Climate change - causes and impacts practice questions.

Which farming practice is the main anthropogenic source of nitrous oxide?

• enteric fermentation in cattle and sheep
• combustion of diesel in farm machinery
• heavy application of nitrogen fertiliser to arable land
• leakage of refrigerant gases from chilled storage buildings

Answer: heavy application of nitrogen fertiliser to arable land

Soil bacteria turn surplus nitrate and ammonium into nitrous oxide through nitrification and denitrification, so the gas escapes whenever more nitrogen is applied than the crop can take up. Enteric fermentation is the dominant source of methane, not of nitrous oxide.

Warming thaws permafrost, so decomposers can break down organic matter that has been frozen for millennia and release methane and carbon dioxide. Within the climate system this loop is best described as

• a negative feedback, because the frozen ground is restored each winter
• a transformation of matter into energy inside the soil storage
• a steady-state equilibrium between the soil and the atmosphere
• a positive feedback, because it amplifies the original warming

Answer: a positive feedback, because it amplifies the original warming

Positive feedback means the output of the loop reinforces the change that started it; it carries no judgement about whether the outcome is desirable. A negative feedback would damp the warming, as extra cloud formed by higher evaporation may partly do.

Which pair of processes accounts for most of the observed rise in global mean sea level?

• thermal expansion of sea water and melting of land-based ice
• thermal expansion of sea water and melting of floating sea ice
• melting of floating sea ice and heavier rainfall over the oceans
• rising ocean salinity and the break-up of ice shelves already afloat

Answer: thermal expansion of sea water and melting of land-based ice

Floating ice already displaces its own mass of water, so its melting adds almost nothing to sea level. The two effective mechanisms are warmer water occupying a greater volume and water transferred from ice sheets and glaciers on land into the ocean storage.

Nitrogen and oxygen make up about 99 % of the atmosphere, yet neither is a greenhouse gas. The reason is that

• their two-atom molecules do not absorb longwave radiation effectively
• they are removed from the air too quickly to accumulate there
• they occur naturally rather than being released by human activity
• they are found only within the lowest few kilometres of the atmosphere

Answer: their two-atom molecules do not absorb longwave radiation effectively

Absorption of infrared radiation depends on how a molecule can vibrate, and simple two-atom molecules of a single element cannot vibrate in the ways required. Molecules of three or more atoms, such as carbon dioxide, methane, nitrous oxide and water, can, which is why trace gases dominate the greenhouse effect despite their tiny concentrations.

Bubbles of air trapped in an Antarctic ice core are of scientific value because they allow researchers to

• date the ice by the radioactive decay of the trapped gases
• measure the atmosphere's composition when that ice formed
• estimate the depth of snow that fell in a single storm
• identify the species of plant that grew nearby at the time

Answer: measure the atmosphere's composition when that ice formed

Snow traps air as it compacts into ice, so each bubble is a sealed sample of the atmosphere of its time; the isotopic composition of the ice itself records temperature. That is how the relationship between carbon dioxide and temperature is known for hundreds of thousands of years before instruments existed.

Which observation most directly indicates that the additional carbon dioxide in the atmosphere comes from fossil fuels rather than from volcanoes or from the ocean?

• The concentration has risen in every single year in which it has been measured
• The rise began at roughly the same time as industrialisation
• The isotopic signature of the air has shifted towards ancient plant carbon
• Mean global surface temperature has risen over the same period

Answer: The isotopic signature of the air has shifted towards ancient plant carbon

Carbon from ancient plant material carries a distinctive isotopic signature, and the atmosphere's carbon has been shifting towards it while the oxygen concentration falls slightly, which is what combustion of that material would do. Timing and correlation are consistent with the fossil source but do not by themselves identify it.