Climate System and the Greenhouse effect

Question 1

Weather vs climate

Answer 1

WEATHER: the conditions of the atmosphere over a short period of time (i.e. days-years).

CLIMATE: how the atmosphere “behaves” over relatively long periods of time (i.e. > 30 years).

Question 2

Climate variability vs climate change

Answer 2

CLIMATE VARIABILITY: variation in the mean state and other statistics of the climate on all spatial and temporal scale.

CLIMATE CHANGE: statistically significant variations of the mean state of the climate or of its variability, typically persisting for decades or longer.

Question 3

Solar insolation

Answer 3

Solar Insolation (SI) is the main forcing of the global climate system.

From SI Earth receives almost all the energy that flows through atmosphere, oceans, land and ice.

The density of energy per m2 differs with latitude, creating thermal gradients that trigger the whole system.

This unequal distribution of incoming solar radiation is aggravated by unequal absorption and reflection by Earth’s surface at different latitudes.

A smaller fraction of the incoming radiation is absorbed at higher latitudes than in the tropics mainly because solar radiation arrives at a less direct angle and snow and ice surfaces at high latitudes reflect more radiation.

Question 4

Albedo effect

Answer 4

The percentage of incoming radiation that is reflected rather than absorbed by a particular surface is referred to as its ALBEDO. The albedo of any surface also varies with the angle at which incoming solar radiation arrives.

• Surface albedos can increase by 75% (from 15% to 90%) when snow-free land areas become covered with snow, and over oceans that become covered by sea ice.
• As a result, a surface that had previously absorbed most incoming radiation will now reflect it away, with significant implications for climate change.

Question 5

Ice Cover

Answer 5

• ICE is one of the most important components of the climate system, because its properties are so different from those of air, water, and land.

• Whereas heat can escape from an unfrozen ocean surface, a cover of sea ice stops the release of heat from the ocean to the atmosphere in winter and causes air temperatures to chill by as much as 30˚C.
• Although freshwater freezes at 0°C, typical seawater resists freezing until it is cooled to -1.9°C. When sea ice forms, it seals off the underlying ocean from interaction with the atmosphere. This change is vital to regional climates.

Question 6

Feedbacks

Answer 6

We speak of a feedback when the results of a process influence the process itself,
amplifying or reducing it.
The climate system works mainly through feedbacks. They are so many and so
interconnected that predicting the evolution of both weather and climate becomes a very arduous task.

There are 2 kind of feedbacks: positive and negative.

Question 7

Albedo -temperature feedback

Answer 7

Initial change -> climate cooling -> increased ice -> less solar radiation absorbed -> greater cooling.

This is an example of the concept of positive feedback: ALBEDO–TEMPERATURE feedback. Climate scientists call this positive feedback. In a larger sense, its net effect is to increase Earth’s overall sensitivity to climate changes. The result of a positive feedback is directed to increase the variation of the system.

Question 8

Example of negative feedback

Answer 8

A simple example of negative feedback is a thermostat. When the heating system reaches the programmed temperature, the thermostat deactivates the process (the heating system). When the temperature drops lower than the settings, the thermostat reactivate the process (the heating) until it reaches again the right temperature.

The result of a negative feedback is directed to maintain unchanged certain conditions.

Question 9

Water cycle

Answer 9

Water is the key to Earth’s climate system. Absorption and storage of solar heat are strongly affected by the presence of liquid water because of its high heat capacity, a measure of the ability of a material to absorb heat.

Question 10

Heat transfer

Answer 10

• The heat energy received and stored in the climate system is exchanged among water, land, and air through several processes.
• Some of the absorbed heat is lost from Earth’s warm surface by long wave back radiation, but most back radiation is trapped by GREENHOUSE gases and radiated back down toward Earth’s surface.
• Another form of heat transfer within the climate system involves the movement of LATENT HEAT. In this case the heat carried by the air is temporarily hidden, latent in the water vapor and then released upon condensation.

