Climate Change – The Science
(updated 28 October 2013)
- 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 consider that this is a significant part of the cause of observed climate change.
- The oceans are a critical part of the climate system, with vastly greater thermal capacity than the atmosphere. A small, slow increase in their temperature is significant.
- Over one-third of human-induced greenhouse gas emissions 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) due to concentrations of certain trace gases and water vapour in the atmosphere. These gases are generally known as greenhouse gases*. Concentrations of some of them have increased steadily during the 20th century and into the 21st, with CO2 rising from under 300 ppm to 400 ppm. A large part of the increase in all greenhouse gases is attributed to human sources, i.e. it is anthropogenic, hence the term ‘anthropogenic global warming’ (AGW).
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 and land and ocean surface. This is natural and what keeps the Earth habitable. Without the greenhouse effect overnight temperatures would plunge and the average surface temperature would be about minus 18°C, about the same as on the moon, which lacks the shroud of our atmosphere. We owe the difference of some 33°C substantially to natural levels of water vapour (60%, or more including clouds) and carbon dioxide in the Earth’s atmosphere.
In respect to enhancing the greenhouse effect, or the likelihood of AGW, the particular issue is focused 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. Atmosphere and oceans are the focus of attention.
A number of indicators suggest that atmospheric warming due to increased levels of greenhouse gases is indeed observable since 1970, despite some masking by aerosols (see below). Global air temperatures do appear to have risen about 0.6oC over the last century, though this has been irregular rather than steady, and does not correlate well with the steady increase in greenhouse gas – notably CO2 – concentrations. The amount, extent and rate of this exceeds 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.
One of these is water vapour, and climate models have assumed that the direct warming effect of CO2 is amplified by water vapour. However, there is doubt about whether in practice this occurs to the extent previously thought.
The oceans have also warmed slightly, affecting climate.
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. Emissions of sulfates are increasingly constrained in most countries. Global sulphate emissions peaked in the early 1970s and decreased until 2000, with an increase since due mainly to increased emissions in China and from international shipping.
Volcanoes have contributed substantially to dust and acid aerosol levels high in the atmosphere. The Mount Pinatubo eruption in 1991 in the Philippines reduced average temperatures about half a degree C. 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.
Global Warming and Climate Change
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 400 ppm. In May 2013 the daily mean concentration of carbon dioxide in the atmosphere of Mauna Loa, Hawaii, the primary global benchmark site, surpassed 400 ppm for the first time since measurements began there in 1958. It has increased by one third in the last 200 years, and half of that in the last 30 years. 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 170-300 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:
||Concentration change, 1800s - 2010
||Proportion of total effect
apart from water vapour (approximate)
280 - 390 ppm
fossil fuel burning, deforestation
0.75 - 1.75 ppm
agriculture, fuel leakage
0 - 0.7 ppb
275 - 310 ppb
15? - 20-30 ppb
Although water vapour has a major influence on absorbing long-wave thermal radiation, its GWP is not calculated since its concentration in the atmosphere varies widely and mainly depends on air temperature. Also its residence time is only about nine days, compared with years for CO2 and methane. It is classed a positive feedback, not a forcing agent for the troposphere. In the stratosphere, water vapour from methane oxidation and possibly from aircraft may be a forcing agent, but the former is included in methane’s GWP.
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* 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 110 million tonnes per year).
* 32.6 billion tonnes in 2011 (8.9 GtC), increasing 3.4% over 2010 (US EIA figures). OECD/IEA in mid 2013 said 2012 global CO2 emission from energy consumption increased 1.4% to 31.6 billion tonnes (8.6 GtC). The IPCC in 2013 estimated 9.5 GtC from fossil fuels and cement production in 2011, 54% above the 1990 emission level.
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 10 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. Substitution of coal by natural gas however requires consideration of methane leakage, and 3% leakage means that the global warming potential from using gas is the same as burning coal.
Estimates of carbon dioxide concentrations in the atmosphere all show substantial increases. Global emissions of energy-related CO2 are projected in several scenarios in the International Energy Agency's (IEA) annual World Energy Outlook reports.
Then there is the question of residence time in the atmosphere. For example methane has about an 11-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 35 billion tonnes per year in 2011), even this is only about four 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 less than half of the anthropogenic emissions show up as increased carbon dioxide levels. Both oceans and some terrestrial ecosystems provide sinks which function as a negative feedback, that is to say they have increased their uptake as the atmospheric concentration has increased. The IPCC summary in 2013 estimated that cumulative fossil fuel and cement production CO2 emission from 1750 to 2011 was about 365 GtC, with another 180 GtC from deforestation and land use change. Of this 545 GtC, about 240 GtC (44%) had accumulated in the atmosphere, 155 GtC (28%) had been taken up in the oceans with slight consequent acidification, and 150 GtC (28%) had accumulated in the terrestrial ecosystems.
