Causes: greenhouse gasses
- The sun warms the Earth through solar radiation. The Earth then radiates this warmth back up into the atmosphere where certain gasses stop heat from escaping to space (Fig. 1 & Video 1).
- These gasses are called climate forcings because they act like a thermostat, helping to regulate the temperature of the atmosphere and with it, the planet.
- Each gas has a different global warming potential (GWP), that is, how much it can potentially contribute to climate change.
- They’re called ‘greenhouse’ gasses (GHGs) because, while the process is slightly different, the effect is similar to that of a greenhouse.
- The more GHGs there are in the atmosphere, the warmer the global climate (Fig. 2).
- Anthropogenic (man-made) activities have added a lot more GHGs to the atmosphere, mostly from burning fossil fuels but also from agriculture, so global temperatures have risen (Fig. 3).
- The climate doesn’t instantly respond to adding excessive GHGs into the atmosphere. But once those gasses are up there, the planet will continue to warm for thousands of years. The more we add, the more we ‘lock in’ the effects for ourselves and future generations.
The main GHGs
A little has a huge impact
Greenhouse gasses in the atmosphere are measured in parts per million (ppm). While the amounts seem small (see Table 1), without GHFs, the average temperature on Earth would be -18°C; a frozen snowball. With just a few hundred ppm of greenhouse gasses in the atmosphere in the year 1850 (before humans began burning large amounts of fossil fuels), the average temperature on Earth was 13.7°C.
Since 1850 (Fig. 2) we’ve added a lot more GHGs. And we’re continuing to add more every year. Fig. 3 shows much this is warming the planet.
One million (1,000,000) molecules of air contains:
~780,000 molecules (parts per million or ppm) of nitrogen (N2)
~210,000 molecules (ppm) of oxygen (O2)
+ non-GHG gasses including helium, hydrogen and trace gasses
+ the following greenhouse gasses:
Year: 1820 (ppm)
Water vapour ~ 3,900
Carbon dioxide ~ 284
Methane ~ 0.774
Nitrous oxide ~ 0.27
Year: 2017 (ppm)
~ 407.9 (415 in 2020)
Some GHGs have a much more powerful global warming potential than others, including entirely new man-made GHGs created through industrial processes (*Table 2).
Global warming potential
25 x stronger
298 x stronger
22,200 x stronger
10,800 x stronger
1,760 x stronger
The above tables are simplified summaries. The chemical interaction of GHGs, how long they survive in the atmosphere and where they’re located in the atmosphere all contribute to their global warming potential over hundreds of years. For full details see the IPCC Report here.
The term ‘climate forcing’ comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy reaching Earth’s atmosphere and the amount that escapes. If more solar energy escapes than arrives, the planet cools (negative RF). Conversely, if less energy escapes than gets in, the planet warms (positive RF). This is due to the The Law of Conservation of Energy, a basic law of thermodynamics, which states that: ‘Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.’
Different climate forcings each determine how much solar energy arrives and escapes.
- Natural Forcings are those that happen through natural changes.
- Anthropogenic Forcings are those due to human activities.
Greenhouses work by convection, while atmospheric heating works by absorption and emission of electromagnetic radiation, preventing absorbed heat from leaving the atmosphere through radiative transfer (RT), which is affected by the individual chemical properties of different greenhouse gasses.
References and further reading
- Ministry for the Environment: New Zealand Emissions Trading Scheme
- Ministry for the Environment: Climate Change Response (Zero Carbon) Amendment Act
- Ministry for the Environment: New Zealand’s Greenhouse Gas Inventory
- Ministry for Primary Industries: Agricultural Inventory Advisory Panel
- Carbon Brief: Why scientists think 100% of global warming is due to humans
- Daily Carbon Tracker
- NCIDSC (National Snow and Ice Data Centre): Climate change in the Arctic
- Science Direct: The Law of Conservation of Energy
- NOAA: Global Monitoring Laboratory – Carbon tracker Moana Loa
- NOAA: Climate forcing
- The Conversation: Hothouse Earth: our planet has been here before—and here’s what it looks like
- IPPC: Chapter 8: Anthropogenic and Natural Radiative Forcing in: Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change
- 2020: Foreign Correspondent ABC News Australia: The hunt for hydroxyl radicals in Antarctica could reveal the secrets of our future climate
- 2019: IPCC: Refinement to the 2006 IPCC Guidelines for National Greenhouse Gas Inventories
- 2019: Solly; Carbon Dioxide Levels Reach Highest Point in Human History Smithsonian magazine
- 2018: Santhanam et al; Greenhouse Gas Sensors Fabricated with New Materials for Climatic Usage: A Review Chemical Engineering 2(3) 38
- 2012: Shakun et al; Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation Nature 484 49-54
- 2010 Oreskes & Conway: Merchants of Doubt; Bloomsbury Press New York
- 2004 Oreskes: Behind the Ivory Tower: The Scientific Consensus on Climate Change Science 306 (5702) 686.
- 2001: Etheridge et al; Law Dome Atmospheric CO2 Data, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series #2001-083. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
- 1856: Foote; Circumstances affecting the heat of the sun’s rays American Journal of Science and Arts; Vol. XXII 377-383