Question 11

Thermal inertia

Answer 11

Differences in amplitude and timing of response between land surfaces and the upper ocean layers are referred to as differences in THERMAL INERTIA.

During the seasonal cycle of solar radiation, ocean surfaces heat and cool slowly and only by small amounts because temperature changes are mixed through a layer 100 m thick.

In contrast, land surfaces heat and cool quickly and strongly because of their low capacity to conduct and store heat.

Question 12

Greenhouse effect

Answer 12

Greenhouse effect, a warming of Earth’s surface and troposphere (the lowest layer of the atmosphere) caused by the presence of water vapor, carbon dioxide, methane, and certain other gases in the air.

The atmosphere metaphorically behaves as a greenhouse, with the greenhouse gases in place of the glass panels. Earth has a NATURAL greenhouse effect, without which life would not be possible. On the other hand, an EXTREME greenhouse effect, as on Venus, would be harmful, since the temperature would increase tremendously.

Without its natural greenhouse effect Earth’s surface temperature would be around -16°C, almost 31°C lower than the actual global mean temperature (14°-15°C).

Question 13

Energy balance

Answer 13

• Earth average surface temperature is practically constant throughout the year. This
  means that the ENERGY BALANCE must be equal to zero. Thus, the whole amount of energy entering the system has to leave the system (235 W/m2).
• This is caused by the presence of several absorption and emission phenomena that occur both in the atmosphere and on the Earth surface.

Atmospheric processes include:
• Reflection by clouds and atmosphere of incoming solar radiation
• Reflection of radiation coming from Earth surface
• Absorption by atmosphere
• Latent heat
• Emission from atmosphere

Processes on the surface include:
• Absorption of incoming solar radiation.
• Heat transfer due to thermals (hot air currents) and evapotranspiration.
• Emission of longwave radiation from the surface.
• Absorption of reflected longwave radiation from the atmosphere radiation.

Question 14

1st principle of thermodynamics

Answer 14

The energy balance must be equal to zero because the law of conservation of energy, known also as the 1st principle of thermodynamics, has to be respected. If in a closed system the temperature and the volume remain the same it means that there is no change in the amount of energy inside the system.

Question 15

Net Irradiance

Answer 15

• The NET IRRADIANCE is the difference between the incoming and the outgoing solar radiation measured at the limit of the troposphere (tropopause) in W/m2.
• An increase in a given greenhouse gas causes a net change in the downward flowing energy minus the upward flowing energy.
• This radiative forcing value is expressed in terms of W/m2 and is defined as the change in net irradiance at atmospheric boundaries between different layers of the atmosphere.
• It depends on two factors: the “potency” or “greenhouse strength” of a given GHG, and the amount of increase of that gas in the atmosphere.

Question 16

Stratification of the atmosphere

Answer 16

• The atmosphere is divided into 4 layers.
• Stratosphere and Troposphere account for the 99,9% of its mass.
• Stratosphere, with its Ozone (O3) layer, blocks most of the incoming ultraviolet radiation.
• 80% of atmospheric gases are in the Troposphere, the layer where we live and where almost all the meteorological phenomena happen.
• Most of the heat exchanges in the atmosphere occur within the first 5 km.

Question 17

Composition of the atmosphere

Answer 17

• The main gases composing a dry atmosphere are nitrogen, oxygen and argon but neither is a greenhouse gas (GHG) because neither traps outgoing radiations.
• Natural GHGs are mainly three: water vapor (normally 1-3%), methane (0,00018%), and carbon dioxide (0,042%)
• Among the rest, with smaller amounts but important peculiar effects, there are nitrous oxide N2O (0,00003%) and ozone O3 (max 0,000007%)

Question 18

Water vapor

Answer 18

• WATER VAPOR is a major factor of Earth’s climatic system and human production, mainly industrial cooling towers for energy production, can not even be compared to the enormous natural production.
• Water vapor concentration ranges from less than 1% in dry areas to more than 3% in wet areas.
• Water vapor is a greenhouse gas BUT it does NOT cause climate change, seen that it is a short-term gas that goes in and out the climatic system at a very fast rate.

