Ch. 18: Earth's Changing Climate

Global Climate Change: 100-150 Years

100 to 150 years: there has been a warming of 0.85 degrees Celsius from 1880 to 2012, but it has been inconsistent. The speed of warming over ~1000 years is fast, the largest increase in temperature of any century during the past 1000 years. Most of the warming has happened over land (closer to the poles), and more in the Northern Hemisphere than the Southern Hemisphere. Scientists blame carbon dioxide and GHG's. As far as precipitation goes, some areas are rainier and some are drier.

Global Climate Change: 1000 Years

1000 years: "Hockey Stick" model. Relatively flat for ~900 years, then a rapid spike in the last 100 years. There is a clear connection between the increasing concentration of carbon dioxide in the atmosphere in the last 100 years leading to warmer temperatures.

Dendrochronography

Dendrochronography is the changes in tree ring thickness. This goes back only ~500 years, the age of the oldest trees. Each year contains a light and a dark ring, an earlywood and a latewood. Wider tree rings represent wet, warm years. Thin tree rings represent cold, dry years. Usually it is necessary to match partial records from multiple trees to obtain a climate record. This is typically used to record short term climate change (hundreds of years).

Global Climate Change: Over 1000 Years Back

Even farther back: Glaciation about 18,000 years ago covered a lot of North America and Western Europe. During the last warm spell, 125,000 years ago, sea level was about 18 feet higher than it is today (if all the ice on Earth today (about 10% of surface), glaciers over poles, melted RIGHT NOW, sea level would rise by 213 feet). There is a consistent pattern of longer cooling (ice ages: cooling trend where polar ice appears and grows, and glaciers advance; glacials; temperatures were 9-18 degrees Farenheit colder than today) and warming between them (interglacials; on average last 10,000 years). The Hothouse Earth was 52 million years ago, 18-27 degrees Farenheit warmer than the Earth now (this is about when alligator fossils were found in Canada). Scale makes a huge difference in these conversations about climate.

Positive vs. Negative Feedback

Feedback loops modify atmospheric processes. Positive feedback enhances natural change, and negative feedback counteracts natural change. Examples of positive feedback mechanism/loops - Ice albedo feedback: temperature increases, ice coverage decreases, albedo decreases, Earth absorbs more heat, and temperature increases all over again. Carbon Dioxide cycle: as temperature increases, oceans release more carbon dioxide, and temperature increases again. Water Vapor-GHG feedback: temperature increases, more water vapor in the atmosphere, temperature increases again. Example of negative feedback mechanism/loop: temperature increases, water vapor increases, precipitation increases in the form of snow, albedo increases and therefore absorption decreases and temperature decreases.

Fossil Evidence

Fossil evidence shows paleoclimates (past climates), and use fossils of organisms to tell us what conditions were like. This goes back ~66 million years, around when the dinosaurs went extinct. This gives evidence of a thick ice sheet at one time throughout the southern continents, rocks forming in tropical conditions (coal swamps), and crocodile fossils in Canada.

Oxygen Isotope Ratios

Oxygen isotope ratios allows for different forms of a chemical (O16 to O18 is the only ratio considered in a sample) to be present in analysis. Oxygen comes in light and heavy varieties (isotopes), and the relative amount of oxygen in water changes with climate.

Tools to show past climate

Proxies are used to represent climate changes; use a variable other than temperature. Fossil Evidence Sediment Cores/Ice Cores Oxygen Isotope Ratios Dendrochronography

How do mountain ranges affect precipitation patterns? What is a rainshadow desert and where does it form?

Rainshadow desert forms on the leeward side of a mountain, and is dryer and warmer than the windward side (where air rises and condenses and forms clouds and storms). Mountain ranges tend to ignite precipitation on the windward side of the mountains, because that is air's method of rising to pass over them.

Main consequences of Climate change?

Rising sea level, possible effects on jet stream and global circulation patterns, longer/earlier and hotter summers, severe droughts in some places and flooding in others, water contamination, retreat of mountain glaciers, changes in ecosystems and extinctions, and so on. Some scientists believe global warming creates more good than harm, for example because carbon dioxide is good for plants and warmer temperatures will allow agriculture in areas that are currently too cold to support it. This is debatable. Surface warming is expected to continue, and precipitation is generally expected to increase in areas where it already prominent.

Sediment Cores/Ice Cores

Sediment cores (calcium carbonate shells, otoliths) and Ice cores are analyzed by drilling down into the sediment/ice and pulling up a sample, and analyzing its date/composition. This goes back ~800,000 years (used in Greenland and Antarctica often). Ice cores sometimes contain trapped air bubbles, which can be analyzed alongside stable isotope ratios and pollen trapped within the layers to infer past climates; the colder the air when the snow fell the more oxygen in the core. Sediment layers can indicate sedimentation rates over time. Remains of organisms, such as diatoms, foraminifera, and pollen within sediment can indicate changes in past climate, since each species has a range of habitable conditions.

How do Sunspots affect climate? What is the Maunder Minimum?

Sunspots are areas of enhanced solar output on the sun, and the time period of more solar output is on a ~11-year cycle. The Maunder Minimum is the period of time from 1645 to 1715 with very few sunspots, that resulted in cooling temperatures.

Milankovich Theory

The Milankovich Theory is that the changes to Earth's orbit amount of sunlight that reaches us. There are 3 separate cyclic movements that combine to produce variation in the amount of solar energy that reaches Earth: Eccentricity-shape of Earth's orbit (~100,000 years), Obliquity-variation in tilt (~41,000 years), and Precession-direction of tilt reverses; wobble around rotation axis (~23,000 years).

Which location is expected to have the largest increase in temperatures?

The poles (particularly the North pole) are expected to have the largest temperature increases, as past patterns have indicated because most warming in the last 100 years has occurred over land, near the poles, and more in the Northern hemisphere than the Southern.

How do Volcanic Eruptions affect climate?

Volcanic eruptions result in a temporary cooling effect on the environment (addition of sulfur dioxide), serving as a natural process of changing the Earth's atmosphere composition. The climatic impact depends on the size and location of the eruption.