Long-Term Changes in Climate Notes

Studying Climate Change Scientists study ancient climates to find out how climates have changed during the history of Earth. Changes in climate may have affected many regions around the world. For example, magnolias and palm trees have been found in Greenland. How is this possible?

Arthropleura is a 2m-long, extinct centipede species that lived during the Carboniferous Period (340-280 million years ago). By studying the plant fossils buried with the Arthropleura fossils, scientists can determine what the climate was like in that part of the world from over 300 million years ago.

Studying Climate Change If scientists find a million-year-old palm tree in the fossil record, they can be sure that that particular tree would require a hot and rainy climate back then, just as palm trees require a hot and rainy climate today. This is known as uniformitarianism. In other words, “the present is the key to the past.”

Studying Climate Change One source of information about ancient climates is pollen. The bottoms of some lakes are covered with thick layers of mud and pollen that fell into the lake over thousands of years. By looking at the layers and determining which plants produced which pollens, the scientists can determine the climates surrounding the lake during different parts of the past.

Studying Climate Change Another source of information about ancient climates is tree rings. A tree ring forms every year that a tree grows. The distance between rings tells scientists how much that tree grew in one year. The amount of growth depends on the age of the tree, the temperature, and the amount of moisture and nutrients available.

Ice Ages An ice age is a period in Earth’s history when glaciers covered large parts of Earth – causing the sea level to drop. Over millions of years, there are warm, interglacial periods and cold, glacial periods. Glaciers transform the landscape by carving massive grooves into the bedrock, moving huge boulders hundreds of kilometers, and depositing huge amounts of sediments.

Ice Ages Most ice ages last 100,000 years or more. The last ice age ended about 10,500 years ago. When that ice melted and the glaciers receded (moved toward the North Pole), the Great Lakes were left behind.

Natural Causes of Climate Change Over thousands of years, Earth’s orbit around the sun shifts a little. The tilt of Earth’s axis also changes slightly. The axis also “wobbles” like a slowly-spinning top. A Yugoslavian scientists named Milutin Milankovitch was the first person to propose that these changes in Earth’s orbit and tilt could result in global climate changes. These are known as Milankovitch Cycles.

Due to the interactions of the sun and other planets, Earth’s orbit shifts between more circular and more elliptical over a 100,000 year cycle. This results in more extreme differences in the seasons when the orbit is more eccentric (more elliptical).

When the tilt is greater, there are hotter summers and colder winters. Every 41,000 years, Earth’s axis moves from a 24.5°tilt to a 22.1 °tilt. When the tilt is greater, there are hotter summers and colder winters. Currently, we are at 23.5° and getting smaller. This should mean that summers are gradually getting cooler and winters becoming warmer, but 20th century measurements show a warming trend with glacial melting. Why?

Over a 26,000 year cycle, Earth’s axis goes through a full precession (or wobble). This means that summers and winters will reverse as the axis changes its tilt direction relative to the sun.

Natural Causes of Climate Change Sunspots are dark, cooler regions on the surface of the sun related to the sun’s magnetic field. When there are many sunspots, Earth’s temperature may increase. Sunspots follow a regular cycle every 11 years. During these 11-year cycles, the sun’s output energy is greatest when the amount of sunspots is at their highest.

Natural Causes of Climate Change When volcanoes erupt, they release massive amounts of gases and ashes into the atmosphere. These materials can block sunlight. As a result, huge volcanic eruptions may lower Earth’s temperatures.

Natural Causes of Climate Change About 225 million years ago, there was a single continent on Earth called Pangaea. A buildup of heat beneath Pangaea would eventually break apart the supercontinent. As the smaller pieces drifted apart (and still do) their climates change in relation to their latitude, proximity to currents and winds, and natural geography (mountains).