METEOROLOGY

GEL-1370

Chapter FourteenChapter Fourteen

Climate ChangeClimate Change

Goal for this ChapterWe are going to learn answers to the following

questions:• How the global climate has changed in the past

• Theories on why the global climate has changed?

If all the ice locked up in glaciers and ice sheets were to melt, what would happen to this town in coastal

south Florida? (65m or 213 ft rise)

If all the ice melts what happens??

Existence of Land Bridge in the Bering Strait

Changing Climate• Glaciers cover ~10% of earth’s land surface (mainly

Greenland and Antarctic ice sheets)• If the all glaciers and ice sheets melt, global sea level

rise will be ~ 65 m (213 feet)• Fossil pollen of a tundra plant in sediment layer in New

England dated to be 12,000 yrs --- climate was much colder; ocean sediment records and ice core records---

• ~18 kyr ago, alpine glaciers extended to river valleys and huge ice sheets covered vast areas of North America and Europe; New and Ohio River Valley were also covered with ice

• Glaciers advanced and retreated 10 times during the last 2 million years – Are we still in the ice age (warmer part of it??)

Past climate changes

• CLIMAP (Climate: Long-range Investigation Mapping and Prediction) brought wealth of information

• Studies on Oxygen isotope ratios --- 18O = [(18O/16O)sam - (18O/16O)sam]/ (18O/16O)samx 1000• Reconstruction of earth’s surface ocean temperature for

various times during the past• Vertical ice cores in Antarctica and Greenland --- info

on past temperature patterns• Glacier formation: Snow accumulates --- compaction ---

recrystallize into ice --- ice begins to move by gravity – glacier is born

• In CLIMAP, the earth’s surface ocean temp for various times in the past was reconstructed;

Sea surface isotherms during August 18 kyr ago and today (Left: 18 kyr ago)

Earth’s changing climate• Global climate was much warmer (8-15° C) than today

in the geological time frame; polar region was ice-free• 700 and 300 myr ago glacial periods existed• 65 million years ago, the earth was warmer than now –

polar ice did not exist; beginning 55 million yrs, the earth entered a long cooling trend and polar ice caps appeared; ~10 myr ago, deep blanket of ice covered Antarctic; snow and ice began to accumulate in high mountain valleys of the Northern Hemisphere, and alpine or valley glaciers soon appeared

• The Pleistocene epoch or the Ice Age began about 2 myr ago; glaciers appeared in the Northern Hemisphere, marking the beginning of the Pleistocene Epoch

Earth Changing Climate – contd.

• Pleistocene: Glaciers advanced and retreated over large portions of North America and Europe

• Between the glacial advances , warmer periods that lasted for 10 kyr or more (interglacial periods)

• During the last glacial maximum (LGM), 18-22 kyr, sea level was 126 m before present --- vast areas exposed; Siberia & Alaska were connected (migration of animals and humans from Asia to North America)

Past climate – contd.• Bubbles of air trapped in the ice --- past composition of

the atmosphere, wind direction and intensity• Dendrochronology: Changes in the thickness of the

rings indicate climatic changes from one year to the next (useful in trees that are stressed by temp or moisture during their growing season)

• Reconstruction of past climate is based on a) lake and ocean sediments; b) loess and other soil deposits c) pollen in the ocean sediments, deep ice caves, soil deposits; d) fossils in sediments; e) oxygen isotope ratios of corals and other carbonate species (foram, etc); f) cave deposits (stalactites & stalagmites)

Pollen concentration vs depth

Coiling pattern vs climate

Oxygen isotope composition

Climate change during the last ~ 2 myr

Climate through Ages – 18 kyr

• Ice began to retreat ~14 kyr ago as surface temp slowly rose; ~11 kyr ago, average temp suddenly dropped and northeastern North America and northern Europe reverted back to glacial conditions (Younger Dryas)

• ~10 kyr ago, the cold spell ended and ~8 kyr, continental ice sheets over North America disappeared;

• ~5-6 kyr BP, climate was ~1°C warmer than at present (Mid-Holocene maximum

• Holocene Epoch: The second epoch of the Quaternary Period, beginning ~10 kyr ago and continuing to the present time

