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The Earth’s Climate System

Climate vs. Weather

• Weather is the state of the atmosphere at a particular time and a particular place.– An example of weather information would be the

temperature at ICC today at 7 p.m.

• Climate is the long-term state of the atmosphere at a particular location.– An example of climate information would that the

normal high in Peoria on August 1 is 83 degrees.– “Normal” conditions is simply a 30-year average of

that particular variable.

Climate vs. Weather

To put it another way…• the climate of your

hometown will determine how many sweaters you have in your closet.

• the weather will determine whether you should be wearing a sweater right now.

Climate Change and the Climate System

• Climate change occurs when the global energy balance between incoming energy from the Sun and outgoing heat from the Earth is upset.

• However, the global climate is also affected by other flows of energy which take place within the climate system.

• This climate system is made up of the atmosphere, the oceans, the ice sheets, living organisms and the rocks, which all affect, to a greater or less extent, the movement of heat around the Earth's surface.

• Earth’s Climate has changed throughout geologic time.

• Note that climate has usually been much warmer than today

• Human have evolved in one of the few cold periods in earth’s history.

• Today we live in a interglacial period during an ice age.

Ice Ages

• Temperature data shows that during the past 450,000 years the Earth's climate has fluctuated between periods of relative warmth and relative cold.

• The colder periods, called glacials or Ice Ages, have usually lasted for 100,000 years, whilst the intervening warmer intervals have been much shorter, lasting about 10,000 years.

• The size of the polar ice caps has repeatedly grown and shrunk with these cycles.

• The most recent glacial period on Earth ended roughly 11,000 years ago.

Recall the evidence for the last glacial period during our

hike

Ice Ages

GlacialGlacial Glacial Glacial

Today’s Interglacial Period

Cause of the Ice Ages• The most likely cause of the 100,000 year glacial cycles is changes

in orbit around the sun.

• The three main orbital changes are : 1. changes in the shape of Earth's orbit, 2. changes in the tilt of Earth's axis, and 3. the wobble of Earth's axis.

• When these orbital conditions are just right, summer insolation is reduced. Note: glacial periods need cooler summers (not cold winters) to enhance the accumulation of glacial ice

Climate Forcing• Mechanisms that can upset the global energy

balance and cause climate change include:

1. Plate Tectonics

2. Orbital Changes (Previously Discussed)

3. Solar Variability

4. Aerosols

5. Greenhouse Gases• These mechanisms "force" the climate to

change. Consequently, scientists call them "climate forcing" mechanisms.

Plate Tectonics• The surface of the earth is broken into numerous

pieces or plates.• Some plates spread apart from each other while

other collide causing earthquakes, volcanic eruptions and the formation of mountains.

• The movement of the plates is very slow, being only a few centimeters each year.

• However, over tens or hundreds of millions of years, both the size and position of land areas can change appreciably.

Plate Tectonics

Plate Tectonics and Global Cooling

• The formation of mountains exposes greater volumes of rock to the effects of chemical weathering.

• Since carbon dioxide is removed during the weathering process, increased rates of mountain building can diminish the strength of the Earth’s natural greenhouse effect, contributing to global cooling.

• The uplift of the Himalaya Mountains due to the collision of India into Asia 50 million years ago, may have contributed to the recent cooling.

Plate Tectonics and Global Warming

• Some long-term (millions of years) climate warming events are directly related to an increase large scale plate tectonic activity known as flood basalts.

• A volcanic flood basalt releases enormous amounts of basalt and CO2, often thousands of cubic miles in a matter of a few hundred years.

Flood Basalts

• The largest Flood Basalt event 250 million years ago.

• The earth rapidly went from an ice age to a hot house at this time resulting in the largest mass extinction in geologic history followed by the time of the dinosaurs.

• This also provided the conditions that lead to the formation of petroleum

Aerosols

• Aerosols are tiny particles suspended in the air that can cause both a warming and a cooling effect in the atmosphere.

• Aerosols naturally, originating from volcanoes, dust storms, forest and grassland fires.

• Human activities such as the burning of fossil fuels and land use changes also generate aerosols.

