What are greenhouse gases and why are they so important?

A number of ‘greenhouse gases’ (GHGs) occur naturally in the atmosphere.  These include: carbon dioxide (CO2); methane (CH4); nitrous oxide (N2O); sulphur dioxide (SO2); carbon monoxide (CO); ozone (O3); and water vapour (H2O).  These primary GHGs are sourced from natural processes, such as:

  • Volcanic eruptions
  • Rock weathering
  • Biological activity

 

Increasingly, human (anthropogenic) activity is also leading to the release of primary GHGs as well as entirely man-made GHCs (such as halocarbons) through:

  • Fossil fuel burning (which particularly enhances the release of CO2)
  • Land use change. Deforestation accounts for approximately 30% of anthropogenic increases in CO2; livestock farming causes increases in CH4; the use of fertilizers leads to increases in nitrous oxides (N2O)
  • Aerosols (which contain, and release into the atmosphere, chlorofluorocarbons – CFCs)

 

These so called ‘greenhouse gases’ received their name as they cause the ‘greenhouse effect’ (Figure 1); a natural phenomenon where atmospheric gases in the troposphere act as a global insulator.  A proportion of solar radiation that reaches earth passes through the atmosphere.  Some of this is absorbed at the land surface, and the remainder is reflected back into space both by the surface and by the atmosphere itself.  A proportion of the radiation that reaches the Earth’s surface is converted to long-wave thermal (infrared) radiation, and transmitted back towards the atmosphere.  Some of this is transmitted into outer space, and some is absorbed by greenhouse gas molecules and reflected back to the Earth’s surface.  In effect, this traps heat between the troposphere and the Earth’s surface.  A greater concentration of greenhouse gases within the atmosphere means that a greater proportion of radiation will be reflected back towards the Earth’s surface.

 

As a natural phenomenon, the greenhouse effect is a process which maintains the temperature of the Earth. Without it, the average temperature on Earth would be about -18°C, in comparison to its +14°C. The greenhouse effect is not synonymous with ‘global warming’.

 

Figure 1.  A schematic representation of the greenhouse gas effect – here demonstrating greenhouse gases as a discrete layer, for ease of understanding. In reality, these gases are mixed with the main atmosphere. Some incoming solar radiation is reflected by the outer atmosphere or by greenhouse gas molecules; some is absorbed at the earth’s surface, where outgoing thermal (infrared) radiation is emitted. Some of this infrared radiation is reflected back to earth by greenhouse gas molecules, some is transmitted beyond the atmosphere to outer space.

Figure 1. A schematic representation of the greenhouse gas effect – here demonstrating greenhouse gases as a discrete layer, for ease of understanding. In reality, these gases are mixed with the main atmosphere. Some incoming solar radiation is reflected by the outer atmosphere or by greenhouse gas molecules; some is absorbed at the earth’s surface, where outgoing thermal (infrared) radiation is emitted. Some of this infrared radiation is reflected back to earth by greenhouse gas molecules, some is transmitted beyond the atmosphere to outer space.

 

Are greenhouse gases causing climate change?

In general terms, greenhouse gases don’t cause climate change per se.  The link between greenhouse gases and current climate change has emerged due to the measured increase in atmospheric greenhouse gas concentrations.  Modern observations show that greenhouse gas concentrations have increased from 280 ppm to 400 ppm since the Industrial Revolution in the mid-18th Century.  It is estimated that, on average, GHG concentrations have increased by 70% from 1970 to 2004 (IPCC, 2007).  Different GHGs have increased at different rates (due to variations in emissions).  CO2 for example has increased by 80% during this period, and in 2004 represented 77% of total anthropogenic emissions.  This is largely due to the increased emissions from energy plants (Figure 2).  Over the last 30 years, GHG emissions have increased on average by 1.6% per year.  CO2 for example has increased by 1.9% per year during this time.  An important factor when assessing greenhouse gas concentrations is their classification into long- and sort-lived GHGs (IPCC, 2007).

 

Long-lived GHGs include CO2, CH4 and N2O.  These gases are chemically stable (in other words, they do not readily react with other substances) and so persist within the atmosphere for decades to centuries or more.  For this reason, these gases have a long-term influence on climate. CO2 is a special case; due to its persistent recycling through the ocean, atmospheric, and biospheric systems it cannot be assigned a firm lifespan within the atmosphere.  Its removal from the atmosphere therefore relies on a range of processes that operate over multiple timescales.

 

Short-lived GHGs include SO2, O3 and CO.  Unlike long-lived GHGs, these are chemically reactive, and are removed from the atmosphere through oxidation (reaction with oxygen within the atmosphere) or by washout in precipitation, for example.  This means that their concentrations within the atmosphere are of limited time spans, and can be highly variable over time.

 

Figure 2.  Sources of direct global CO2 emissions from 1970 to 2004 (Source: IPCC 2007, adapted from Olivier et al., 2006).

Figure 2. Sources of direct global CO2 emissions from 1970 to 2004 (Source: IPCC 2007, adapted from Olivier et al., 2006).

 

If we consider greenhouse gases over longer timescales, we can begin to assess whether present greenhouse gas levels are within the limits expected from natural climate cycles, or whether they are significantly different to conditions experienced in the past.  One of the most reliable ways to investigate atmospheric gas concentrations during the Quaternary is to analyse trace gases that have been trapped within polar ice sheets.  Ice cores extracted from the ice sheets contain gas bubbles that have been preserved since the layers of ice were formed (Figure 3).  A number of ice cores have been obtained from the Greenland and Antarctic ice sheets, extending back as far as 150,000 (Raynaud et al., 1993) and 800,000 years (Jouzel et al., 2007; Luthi et al., 2008), respectively.  These studies suggest that, whilst greenhouse gas concentrations have fluctuated in phase with the major glacial-interglacial cycles, the current levels of CO2, CH4 and N2O are unprecedented over the last several hundred thousand years.

 

Figure 3.  Schematic diagram of the process leading to the preservation of atmospheric trace gases within polar ice sheets.

Figure 3. Schematic diagram of the process leading to the preservation of atmospheric trace gases within polar ice sheets.

 

Greenhouse gases are therefore a vital part of the Earth system.  Without them, conditions on earth would be unrecognisable.  The vast increases in measured GHG concentrations since the Industrial Revolution have led to considerable concern about their role in rapid environmental change.  The real challenge for scientists is to establish the impacts that these increased GHG concentrations will have on the functioning of the earth systems (such as the atmosphere, hydrosphere and biosphere), and how we can address future environmental scenarios.