CDS (I) 2024 General Knowledge · Question 38 of 120

Correct — D, Fall of temperature due to increased particulates. Global dimming is a decline in the amount of sunlight reaching the Earth's surface, caused by atmospheric particulates and predominantly by sulphate aerosols from burning fossil fuels. It works two ways. Directly, the particles reflect sunlight back to space like tiny mirrors. Indirectly, they act as cloud condensation nuclei, producing clouds made of smaller and more numerous droplets, which are more reflective still. Less sunlight arriving at the surface means less energy absorbed, so the surface cools — which is why aerosols are described as having masked part of the warming already caused by greenhouse gases, with the most polluted regions even cooling during the 1970s.

• (a)Fall of atmospheric pressure due to increased particulates — Global dimming is about radiation, not pressure. Suspended particles are far too sparse by mass to alter surface atmospheric pressure in any measurable way.
• (b)Raise of atmospheric pressure due to increased particulates — Same category error as option (a), in the opposite direction. The phenomenon is defined by how much sunlight reaches the ground, not by any pressure change.
• (c)Raise of temperature due to increased particulates — Gets the sign wrong. Most aerosols scatter and reflect sunlight and therefore cool the surface; black carbon is the exception that absorbs and warms, but it is not what global dimming names.

Sunlight reaching the surface was measured to be falling at roughly 4 to 5 per cent per decade from the late 1950s to the 1980s, a trend named global dimming. The cause is aerosol pollution, chiefly sulphate particles from coal and oil combustion, working through a direct reflective effect and an indirect cloud-brightening effect. Since the 1980s the trend has reversed in many regions as sulphur emissions were controlled, a reversal known as global brightening — which unmasks some of the greenhouse warming the aerosols had been hiding.

Read the four options as a two-by-two grid: two of them change pressure, two change temperature, and each pair splits into a rise and a fall. Deciding the axis first is quicker than arguing about the sign, and 'dimming' is a word about light, hence about the energy budget, hence about temperature. Then the sign follows from the mechanism — anything that reflects sunlight away cools what lies beneath it. Two nuances are worth carrying. Black carbon, or soot, absorbs rather than reflects and therefore warms, so not every particulate cools. And global dimming does more than cool: by lowering evaporation it has interfered with the water cycle and has probably reduced rainfall in some regions, a mechanism invoked in studies of monsoon weakening. The same physics underlies proposals for solar radiation management, which would inject sulphate aerosols deliberately.

• Global dimming is a decline in the amount of sunlight reaching the Earth's surface, caused chiefly by sulphate aerosols.
• Aerosols reflect sunlight directly and also act as cloud condensation nuclei, making clouds more reflective.
• Observed dimming ran at about 4 to 5 per cent per decade from the late 1950s to the 1980s, then reversed as global brightening.
• Aerosol cooling has masked part of the warming caused by greenhouse gases, with the most polluted regions cooling in the 1970s.
• By lowering evaporation, global dimming has weakened the water cycle and probably reduced rainfall in some regions.
• Black carbon is the exception among particulates — it absorbs sunlight and warms.

• Reading 'dimming' as something to do with pressure or visibility rather than with the surface energy budget.
• Assuming all particulates cool; black carbon absorbs sunlight and warms.
• Treating global dimming as the opposite of global warming rather than as a partial mask on it.

Asked as a one-line definition item where the options vary two things at once — the quantity affected and the direction of change.

• practice — not a real PYQ

  The cooling effect of sulphate aerosols on the Earth's surface is best described as

  • (a)a permanent reversal of global warming
  • (b)a masking of part of the warming caused by greenhouse gases
  • (c)an increase in the greenhouse effect
  • (d)a depletion of stratospheric ozone

  Answer(b) a masking of part of the warming caused by greenhouse gases — which is why cutting aerosol pollution unmasks warming already committed.

• practice — not a real PYQ

  Which one of the following atmospheric particulates warms rather than cools the surface?

  • (a)Sulphate aerosol
  • (b)Sea salt aerosol
  • (c)Black carbon
  • (d)Volcanic ash in the stratosphere

  Answer(c) Black carbon — soot absorbs incoming sunlight instead of reflecting it, so it has a warming effect.