For this reason, is not seen as a long-term forcing element but only as an important part of the climatic system involved in different kind of feedbacks. The last IPCC report estimates that water vapor is responsible for 60% to 80% of the world’s natural greenhouse effect.

Question 19

Carbon dioxide

Answer 19

CARBON DIOXIDE is a very common molecule. It is produced by respiration, combustion, decomposition of organic matter, vulcanoes emissions and dissolution of carbonate rocks

Main anthropogenic sources are deforestation, industries, combustion of fossil fuels

To facilitate global calculations of GHGs a relative scale has been created: the GWP (Global Warming Potential). In this system all GHGs are converted in CO2 equivalents.

THE GWP OF CO2 IS 1

Direct measurements of atmospheric CO2 concentration have been recorded since 1958. In that time, the concentration has risen from 315 ppm to 408 ppm (in 2019).

• Nowadays the problem is not so much the amount of the CO2 (GHGs) increase but its SPEED. If we take a look at pre-historic times and different interglacial periods, we can see how CO2 levels have been as high as today or even higher.
• The point is that the industrial era increase in GHGs is really fast, compared to previous times. If before even relatively small changes in the amount of CO2 were taking place in millennia, the actual rising is a matter of some decades.
• This make things completely different and almost not comparable to past climate changes seen that the Earth system, with all its various cycles and feedbacks, does not have time to adjust to the changing conditions, thus breaking the autoregulating mechanisms.

Question 20

Methane CH4

Answer 20

METHANE is emitted naturally mainly from marshes and decomposing of organic matter in reducing environments.

It has a GWP of 25 and it is a strong greenhouse gas, though its amount in the
atmosphere is well under CO2 levels.

Humans increase methane emission mainly through gas extraction and leakage
and zoological mass production.

• Direct measurements of methane concentration began later than the corresponding carbon dioxide measurements. However, ice cores also provide
scientists with methane concentration data extending back many thousands of years.
• Pre-industrial levels of methane were around 700 ppb in 1750. By 1998, methane concentration had risen to 1,745 ppb; a whopping 149% increase over preindustrial levels.

Question 21

Nitrous Oxide N2O

Answer 21

• NITROUS OXIDE (N2O), know also as “laughing gas” is emitted during agricultural
and industrial activities and combustion of solid waste and fossil fuels.
• It is a strong greenhouse gas seen that its GWP is around 298. Fortunately its
amount in the atmosphere is small (0,00003%), even if increasing due to human
activities.

Question 22

Ozone O3

Answer 22

OZONE (O3) is an important atmospheric gas It is a highly oxidant triatomic molecule, very dangerous to life in the troposphere (where there is the 10% of atmospheric ozone).

At the same time, vital to life considered its UV shielding role in the stratosphere
(the remaining 90%).

This important shielding role has been endangered in the last decades by the effect
of Chlorofluorocarbons (CFC).

At the same time, vital to life considered its UV shielding role in the stratosphere
(the remaining 90%).

Troposphere ozone acts as a GHG trapping IR radiation, but it has mainly a local effect. This because, even if its GWP and radiative forcing is pretty high, its tropospheric amount is really low even in polluted areas and its lifetime is short (around 20 years) and its concentration is localized. After 20 years its forcing effect ceases.

Non natural O3 is mainly produced accidentally by electrical machines and it is a source of both outdoor and indoor pollution, already regulated by specific limits.

Question 23

Chlorofluorocarbons (CFC)

Answer 23

Chlorofluorocarbons (CFC) are a family of organic compounds discovered at the end of the 19th century and mass produced since the ‘20s. They are also commonly known by the DuPont trade name Freon.

