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For millions of years, gases in Earth’s atmosphere have trapped heat from the sun to create an environment that allows the development of complex life forms like ourselves. The atmosphere stretches up to 10,000 km high, but 80% of its mass is in the Troposphere – a layer between 7 and 15 km above ground that includes the air we breathe, the clouds we see, and all the greenhouse gases (GHGs) that keep our planet habitable.
Greenhouse gases trap the heat of the sun radiated back from the surface of the earth. Without them, the average temperature of the planet would be -20°C. The most important GHGs are water vapour, carbon dioxide (CO2), nitrous oxide (N2O), methane and ozone. These gases have different qualities of heat absorption and longevity, with methane 23 times more effective at retaining heat than CO2, and N2O 296 times more effective.
Water vapour makes up the vast majority of GHGs at 97%, and of the manmade (anthropogenic) gas emissions, CO2 makes up 79%, Methane 11% and N2O 7%. Overall, GHGs account for just 0.1% of Earth’s atmosphere. CO2 stays in the atmosphere for between 300 and 1,000 years; N2O persists for 121 years, and water vapour for just over a week. The reason that oxygen and nitrogen are not GHGs is that their molecular structure is not affected by infrared radiation and don’t vibrate at the same wavelength so cannot reflect or retain it in the form of heat.
The emissions problem
That’s the science, but what has put this delicate balance out of kilter? For 250 years since the Industrial Revolution, we’ve been burning fossil fuels and releasing carbon dioxide into the atmosphere at a growing rate. This has resulted in a 50% increase in atmospheric CO2, which has increased heat absorption and caused temperatures to rise by nearly 1°C over the last century. If humanity continues on its present course, this temperature rise is predicted to be over 3% by 2100, and this increase will have catastrophic effects on the ability of life to exist here.
What the additional GHGs have done is to increase the efficiency of the blanket of gas that surrounds our planet, like adding insulation to a building to keep the interior warmer. However, we have no windows to open to regulate our temperature, so we are forced to reduce the anthropogenic production of GHGs caused by burning fossil fuels in an effort to stabilise our environment. This is why we must achieve the net zero carbon emission targets set out in the Paris Agreement, otherwise it may be too late for us to find another way to survive.