The Critical Zone



Paleoclimatology, the study of ancient climates, has become increasingly recognized as a socially relevant tool for unraveling the causes and consequences of ongoing and future climate change. Scientists use the knowledge and insight gained from reconstructing ancient episodes of climate change to better understand how Earth's future climate may behave during climate states not experienced during recorded human history. In addition, a complete understanding of natural climate variability allows us to better identify and understand the role of human activity on climate change. For example, we know that ancient episodes of warmth have occurred in which crocodile-like reptiles and subtropical forests thrived throughout the high arctic. During these times, the atmospheric hydrologic cycle was amplified, the quantity of precipitation was elevated and more broadly distributed, and weathering and soil formation were intensified with tropical soil-forming conditions extending into the mid- to high-latitude regions of the planet.

Did you know . . . ?

Tim and colleagues are working to reconstruct one such interval in Earth history the Paleocene-Eocene Thermal Maximum.

To begin to understand the ties between this work and the carbon cycle, view this video from Svalbard.

Video: A Visit to Svalbard, Norway (02:15)

Click here for a transcript of the A Visit to Svalbard, Norway Video.


TIM WHITE: I'm Tim White. I'm a sedimentary geologist and a paleoclimatologist at Penn State. And I've come here for a variety of reasons. One, to look at siderite-bearings paleosols that I use to reconstruct ancient atmospheric hydrologies during greenhouse warming episodes. I do that work with my colleague Dave Pollard and Chris Poulsen, who do the modeling of the data that I generate.

This is the Longyearbyen Glacier. We've been spending at least three full days hiking up along the lateral moraine. To the right, you can see the old coal workings from the early 1900s. And at the very most top hill are some of the formations that we've been working on.

You can see all the relics of mining. And maybe even now, you can see that there are red colors up on the slope. Those are places where the coal seams burn, as a result of German shelling of this mining town during World War II.

This is the only coal mine in Scandinavia, even today. So at the time, it was a very important part of the allied economic strength, if you can call it that. So the Germans were determined to knock out whatever they could.

They still burn the coal today, and that's what powers this entire town. And you can even see a little bit of-- well, I don't want to call it smog, but you can see smoke emanating from the coal-burning power plant right there. And it's kind of funny, being here near the top of the world, seeing coal being burnt, while we're here to study one of the more extreme greenhouse episodes in Earth history.


Another example is in the Sahara Desert, where geologists have found ample evidence for the presence of widespread, deep freshwater lakes throughout the desert region as recently as 9,000 years ago, evidence for profound climate change in the intervening time. Imagine the difference this environmental change had on the habitability and presence of life in the region (learn more . . .).

In some places in Pennsylvania, deposits from these episodes are visible at the surface; in other portions of the state, relict glacial deposits attest to the fairly recent presence of thick and vast ice sheets that extended far to the north into Canada a short 18,000 years ago.

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Watch this!

The record of paleoclimate variation in Pennsylvania includes very ancient paleosols formed during episodes of tropical climate conditions to the more recent deposits of vast ice sheets that extended far to the north into Canada. To learn more view the following video.

Video: Paleoclimatology and Pennsylvania's Ancient Climate (04:19)

Click here for a transcript of the Paleoclimatology and Pennsylvania's Ancient Climate Video.


DR. TIM WHITE: Paleoclimatology is the study of ancient climates. In our next lesson, we'll consider aspects of paleoclimatology that involve computer modeling, and you'll meet one of my colleagues who uses computer models to understand ancient climates.

But now I'd like to show you some of the geologic evidence for past climate change that serves as the backdrop from which modelers ply their trade. Earth has undergone many drastic shifts in climate outside those experienced by humanity. For example, here in middle-latitude Pennsylvania, we experience a moist, continental climate that sustains our expansive deciduous forests and agricultural pursuits.

However, Pennsylvania has not always experienced an equable climate. 360 to 370 million years ago, during the Devonian Period, portions of Pennsylvania were covered by a broad, coastal plain next to a large inland sea that existed to the west in the interior of North America. North America was positioned in the Southern Hemisphere at this time.

Notice the gray to green resistant layers behind me, and these reddish recessive layers right here at my feet. The resistant layers are sandstones that were deposited in river channels. Whereas these recessive layers were deposited in the ancient floodplain, where soils formed during that time.

Notice these nodules within the reddish recessive layers. These nodules are calcite formed in soils in the ancient floodplain of the Devonian. These nodules help us to identify these paleosols as calcic Aridisols formed in a much dryer climate than Pennsylvania experiences today-- perhaps similar to West Texas or northwestern Argentina now.

If you need to, return to the lesson on soils and review the modern Aridisol distribution map. You'll find that Aridisols are developed within and along the fringes of many of our modern deserts. And so this ancient setting was much dryer than the climate we experience today, and, in part, can be described by North America's position south of the equator during this time.


We're in the so-called barrens just northwest of State College, and we're trying to make our way over to that little exposure there, which is in an old bauxite mining pit from the 1870s.

In contrast to the red Devonian calcic Aridisols that we just viewed, these paleosols behind me are oxisols formed sometime between about 15 and 50 million years ago in a very warm and wet tropical setting. Notice the light gray to white color of the soil matrix. The tropical weathering conditions were intense enough to leech most of the elements and nutrients from the soil profile, hence the dull or plain color of the matrix.

You can also see that the profile's much thicker than Devonian paleosols, or any of the soils that we visited in the Introduction to Soils video. This thickness is a tribute to the intense, tropical weathering conditions that existed here deep in the past.

During the time when these paleosols formed, Pennsylvania was located approximately in the same position as it is today. So the presence of these oxisols here indicate that tropical weathering conditions were much more widespread during this deep-time episode of global warmth.