's included in what they call natural forcing. But it shows basically a flat and even cooling climate after 1940, and missing entirely the?decrease here caused by the sulfates, and then the increase caused by the eventual dominance of the CO2.

This is the same sort of thing applied continent by continent, global, global land, and global ocean. And in each case the blue is models using only natural forcings. And the reddish one is models using anthropogenic forcings. And the difference is dramatic, especially in regards to this most recent rise since 1975. So the models seem to be giving a consistent view in that it's these anthropogenic things, especially CO2, that's causing the final rise there.

The factors that seem to have the biggest influence on 20th century climate. The rise in carbon dioxide level. Sulfate emissions from coal burning. Major volcanic eruptions. I should have added also heat storage. Because as you saw from your data, the oceans are warming less than the continents, and because the winds are transporting heat back and forth between the continents and the oceans, the oceans are probably limiting the rate of warming of the continents to some extent, as well. The point is the oceans have such a large heat storage capacity that even though we're doing these things to the climate, in order to warm up the earth, you've got to warm up the oceans first. And they have such a large heat capacity that that's going to limit the amount of heating you can get because of that heat storage effect. That's going to give you a lag.?

One way to understand what's going on over the last 100 or so years is to look at the radiative forcings. This is, again, from the IPCC report. Radiative forcing is defined as the excess radiation coming down to the surface of the earth since the industrial revolution. Let's say, over the last 100 or 150 years. And it could be either long wave forcing or short wave forcing.

The radiative forcing in watts per square meter caused by the CO2?increase over that period of time. It's about 1.5 watts per square meter. That's a long wave forcing.?So that's radiation from the earth's surface gone up to the atmosphere, been absorbed, and been reemitted back down to the earth's surface as thermal infrared radiation.?

There are some other greenhouse gases that have been increasing over that same period of time. It includes methane, N2O, and some halocarbons. And that is perhaps another watt per square meter. Ozone has been increasing slightly in the troposphere. Not as important as the others, but maybe 0.3 watts per square meter.

At the same time there have been these aerosol effects from the burning of coal, giving sulfate particles in the atmosphere. And that's tended to cool the climate significantly. If you add all these up, you get total net anthropogenic. The greenhouse gases dominate, but the aerosols partly reduce this. If I added up these three, they'd be out to here or so. Instead, the net's only about 1.5.

So there's been a partial cancellation between the anthropogenic effects from putting greenhouse gases into the atmosphere and the anthropogenic effects from putting sulfate and aerosols into the atmosphere. Giving a net radiative forcing over the last 100 years of about one and a half watts per square meter excess radiation reaching the surface of the earth, mostly in the form of long wave infrared radiation.

Before we leave the subject of the last 100 years, let's look at some of the other things that have been changing over that period of time. Sea level has been rising. Not as rapidly as it did as we came out of the last ice age, but still rising. The rate here is about three centimeters per decade. So every 10 that goes by, we raise sea level about that much. It's been pretty steady.

And our estimates are that this is approximately half due to the melting of glaciers putting more water into the ocean basins, and the other half is due to the warming of the water that's already in the oceans--when you warm water you make it expand, and so the top of the ocean rises a little bit because you've increased the volume of the existing water in the oceans. So each has contributed about half to this total increase.

There's another curious result of the warming, of the CO2?addition, that I wanted to point out you. Adding CO2?to the atmosphere increases the greenhouse effect. We know that. It absorbs outgoing long wave radiation and then re-emits it back down to the surface, causing a warming of the surface. But adding CO2?to the stratosphere does something different. Because it's a greenhouse gas, it can radiate. And it tends to radiate more energy to space than was going on before you added that extra CO2.

So the models tell us this. We knew this was going to happen. But the data confirms that the result of adding CO2?to the atmosphere, while it warms the troposphere and the surface of the earth, it cools the stratosphere.?This is sometimes referred to as a line of evidence known as fingerprint evidence. In other words, if you change CO2,and the models tell you something about the nature of that change--like warming the troposphere and cooling the stratosphere--and then you see that happening in the data, that tends to confirm those theories.

Let's look at the data. This is, again, with a mean removed. These are degrees C departures. The time schedule is from 1980 to 2010, and it's for stratospheric temperatures. And indeed, you find some sudden warmings connected with those volcanic explosions. If you look at those two dates, they correspond to El Chichon and Pinatubo. But generally there's been a cooling of the stratosphere in rough agreement with what the models would predict for adding CO2?to the atmosphere.

So if someone says, well, we're not getting global warming because the stratosphere is cooling, well, that's true. The stratosphere has been cooling over the last 20 or 30 years. But that doesn't negate the fact that the troposphere and the surface of the earth have been warming over that same period of time.

It seems like the leaves are falling off the trees later and later each year.?Here's a study that was done in Indiana. And they’ve got--the year is plotted on the x-axis. 1998 to 2008. And on the y-axis is--and this one is the day of year. DOY means day of year. So January 1 is day one. December 31 is day 365. And it's a way to keep track of where you are in the year without using months and dates and things like that, just a straight day of year.

This shows two measures. One is they measured the net carbon uptake by this forest. And the other is they looked at the forest and gave it a greenness index. How green do the leaves appear.?So here's the start of the deciduous growing cycle. Based on carbon uptake, it was about year 115 or 118, and it stayed relatively flat over this decade of study. According to the greenness index it started a little bit earlier, but it was also flat during the 10 year period of time.

But if you look at the end, if you look at the fall season--day of year 260, 280, 300--you see a real trend. So the forests are keeping their carbon uptake going longer, their leaves aren't changing from green to colored until later, and that slope in both ways of measuring it--by carbon uptake or by greenness index--is about three days per year. In other words, every year, three days later. That's an enormous change. So that over the 10 year period of study, there's been about a 20 day—20 or 30 day delay in this.

Observed Changes during the 20th?Century [00:40:07]

So some of the observed changes over the last—over the 20th century. Warming largest in the northern hemisphere continents. Global warming of about 0.8 degrees Celsius. Earlier in the course we've talked about the arctic sea ice reduction. But remember, the antarctic ice hasn't changed. The antarctic sea ice hasn't changed.

Most of the mountain glaciers are in retreat. We talked earlier about the Greenland ice sheet reducing its mass. Sea level rise--I showed you that diagram here. Stratospheric cooling, and the deciduous growing season is longer. So all those changes have been noted over the 20th century, especially over the last 25 or 30 years or so.