Weather and Climate Lecture 7 Climatic Change Are we living in an ice age??

Weather and Climate Lecture 7

Climatic Change

Are we living in an ice age??

Introduction

Climate: statistical properties of atmospheric variables:– Temp– Precipitation– Wind, etc– Relatively constant over a period of 35 years

Climate change

A change in any statistical property of the atmosphere

Can still occur without any change in annual average precipitation– Timing of heavy rainfall or drought– Snow as precipitation instead of rain, are all

indicators of climatic change

Earth’s Past climates

Present climate on earth highly ‘unusual’– Today, Arctic sea mostly frozen, ice sheets over bulk of

Antarctica– Throughout earth’s history, palaeoclimatic studies:

Earth was predominantly warm throughout Punctuated with brief ice ages Warm periods lasted up to billions of years Ice ages lasted a hundred million years at the most Can be said that we are currently in an ice age

Reasons for climatic change

Sunspot Activity– Dark regions which form on the photosphere of the

sun– Diameters of about 10,000km, 1500 deg C cooler

than surrounding– Form as a result of strong magnetic fields; peak

every 11 years– Cause periods of less insolation and therefore less

intensity and temperatures, therefore possible climate change

Reasons for climatic change

Earth Geometry and Milankovitch cycles– Geometry of the earth varies through time– Amounts of insolation received as geometry varies

will affect the amount of insolation, hence temperatures and therefore climate

– 3 ways in which earth’s geometry varies, known as Milankovitch cycles

Milankovitch cycles

Earth’s orbit around the sun changes from being near-circular to more elliptical on a cycle of about 1 million years

The earth rolls or wobbles slightly on its axis on a cycle of 40,000 years

Axis of the earth moves round slowly on a cycle of 20,000 years, affecting the distance of different areas from the sun on midsummer’s day

Milankovitch Cycles

Cycles mainly change the distribution of energy over the earth’s surface

Have a large effect on seasonality without changing the actual amount of energy within the season

There must therefore be other factors at play in contributing to climate change

Plate Tectonics movement of plates

M move land areas into different climatic regions if moved to the poles, or surrounding, encourages ice

sheets to develop may therefore prevent warm ocean currents from

reaching the poles, so snowfall is less inclined to melt and can accumulate to form ice sheets

climate models also show that without the Himalayas and the Tibetan Plateau, the Indian monsoon would not occur

for almost 250 million years the earth’s climate was warm and wet, then about 40 million years ago, a marked cooling began

coincided with major plate movements, including the merging of India with Asia that led to the uplift of the Tibetan Plateau

El Nino

- El Nino Southern Oscillations (ENSOs) - occur once every 2 to 9 years, triggered by a

reversal in the normal westward flow of the Trade winds and ocean currents that flow across the tropical Pacific from the Americas towards Asia

- produces unseasonable weather throughout the globe

La Nina

- like ENSOs, occur at irregular intervals, but roughly half as frequently as El Nino

- brings about climatic effects opposite of El Nino

- warmer, wetter weather in western Pacific - colder, drier conditions prevail in the tropical

eastern Pacific

Volcanic Activity

principally has a cooling effect on the earth’s climate

volcanic ash in atmosphere leads to the alteration of the earth’s albedo (increases), and increase in reflected energy

gases emitted may also react with ozone in upper atmosphere and destroy ozone; implications for global warming as ozone also reflects insolation

Deforestation

- removal of trees and forests on a large scale can change the microclimate

- less trees, more carbon dioxide (greenhouse gas), higher amounts of heat re-radiated after absorption of heat by carbon dioxide, higher temperatures

- however, debatable

Pollution

- burning of fossil fuels and emission of gases

- absorb outgoing energy

- overall warming effect on the earth, but debatable

Evidences for climate change

The evidences for climate change stem from: - Glaciological Evidence - Geological Evidence

- Biological Evidence


The following glaciological evidences suggest climatic change

Retreat of Glaciers– Study of glacial retreat in Northern Hemisphere– Equated to the melting of glaciers– Reflection of a general rise in temperatures


Study of Ice cores and isotope analysis– Ice cores from Greenland and Antarctic icesheets,

as well as Alpine glaciers– Oxygen isotope analysis: – At the end of the Pleistocene, temp over Greenland

warmed about 9 deg C


In addition, study of the amount of carbon dioxide locked in ice cores also show climatic change– Past periods of high temperatures coincide with high

carbon dioxide concentrations– Glacial periods with reduced carbon dioxide

concentrations– Ice cores have shown different layers of different

concentrations of carbon dioxide, hence suggesting climatic change


• Geological Evidence is widely used to indicate past climate changes

• Drainage Patterns

•The extent and size of ancient drainage patterns and fluvial deposits in north Africa

•- cannot be explained by present climates

- reflect past, wetter climates known as pluvials

Evidences for climatic change

Sediment layers of streams and rivers– Size of material in sediment layers determined by

capacity and competence of streams– Determined by speed and volume of discharge, in

turn affected by climate– By examining the layering, or stratigraphy, some

inference of periods of low and high precipitation episodes can be determined


Biological Evidence like plant growth is strongly related to climate

Fluctuations in:– Temperature, sunlight, precipitation– Will have an effect on vegetation type and growth– Scientists have used this information to study past

climates


Dendrochoronology– Involves the study of the annual growth of tree rings– Number of rings within the trunk tells the age,

forming concentric rings– Relative spaces between each ring suggests how

much the trunk has increased in size in a year


Dendrochoronology– Width of each ring:

Stressful conditions will retard the growth of these rings

Favourable conditions, relatively think ringsCores from tree trunks which have lived for

thousands of years:Variations in relative spaces between rings,

suggesting changes in climate


Pollen Analysis– Based on the idea that as climate changes, so will

the dominant species of plants in the community– Study of peat layers where marshes and bogs are

found:– Different species of pollen grains found in different

layers of peat– Suggests a transition of one type of dominant plant

species to another as a result of climate change

Documents

Weather and Climate Lecture 7 Climatic Change Are we living in an ice age??