News stories about the climate crisis often contain mentions of greenhouse gases, and the greenhouse effect. Whilst most will find the analogy easy to understand, what exactly are these gases, and why are they contributing to the warming of the Earth? 1. What is the greenhouse effect? In a greenhouse, sunlight enters, and heat is retained. The greenhouse effect describes a similar phenomenon on a planetary scale but, instead of the glass of a greenhouse, certain gases are increasingly raising global temperatures. The surface of the Earth absorbs just under half of the sun's energy, while the atmosphere absorbs 23 per cent, and the rest is reflected back into space. Natural processes ensure that the amount of incoming and outgoing energy is equal, keeping the planet's temperature stable. However, human activity is resulting in the increased emission of so-called greenhouse gases (GHGs), which, unlike other atmospheric gases such as oxygen and nitrogen, becomes trapped in the atmosphere, unable to escape the planet. This energy returns to the surface, where it is reabsorbed. Because more energy enters than exits the planet, surface temperatures increase until a new balance is achieved. 2. Why does the warming matter? This temperature increase has long-term, adverse effects on the climate, and affects a myriad of natural systems. Effects include increases in the frequency and intensity of extreme weather events — including flooding, droughts, wildfires and hurricanes — that affect millions of people and cause trillions in economic losses. "Human-caused greenhouse gas emissions endanger human and environmental health," says Mark Radka, chief of the UN Environment Program's (UNEP) Energy and Climate Branch. "And the impacts will become more widespread and severe without strong climate action." GHG emissions are critical to understanding and addressing the climate crisis: despite an initial dip due to COVID-19, the latest UNEP Emissions Gap Report shows a rebound, and forecasts a disastrous global temperature rise of at least 2.7 degrees this century, unless countries make much greater efforts to reduce emissions. The report found that GHG emissions need to be halved by 2030, if we are to limit global warming to 1.5°C compared to pre-industrial levels by the end of the century. 3. What are the major greenhouse gases? Water vapor is the biggest overall contributor to the greenhouse effect. However, almost all the water vapor in the atmosphere comes from natural processes. Carbon dioxide (CO2), methane and nitrous oxide are the major GHGs to worry about. CO2 stays in the atmosphere for up to 1,000 years, methane for around a decade, and nitrous oxide for approximately 120 years. Measured over a 20-year period, methane is 80 times more potent than CO2 in causing global warming, while nitrous oxide is 280 times more potent. 4. How is human activity producing these greenhouse gases? Coal, oil, and natural gas continue to power many parts of the world. Carbon is the main element in these fuels and, when they're burned to generate electricity, power transportation, or provide heat, they produce CO2. Oil and gas extraction, coal mining, and waste landfills account for 55 per cent of human-caused methane emissions. Approximately 32 percent of human-caused methane emissions are attributable to cows, sheep and other ruminants that ferment food in their stomachs. Manure decomposition is another agricultural source of the gas, as is rice cultivation. Human-caused nitrous oxide emissions largely arise from agriculture practices. Bacteria in soil and water naturally convert nitrogen into nitrous oxide, but fertilizer use and run-off add to this process by putting more nitrogen into the environment. Fluorinated gases — such as hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride — are GHGs that do not occur naturally. Hydrofluorocarbons are refrigerants used as alternatives to chlorofluorocarbons (CFCs), which, having depleted the ozone layer, were phased out thanks to the Montreal Protocol. The others have industrial and commercial uses. While fluorinated gases are far less prevalent than other GHGs and do not deplete the ozone layer like CFCs, they are still very powerful. Over a 20-year period, the global warming potential of some fluorinated gases is up to 16,300 times greater than that of CO2. 5. What can we do to reduce GHG emissions? Shifting to renewable energy, putting a price on carbon, and phasing out coal are all important elements in reducing GHG emissions. Ultimately, stronger emission-reduction targets are necessary for the preservation of long-term human and environmental health. "We need to implement strong policies that back the raised ambitions," said Radka. "We cannot continue down the same path and expect better results. Action is needed now." During COP26, the European Union and the United States launched the Global Methane Pledge, which will see over 100 countries aim to reduce 30 percent of methane emissions in the fuel, agriculture and waste sectors by 2030. Despite the challenges, there is reason to be positive. From 2010 to 2021, policies were put in place to lower annual emissions by 11 gigatons by 2030 compared to what would have otherwise happened. Individuals can also join the UN's #ActNow campaign for ideas to take climate-positive actions. By making choices that have less harmful effects on the environment, everyone can be a part of the solution and influence change. Speaking up is one way to multiply impact and create change on a much bigger scale. UNEP's role in reducing GHGs UNEP has outlined its six-sector solution, which can reduce 29–32 gigatons of carbon dioxide by 2030 to meet the 1.5°C warming limit. The six sectors identified are: energy; industry; agriculture and food; forests and land use; transport; and buildings and cities. UNEP also maintains an online "Climate Note," a tool that visualizes the changing state of the climate with a baseline of 1990. Through its other multilateral environmental agreements and reports, UNEP raises awareness and advocates for effective environmental action. UNEP will continue to work closely with its 193 Member States and other stakeholders to set the environmental agenda and advocate for a drastic reduction in GHG emissions. — UN News