Where does the heat end up?
The focus of attention regarding global warming has been the atmosphere, where the heat is initially retained. However, more recently attention has turned to the oceans, whose thermal capacity is well over one hundred times that of the atmosphere. During the last decade many more measurements with higher accuracy have been made of temperatures in the upper layers of the ocean and in some parts of the deeper ocean. These have shown a slow but steady temperature rise broadly consistent with the increase in warming at the ocean’s surface due to human influences, especially the release of greenhouse gases. Most of the net energy increase in the climate system in recent decades is stored in the oceans.
In the atmosphere, some warming of the troposphere is evident since the mid-20th century, though there has been an apparent pause in warming since 1998, associated with a strong El Nino-Southern Oscillation (ENSO) event then. ENSO has a major effect on regional rainfall.
Defining climate change prospects, effects and mitigation
The outcome of any significant climate change will be varied rather than simply an overall increase in average or nocturnal temperatures. Climate researchers have designed models to predict the longer-term consequences both in air and ocean circulation patterns. These reproduce observed continental-scale surface temperature patterns and trends over many decades, including the more rapid warming since the mid-20th century and the cooling immediately following large volcanic eruptions, thus giving a range and probability of climatic impacts on different regions of the world. The models are constantly being refined, and in 2013 the IPCC noted “differences between simulated and observed trends over periods as short as 10 to 15 years (e.g. 1998 to 2012)”, ie shorter term than the models. Climate is defined as the statistical average of weather over a long period, typically 30 years.
The science behind the politics of climate change 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. The Fifth Assessment Report in 2013 repeated the call for a global agreement to limit carbon emissions, though it did slightly adjust downward the likely effects of increased CO2 levels.
Each report is published in three parts. The first details the physical scientific basis for climate change. The second covers the impacts of climate change, the options for adaptation and identifies where people and the environment are most vulnerable. The third part identifies options for mitigation of climate change.
A synthesis of all three reports, including a Summary for Policy Makers, is published.
The first part of each 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 Fifth Report (2013) findings were:
- More than halft of the observed increase in globally averaged temperatures since the mid-20th century is extremely likely (95%+ probability) due to human influence, notably the observed increase in anthropogenic greenhouse gas concentrations.
- Greenhouse gases contributed a global mean surface warming likely (66%+ confidence) to be in the range of 0.5°C to 1.3°C over the period 1951−2010.
- More than 60% of the net energy increase in the climate system was stored in the upper ocean (0-700 m) from 1971 to 2010, and about 30% is stored in the ocean below 700 m.
- Anthropogenic influences likely contributed to the retreat of glaciers since the 1960s and to the diminution of the Greenland ice sheet since 1993.
- Multiple lines of evidence support very substantial Arctic warming since the mid-20th century, and anthropogenic influences have very likely contributed to Arctic sea ice loss since 1979.
- Global average sea level rose at an average rate of 2.0 mm per year over 1971 to 2010. The rate was faster over 1993 to 2010, about 3.2 mm per year.
- It is very likely that there is a substantial anthropogenic contribution to the global mean sea level rise since the 1970s. This is based on the high confidence in an anthropogenic influence on the two largest contributions to sea level rise – thermal expansion and glacier mass loss.
- 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 methane has increased from a pre-industrial value of about 715 ppb to 1820 ppb in 2011.
- The combined radiative forcing due to increases in carbon dioxide, methane, nitrous oxide and halocarbons is +2.83 W/m2, and its rate of increase during the industrial era is very likely to have been unprecedented in more than 10,000 years.
In the Fifth report in 2013, four scenarios for future carbon emissions to 2100 ranged from means of 270 GtC, assuming substantial cuts in emissions and correlated with best-case radiative forcing of 2.5 W/m2 , to 1685 GtC correlated with 8.5 W/m2 radiative forcing. Accordingly, it predicted that, based on the range of scenarios, by the end of the 21st century climate change will result in :
- Global surface temperature change is likely to exceed 1.5°C relative to 1850 to 1900 for two scenarios, be about 2ºC in one, and approach 4ºC in the other.
- A sea level rise most likely to be 47-63 cm, due more to thermal expansion than retreating glaciers and Greenland ice cap.
- Arctic summer sea ice disappearing in second half of century in all but hte lowest scenario.
- It is virtually certain that there will be more frequent hot and fewer cold temperature extremes over most land areas on daily and seasonal timescales as global mean temperatures increase. It is very likely that heat waves will occur with a higher frequency and duration. Occasional cold winter extremes will continue to occur.
- It is virtually certain that near-surface permafrost extent at high northern latitudes will be reduced as global mean surface temperature increases.
