Studying Past Earth Climates

The natural history of this planet is fraught with biological changes.  Scientists seek to determine the reasons behind these changes by reconstructing past environmental conditions.  Reconstructing ancient environments without direct observational record can be challenging.  Fortunately, some very important clues can be found in the sediments of lakes and peat bogs.  These clues, known as climate proxies, can be used to reconstruct past climate conditions. Scientists who study past Earth climates and how they have changed are called paleoclimatologists.  By comparing data collected by paleoclimatologists with modern scientific analysis, researchers believe they can form trends that can help them accurately predict future environmental changes. 

One method that paleoclimatologists use to determine the environmental conditions of Earth’s history is pollen analysis.  This is the analysis of the distribution of pollen grains in sediments of peat bogs, bogs formed when mosses, herbs, and other plants rot in acidic water.  Scientists began studying the sediments from peat bogs for pollen concentrations in the early 1900s.

Peat bog sediments can provide climate data from as far back as 20,000 years.  The type of pollen which is most abundant within a sample from a particular layer gives information about what type of tree was most abundant during that time period, and reveals information about the climate.  For example, high levels of pollen from spruce or fir trees indicate warm conditions while high levels of pollen from beech trees indicate periods of cool temperatures.  Natural factors that could cause large scale temperature changes are volcanic eruptions, changes in the Earth’s orbit, and activity on or within the sun that would alter the intensity of sunlight. 

Peat bogs also contain fossilized leaves that can tell paleoclimatologists about changes in levels of carbon dioxide in the atmosphere throughout Earth’s history.  Leaves have pore-like openings called stomata on their surfaces.  The number of stomata tells paleoclimatologists about carbon dioxide levels during a particular period.  Low numbers of stomata on leaves indicate high levels of carbon dioxide in the atmosphere.  Carbon dioxide is a greenhouse gas, and an abundance of it leads to rising global temperatures.  Recent studies of leaf stomata have shown that atmospheric levels of carbon dioxide are the highest they have been in the past 16 million years.

Sediment from lakes, which also contains leaves and pollen, is usually less disturbed and can provide more extensive information about past temperatures and carbon dioxide levels than peat bogs.  However, lake sediments also reveal information about water temperatures and climate change.  It was not until the 1990s that scientists began studying lake beds ingreat detail. 

One way that paleoclimatologists can collect information about water temperatures is by studying lake beds for the fossilized shells of microbial aquatic organisms called diatoms for oxygen isotopes.  The most common oxygen isotope in nature is called oxygen-16, because an atom of it has eight protons and eight neutrons in its nucleus.  Although all atoms of oxygen have eight protons, they can have as many as ten neutrons.  Diatoms tend to incorporate a greater number of heavy isotopes in their shells because water containing heavy isotopes condenses faster than water containing light isotopes and is more abundant in lakes.  Shells formed in cold waters have greater proportions of heavy isotopes than normal, so a layer that contains shells abnormally high in heavy isotopes is indicative of cold temperatures.  Examining differences in shell compositions at different layers and different points on the globe helps paleoclimatologists determine how weather patterns and ocean currents have changed over time.

Paleoclimatologists can also determine water conditions by examining populations of organisms such as midge flies, which flourish in cold climates.  A study of an Arctic lake conducted by researchers at the University of Colorado at Boulder in 2009 showed declining midge fly populations in the lake.  In fact, two species which had adapted to the coldest temperatures in the region had disappeared completely.  The study also revealed recent increases in a species of diatom that was previously rare in the region.  All this comes at a time when natural processes should lead to cooling in the Arctic.

Information about climate changes can be obtained by examining varves in lake beds.  Varved sediments serve as a record of environmental change in a region.  They are formed by clay and silt settling at the bottom of lakes.  Varve thickness is a function of the amount of clay or silt that is blown into a region.

In addition to trying to understand the natural causes of environmental change, paleoclimatologists use the data they collect from lake and bog sediments to try to understand the influence of humankind on changes in climate.  Scientists have already determined that the global average temperature has risen by two degrees in only 400 years.  This has resulted in a recent surge in strong hurricanes near the equator, wildfires, droughts, and increased temperatures near the poles.