Climate Change Before Humans
Earth's climate has been changing for the entire 4.6 billion years of its history. This change occurs due to many different types of processes which occur at different speeds. In Module 2, you were introduced to the concept of rates of change at geologic time scales, and will revisit that concept as we begin to look at climate change.
Prior to the human population becoming so large that it could substantially impact and change Earth (basically, prior to about 200 years ago), there were many processes and global feedbacks already in place that produced climate change, particularly at global scales. How do scientists know about these changes and processes? How do they measure rates of change of paleoclimates (meaning, climates before recorded history)? They use proxy records and models to understand what happened before humans were around to observe and record weather observations and climate changes.
One type of proxy record that is widely used by scientists to understand past climates is called a sediment core. Found in lakes, springs, and even the ocean shelves along coastlines, sediment cores are deposits of sediment laid down one layer on top of another for thousands of years. Trapped in the sediments are two particularly useful pieces of data: pollen and charcoal. Pollen tells us what kind of vegetation dominated the landscape around the lake or spring during a certain time period, and since scientists can look at what climates those same forests, shrublands, and grasslands are associated with today, they can infer what the climate of an area was for a given time based on what the vegetation was.
Charcoal is also useful because it allows scientists to identify when there were periods where many wildfires burned. In most regions, wildfires only occur during warm, dry conditions, so scientists can develop timelines of when conditions were dryer and warmer or cooler and wetter for a location.
Prior to the human population becoming so large that it could substantially impact and change Earth (basically, prior to about 200 years ago), there were many processes and global feedbacks already in place that produced climate change, particularly at global scales. How do scientists know about these changes and processes? How do they measure rates of change of paleoclimates (meaning, climates before recorded history)? They use proxy records and models to understand what happened before humans were around to observe and record weather observations and climate changes.
One type of proxy record that is widely used by scientists to understand past climates is called a sediment core. Found in lakes, springs, and even the ocean shelves along coastlines, sediment cores are deposits of sediment laid down one layer on top of another for thousands of years. Trapped in the sediments are two particularly useful pieces of data: pollen and charcoal. Pollen tells us what kind of vegetation dominated the landscape around the lake or spring during a certain time period, and since scientists can look at what climates those same forests, shrublands, and grasslands are associated with today, they can infer what the climate of an area was for a given time based on what the vegetation was.
Charcoal is also useful because it allows scientists to identify when there were periods where many wildfires burned. In most regions, wildfires only occur during warm, dry conditions, so scientists can develop timelines of when conditions were dryer and warmer or cooler and wetter for a location.
Another type of proxy record used by scientists to reconstruct past climates is an ice core. In glaciers, air bubbles get trapped when snow falls and then turns into ice (as shown in the picture at left); these bubbles contain information about the atmosphere from thousands of years ago and are laid down in layers much the same as sediments are. Scientists are able to extract these cores with the bubbles intact from places with thick ice sheets, like Antarctica and Greenland. They then carefully slice and analyze the air in the bubble to reconstruct past conditions.
From pollen cores, ice bubbles, and by mapping the landforms that are only created by and associated with glaciers, scientists have determined how the global climate has changed since the end of the last ice age. Log in to the support website for the textbook and go to the Study Area. Click on Interactive Animations, and then under Chapter 8, select the animation titled "End of the Last Ice Age." Watch the animation first, then use the animation to answer the following questions.
1) At the peak of the last ice age about 21,000 years ago (denoted 'ka' in the animation), what continent was covered by the largest extent of ice?
2) At the peak of the last ice age, did the ice sheet extend further south in the eastern half of the US or the western half? What states would you expect to find evidence left behind by the continental ice sheet today?
3) How many thousand years ago did the Beringia land bridge between present-day Alaska and Russia disappear, ending the migration of humans from eastern Asia into North America?
4) Approximately how many thousand years ago did the percent of pine forest peak in the northeastern US?
5) Compare the two maps: extent of last glacial maximum and distribution of pine forest. How would you describe the spatial relationship (i.e., the overlap) between the extent of the continental glacier and the area where percent pine forest peaked? Are they closely related? Not related at all?
6) How extensive are pine forests in the eastern US today? Why do you think pine forest density diminished to its present extent from the peak? What might be a hypothesis you could test?
1) At the peak of the last ice age about 21,000 years ago (denoted 'ka' in the animation), what continent was covered by the largest extent of ice?
2) At the peak of the last ice age, did the ice sheet extend further south in the eastern half of the US or the western half? What states would you expect to find evidence left behind by the continental ice sheet today?
3) How many thousand years ago did the Beringia land bridge between present-day Alaska and Russia disappear, ending the migration of humans from eastern Asia into North America?
4) Approximately how many thousand years ago did the percent of pine forest peak in the northeastern US?
5) Compare the two maps: extent of last glacial maximum and distribution of pine forest. How would you describe the spatial relationship (i.e., the overlap) between the extent of the continental glacier and the area where percent pine forest peaked? Are they closely related? Not related at all?
6) How extensive are pine forests in the eastern US today? Why do you think pine forest density diminished to its present extent from the peak? What might be a hypothesis you could test?