Laura Nielsen for Frontier Scientists –
We can't use time machines to go back and take the Earth's temperature during ancient times, yet we need past records of climate data to help calculate Earth's history, where we are now, and what our planet will look like going forward. Paleoclimatology studies ancient climates with the use of proxy data, data from natural sources that we can seek out today to learn more about the past.
Clams build their shells larger layer by layer from calcium carbonate (CaCO3). When scientists analyze shellfish shells, they measure oxygen isotopes within the calcium carbonate and use them to track what temperature the water was at the time the layer was formed. Since some species of clams can live hundreds of years and shells grow continuously, studying mollusks and mollusk fossils is great way to uncover localized variations in the ocean's temperature. Corals can be used in much the same way.
Foraminifera, shelled microorganisms, also build their shells of calcium carbonate. Some types float in open water, others live and die at the bottom of the sea or on lake beds. The data found in their shell remains can map climate conditions, and it's a great proxy source because Foraminifera can live all over the world. They've been around for the last 500 million years.
The tree rings inscribed in tree trunks tell a tale of climate history. Years of plenty, years of drought, and the occurrence of wildfire are laid out by tree rings. At the advent of the last great ice, age, the boreal forests of North America shifted south over many hundreds of years as ice expanding from the north made northern climates less favorable to growth. A given tree species grows best in a particular climate; if we know the climate the tree favored, tree remains can tell us what past climates were like. Even living trees give clues: many of California's mighty sequoias show burn scars from old wildfires that charred their trunks.
Plant pollen, fossilized or preserved, can help reconstruct past climate conditions — plant species thrive under particular climate conditions with temperatures and moisture levels that favor their species. If local plant populations faded out and were replaced by different dominant species, it's a good sign that ideal growing conditions and the climate changed. Every species' pollen has a unique shape. And plant remains tell a story about where Earth's carbon rests. During photosynthesis, different species of plants take in different amounts of carbon from the atmosphere. A very dry climate favors plant species that take on more carbon (the first organic compound formed during their photosynthesis has 4 carbon atoms). In contrast, a wet climate favors species which take on less carbon (forming compounds with 3 carbon atoms) but which must sacrifice water through minuscule openings in their leaves called stomata while they perform photosynthesis.
Hyrax are small mammals that eat only plants. They live in family colonies, and like to urinate in the same location over and over again. Over many generations, the dried urine, hyraceum, builds up in layers, trapping evidence of plants and plant pollens in the layers. Since Hyrax have existed for 37 million years, deposits of fossilized hyraceum let scientists study what local plant life was like long ago, giving important clues about climate.
The woolly mammoth became extinct about 4,000 years ago. The mammoths' stories and fossils provide a model of how animal populations might react to changing climate. Mammoths lived on grass and tiny willow plants in the ancient Arctic. Then, about 12,000 years ago, a warming climate (transitioning much much slower than today's climate!) transformed their habitat, introducing wetlands, peat bogs, and eventually boreal forests. The mammoths weren't adapted to graze on the different plant life, and they faced competition for resources from animals that did favor those plant biomes. Mammoths were also hunted down by humans. Scientists have used radiocarbon-dated fossils of mammoths and ancient plants from Siberia to map the mammoths' changing habitat over time.
Ice cores & sediment cores
Another record of the past rests in Earth's ice caps. Researchers drill a cylindrical tube into thick ice and extract ice cores, long pillars of ice. When examined closely, the ice cores show a pattern of dark compact winter ice and lighter layers of summer ice. The layers reveal the amount of local precipitation from every year. The ice traps and preserves bubbles of ancient atmospheric gas as well as dust, pollen, and ash from wildfires and volcanic eruptions. The European Project for Ice Coring in Antarctica (EPICA) collected Antarctic cores that help detail yearly climate for the last 800,000 years.
For an even more ancient proxy record, scientists can look to sediment cores. Lake El'gygytgyn lies in Siberia, Russia, marking the site of a meteorite impact that slammed into Earth 3.6 million years ago. The impact formed a hard layer of bedrock. So when scientists extracted sediment cores from the bottom of the lake, they could be certain the sediment (mud composed of dead plant matter, tiny ancient animals, fossils, algae pollen, dirt, and ash) represented a record stretching back 3.6 million years ago. Sediment cores drawn from the ocean bottom can be even older.
Evidence of volcanic eruptions help us keep a tab on climate influences. When a volcano erupts, it releases particulate matter and sulfur dioxide gas high into the atmosphere. The sulfur dioxide reacts with water vapor in the air and forms sulfuric acid, tiny dark aerosol particles. These dark particles reflect sunlight back into space, meaning less sunlight reaches the ground. After a big enough eruption, the surrounding region can experience lower temperatures as well as crop failures because of the stunted sunlight. Volcanic eruptions thus have influenced historic climate, and could do so in the future. Volcanic ash found in archaeological sites can help date the site, because ash can be tested and linked to specific historic eruptions.
Understanding our climate past through paleoclimatology and proxy data helps us build models of Earth's climate future.
Frontier Scientists: presenting scientific discovery in the Arctic and beyond
- 'Proxy Data: A Paleo Perspective' National Climatic Data Center, National Oceanic and Atmospheric Administration (2008)
- 'Researchers study woolly mammoth for clues on climate change' Amanda Alvarez, Journal Sentinel (2012)