Lecture 9: Contemporary climate change

Question 1

RADIATIVE FORCING

Answer 1

• Radiative Forcing is one of the most commonly used climate change metrics. It is used extensively in the IPCC report.
• “Radiative forcing is a measure of how the energy balance of the Earth-atmosphere system is influenced when factors that affect climate are altered.” (IPCC, 2007)
• It allows the inter- comparison between different potential climate change mechanisms by considering their instantaneous value.

Question 2

CARBON DIOXIDE CHANGE SINCE PRE-INDUSTRIAL TIMES

Answer 2

• Carbon dioxide has increased from a pre- industrial value of about 280 ppm to
396 ppm in 2013
 increase of over 115 ppm since pre-industrial times (over 40% increase)
• The atmospheric concentration of carbon dioxide in 2013 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores
• The annual carbon dioxide concentration growth rate was larger during the last 10 years (2000– 2010 average: 1.9 ppm per year), than it has been since the beginning of continuous direct atmospheric measurements. In 2012, this was 2.66 ppm per year.
• The primary source of the increased atmospheric concentration of carbon dioxide since the pre- industrial period results from fossil fuel use, with land-use change providing another significant but smaller contribution

Question 3

ATMOSPHERIC METHANE (CH4)

Answer 3

• Principal component of natural gas
• Greenhouse effect of methane molecule is much higher than that of carbon dioxide
• It is chemically active and plays an important role in the formation of tropospheric ozone, another greenhouse gas
• Emission totals are associated with much more uncertainty than those of CO2 emissions
• Principal loss process is chemical destruction (oxidation), involving sunlight, oxygen,
ozone, and water vapour
• Atmospheric lifetime is approximately 9 years

Question 4

METHANE CONCENTRATIONS

Answer 4

• Methane concentrations in the atmosphere have more than doubled since 1800
• Recently the rate of increase has become smaller, presumed to be partly a result of better leakage control or due to changes to the oxidising capacity of the atmosphere (exact reasons not completely understood).• Large amounts of methane are stored in form of hydrates (methane clathrate, methane ice) beneath sediments of the ocean floor
• This has given rise to economic interests as a potential new source of power
• Should ocean temperatures increase the methane could be released into the atmosphere

Question 5

METHANE PLUMES IN THE OCEANS

Answer 5

• Recent find of methane plumes in Arctic waters (near Svalbard)
• RRS James Clark Ross in 2008
• Sonar image of bubbles emanating from the sea bed.
• Strength of acoustic response is given by colour
(blue: weak, red: strong)
• Plumes are deflected towards the North by West Svalbard current

Question 6

OZONE LAYER

Answer 6

• The altitude range 20-30 km with higher ozone values (10 ppmv = 10,000 ppbv) is referred to as the “ozone layer” and it contains about 90% of atmospheric ozone which is produced via photolysis of molecular oxygen.
• Tropospheric ozone is an air pollutant (in contrast to stratospheric ozone) and is produced from oxidation of hydrocarbons in the presence of nitrogen oxides (involving the photolysis of nitrogen dioxide).
• Typical mixing ratios are 10-40 ppbv.

Question 7

A LITTLE BIT ON TROPOSPHERIC OZONE FORMATION

Answer 7

 Ozone is formed in the presence of sunlight – no ozone formation takes place at night!
 “Ingredients”:
A polluted atmosphere – a mixture of
o Carbon Monoxide (CO) or Volatile Organic Compounds (VOCs) plus
o water vapour (more precisely: oxides of hydrogen) plus
o Nitrogen Oxides (NOx) plus
o sunlight
results in the formation of ozone
 Pollutants must be present in the right concentration – the right concentration of NOx is particularly important
 The urban environment usually contains pollutants in concentrations that favour the formation of ozone
 Ozone production involves a sequence of several chemical reactions – a reaction chain
 Troposphericozoneconcentrationsare typically of the order of 10–40 ppbv

Question 8

FORMATION OF STRATOSPHERIC OZONE

Answer 8

• Ozone forms in the stratosphere (20–50 km altitude) through photodissociation of the oxygen molecule
• This process requires shortwave solar radiation (l

Question 9

GLOBAL SATELLITE MAPS OF TOTAL OZONE

Answer 9

• The total content of ozone in the atmosphere (a vertical “ozone column”) is measured in Dobson Units (DU).
• If you compress the vertical ozone column to standard temperature and pressure then a 1 mm thick ozone layer would correspond to 100 DU.

Question 10

CHEMICAL PROCESSES IN OZONE DEPLETION

Answer 10

In the stratosphere the halogen- containing source cases (CFCs) which are chemically inert at lower altitudes are broken up by UV light (photolysed) and the halogen atoms react to form reactive halogen gases and “reservoir gases”

In polar stratospheric conditions (on the surface of PSCs) the formation of the reactive halogens is particularly effective and therefore ozone depletion is strongest at this location

Stratospheric ozone is then destroyed through catalytic cycles

Question 11

CONDITIONS FOR OZONE DEPLETION

Answer 11

At higher latitudes, ozone destruction a function of stratospheric temperature: Lower temperatures = less depletion due to slower reactionsAtmospheric Conditions for Efficient Ozone Depletion at the poles:
Low temperatures (formation of PSC)
Isolated conditions (chemical processes)
Polar Stratospheric Clouds (reaction on surface of cloud crystals increases destruction efficiency by 100×

Question 12

OZONE DEPLETION (PROFILE AND MAP)

Answer 12

• Ozone Depletion is strongest in polar regions during the onset of spring when sunlight reaches the polar regions again.
• Ozone Depletion occurs globally but it is most efficient in polar regions as meteorological conditions there enhance the depletion process.

Question 13

OZONE HOLE: LATEST DATA

Answer 13

• At the Earth Summit in Rio in 1992 agreement was reached on controlling CFC emissions.
• Monitoring by TOMS shows that the area of the ozone hole has now stabilized because of reduced emissions.
• It is predicted to start decreasing in the near future.
• BUT the minimum amount of ozone has been decreasing for some time, with some recent signs of recovery.
• The short term variations confuse the picture. They are caused by abnormal weather conditions.

Question 14

ENVIRONMENTAL IMPACTS OF OZONE DEPLETION

Answer 14

• The decrease in the stratospheric ozone layer has a small cooling effect on global temperatures. As stratospheric ozone is not an efficient greenhouse gas (in contrast to tropospheric ozone) the impact has been rather small in spite of the magnitude of depletion.
• UV radiation has significantly increased due to stratospheric ozone loss. Particularly in countries in the Southern Hemisphere (Australia, South Africa, Argentina) the impact proportionally larger  skin cancer, plant damage. Many hundreds of thousands of extra cases of skin cancer (UN, 2006).

Question 15

RECOVERY OF GLOBAL OZONE

Answer 15

• Full recovery is expected in mid-latitudes by 2050, or perhaps earlier. Recovery in the Antarctic will occur somewhat later (based on model projections).
• Large uncertainties due to uncertainties associated with future climate change.
• Influence on ozone will shift from halogen emission to CO2 and associated warming of the
troposphere/cooling of the stratosphere