• ~5 kyr BP, cooling trend set in, alpine glaciers returned, but not continental ice sheets

Average air temp variations for the past 18 kyr

Average temp variations of eastern Europe for the last 1200 yrs

Climate during the last 1 kyr

• Ice core date from Greenland reveal rapid shifts in climate (from ice stage conditions to a warm state)

• ~1200 AD, mild climate of western Europe changed to extreme variations; climate grew stormy; great floods and great droughts occurred; cold winters followed by relatively warm ones; several famines during the 1300s

• Little Ice Age: Period from ~1550 to 1850 when average global temperatures were lower (0.5°C), and alpine glaciers increased in size and advanced down mountain canyons

• 1816: Unusually cold summer followed by a bitterly cold winter

Climate during the last 1-kyr – contd.• 1970s – early 1980s: Average yearly temp showed

considerable fluctuation from year to year & from region to region, with overall trend pointing to warming

• Warning trend continued into the 1990s• 20th century warming is ~0.7°C; warming is not uniform;

greatest warming over the mid-latitude continents in winter and spring while a few areas (North Atlantic Ocean) have actually cooled in recent decades; US has experienced little warming

• Warming of 0.7°C is quite significant, as the global temp have varied no more than 1.5°C during the past 10,000 yrs

• Part of the warming is due to an enhanced Greenhouse effect caused by increasing levels of greenhouse gases

Causes of Climate Change• Intricate relationship between atmosphere-ocean-

continent coupling --- not fully known• Feedback mechanisms is not fully characterized • Positive Feedback: Initial increase in temperature is

reinforced by the other process • Example: Earth has entered a slow warming trend ---

temperature rises --- ocean water evaporates rapidly --- water vapor content in the atmosphere increases --- more IR energy is absorbed --- leads to warming --- more water vapor evaporation --- and so on – Water vapor-temperature rise feedback – Such a chain reaction is called ‘Runaway greenhouse effect’ – no evidence that a runaway greenhouse effect ever occurred on earth!

Causes of climatic change – contd.

• There are checks and balances; for example, when air temp increases, outgoing IR energy increases and this helps to slow the temp change and stabilize the climate

• Example for negative feedback: Air warms --- more evaporation --- more moisture (and precipitation) --- global low cloudiness increases --- large percentage of incoming sunlight is reflected --- less energy to heat the surface --- cooling

• All feedback mechanisms work simultaneously in both directions; example: increase in global surface air temp --- to melt snow and ice more --- decrease in albedo --- more solar radiation to reach the surface

Climate change and feedback mechanisms

• Lower temp --- greater snow cover --- increase in albedo --- more light reflected back --- further drop in temp ---runaway ice age --- not found on the earth

• CLIMATE CHANGE, PLATE TECTONICS, & MOUNTAIN-BUILDING:– Glacial features found near the sea level in Africa today – area

underwent a period of glaciation hundreds of millions of years ago

– Fossil remains of tropical vegetation can be found under layers of ice in polar regions today

If land masses are redistributed (concentrated in middle & high latitudes), ice sheets are likely to form (more light reflected --- cooling) –influence the path of ocean currents --- affect the transport of heat & wind

Relationship between global change and plate tectonics

• Rate at which plates move (currently 1-10 cm/yr) might be related to the climatic changes took place in millions of years; rapid spreading, a relatively wide ridge forms, causing sea level to rise relative to the continents; at the same time, an increase in volcanic activity vents large quantities of CO2 into the atmosphere --- increased greenhouse effect --- increase in global temperature

• Increased temperature leads to high precipitation --- increased chemical weathering and hence faster removal of CO2

• Mountain building processes lead to collision of two continental plates --- can have a marked influence of global circulation patterns (e.g., Tibetan Plateau)

Geographical distribution of landmasses 180 myr ago (a) & today (direction of continental movement)