• In 1974 the real danger of CFC was discovered. They have a low reactivity that gives them a long atmospheric lifespan (around 100 years).
• Some fluorocarbons have GWP values of more than 1000 or even more than 10000, thus they potentially are really strong greenhouse gases even in low concentrations.
• Once they get to the stratosphere, UV rays break their C-Cl bonds and
set the Chlorine free. Cl- behaves in a really aggressive way toward ozone molecules.
• The danger mechanism of ozone depletion and the «Ozone hole» over Antarctica were discovered in 1974.
• Since 1987 the Montreal protocol is strongly phasing out CFC production and use all around the world.

Question 24

The physicochemical mechanism

Answer 24

• Atoms and molecules can absorb electromagnetic radiation, but only at certain energies (wavelengths).
• Very energetic photons (UV to X-ray region of the spectrum) may cause an electron to be ejected from the molecule (ionization), but photons in the infrared (IR) region of the spectrum have much less energy than photons in the Visible or UV regions of the electromagnetic spectrum. They can excite vibrations in molecules.
• This extra kinetic energy may then be transmitted to other molecules such as oxygen and nitrogen and causes a general heating of the atmosphere.

Question 25

Why CO2 is such a strong greenhouse gas?

Answer 25

• The climate change forcing strength of a greenhouse gas is directly proportional with its:

1 – infrared radiation capturing capacity
2 – amount in the atmosphere
3 – lifetime

• CO2 has a really long atmospheric lifetime: centuries. This means that excesses produced today can not be re-absorbed by the system in time to preserve the balance, but they have time to force a change in the system and move its equilibrium.
• This is due to the characteristics of the Carbon Cycle.

Question 26

CO2 removal

Answer 26

The 3 main mechanisms through which CO2 is removed from the atmosphere are:
1 – photosynthesis
2 – chemical weathering and ocean sedimentation
3 – shell formation and ocean sedimentation

Question 27

Photosynthesis

Answer 27

• Photosynthesis is an important chemical reaction that needs energy, water and CO2 to happen. In this way CO2 is taken from the atmosphere and the Carbon stored into organic compounds. This way of removing CO2 is effective and important but has a defect: most of the times the organic matter produced is oxidated again within some years, maximum centuries.
• Thus the lifetime of organic carbon is too short (as for water vapor) to really allow
photosynthesis to be a decisive forcing factor against global warming.
• The only way to exploit photosynthesis against global warming is to increase forest coverage consistently, at least at pre-historic levels. However, this is a not reachable target. We can only try to slow deforestation down.

Question 28

Chemical weathering

Answer 28

CO2 CHEMICAL WEATHERING happens when CO2 dissolves in water, mainly rains, turns to carbonic acid: CO2 + H2O =H2CO3

and attacks carbonatic rocks creating calcium bicarbonate:
H2CO3 + CaCO3 = Ca(HCO3)2

and/or attacks silicatic rocks, creating CaCO3and the remaining quartz.

The products of these reactions are occasionally washed to the sea and buried with the other sediments at the bottom of the oceans.

The two main issues of this removal system are:
1 – the constant need of fresh rock surface (the removal is faster in mountains areas)
2 – the slowness of the depositing and subducting processes

Question 29

shell formation and ocean sedimentation

Answer 29

• Oceans are the world’s biggest CO2 sink. CO2 is highly soluble in water and its solubility increase with lower temperatures.
• Once in water it can assume different shapes: calcium carbonate, carbonic acid or simply remains CO2.

• One of the most important ways in which oceanic carbon (therefore CO2) is then
sequestered is in the formation of foraminifera’s shells.
• Foraminifera are microscopics unicelular «animals» (better protists) that live both in salty and fresh water (different species) all around the world.
• Their characteristic is the formation of this carbonatic shell that, after the death of the internal cell, sinks to the bottom of the seas and becomes sediment.