• Aerosols do not produce long-term change because they leave the atmosphere not long after they are emitted.

Reflective Aerosols

• Reflective aerosols include sulfur and nitrogen compounds released during the combustion of biomass (wood) and fossil fuels, and during the volcanic eruptions.

• Because these aerosols reflect sunlight back into space, they have a "direct" cooling effect by reducing the amount of solar radiation that reaches the surface.

• The magnitude of this cooling effect depends on the size and composition of the aerosol particles.

Aerosols and Volcanic Eruptions

• Volcanic aerosols (ash and sulfur gasses) tend to block sunlight and contribute to short term cooling (one or two years).

• The 1991 Pinatubo eruption caused the globally averaged surface temperature to cool less than 1°F.

Black Carbon Aerosols • Black carbon (BC) or soot is formed through the

incomplete combustion of fossil fuels and wood.• BC warms the planet by absorbing heat in the

atmosphere and by reducing albedo when deposited on snow and ice.

• Estimated to be the second largest short-term contributor to global warming after green house gasses.

• Because BC remains in the atmosphere only for a few weeks, reducing BC emissions may be the fastest means of slowing climate change in the near-term.

Solar Variability

• Changes occurring within (or inside) the sun can affect the intensity of the sunlight that reaches the Earth's surface.

• The intensity of the sunlight can cause either warming (for stronger solar intensity) or cooling (for weaker solar intensity).

• Short term effect on earth’s climate: 10s to 100s of years.

Solar Variability• Sunspots are areas of lower temperature on the

surface of the sun that appear dark.• Despite the fact that sunspots are areas of lower

temperature on the Sun, they actually increase the amount of solar radiation leaving the Sun. – More Sunspots = More Solar Radiation

• The large dark sunspot visible in this image is 20 times as large as the Earth!

Solar Variability

• Sunspots follow an 11-year cycle in their activity, known as the solar cycle.

Solar Minimums• There have also been disruptions in this pattern. • The largest well-documented disruption was an

era that lasted from about 1645 to 1715 during which almost no sunspots were seen.

• This long lull is known as the Maunder Minimum which corresponds to a period of cooler climate called the Little Ice Age.

Greenhouse Gasses• Even though they are not very abundant,

greenhouse gases are an important forcing mechanism.

• In a very rough approximation the following trace gases contribute to the greenhouse effect:– 60% water vapor– 20% carbon dioxide (CO2)– The rest (~20%) is caused by methane (CH4), nitrous

oxide (NO), and tropospheric ozone (O3).

• Water is the most efficient at trapping radiant heat and there is more water vapor in the atmosphere than any of the other trace greenhouse gases. However…

A Few Words about Water Vapor..

• …water vapor does not accumulate in the atmosphere over the multi-year periods that other greenhouse gases do.

• Water stays in the atmosphere for a few days, while other carbon-based greenhouse gases linger for decades or centuries.

• Changes in water vapor are not a long term climate forcing mechanism, rather water vapor works as a feedback mechanism that enhances climate change.

Greenhouse Gasses

• Natural carbon-based gases (CO2 and CH4) are the most important climate forcing greenhouse gases in the atmosphere.– CO2 is released into the atmosphere via volcanic

eruptions, natural biological activity (respiration, decomposition, etc), or fuel combustion.

– CH4 is released directly from decomposition of organic material or from the melting of frozen methane deposits such as ocean methane hydrates or Artic Permafrost.

• The natural release of greenhouse gasses stabilizes the Earth’s temperature and keeps it from moving too far in one direction or another.

Rates of Climate Change

• While the weather can change in just a few hours, climate changes over a wide variety of timeframes.

• The longest periods of global climate change occur over hundreds of millions of years, in response to episodes of continental drift and flood basalt activity (both associated with plate tectonics).

Rates of Climate Change

• Within such long term cycles there exist much shorter climatic fluctuations over tens and hundreds of thousands of years, driven by changes in the Earth's orbit around the Sun.

• Over the shortest time scales of centuries, decades and even individual years, global climate is influenced by solar variability, aerosols emissions and changes in atmospheric greenhouse gas concentrations.

