The climate system is a complex, interactive system consisting of the atmosphere, land surface, snow and ice, oceans and other bodies of water, and living things. The atmospheric component of the climate system most obviously characterises climate; climate is often defined as ‘average weather’. Climate is usually described in terms of the mean and variability of temperature, precipitation and wind over a period of time, ranging from months to millions of years (the classical period is 30 years). The climate system evolves in time under the influence of its own internal dynamics and due to changes in external factors that affect climate. External factors include natural phenomena such as volcanic eruptions and solar variations, as well as human-induced changes in atmospheric composition. Solar radiation powers the climate system. There are three fundamental ways to change the radiation balance of the Earth: 1) by changing the incoming solar radiation (e.g., by changes in Earth’s orbit or in the Sun itself); 2) by changing the fraction of solar radiation that is reflected (called ‘albedo’; e.g., by changes in cloud cover, atmospheric particles or vegetation); and 3) by altering the longwave radiation from Earth back towards space (e.g., by changing greenhouse gas concentrations). Climate, in turn, responds directly to such changes, as well as indirectly, through a variety of feedback mechanisms.
Climate change and global warming
Global warming refers to the recent and ongoing rise in global average temperature near Earth's surface. It is caused mostly by increasing concentrations of greenhouse gases in the atmosphere. Global warming causes climate patterns to change. However, global warming itself represents only one aspect of climate change.
Climate change refers to any significant change in the measures of climate lasting for an extended period of time. In other words, climate change includes major changes in temperature, precipitation, or wind patterns, among other effects, that occur over several decades or longer.
The main greenhouse gases are carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O). Carbon dioxide emissions are primarily released through the burning of fossil fuels such as coal, oil, gas and peat. In addition, carbon dioxide is emitted through poor land management and land use changes such as deforestation and urbanisation. The main sources of methane and nitrous oxide emissions in Ireland are from the agriculture sector including livestock, manure management and application of fertilisers. Over the last 200 years, the concentrations of the main heat-trapping greenhouse gases have increased significantly in our atmosphere; carbon dioxide by 40%, methane by 150% and nitrous oxide by 20%. These three gases are now higher than they have been for at least 800,000 years. The consequences of this are most clearly evident in the global temperature records, which show that, on average, the global temperature has increased by 0.85°C since records began in the mid-19th century. Continued emissions at or above current levels will cause further warming and result in changes in the global climate system during the 21st century that would very likely be larger than those observed during the 20th century.
Causes of Climate Change
Earth's temperature depends on the balance between energy entering and leaving the planet’s system. When incoming energy from the sun is absorbed by the Earth system, Earth warms. When the sun’s energy is reflected back into space, Earth avoids warming. When absorbed energy is released back into space, Earth cools. Many factors, both natural and human, can cause changes in Earth’s energy balance, including:
- Variations in the sun's energy reaching Earth
- Changes in the reflectivity of Earth’s atmosphere and surface
- Changes in the greenhouse effect, which affects the amount of heat retained by Earth’s atmosphere
These factors have caused Earth’s climate to change many times.Recent climate changes, however, cannot be explained by natural causes alone. Research indicates that natural causes do not explain most observed warming, especially warming since the mid-20th century. Rather, it is extremely likely that human activities have been the dominant cause of that warming.
The greenhouse effect is a natural process that warms the Earth’s surface. When the Sun’s energy reaches the Earth’s atmosphere, some of it is reflected back to space and the rest is absorbed and re-radiated by greenhouse gases.
Greenhouse gases include water vapour, carbon dioxide, methane, nitrous oxide, ozone and some artificial chemicals such as chlorofluorocarbons (CFCs).The absorbed energy warms the atmosphere and the surface of the Earth. This process maintains the Earth’s temperature at around 33 degrees Celsius warmer than it would otherwise be, allowing life on Earth to exist.
Enhanced Greenhouse Effect
The problem we now face is that human activities – particularly burning fossil fuels (coal, oil and natural gas), agriculture and land clearing – are increasing the concentrations of greenhouse gases. This is the enhanced greenhouse effect, which is contributing to warming of the Earth.
What was the Past?
Over the last several hundred thousand years, CO2 levels varied in tandem with the glacial cycles. During warm "interglacial" periods, CO2 levels were higher. During cool "glacial" periods, CO2 levels were lower. The heating or cooling of Earth’s surface and oceans can cause changes in the natural sources and sinks of these gases, and thus change greenhouse gas concentrations in the atmosphere. These changing concentrations are thought to have acted as a positive feedback, amplifying the temperature changes caused by long-term shifts in Earth’s orbit.
Estimates of the Earth’s changing CO2 concentration (top) and Antarctic temperature (bottom), based on analysis of ice core data extending back 800,000 years. Until the past century, natural factors caused atmospheric CO2 concentrations to vary within a range of about 180 to 300 parts per million by volume (ppmv). Warmer periods coincide with periods of relatively high CO2 concentrations.
Other Climate Forces
Particles and aerosols in the atmosphere can also affect climate. Human activities such as burning fossil fuels and biomass contribute to emissions of these substances, although some aerosols also come from natural sources such as volcanoes and marine plankton.
Black carbon (BC) is a solid particle or aerosol, not a gas, but it also contributes to warming of the atmosphere. Unlike GHGs, BC can directly absorb incoming and reflected sunlight in addition to absorbing infrared radiation. BC can also be deposited on snow and ice, darkening the surface and thereby increasing the snow's absorption of sunlight and accelerating melt.
Sulfates, organic carbon, and other aerosols can cause cooling by reflecting sunlight.Warming and cooling aerosols can interact with clouds, changing a number of cloud attributes such as their formation, dissipation, reflectivity, and precipitation rates. Clouds can contribute both to cooling, by reflecting sunlight, and warming, by trapping outgoing heat.
Indicators of Climate Change
There are many indicators of climate change. These include physical responses such as changes in the following: surface temperature, atmospheric water vapour, precipitation, severe events, glaciers, ocean and land ice, and sea level.
The historical tide gauge record shows that the average rate of global mean sea level rise over the 20th century was 1.7 ± 0.2 mm yr–1. This rate increased to 3.2 ± 0.4 mm yr–1 since 1990, mostly because of increased thermal expansion and land ice contributions. Although the long-term sea level record shows decadal and multi-decadal oscillations, there is evidence that the rate of global mean sea level rise during the 20th century was greater than during the 19th century