Climate change: Warmer... but why? | Upsetting the balance | Mammals on the move
Connecting climate and disease | How do sea shells record temperature change? |Taking Earth's temperature


Is global warming due to human activities that have changed the chemical composition of the atmosphere, enhancing Earth’s natural greenhouse effect? One way to answer this question is by comparing climate records from after 1750, when the Industrial Revolution began, with those from earlier times. But people have been measuring and recording temperature and precipitation for only the past century or so.

So, how can we figure out what the climate has been for the past 500 years?


Reconstructing Earth's temperature history

Since the early 1990s University of Michigan Professor Henry Pollack and Research Scientist Shaopeng Huang have been reconstructing the recent history of Earth's temperature by taking the temperature of rocks under Earth's surface.

Here’s how it works: As Earth’s atmosphere warms, rocks in the uppermost part of the crust also warm, and the heat gradually spreads deeper into the rock beneath. Rocks quickly “forget” short-term or seasonal ups and downs, but “remember” temperature changes that take place slowly over hundreds of years.


Collecting data from lots of holes

Pollack and Huang record temperatures at different depths and then interpret the readings to reconstruct the history of surface temperatures that produced the present-day rock temperature profile.

In this photograph taken in South America, Professor Pollack and his crew are taking readings from the drill hole thermometer.

The red triangles on the globe show locations of the drill holes that have been measured.





Climate change goes deep

Pollack and Huang have found that Earth’s warming is detectable down to a depth of over 200 meters (600 feet). Dr. Huang says, “Some people say you’d have to be hiding under a rock to be unaware that Earth is heating up. Actually, you’d have to be hiding under 200 meters of rock!” A good understanding of the changes in land mass temperature will help us uncover more subtle effects of recent global warming.

The orange dots on this graph show temperature measurements in three boreholes in India. The curving of the upper parts toward higher temperatures is a response to Earth’s surface warming. The linear increase of temperature in the deeper parts of the holes is the undisturbed geothermal gradient. The blue dashed line is the extrapolation of the geothermal gradient up to the surface, and represents what temperatures were prior to the onset of the warming.



Cores taken from the drill holes are sectioned and numbered to keep track of the type of rocky material that has been sampled, and the depths at which they occurred. (Click images for higher resolution)

Verdict: It's getting hotter, and we're the cause

Using data from several hundred sites around the world, Pollack and Huang have documented a world-wide continental land mass temperature increase of about 1 degree Celsius (1.8 degrees Fahrenheit) over the past 500 years. When the recent climate change is placed in the context of longer-term trends, the 20th century stands out as the warmest century of the past five.


This graph is a borehole-based global climate reconstruction for the past 500 years, with comparison to the meteorological record (red line). The results of this climate reconstruction are consistent with those based on other data, such as tree-rings, pollen, and ice cores from glaciers.

A very deep sample

This sample from a drill hole comes from near the bottom of the deepest hole ever drilled in Michigan, going down more than 17,000 feet. That's over three miles!
thermometer rendering

An Earth thermometer

Pollack and Huang take Earth’s temperature with digital thermometers much like the ones we use to check for fever when we’re sick. In the piece of equipment you see here, the thermometer sticks out from the bottom of the brass housing, and is protected by a wire frame. The thermometer unit is lowered down a hole bored into Earth from the surface, hundreds or thousands of feet deep. An electrical cord comes from the thermometer and connects to a digital readout unit at the surface.











Climate change: Warmer... but why? | Upsetting the balance | Mammals on the move
Connecting climate and disease | How do sea shells record temperature change? |Taking Earth's temperature