Part 6. Current, Past, and Future Climates

Chapter 16

Climate Changes: Past and Future

IntroductionClimate change is the change in the statistical properties of one or more atmospheric variables

• Climate changes on many different time scales• Climate change is greatest at the Earth’s poles

and least in tropical regions• Understanding climate change requires

understand the physical cause or causes of the climate change

The Geologic Column

Human history starts

Dinosaurs are wiped out

Major extinction of life

First multicell animals

High sea level stands during the Paleozoic

High sea level stands during the Cretaceous

Pleistocene ice ages

Age of Earth - 4.6 billion years

Past climates in Earth history can be inferred from geologic and fossil evidence

Warm Intervals and Ice Ages• For most of Earth’s history, climate was 5-

15oC warmer than present, and ice was rare• Brief cold ice ages interspersed generally

warm climate– Over past 2.5 billion years, ice ages

occurred only 10-20% of the time

The Earth began a gradual cooling phase about 55 Mya. Ice accumulated on Antarctica about 34 Mya.

By 10 Mya, Antarctica was covered with ice. By 4 Mya, so was Greenland.

The Earth has demonstrated regular glacial/interglacial cycles.

The Earth currently is in a warm phase. The last glacial phase peaked about 20,000 years ago.

The last interglacial may have been the warmest time in Earth history

• Peaked ~ 125,000 years ago• Air temperature about 2°F to 5°F warmer

than present• Sea levels about 20 feet higher than at

present

Pollen diagrams detail past vegetation and climate information

Ice extent during the last glaciation

During the last glaciation, North America was covered with ice more than 2 miles thick in places; the ice extended as far south as St. Louis, Missouri. Sea level was more than 300 feet lower than today. The extent and thickness of the ice is estimated from geologic evidence.

Ice extent about 20,000 years ago; the glacial ice started retreating about 15,000 years ago; the ice readvanced about 13,000 years ago for about 1200 years during the Younger Dryas period

Ice extent about 20,000 years ago; the ice left New England about 12,000 years ago

Earth Temperatures for the last 1000 years

Medieval warm periodLittle ice age

Current warming

July moisture Tidewater region of Virginia and North Carolina

Historic climate; dry periods affected early North American settlements

The shorter term Dansgaard-Oeschger cycles and longer-term Bond cycles attempt to explain the regularity of warming and cooling events during the past 150,000 years. These cycles are probably caused by changes in ocean circulation, atmospheric circulation, and insolation.

Factors Involved in Climatic Change• Variations in

– Insolation intensity– Earth’s orbit– Land surface changes– Atmospheric and aerosol composition

Variations in solar output• Solar output regularly changes

– 0.1-0.2% change due to sunspots– 11 year cycle for sunspots

• The Maunder Minimum was a period of few sunspots and lower solar activity around the year 1600

– The Little Ice Age occurred during the Maunder minimum

– Links to the quasi-biennial oscillation (QBO)--changes in stratospheric tropical winds associated with changes in sunspots

Early faint Sun paradox• The geologic record shows warmer early

Earth temperatures, but astrophysical models show that the sun was about 1/3 weaker than today

– The early warmth was probably caused by greater CO2 concentrations in the early Earth atmosphere

Milankovitch Cycles -- Precession

Milankovitch cycles -- regular natural variations in the Earth’s orbit around the sun

– Obliquity -- 41,000-year period – Eccentricity -- 100,000-year period– Precession -- 27,000-year period

Changes in land configuration and surface characteristics• Plate tectonics gradually changes the

configurations of the mountains and oceans• Mountain building and land erosion affect

climate over geologic time• Land use changes such as deforestation and

desertification change albedo, surface temperatures, and water balance

Changes in atmospheric aerosols affect the amount of solar energy that can reach the Earth’s surface• Major volcanic eruptions inject great

amounts of aerosols into the atmosphere over days or weeks, leading to temporary climate cooling

• Residence times of tropospheric aerosols is a few years

• Residence times of stratospheric aerosols is a few decades

Mt. Pinatubo aerosols

Ship tracks over thePacific leave clouds in their exhaust trails (excess condensation nuclei)

Changes in radiation-absorbing gases

• Anthropogenic contributions of CO2

– Increased exponentially since the mid 19th century due to fossil-fuel burning

– Increased CO2 concentrations leads to increased atmospheric absorption of IR radiation

– Increased anthropogenic greenhouse gases in the atmosphere can lead to increased atmospheric water vapor (the most important greenhouse gas)

• Exchange of CO2 between the atmosphere and ocean

– Current CO2 emission rates increasing 3.5 ppm/yr

– The oceans are a major absorber of CO2 due to oceanic biota photosynthesis and solution of CO2 in the water

– Only about 1/2 of the anthropogenic CO2 emission ends up in the atmosphere (where does the other 1/2 go?)

Feedback mechanisms are systems in which changes in one variable lead to changes in another• Feedback mechanisms can be

– Negative, where the feedback acts to inhibit further change in a variable

– Positive, where the feedback acts to magnify further change in a variable

Examples of feedbacksIce-albedo feedback (positive feedback)• Ice cover affects global albedo

Evaporation of water vapor (positive feedback)• Water vapor is a greenhouse gas

Ocean-atmospheric interaction (positive or negative feedback)• Ocean levels change through thermal

expansion and glacial melting

Sea ice

Computer models of global climate change give predictions of what future climate might be. They take into account the climate/ocean feedback mechanisms that are known. This climate prediction is for double the atmospheric CO2 over current values.

This climate prediction is for double the atmospheric CO2 over current values. While global warming would be greatest at the poles, changes in the precipitation patterns would be much more diverse across the Earth.

End of Chapter 16

Understanding Weather and Climate

4th Edition

Edward Aguado and James E. Burt