Abrupt Climate Change

• An abrupt climate change occurs when the climate system is forced to cross some threshold, triggering a transition to a new state at a rate determined by the climate system itself and faster than the cause.

• An abrupt climate change takes place over a few decades or less, persists for at least a few decades, and causes substantial disruptions in human and natural systems.

• Abrupt climate changes may occur over a region, a hemisphere, or the entire globe.

Abrupt Climate Change• For a visual analogy of an abrupt climate change, imagine

a landscape with two valleys and a ball sitting in one of these valleys (animated below).

• A gradual push (due to forcing mechanism) is given to the ball and it begins to roll up the hill. If the push is not strong enough, the ball stops midway up the hill and rolls backward to its original stable position. This is called climate variability

Stable StateUnstable State

Abrupt Climate Change

• With a stronger push, the ball rolls up the hill and, suddenly, the ball tops the hill (unstable state) and rolls down the other side into the second valley. An abrupt change to a new stable state has occurred.

• Note that the new “stable state is at a higher level (temperature) than the former stable state.

Stable StateUnstable State

Climate Feedback• A feedback is an indirect (or "secondary")

change within the climate system that occurs in response to a forcing

• There are many feedback mechanisms in the climate system that can either amplify (‘positive feedback’) or diminish (‘negative feedback’) the effects of climate change. – A positive feedback is a process in which an initial

change will bring about an additional change in the same direction.

– A negative feedback is a process in which an initial change will bring about an additional change in the opposite direction.

• An example of a simple positive feedback in everyday life is the growth of an interest-earning savings account. As interest is accrued the principal will begin to grow. As the principal grows, even more interest will be accrued, quickening the rate of principal growth.

• An example of a simple negative feedback is your body's cooling mechanism. When your body temperature rises, you begin to sweat. The evaporation of this sweat from your skin cools your body and your temperature returns to normal.

Climate Feedback

Examples of Feedbacks when

atmospheric temperature

increase

• Warming = Positive Feedback

• Cooling = Negative Feedback

Feedback and the Climate System

• In Positive Feedbacks, a small initial forcing can yield a large change by pushing the climate system toward and unstable state.

• Negative Feedbacks, on the other hand, stabilize the climate system by bringing it back to its original state.

Positive Feedback

Negative FeedbackNote: It is positive, rather than negative feedbacks that contribute to abrupt climate changes.

Climate Feedback Mechanisms

1. Ocean – Carbon Dioxide Feedback

2. Water Feedback

3. Ice-Albedo Feedback

4. Methane Feedback

5. Cloud Feedback

6. Biological Feedback

7. Ocean Circulation Feedback

Ocean – Carbon Dioxide Feedback

• Carbon Dioxide dissolved in the oceans plays an important positive feedback role in climate change.– Recall: Cold water can contain more CO2 than warm water.

• If the climate warms, CO2 evaporates more readily from the ocean into the atmosphere. This increases global temperature which leads to even more CO2 evaporation from the oceans.

• If the climate cools, atmospheric CO2 dissolves and accumulates in the oceans. This reduces global temperature and leads to even cooler oceans and more CO2 dissolution.

Ocean – Carbon Dioxide Feedback

Note the positive correlation between atmospheric CO2 and global temperature over the past 450,000 years.

Water Vapor Feedback

• Water vapor in the atmosphere plays a positive feedback role in climate change.

• Warmer temperatures will lead to an increase in water evaporating from the oceans. This increase in water vapor in the atmosphere results in even warmer temperatures (water is a greenhouse gas).

• In addition, because the air is warmer, the relative humidity can be higher (air is able to 'hold' more water when its warmer), leading to more water vapor in the atmosphere.

Ice-Albedo Feedback

• Water vapor in the atmosphere plays a positive feedback role in climate change.

• Ice-covered surfaces have a high albedo, thus they reflect more solar energy than ice-free surfaces.

• Warmer temperatures reduce global snow and ice cover, this the warming will be enhanced because more solar energy will be absorbed.