During times of rapid spreading, increased volcanic activity may promote global warming by increased CO2

emission

Milankovitch Theory• Three separate cyclic movements combine to produce

variations in the amount of solar energy that falls on the earth– Changes in the eccentricity of the earth’s orbit (100,000 yrs);

the difference in distance is only 3% while the increase in the solar energy is ~7% received at the top of the atmosphere from July to January; when the difference in distance is 9%, the difference in solar energy will be ~20%

– Precession (23,000 yrs) of the equinoxes; as the earth rotates on its axis, it wobbles like a spinning top; Presently, the earth is closer to the sun in January and farther away in July. Due to precession, the reverse will be true in ~11,000 yrs

– Changes in the tilt of earth (41,000 yrs); present tilt is 23½°, but during the 41,000 yr cycle it varied from about 22° to 24½° ; smaller the tilt, less variations between seasons

Earth’s orbital changes (eccentricity) – 100 kyr cycle

Milankovitch theory

(a): Changes in the tilt (41 kyr); (b): presently earth is closer to the sun when N.Hemisphere is in winter; (c) 11 kyr from

now, earth will be closer to sun in July when N. Hemisphere is in summer

Past climate changes

• CLIMAP project results: during the past 800 kyr, ice sheets have peaked about every 100 kyr; superimposed on this are smaller advances that show up at intervals of about 41 kyr and 23 kyr

• CO2 levels were about 30% lower during glacial periods than during interglacial periods; ice core studies also reveal CH4 follows CO2 pattern

• Why CO2 levels have varied as glaciers expanded and contracted – Due to changes in the biological productivity (phytoplankton productivity)

• Changing levels of CO2 indicate a shift in ocean circulation patterns---changes in the precipitation & evaporation rates & wind intensity and direction

Climate Change - contd• Other factors that can affect temperature variations are:

– Dust and other aerosol contents in the atmosphere

– Changes in the albedo of the ice sheets and clouds

– Isostatic changes due to depression by ice (during interglacial period)

– Presence of other trace gases

Aerosol sources: Factories, auto emissions, wild fires, dust storms, etc; some particles (soil dust & sulfate particles) reflect and scatter incoming sunlight while others (smoky soot) absorb sunlight – warming

Overall net effect of human-induced aerosols on climate is to cool the climate

Sulfate aerosols: serve as cloud condensation nuclei, and scatter and reflect incoming sunlight

Volcanic eruption and climate

• Sulfur-rich gases have the greatest impact• Two largest volcanic eruptions in the last 100 yrs are: El

Chichòn in Mexico (April 1982) & Mount Pinatubo in Philippines (June 1991)

• Mathmatical models predict global mean temp can drop by 0.2 to 0.5°C

• Acidic ice was uncovered from ~AD 1350 to ~ 1700 – corresponding to Little Ice Age – sulfur-rich volcanic eruptions may have played an important role in triggering this cool period? Other cool periods were also triggered by such events??

Changes in average global air temp from 1990-1992 after eruption of Mt. Pinatubo, June 1991; temp decreased by

~0.5°C

Variations in Solar Output

• 11-year sunspot cycle• Sun’s energy output appears to change slightly with

sunspot activity• Maunder minimum: Between 1645 and 1715, there were

few sunspots; this period is called so• Changes in sun’s energy output – cause for climate

change??

CO2, Greenhouse effect and global warming

• Annual average in 1999 368 ppm; 1.5 ppm/yr increase; 500 ppm by end of this century

• Increase in CH4, N2) CFCs effect ~ CO2 in their ability to increase greenhouse effect

• Temperature increase of 1-3.5°C

Human activity contributes to global warming

Ocean conveyor belt in the N. Atlantic

chapter –14- Summary• Variations in the oxygen-18/16 ratios-climate info• CLIMAP Project; global climate change in the past• Glaciers – glacial and interglacial periods• Effect of volcanoes on global change• Positive and negative feedbacks• Milankovitch theory – 3 periodicities; factors that cause• Sunspot cycle and climatic changes• Maunder minimum

• CO2 levels during glacial and interglacial periods

• Relationship between increased temp and amount of pptn

• Fertilization effect, ocean as a source/sink of CO2