EPA - Ireland's Environment, An Integrated Assessment - 2020

Ireland’s Environment – An Integrated Assessment 2020 2. The Causes of Climate Change Enhanced levels of atmospheric GHGs, particularly carbon dioxide, have changed the Earth’s energy balance, resulting in less thermal energy/heat being lost to space. This is causing global warming, which is observed as increased global average temperatures, changes in precipitation patterns, mean sea level rise and changes in the character of weather extremes. The Earth’s energy balance (i.e. the balance between the amount of energy it receives from the Sun and that lost to space) has remained relatively stable for millennia. This balance is regulated by relatively small amounts of gases in the atmosphere, known as GHGs, as well as microscopic particles, dusts and clouds. The most rapid and dramatic changes to the energy balance have occurred when major volcanic eruptions have launched large amounts of material into the upper atmosphere, where it can reside for years. This material reflects sunlight back to space and causes regional and global cooling (e.g. 1816 was known as the ‘year without a summer’, when the average global temperature decreased by 0.4- 0.7°C, with major food shortages experienced across the Northern Hemisphere). The other principal source of such particulates is human activities, specifically combustion for heating, transport and industry. Figure 2.1  Atmospheric concentrations of the GHGs carbon dioxide (CO 2 , green), methane (CH 4 , orange) and nitrous oxide (N 2 O, red), determined from ice core data (dots) and from direct atmospheric measurements (lines) (Source: IPCC, 2014, Figure SPM.1 (panel (c)) 800 1000 1200 1400 1600 1800 CH 4 (ppb) 270 280 290 300 310 320 330 N 2 O (ppb) 280 300 320 340 360 380 400 CO 2 (ppm) 1850 1900 1950 2000 Year Greenhouse gases, however, are different: as gases they are invisible and they reside in the atmosphere for years to centuries. The atmospheric concentrations of the main GHGs (i.e. carbon dioxide, methane and nitrous oxide) remained relatively stable for at least 800,000 years until the start of the industrial revolution in the 18th century. Since then they have increased at an unprecedented rate, reaching levels that have not existed on Earth for, in all likelihood, millions of years. Carbon dioxide concentrations have increased by 40 per cent since pre-industrial times (Figure 2.1). The global average atmospheric carbon dioxide in 2019 was 409.8 parts per million (ppm for short), with a range of uncertainty of plus or minus 0.1 ppm. Carbon dioxide levels today are higher than at any point in at least the past 800,000 years. 1 This is primarily due to fossil fuel emissions but also land use changes, which release carbon from biomass and soils. This increased energy in the Earth’s climate system is driving changes that are observed across the world’s continents, islands and oceans (IPCC, 2014, 2018, 2019a). Carbon dioxide is the largest and most important contributor to climate change. Methane, nitrous oxide, other gases and ozone are also important GHGs. Carbon dioxide is particularly important owing to its role in the global carbon cycle, which is central to life on Earth. This cycle is being significantly disrupted by the combustion of fossil fuels. As a consequence, carbon dioxide is accumulating in the atmosphere, where it is the key driver of global climate change. 1 https://www.climate.gov/news-features/understanding-climate/climate- change-atmospheric-carbon-dioxide#:~:text=The%20global%20 average%20atmospheric%20carbon,least%20the%20past%20 800%2C000%20years (accessed 8 October 2020) 36