• Likewise, during an ice age, the cooling is enhanced because more solar radiation will be reflected back into space due to an increase in global snow and ice cover.

Methane Feedback

• Recall that methane is 60 times more powerful than CO2 as a greenhouse gas but only remains in the atmosphere for about ten years and so looses it's greenhouse effect quickly compared to CO2 which remains in the atmosphere for 100 years.

• Methane can be “frozen” as methane hydrate deposits in the cold, high pressure environment at the bottom of the ocean.

• It is estimated that there is more carbon locked away as ocean methane hydrates than all of the oil and gas reserves of the world combined.

Methane Feedback - PETM• Fifty-five million years ago global temperatures

rapidly increased by at least 5 to 7°C in just a few thousand years. This event is called the Paleocene-Eocene Thermal Maximum (PETM)

• The theory to explain the PETM is that gradual global warming due to some natural cause (flood basalts) had resulted in an increase in global atmosphere and ocean temperatures.

• This caused methane hydrate deposits in the oceans to melt, releasing methane into the atmosphere and accelerating the rate of warming in a sudden, runaway fashion.

Methane FeedbackArctic Permafrost

• Methane can also be trapped by permafrost layers which over-lay lower unfrozen layers of vegetable material that is decaying and producing methane which remains trapped by the frozen permafrost on top.

• If the permafrost layer were to melt then the methane in the layers below would escape into the atmosphere.

• Given the vast areas of permafrost in the Arctic Regions there is a significant potential for methane to be released if the permafrost melted as a result of global warming.

Cloud Feedback• Clouds can produce both positive and negative

feedback in the climate system• An increase in global temperature will increase

evaporating from the oceans which leads to the formation of clouds.

• Negative Feedback: Low, thick clouds primarily reflect solar radiation and cool the surface of the Earth.

• Positive Feedback: High, thin clouds primarily transmit incoming solar radiation; at the same time, they trap some of the outgoing infrared radiation emitted by the Earth and radiate it back downward, thereby warming the surface of the Earth.

Biological Feedbacks

• The main biological feedbacks in the climate system are negative.

• An increase in global temperature will lead to more vegetation on land and plankton in the oceans, both of which can lead to a decrease of atmospheric CO2 and lower global temperature– Terrestrial vegetation removes CO2 from atmosphere

by photosynthesis and by enhancing chemical weathering.

– Microscopic plankton utilize CO2 dissolved in seawater to manufacture their shells. The oceans replace the this carbon dioxide by "sucking" down the gas from the atmosphere.

Ocean Circulation Feedback

• The thermohaline circulation is the part of the global ocean circulation that is driven by geographic differences in the density of sea water, which are controlled by temperature (thermal) and salinity (haline).

• In the North Atlantic this circulation transports warm and salty water from the tropics to the north (See Next Slide).

• There, the water cools and releases heat to the atmosphere, warming the North Atlantic region.

Ocean Circulation Feedback

Ocean Circulation Feedback

• Once the water loses heat, it becomes cooler and more dense, sinking into the deep ocean.

• This deepwater flows slowly southward (~0.1 m/s) near the bottom of the ocean basins and gradually returns to the surface as a result of wind-driven upwelling near Antarctica and slow diffusive upwelling over the rest of the global ocean.

• It then joins near-surface currents to be returned to the areas of deepwater formation.

Ocean Circulation Feedback

• Warmer temperatures can melt the glacial ice in the North Atlantic Ocean

• The fresh melt water would dilute the salinity (lower the density) of surface waters and thus reduce sinking of the surface water into the deep ocean.

• This would slow the ocean conveyer belt that brings warm water from the equator to the poles and result in cooling.

The Younger Dryas

• Near the end of the last glacial period 13,000 years ago, massive amounts fresh water from the melting glaciers disrupted the ocean circulation in the north Atlantic.

• This disruption may have caused a period of rapid global cooling known as the Younger Dryas.

• This abrupt climate change episode lasted 1,000 years and may have lead to the extinction of the mammoth and other large mammals.

• If an increase in global temperature leads to:

• Warming = Positive Feedback

• Cooling = Negative Feedback