Chapter 16Chapter 16

Climate Changes:Climate Changes:Past and FuturePast and Future

Climate change can be defined as a change in any statistical property of the atmosphere,

such as a change in mean temperature.

However, changes in climate may occureven though the mean values of precipitation,

temperature, and wind remain the same over time.

The geologic column.

For most of its life, Earth has been largely free ofpermanent (year-round) ice. It is a warm planet,

punctuated by perhaps seven relatively brief ice ages.

The warm times persist for hundreds of millions of years tobillions of years, whereas the ice ages last on the order of

tens of millions of years to perhaps a hundred million years.

Oscillations in temperature and ice cover are calledglacial/interglacial cycles.

In the depths of the last glaciation, around 20,000 years ago,land ice covered much more area as seen in the map above.

Sea level was about 120 m lower than it is now, so that a land bridge existed between Siberia and Alaska.

Warming began about 15,000 years ago, interrupted about2,000 years later by the Younger Dryas, a time when

colder conditions returned for about 1,200 years.

11,800 years ago another period of abrupt warmingbegan bringing climate into the present interglacial.

There is evidence that the period a.d. 900–1200 was warmin the North Atlantic. This Medieval Warm Period,coincides with the Viking settlement of Greenland.

The so-called Little Ice Age, from 1450 to 1850,was a cold period for western Europe as alpine glaciersadvanced and temperatures fell by about 0.5 to 1°C.

Persistent oscillations emerge for every time period goingback 500,000 years. These millennial-scale oscillations appear at intervals of 6000, 2600, 1800, and 1450 years.

Such oscillations suggest that the Earth-atmospheresystem has a tendency to flip back and forth between

warm and cold states, independent of long-term climate.

The Maunder Minimum was a period of minimal sunspotactivity between about 1645 and 1715, which coincided

with one of the coldest periods of the Little Ice Age.

However, there have been episodes in which variationsin sunspot activity did not coincide with climate change.

The quasi-biennial oscillation (QBO) refers to a patternof stratospheric winds over the Tropics that tend to reverse

their direction in approximately two-year cycles.

When the QBO is in its west-to-east modethere appears to be a relationship between the number of sunspots and winter conditions over northern Canadaas surface pressure rises and falls with sunspot number.

When the QBO is in its east-to-west phase, no such connection is evident.

Three astronomical factors influence the timing and intensity of the seasons:

eccentricity in the orbit,

Earth’s axial tilt off the perpendicularto the plane of the orbit,

and the timing of aphelion and perihelion relative to the timing of the equinoxes.

These three factors make up the Milankovitch cycles.

The eccentricity of Earth’s orbit changes cyclically with acycle of about 100,000 years being especially prominent.

Over about the last 15,000 years, there has been a steadydecrease in eccentricity, which will continue for 35,000 years.

Obliquity is the tilt of Earth’s axis, which also varies cyclicallywith a dominant period of about 41,000 years during which

it varies between 22.1° and 24.5° off the perpendicular.

The most recent peak in obliquity occurred 10,000 years ago,thus, we are about midway in the half cycle

from maximum to minimum obliquity.

Precession is the change in the orientation of the Earth’s axis.Combined with changes in the orientation of the elliptical orbit,

the result is a 23,000-year cycle in radiation.

The breakup of Pangaea (the early supercontinent) and theslow movement of the resultant continents undoubtedly

caused major climatic changes because all the factors thataffect climate variables were themselves greatly affected

by the movement of the continents.

Atmospheric turbidity refers to the amount of suspendedsolid and liquid material (aerosols) contained in the air.

Aerosols directly affect the transmission and absorption ofboth solar and infrared radiation. Aerosols can also affect

climate indirectly as cloud condensation nuclei.

Ship tracks appear as white streaks embedded in a low-levelcloud deck of speckled light gray clouds. The image shows an areajust offshore of the U.S. with small amounts of cloud-free ocean.

Numerical models indicate that increasedtropospheric aerosol contents have the net effect

of reducing surface temperatures globally.

Stratospheric aerosols tend to be smallerand have lower terminal velocities.

Because they are small, the reduction in solar radiationreaching the surface exceeds the gain in longwave radiation.

Since the middle of the nineteenth century, there has beenan exponential increase in the input of carbon dioxide

to the atmosphere by fossil fuel consumption.

However, carbon dioxide is only one of severalanthropogenic greenhouse gases that absorb outgoing

longwave radiation. Methane, nitrous oxide, andchlorofluorocarbons are also effective absorbers

whose contents are currently increasing in the atmosphere.

There is a constant exchange of carbon dioxide between the ocean and atmosphere with the ocean

acting as a net sink for the greenhouse gas.

Photosynthesis by marine plants removes carbon and settlingplant and animal remains transfer carbon downward.

If the removal rate were to decrease, there would be anaccelerated increase in atmospheric carbon dioxide levels.

Feedbacks refer to the interconnections of different components of the atmospheric systems.

In a simple system that consists of only two variables,changes in either one affect the state of the other.

If there is a change in the first variable, it will produce a change in the second,

which will in turn affect the first variable.

Negative feedbacks inhibit further change and areself-regulating. When the second variable responds to

initial change in the first variable, its responsewill suppress further change in the first.

Positive feedbacks amplify change in the initial variable.The response of the second variable causes the

initial change to grow in a snowball effect.

Among the feedback mechanisms affecting climate are:

Ice-Albedo FeedbackEvaporation of Water Vapor

Ocean-Atmosphere InteractionsAtmosphere-Biota Interactions

Insights into paleoclimates or past climates are based oninformation left in the geological and biological records.

There are a number of techniques for studying past climates.

Oceanic DepositsIce Cores

Remnant LandformsPast Vegetation

Relic Soils

Remnant Landforms

Layers of sediment composed of large material canindicate the occurrence of heavy precipitation in the past.

Pollen diagrams provide information on past vegetation at a site,which is useful for determining past climates.

Past Vegetation

The final chapter examinesThe final chapter examinesatmospheric optics.atmospheric optics.