Modern Ancient Temperatures

OK, no need to torture me, I confess it—I’m a data junkie.

And when I see a new (to me at least) high-resolution dataset, my knees get weak. Case in point? The temperature dataset of the Colle Gnifetti ice core. It has a two-year resolution thanks to some new techniques. Better, it stretches clear back to the year 800. And best, it extends up to near the present, 2006. This lets us compare it to modern datasets. The analysis of the ice core dataset is described in Temperature and mineral dust variability recorded in two low-accumulation Alpine ice cores over the last millennium by Pascal Bohleber et al.

Let me start with where Colle Gnifetti is located. Unusual among ice core records, it’s from Europe, specifically in the Alps on the border of Switzerland and Italy.

Figure 1. Location of the ice cores in the study.

This is good because some of the longest thermometer-based temperature records are in Europe.

One interesting thing about the site is that usually, ice core drilling occurs at the literal ends of the earth, in Antarctica and Greenland and the like. But this site is not far from the foot of the Margherita Hut, which is at over 4500 metres elevation.

Figure 2. The best advertisement ever for becoming a glaciologist.

Now, I wanted to see how well the ice core records matched up with the temperature records. So I calculated three records from the Berkeley Earth land-only dataset. (I used the land-only dataset because I’m comparing with a location on land. However, there is only a minimal difference from using their “land and ocean” dataset.)

The first record I calculated is the global temperature anomaly. Next is the northern hemisphere temperature anomaly. Finally, I looked at a 6° longitude by 4° latitude roughly square box around Colle Gnifetti itself. 

Curiously, of these three the best match is with the northern hemisphere data. Figure 2 below shows the comparison. I’ve linearly adjusted the ice core data to give the best fit to the Berkeley Earth data. 

Why linearly adjust it? Because the variance of a single ice core record at one location high on a mountain is different than the variance of e.g. the northern hemisphere average land temperature. This allows us to compare the records to the same scale.

So here is the comparison between thermometer data and the recent end of the Colle Gnifetti ice core data. Data sources are listed on the graph.

Figure 3. Berkeley Earth land-only temperatures and Colle Gnifetti ice core temperatures. Ice core temperatures have been linearly adjusted to give the best fit to the modern data, by multiplying them by about 1.6 and subtracting about 0.2°C. The background is the drilling hut at Colle Gnifetti

Man, I love it when two totally separate datasets line up in such an excellent fashion. I’d say that the Colle Gnifetti ice core tracks the northern hemisphere temperature very well. The only real anomaly is recent when the two diverge. No idea what that’s about. The answer may be in either dataset. But look at the excellent agreement of the large peaks and swings in the earlier part of the dataset.

So now that we have the paleo dataset aligned and variance-matched with the modern dataset, we can take a look at the ice core record of temperature variation over the entire time span of the data.

Figure 4. Colle Gnifetti ice core temperature, linearly adjusted to best fit modern data as shown in Figure 3. The background is the drilling area, lower right is the drilling hut at Colle Gnifetti

Now, this is a most fascinating temperature dataset. You can see the slow descent from about 1400-1500 into the Little Ice Age, bottoming out at around 1700.

The total range of the fast temperature swings is also quite interesting. For example, in the fifty years from 1190 to 1240, the temperature dropped by 2.3°C.

And the steepness of the natural warming trends is instructive. In 35 years, from 850 to 885, the temperature rose by 2.3°C. Zowie!

To take another look at the warming and the cooling, here’s a graph of the thirty-year trailing trends in the data.

Figure 5. 30-year trailing trends of the temperature record of the Colle Gnifetti ice core.

Current 30-year trailing temperature trends are on the order of 0.1 – 0.2°C/decade. But as you can see, this rate of warming is hardly unusual in the record. Indeed, twice in the period of record the trend has been four times as large as in the modern era. And the current rate of warming has been exceeded many other times in the past.

So … what kind of conclusions and questions can we draw from the data? Let me toss Figure 4 up again for reference.

Figure 4 (repeated).

First, what the heck caused the big swings in temperature? The earlier part of this paleo record shows swings that are both much larger and much faster than anything in modern times. Call me crazy, but on a planet with naturally occurring warmings of e.g. over two degrees C in 35 years from 850 to 885, I’m not seeing how people can possibly rule out such natural swings in looking at the relatively featureless and certainly in no way anomalous modern record.

This brings up my recommendation for the field of climate science—stop projecting the future and start reflecting on the past. Until we can explain things like why temperatures crashed around the year 1200, falling by 2.3°C in a mere fifty years, and then bouncing right back, we have NO BUSINESS MAKING PROJECTIONS. 

But I digress … next, we see warming in the data starting at the end of the Little Ice Age, around the year 1700. It occurs in two waves. The first rise in temperature, from about 1700 to 1790, is both faster and larger than the succeeding rise, which was from about 1820 to the present.

In fact, the modern temperature rise, supposedly fueled by CO2, is the slowest rise of the various longer-term temperature increases in this paleo record. Every other temperature rise is steeper … say what? I thought CO2 was supposed to be driving faster warming, but out here in the real world, there’s slower warming.

Next, in this ice core record, the peak of the later “Medieval Warm Period” around 1190 is about the same temperature as at present. However, the earlier peak around 920 is about half a degree warmer than that. It seems that current temperatures are not as unusual as is often claimed.

Finally, the caveats. The main caveat is the underlying assumption of invariability—that ceteris paribus, the past is operating under the same rules as the present. 

For example, to linearly adjust the modern end of the ice core data to be best fit to the modern temperature data you multiply it by about 1.6 and subtract about 0.2. The figure above assumes that the same thing is true in the past. This is a very reasonable assumption, we know no reason why it wouldn’t be so … and yet …

Next caveat? it’s only one study. I’d be happy to see more using the improved methods that give biennial resolution.

However, given those caveats, I find it a most instructive dataset.

Here on the Northern California coast, summer is in full swing. Many days, the inland valleys heat up. The heated air rises, pulling cool foggy air in from the cold nearby ocean. This cools the entire sea-facing side of the coastal mountain range, including our couple acres … so today it’s cool and foggy here.

I greatly enjoy the local symmetry. It gets hotter in one place … and it gets colder in another place. Lovely.

Figure 6. Satellite view of where I live, about 6 miles (10 km) inland from Bodega Bay, on the ocean-facing side near the top of the first big ridge in from the coast. Blue flag in the large patch of redwood forest marks the location of our house.

The layer of fog isn’t all that thick, typically maybe a couple thousand feet (600m). This leads to a curious acoustic phenomenon. Sounds down along the coast get “tunnel ducted” all the way up the hill. So even though the ocean is six miles (10km) away from our house as the crow flies, on certain foggy days we can hear the waves breaking on the shore. And sometimes, we can even hear the foghorn out at the end of the breakwater in Bodega Bay, my old commercial fishing home port, calling out its endless paean to the souls of those poor fisherwomen and men who never came back home to their loved ones …

Stay well, dear friends. Life is short, be sure to take the time to take the time.

w.

Further Reading: It’s instructive to compare the listed temperatures with the data in A Chronological Listing of Early Weather Events.

As Usual: I ask that when you comment you quote the exact words you are discussing, so that we can all follow the bouncing ball and avoid misunderstandings.