Global Warming - the science

(May 2007)

The greenhouse effect occurs naturally, providing a habitable climate.
Atmospheric concentrations of some of the gases that produce the greenhouse effect are increasing due to human activity and most of the world's climate scientists believe this causes global warming.
Over one-third of human-induced greenhouse gases come from the burning of fossil fuel to generate electricity. Nuclear power plants do not emit these gases.

The "greenhouse effect" is the term used to describe the retention of heat in the Earth's lower 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

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 18^oC, about the same as on the moon.

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. 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.

The result of all this is 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.

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 0.6^oC 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, the seven warmest years on record have been in the last decade. However, the climate is a complex system and other factors influence global temperatures.

Balancing Factors

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. However, in many countries there are now programmes to reduce sulphur dioxide emissions from power stations, as these emissions cause acid rain, so the impact of this balancing factor will reduce.

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 counter nearly half the heating effect due to anthropogenic greenhouse gases. As emissions of sulfates reduce and this balancing factor diminishes the rate of temperature increase due to greenhouse gases may increase.

Global Warming

There is clear evidence of changes in the composition of the greenhouse gases in the lower atmosphere. Ice core samples show that both carbon dioxide and methane levels are higher than at any time in the past 160,000 years.

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:

Greenhouse gas	Concentration, 1800s - 2000	Anthropogenic sources	GWP	Proportion of total effect (approximate)
carbon dioxide	280 - 370 ppm	fossil fuel burning, deforestation	1	60%
methane	0.75 - 1.75 ppm	agriculture, fuel leakage	21	20%
halocarbons	0 - 0.7 ppb	refrigerants	3400+	14%
nitrous oxide	275 - 310 ppb	agriculture, combustion	310	6%
ozone	15? - 20-30 ppb	urban pollution

Sources, Residence and Sinks

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 26 billion tonnes per year, 7.2 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. 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 would rise by a third - 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 in the next century all show substantial increases. Global emissions are expected to be about 50% higher in 2010 than in 1990.

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.

Climate Change

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.

Source: IAEA Bulletin 42,2; 2000

Global Warming Concerns Firm Up

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 September 2001.

In 2007 the IPCC is publishing the results of their Fourth Assessment Report. This is being published in three parts. The first, which was published in February, details the physical scientific basis for climate change. The second, published in April 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, published in May, identifies options for mitigation of climate change. A synthesis of all three reports, including a Summary for Policy Makers, will be published in November.

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:

Eleven of the last twelve years (1995-2006) rank among the 12 warmest years in the instrumental record of global surface temperature (since 1850).
Most of the observed increase in globally averaged temperatures since the mid-20th century is very likely (90%+ probability) due to the observed increase in anthropogenic greenhouse gas concentrations.
The average temperature of the global ocean has increased to depths of at least 3000 m and that the ocean has been absorbing more than 80% of the heat added to the climate system. Such warming causes seawater to expand, contributing to sea level rise.
Mountain glaciers and snow cover have declined on average in both hemispheres. Widespread decreases in glaciers and ice caps have contributed to sea level rise
Global average sea level rose at an average rate of 1.8 mm per year over 1961 to 2003. The rate was faster over 1993 to 2003, about 3.1 mm per year.
Average Arctic temperatures increased at almost twice the global average rate in the past 100 years.
More intense and longer droughts have been observed over wider areas since the 1970s, particularly in the tropics and subtropics.
Widespread changes in extreme temperatures have been observed over the last 50 years. Cold days, cold nights and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent
The global atmospheric concentration of carbon dioxide has increased from a pre-industrial value of about 280 ppm to 379 ppm in 2005. The atmospheric concentration of carbon dioxide in 2005 exceeds by far the natural range over the last 650,000 years (180 to 300 ppm) as determined from ice cores.
The primary source of the increased atmospheric concentration of carbon dioxide since the pre-industrial period results from fossil fuel use, with land use change providing another significant but smaller contribution. Annual fossil carbon dioxide emissions increased from an average of 23.5 Gt CO2 per year in the 1990s, to 26.4 Gt CO2 per year in 2000-2005.
The global atmospheric concentration of methane has increased from a pre-industrial value of about 715 ppb to 1732 ppb in the early 1990s, and is 1774 ppb in 2005.
The combined radiative forcing due to increases in carbon dioxide, methane, and nitrous oxide is +2.30 W/m², and its rate of increase during the industrial era is very likely to have been unprecedented in more than 10,000 years.

The IPCC predicts that, based on a range of scenarios, by the end of the 21st century climate change will result in :

A probable temperature rise between 1.8°C and 4°C, with a possible temperature rise between 1.1°C and 6.4°C.
A sea level rise most likely to be 28-43cm
Arctic summer sea ice disappearing in second half of century
An increase in heatwaves being very likely
A likely increase in tropical storm intensity.

The second part of the 2007 report deals 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 deals 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 agree that major changes are required, to adopt low-carbon energy technologies. It says 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'.

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.

Carbon Dioxide Emissions Avoided by Nuclear Energy

(2007 data is similar)

	Nuclear generation 2005		OPERABLE at May 2006		Approx CO2 avoided per year
Country	billion kWh	% e	No.	MWe	million tonnes
Argentina	6.4	6.9	2	935	6
Armenia	2.5	43	1	376	6
Belgium	45.3	56	7	5728	45
Brazil	9.9	2.5	2	1901	9
Bulgaria	17.3	44	4	2722	17
Canada*	86.8	15	18	12595	85
Chinese
mainland	50.3	2	10	7587	50
Taiwan	38.4	20	6	4884	36
Czech Rep.	23.3	31	6	3472	23
Finland	22.3	33	4	2676	22
France	430.9	79	59	63473	430
Germany	154.6	31	17	20303	154
Hungary	13	37	4	1755	13
India	15.7	2.8	15	2993	15
Japan	280.7	29	55	47700	280
Korea RO (S)	139.3	45	20	16840	139
Lithuania	10.3	70	1	1185	10
Mexico	10.8	5	2	1310	10
Netherlands	3.8	3.9	1	452	3
Pakistan	2.4	2.8	2	425	2
Romania	5.1	8.6	1	655	5
Russia	137.3	16	31	21743	130
Slovakia	16.3	56	6	2472	16
Slovenia	5.6	42	1	676	5
South Africa	12.2	5.5	2	1842	12
Spain	54.7	20	8	7442	50
Sweden	69.5	45	10	8938	69
Switzerland	22.1	32	5	3220	22
Ukraine	83.3	49	15	13168	83
UK	75.2	20	23	11852	75
USA	780.5	19	103	98054	780
	2,626	16	441	369,374	2600

Sources: WNA to 31/5/06, IAEA- for electricity production
Operating = Connected to the grid
MWe = Megawatt (electrical as distinct from thermal), kWh = kilowatt-hour

Basis: 1 billion kWh would require 409,000 tonnes black coal with 67% carbon.
World carbon dioxide emissions from electricity generation are about 9500 million tonnes per year (most from coal). Electricity contributes about 40% of total world CO₂ emissions.