- Most aspects of climate change will persist for many centuries even if emissions of CO2 are stopped. This represents a substantial multi-century climate change commitment created by past, present and future emissions of CO2.
The second part of each 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 each report 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 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 Fourth report acknowledged that nuclear power is now and will remain a 'key mitigation technology'. IEA projections support this.
It said 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 increase to perhaps 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 December 2011 a report from the Global Carbon Project (GCP), a research consortium, pointed out that atmospheric CO2 concentration had reached 390 ppm at the end of 2010, 39% above that at the start of the industrial revolution in 1750. Emissions from fossil fuels in the last decade were 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 some 55% of CO2 emissions. In 2010 CO2 emissions from fossil fuels and cement production were 33.4 +/- 1.8 Gt CO2, 41% of these from coal and 34% from oil. These emissions were the highest in human history and 49% higher than in 1990 (the Kyoto reference year). Coal burning was responsible for 52% of the fossil fuel emissions growth in 2010 (gas 23% and liquid 18%). CO2 emissions from deforestation and other land use change were 3.3 +/- 2.6 Gt CO2 (0.9 +/- 0.7 GtC) in 2010, leading to total emissions (including fossil fuel and land use change) of 36.7 +/- 3.3 Gt CO2. According to GCP these ended up 50% in atmosphere, 26% in biomass and 24% in oceans.
Geological context and perspective
The Earth's climate has changed over millions of years, and there have been times when CO2 levels were higher than today. Evidence for climate change is preserved in a wide range of geological settings, including marine and lake sediments, ice sheets, fossil corals, stalagmites and fossil tree rings. The following information comes from a 2010 position statement from the Geological Society of London.
The Earth’s climate has been gradually cooling for most of the last 50 million years. At the beginning of that cooling (in the early Eocene), the global average temperature was about 6-7ºC warmer than now. About 34 million years ago, at the end of the Eocene, ice caps coalesced to form a continental ice sheet on Antarctica. In the northern hemisphere, as global cooling continued, local ice caps and mountain glaciers gave way to large ice sheets around 2.6 million years ago.
Over the past 2.6 million years (the Pleistocene and Holocene), the Earth’s climate has been on average cooler than today, and often much colder. That period is known as the ‘Ice Age’, a series of glacial episodes separated by short warm ‘interglacial’ periods that lasted between 10,000-30,000 years. We are currently living through one of these interglacial periods. The present warm period (known as the Holocene) became established only 11,500 years ago, since when our climate has been relatively stable. Although we currently lack the large Northern Hemisphere ice sheets of the Pleistocene, there are of course still large ice sheets on Greenland and Antarctica.
Global sea level is very sensitive to changes in global temperatures. Ice sheets grow when the Earth cools and melt when it warms. Warming also heats the ocean, causing the water to expand and the sea level to rise. When ice sheets were at a maximum during the Pleistocene, world sea level fell to at least 120 metres below where it stands today. Relatively small increases in global temperature in the past have led to sea level rises of several metres. During parts of the previous interglacial period, when polar temperatures reached 3-5°C above today’s, global sea levels were higher than today’s by around 4-9 metres.
Relatively rapid global warming has occurred in the past. About 55 million years ago, at the end of the Paleocene, there was a sudden warming event in which temperatures rose by about 6º C globally and by 10-20º C at the poles. Carbon isotopic data show that this warming event (called by some the Paleocene-Eocene Thermal Maximum, or PETM) was accompanied by a major release of 1500-2000 billion tonnes or more of carbon (5550-7400 billion tonnes or more of CO2) into the ocean and atmosphere. This injection of carbon may have come mainly from the breakdown of methane hydrates beneath the deep sea floor, perhaps triggered by volcanic activity superimposed on an underlying gradual global warming trend that peaked some 50 million years ago in the early Eocene. CO2 levels were already high at the time. It took the Earth’s climate around 100,000 years or more to recover, showing that a CO2 release of such magnitude may affect the Earth’s climate for that length of time.
Recent estimates suggest that at times between 5.2 and 2.6 million years ago (during the Pliocene), the carbon dioxide concentrations in the atmosphere reached between 330 and 400 ppm. During those periods, global temperatures were 2-3°C higher than now, and sea levels were higher than now by 10-25 metres, implying that global ice volume was much less than today. There were large fluctuations in ice cover on Greenland and western Antarctica during the Pliocene, and during the warm intervals those areas were probably largely free of ice.
IPCC Fourth Assessment Report 2007
IPCC Fifth Assessment Report 2013
OECD/NEA World Energy Outlook - annual.
Global Carbon Project 2008, Carbon Budget 2007.
Smith S.J. et al, 2011, Anthropogenic sulphur dioxide emissions: 1850-2005, Atmos. Chem. Phys., 11, 1101–1116.