Pariksha SaathiLesson 7 of 10 · Environment and Ecology
Climate Change, the Greenhouse Effect and Ozone Depletion
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The greenhouse effect
Sunlight reaches the Earth mostly as short-wave radiation and warms the surface, which then gives off heat as long-wave infrared radiation. Greenhouse gases in the atmosphere absorb part of this infrared and send it back, keeping the planet warm. This natural effect is essential: without it the Earth's average surface temperature would be about minus 18 degrees Celsius instead of about 15 degrees Celsius. The problem is the enhanced greenhouse effect, the extra warming from gases added by human activity since the Industrial Revolution. The Swedish scientist Svante Arrhenius calculated in 1896 that a change in carbon dioxide could change the Earth's temperature, and since 1958 Charles David Keeling's measurements at Mauna Loa in Hawaii have recorded a steady rise of carbon dioxide (the Keeling curve) above its pre-industrial level of about 280 parts per million.
The greenhouse gases
Water vapour is the most abundant natural greenhouse gas, but the concern is the gases human activity adds. Carbon dioxide comes from burning coal, oil and gas and from deforestation; methane from paddy fields, cattle, landfills and wetlands; nitrous oxide from nitrogen fertilisers; and chlorofluorocarbons (CFCs), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulphur hexafluoride (SF6) from industry. The Kyoto Protocol's original basket has six: CO2, CH4, N2O, HFCs, PFCs and SF6; the Doha Amendment of 2012 added nitrogen trifluoride (NF3) for its second period. The NCERT textbook gives the relative contribution to warming as carbon dioxide 60 per cent, methane 20, CFCs 14 and nitrous oxide 6 per cent. Each gas has a global warming potential (GWP) measured against carbon dioxide, which is taken as 1, and SF6 has the highest GWP among the Kyoto gases.
Effects of global warming
Warming melts glaciers and ice sheets and makes sea water expand, so the sea level rises and threatens low-lying islands such as the Maldives and Tuvalu, deltas such as the Sundarbans, and coastal cities. Himalayan glaciers such as Gangotri are retreating, which affects the rivers they feed. Heat waves, droughts, very heavy rainfall and intense cyclones become more frequent; monsoon patterns become less reliable; and crops and fisheries suffer. The oceans absorb part of the carbon dioxide and become more acidic, which, with warmer water, causes coral bleaching.
The ozone layer
Ozone (O3) is a molecule of three oxygen atoms. Most of it lies in the stratosphere, roughly 15 to 35 km above the Earth, where ultraviolet rays split oxygen molecules and the free atoms join other oxygen molecules to form ozone. This layer absorbs most of the Sun's harmful ultraviolet-B (UV-B) radiation, so it is called good ozone, while ozone at ground level is a harmful pollutant in smog. Ozone is measured in Dobson Units (DU); a normal column is about 300 DU, and an area where it falls below 220 DU is called an ozone hole. More UV-B at the ground causes skin cancer, cataract and snow blindness, weakens immunity, and harms phytoplankton and crops.
Ozone depletion and the ozone hole
CFCs, once used in refrigerators, air conditioners, aerosol sprays and foam, are stable near the ground and slowly drift up to the stratosphere. There ultraviolet rays break them and release chlorine atoms, which destroy ozone in a chain reaction, so one chlorine atom can destroy many thousands of ozone molecules. Halons from fire extinguishers release bromine, which is even more destructive, and carbon tetrachloride, methyl chloroform and methyl bromide also deplete ozone. Mario Molina and F. Sherwood Rowland warned about CFCs in 1974 and shared the 1995 Nobel Prize in Chemistry with Paul Crutzen. The Antarctic ozone hole was reported in 1985 by Joseph Farman and colleagues of the British Antarctic Survey. It forms every southern spring, in September and October, because very cold polar stratospheric clouds inside the polar vortex speed up the chlorine reactions. The Montreal Protocol of 1987 phased out CFCs, and 16 September is World Ozone Day.
Carbon footprint
A carbon footprint is the total greenhouse gas emission caused directly and indirectly by a person, organisation, event or product, expressed in tonnes of carbon dioxide equivalent. It can be reduced by saving energy, using public transport, switching to renewable power and cutting waste. Related exam terms: black carbon is soot from incomplete burning that darkens snow and ice and speeds melting; green carbon is the carbon stored in forests; blue carbon is the carbon stored in mangroves, seagrass and salt marshes.
Carbon credits and offsets
One carbon credit is a tradable certificate equal to one tonne of carbon dioxide, or its equivalent, reduced or removed. Under cap and trade, a government fixes a limit on total emissions; a company that emits less than its allowance can sell the surplus to one that emits more. The Kyoto Protocol created three market mechanisms: international emissions trading, Joint Implementation and the Clean Development Mechanism (CDM), under which developed countries earned Certified Emission Reductions (CERs) by funding clean projects in developing countries such as India, a major host of CDM projects. A carbon offset lets someone balance their own emissions by paying for an equal reduction elsewhere. Carbon neutral, or net zero, means emissions are balanced by removals.
Key facts
- Without the natural greenhouse effect the Earth's average surface temperature would be about minus 18 degrees Celsius instead of about 15 degrees Celsius.
- Keeling curve: carbon dioxide measured at Mauna Loa, Hawaii, since 1958; the pre-industrial level was about 280 ppm; Arrhenius linked CO2 and temperature in 1896.
- Kyoto's original basket of six gases: CO2, CH4, N2O, HFCs, PFCs, SF6 (NF3 added by the Doha Amendment, 2012); SF6 has the highest global warming potential among them.
- NCERT share of warming: CO2 60 per cent, methane 20, CFCs 14, nitrous oxide 6 per cent; methane comes from paddy fields, cattle and landfills.
- Ozone layer: in the stratosphere, about 15 to 35 km up; absorbs UV-B; measured in Dobson Units (normal about 300 DU; ozone hole below 220 DU).
- Ozone destroyers: CFCs (chlorine), halons (bromine), carbon tetrachloride, methyl chloroform, methyl bromide; one chlorine atom destroys thousands of ozone molecules.
- Ozone hole over Antarctica reported in 1985 (Farman, British Antarctic Survey); Molina, Rowland and Crutzen won the 1995 Nobel Prize in Chemistry.
- Carbon types: black carbon = soot; green carbon = forests; blue carbon = mangroves, seagrass and salt marshes.
- One carbon credit = one tonne of CO2 equivalent; Kyoto mechanisms: emissions trading, Joint Implementation, Clean Development Mechanism (CERs).
Quick revision
Natural greenhouse effect keeps Earth at about 15 C instead of minus 18 C; Arrhenius 1896; Keeling curve from 1958, pre-industrial CO2 280 ppm; Kyoto's original six gases; NCERT shares CO2 60, CH4 20, CFCs 14, N2O 6 per cent; ozone in stratosphere 15-35 km, Dobson Units, hole below 220 DU; CFC chlorine, halon bromine; ozone hole reported 1985; Nobel 1995; carbon credit = 1 tonne CO2e; CDM gives CERs.