The Climate System (lecture 2)

Question 1

Why are Atlantic surface waters saltier than the Pacific? Is there Deep Water Formation as a result?

Answer 1

In the mid-latitudes, the mountain ranges across North and South America retard water transport into the Atlantic by the westerlies through orographic forcing.
In the tropics, the trade winds are not obstructed and are able to transport moisture to the Pacific.
Pacific waters are fresher and more buoyant, and there is not deep water formation as a result.

Question 2

Why is it not possible that the warming observed in the atmosphere during the past 50 years was caused by changes in the ocean circulation?

Answer 2

If ocean circulation were to be responsible, then the warming surface waters would have to coincide with cooler subsurface ocean waters. Temperature measurements do not reveal this cooling.

Question 3

Where does deep water formation occur?

Answer 3

In the high latitudes - the North Atlantic (in the Labrador and Greenland-Norwegian Sea - forming NADW) and in the Southern Ocean in the Wedell and Ross sea (AABW) - as salinity increases during sea ice formation.

Question 4

What does the acceleration of the atmospheric component of the hydrological lead to? How would this affect the AMOC?

Answer 4

The subtropics (in the Atlantic) become saltier. The North Pacific and Southern Ocean become fresher.
Freshening (and warming -  a region of maximum heat uptake in the North Atlantic) of surface waters reduces density, decreasing deep water formation.

Question 5

When was a weakening AMOC initially predicted?

Answer 5

(Manabe and Stouffer, 1993)

Question 6

Where is the weakening AMOC observed?

Answer 6

In the subtropical Atlantic (Smeed et al., 2014).

Question 7

Why can the cause of the AMOC reduction not be attributed?

Answer 7

Data has only been acquired since 2004. Therefore, it cannot be concluded whether it is simply natural variability rather than anthropogenic greenhouse gas emissions.

Question 8

What causes the greenhouse effect?

Answer 8

The atmosphere is relatively transparent to incoming shortwave solar radiation, but certain gases (carbon dioxide, water vapour, nitrous oxide, halocarbons etc.) are relatively opaque to the outgoing long wave (IR) terrestrial radiation (absorption). The greenhouse gases are those that interact with (absorb) the IR radiation due to the presence of a dipole moment (permanent or engendered by certain vibrational modes of heteronuclear molecules). Emission in all directions - heat flux to the surface.

Question 9

Is absorbed solar radiation or (downward) terrestrial radiation the greatest heat flux to the surface?

Answer 9

Terrestrial is more than twice solar - this is why the Earth is approximately 30 degrees warmer due to the greenhouse effect.

Question 10

What are the factors that constitute radiative forcing? Are they positive or negative?

Answer 10

Greenhouse gas concentrations - positive (although there is a decreasing return)
Aerosols (both anthropogenic and natural) - negative
Solar irradiance - positive

Question 11

Compare methane and carbon dioxide as greenhouse gases in terms of Global Warming Potential (considering lifetime and concentrations).

Answer 11

Even though methane concentration has doubled (from 700 ppb preindustrial to 1900 ppb) and more potent per molecule, atmospheric concentrations are 2 order of magnitude smaller and lifetime ~10 years while carbon dioxide tends to be longer - carbon dioxide can accumulate.

Question 12

Methane - radiative forcing since industrial revolution?

Answer 12

0.5 Wm-2 (2016)

Question 13

Carbon dioxide - radiative forcing since industrial revolution?

Answer 13

1.9 Wm-2 (2016)

Question 14

What are the anthropogenic and natural processes that deliver aerosols into the atmosphere? Estimate the radiative forcing.

Answer 14

Deforestation and burning of the timber, chemtrails from planes, greenhouse gas pollutants (e.g., sulphur dioxide, soot etc.) : -0.5 to -1.5 Wm-2
These compensate for the positive anthropogenic forcing from greenhouse gas emissions.
[Dust storms and] volcanic eruptions (e.g., sulphuric acid aerosols produced through the oxidation of sulphur dioxide ejected into stratosphere)
: up to -4 Wm-2

Question 15

What are the direct and indirect effects of aerosols?

Answer 15

Direct - (scattering aerosols) reflect incoming shortwave radiation. Although absorbing aerosols (e.g., soot) - black carbon on snow - lower albedo.
Indirect - serve as cloud condensation nuclei - brighter clouds or more of them - clouds reflect incoming solar radiation back to space (higher albedo) - aerosol-cloud interactions

Question 16

Solar irradiance - radiative forcing since industrial revolution? Give an example of sunspot cycle.

Answer 16

0.25 Wm-2 e.g., Schwabe 11 year sunspot cycle.

Question 17

What are the feedbacks associated with these forcings? Positive or negative?

Answer 17

Planck feedback - overwhelming negative
Water vapour feedback - positive
Lapse rate feedback - negative
Ice-albedo feedback - positive e.g., sea ice or black carbon on snow.

Question 18

What is the lapse rate feedback?

Answer 18

With increased water vapour, there is increased heating of the upper troposphere (and cooling of the surface) through latent heat flux - where condensation occurs.
A reduced in the lapse rate (the change in temperature with altitude).

Question 19

How can climate sensitivity be defined?

Answer 19

The change at equilibrium in global surface temperate associated with a doubling of carbon dioxide concentrations.

Question 20

Quantify the climate sensitivity?

Answer 20

Estimated between 1.5-4.5 K

Question 21

Why is there such a large uncertainty in climate sensitivity?

Answer 21

The cloud feedback - cloud altitude and classification.

Question 22

What is the role of sea ice?

Answer 22

It acts has a ‘lid’ on the ocean - a physical barrier to the exchange of heat and carbon dioxide with the atmosphere.

Question 23

What are the standardisation methods in climate variable measurements?

Answer 23

Use of a Stephenson screen. 
Level ground (frost hollow). 
No trees (shade), buildings (warming), or steep ground (exposed to winds).

Question 24

What effect does climate change have on the jet stream?

Answer 24

The jet stream is destabilised - meanders - and permanent shift - migration

Question 25

What role does the jet stream have in climate?

Answer 25

It influences weather systems over weeks and months - intensity and location

Question 26

Where is the North Atlantic jet predicated to shift? Compass direction. When?

Answer 26

North

Outside of natural variability by 2060 - under a strong warming scenario

Question 27

What are the implications of a migrating jet stream?

Answer 27

Extreme weather events e.g., cold outbreaks, droughts and heat waves. over a longer period of time.

Question 28

What are the consequences of the North Atlantic jet stream shifting further north? Osman et al.

Answer 28

Movement of moisture away from dry, Iberian Peninsula to Scandinavian region.
More droughts and heat waves in southern Europe and the eastern US and More rain and flooding are expected in northern parts of Europe and Scandinavia

e.g., famine, Iberian Peninsula, 1374, the jet stream was situated unusually far north.

Question 29

What slows the jet stream?

Answer 29

Warmer polar temperatures - reduced temperature difference north and south of jet stream- reduced velocity
Meandering - reduced velocity

Question 30

What causes the jet stream to migrate?

Answer 30

Arctic warming (polar amplification)