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(October 2008)
The "greenhouse effect" is the term used to describe the retention of heat in the Earth's ³ atmosphere (troposphere). In colloquial usage it often refers to the enhanced global warming which is considered likely to occur because of the increasing concentrations of certain trace gases in the atmosphere. These gases are generally known as greenhouse gases*. Concentrations of them have increased significantly during the 20th century, and a large part of this increase is attributed to human sources, i.e. it is anthropogenic.
* or more specifically as radiative gases.
Furthermore, although most sources of anthropogenic emissions can be identified in particular countries, their effect is in no way confined to those countries, - it is global.
The greenhouse effect itself occurs when short-wave solar radiation (which is not impeded by the greenhouse gases) heats the surface of the Earth, and the energy is radiated back through the Earth's atmosphere as heat, with a longer wavelength. In the wavelengths 5-30µm a lot of this thermal radiation is absorbed by water vapour and carbon dioxide, which in turn radiate it, thus heating the atmosphere. This is what keeps the Earth habitable. Without the greenhouse effect overnight temperatures would plunge and the average surface temperature would be about minus 18oC, about the same as on the moon, which lacks the shroud of our atmosphere.
The particular problem arises in the 8-18µm band where water vapour is a weak absorber of radiation and where the Earth's thermal radiation is greatest.* Increased concentrations of CO2 and other radiative gases here mean that less heat is lost to space from the Earth's lower atmosphere, and temperatures at the Earth's surface are therefore likely to increase.
* Part of this "window" (12.5-18µm) is largely blocked by carbon dioxide absorption, even at the low levels originally existing in the atmosphere. The remainder of the "window" coincides with the absorption proclivities of the other radiative gases: methane, (tropospheric) ozone, CFCs and nitrous oxide. It also appears that increased levels of carbon dioxide will increase the capture of heat in its absorption band to some, perhaps significant, extent.
A number of indicators suggest that warming due to increased levels of greenhouse gases is indeed observable since 1980, despite some masking by aerosols (see below). One problem is that while global air temperatures do appear to have risen about 0.6oC over the last century, this has been irregular rather than steady, and does not correlate well with the steady increase in greenhouse gas concentrations. While the amount is consistent with natural climate variability, some of the warmest years on record have been in the last decade. However, the climate is a complex system and other factors influence global temperatures.
The major role of water vapour in absorbing thermal radiation is in some respects balanced by the fact that when condensed it causes an albedo effect which reflects about one third of the incoming sunlight back into space. This effect is enhanced by atmospheric sulfate aerosols and dust, which provide condensation nuclei. Nearly half the sulfates in the atmosphere originate from sulfur dioxide emissions from power stations and industry, particularly in the northern hemisphere.
In recent decades volcanoes have contributed substantially to dust and acid aerosol levels high in the atmosphere. While at lower levels in the atmosphere sulfate aerosols and dust are short-lived, such material in the stratosphere remains for years, increasing the amount of sunlight which is reflected away. Hence there is, for the time being, a balancing cooling effect on the earth's surface. In the northern hemisphere the sulfate aerosols are estimated to counter nearly half the heating effect due to anthropogenic greenhouse gases.
However, in many countries there are now programs to reduce sulphur dioxide emissions from power stations, as these emissions cause acid rain. Hence this balancing factor will diminish and the rate of temperature increase due to greenhouse gases may consequently increase. Removing the sulfate aerosols is likely to be equivalent to increasing CO2 levels by 100ppm, according to a senior UK climate scientist.
There is clear evidence of changes in the composition of the greenhouse gases in the lower atmosphere, with CO2 in particular steadily increasing to its present level of about 380 ppm. Ice core samples show that both carbon dioxide and methane levels are higher than at any time in the past 650,000 years - CO2 there being 180-300 ppm.
Estimates of the individual contribution of particular gases to the greenhouse effect, - their Global Warming Potential (GWP), are broadly agreed (relative to carbon dioxide = 1). Such estimates depend on the physical behaviour of each kind of molecule and its lifetime in the atmosphere, as well as the gas's concentration. Both direct and indirect effects due to interaction with other gases and radicals must be taken into account and some of the latter remain uncertain:
Relating these atmospheric concentrations to emissions, sources and sinks is a steadily evolving sphere of scientific inquiry. Certain inputs to the atmosphere can be discerned and readily quantified, - carbon dioxide from fossil fuel burning (about 28 billion tonnes per year, 7.65 GtC) and CFCs from refrigerants for instance. Others, such as methane sources, are less certain, though about one fifth of the methane emissions appear to be from fossil sources (coal seams, oil and natural gas, about 100 million tonnes per year).
Electricity generation is one of the major sources of carbon dioxide emissions, providing about one third of the total and one half of the increase expected 2005-30. Coal-fired generation* gives rise to twice as much carbon dioxide as natural gas per unit of power at the point of use, but hydro, nuclear power and most renewables do not directly contribute any. If all the world's nuclear power were replaced by coal-fired power, electricity's carbon dioxide emissions (now about 9.5 billion tonnes per year) would rise by a quarter - about 2.5 billion tonnes per year. Conversely, there is scope for reducing coal's carbon dioxide contribution by substituting natural gas or nuclear, and by improving the efficiency of coal-fired generation itself, a process which is well under way.
* in developed countries, with average 33% thermal efficiency. The difference is greater considering developing countries' average 25% efficiency.
Estimates of carbon dioxide concentrations in the atmosphere all show substantial increases. Global emissions of energy-related CO2 would be about 57% higher in 2030 than in 2005 under the International Energy Agency's (IEA) reference scenario (27% under alternative scenario and 68% under high-growth scenario). The IEA's alternative policy scenario involves a 27% reduction in energy intensity for power generation.
Then there is the question of residence time in the atmosphere. For example methane has about an eleven year residence time before it is oxidised to carbon dioxide. Hydroxyl (OH) radicals are the main means of this oxidation. Carbon dioxide has a much longer residence time in the atmosphere, until it is either used up in photosynthesis or absorbed in rain or oceans.
Finally, in relating emissions to atmospheric concentrations, there is the question of sinks, or natural processes for breaking down or removing individual gases, particularly carbon dioxide. While the increase in carbon dioxide concentrations is remarkable, and the rate of anthropogenic emissions considerable (some 30 billion tonnes per year), even this is only about three percent of the natural flux between the atmosphere and the land and oceans. This perspective is important as a reminder that only a very small change to natural processes is required to compensate for (or exacerbate) anthropogenic emissions.
In fact, study of the atmospheric carbon cycle shows that only about half of the anthropogenic emissions show up as increased carbon dioxide levels. This puzzle is not fully explained, but it seems that some terrestrial sinks are functioning as a negative feedback, that is to say they have increased their uptake as the atmospheric concentration has increased. The oceans are a major sink.
Source: IAEA Bulletin 42,2; 2000
The outcome of any significant global warming will be various changes in climate rather than simply an overall increase in average or nocturnal temperatures. Climate researchers have designed models to predict the consequences both in air and ocean circulation patterns. These give a range and probability of climatic impacts on different regions of the world.
The science behind the politics of global warming took a step forward and also ratcheted up concerns with the release of the Third Assessment Report from the UN's Intergovernmental Panel on Climate Change (IPCC), in 2001.
The Fourth Assessment Report in 2007 further reduced uncertainties and led to calls for action. This was published in three parts. The first detailed the physical scientific basis for climate change. The second covered the impacts of climate change, the options for adaptation and identified where people and the environment are most vulnerable. The third part of the report identified options for mitigation of climate change.
A synthesis of all three reports, including a Summary for Policy Makers, was published in November 2007.
The first part of the Fourth Assessment report on the science relating to climate change concluded that the evidence that human-derived greenhouse gas emissions had already had an impact on the climate had strengthened. Furthermore, there was greater confidence in predictions of the impacts of future greenhouse gas emissions.
Among the findings were:
The IPCC predicts that, based on a range of scenarios, by the end of the 21st century climate change will result in :
The second part of the 2007 report dealt with impacts, adaptation and vulnerabilities. It concludes that climate change will have significant impacts including increased stress on water supplies and a widening threat of species extinction.
The third part of the report in May 2007 dealt with the mitigation of climate change, outlining the prospects and options for change, particularly in the energy sector, which accounts for 60% of emissions. It was signed off by over 100 countries which agreed that major changes are required, to adopt low-carbon energy technologies. It said that a key to achieving this is putting a price on carbon emissions, particularly from power generation. The report acknowledges that nuclear power is now and will remain a 'key mitigation technology'. IEA projections support this.
It says that the most cost-effective option for restricting the temperature rise to under 3°C will require an increase in non-carbon electricity generation from 34% (nuclear plus hydro) now to 48 - 53% by 2030, along with other measures. With a doubling of overall electricity demand by then, and a carbon emission cost of US$ 50 per tonne of CO2, nuclear's share of electricity generation is projected by IPCC to grow from 16% now to 18% of the increased demand. This would represent more than a doubling of the current nuclear output by 2030. The report projects other non-carbon sources apart from hydro contributing some 12-17% of global electricity generation by 2030.
These projected figures are estimates, and it is evident that if renewables fail to grow as much as hoped it means that other non-carbon sources will need to play a larger role. Thus nuclear power's contribution could triple or perhaps quadruple to more than 30% of the global generation mix in 2030. The report also states that costs of achieving any overall target for atmospheric greenhouse gas concentrations would increase if any generation options were excluded. Clearly, any country excluding or phasing out nuclear energy is raising the overall cost of meeting emission reduction targets. This runs counter to the economic objectives of sustainable development.
In September 2008 some of these figures were updated in a report from the Global Carbon Project, a research consortium. It pointed out that atmospheric CO2 concentration had reached 383 ppm at the end of 2007, 37% above that at the start of the industrial revolution in 1750. Emissions from fossil fuels in the current decade are increasing four times faster than in the 1990s. In 2006 China passed the USA as the largest CO2 emitter, and India is projected to overtake Russia as the third largest. Developing countries now account for 53% of CO2 emissions. In 2007 CO2 emissions were some 36 billion tonnes, with fossil fuels and cement production contributing 31 Gt of this.
Main References:IPCC Fourth Assessment Report 2007OECD/NEA World Energy Outlook 2007.
Global Carbon Project 2008, Carbon Budget 2007.http://www.esrl.noaa.gov/gmd/ccgg/trends