9.6 Option – Organic Geology – A Non-renewable Resource
Greenhouse Effect
Over the last two or three centuries, scientists have added greatly to our knowledge about the processes that influence the temperature of the atmosphere not only for our own planet Earth but also for other planets within our Solar System. This knowledge is summarised in Figure 1 below.
Figure 1.An idealised model of the natural greenhouse effect (Source: IPCC 2007 b p.98 ).
The Sun is the source of almost all the energy entering the Earth's upper atmosphere. This energy from the Sun is mainly in the form of ultraviolet, visible and near-infrared radiation. About 30% of this short wavelength radiant energy from the Sun is reflected straight back into space either by clouds and aerosols in the atmosphere or by reflective surfaces (especially snow and ice fields) at ground and sea level.
The rest of the radiant energy received from the Sun – i.e. that which is not reflected back - is absorbed by the atmosphere (19% of the total) and by the land and oceans at ground level ( 51% of the total).This absorbed energy warms the Earth’s surface and lower atmosphere (i.e. it increases the motion of their atoms and molecules). Eventually this absorbed energy is released (e.g. by convection or as latent heat of evaporation) or is re-emitted by the Earth’s surface as heat radiation at longer wavelengths (i.e. far-infrared radiation) that are invisible to the naked eye (but can be sensed by our skin as heat.
While some of this longwave thermal energy emitted from the Earth’s surface escapes directly out into space, some is also absorbed by trace gases in the atmosphere. Of the longwave thermal radiation intercepted by the atmosphere in this way, more than 75% can be attributed to the action of these so-called “greenhouse gases” which occur naturally in the atmosphere but which increase in concentration as a result of certain human activities. Greenhouse gases – gases like water vapour, carbon dioxide, methane, nitrous oxide and some gaseous compounds of fluorine - make up less than 1% of the total atmosphere of Earth but because of particular features of their molecular structure they are powerful absorbers of radiant energy (i.e. far-infrared radiation). Absorbing this energy ‘excites’ the motion of the gas molecules, warming the atmosphere. The energy is then re-emitted in all directions. Again some of this energy is lost to space (38%), but much is directed back to the Earth’s surface and lower atmosphere (62 %) to warm it still further.
A more detailed quantitative version of this energy balance is presented in Figure 2.
Figure 2: Estimate of the Earth’s annual and global mean energy balance. Over the long term, the amount of incoming solar radiation absorbed by the Earth and atmosphere is balanced by the Earth and atmosphere releasing the same amount of outgoing longwave radiation. About half of the incoming solar radiation is absorbed by the Earth’s surface. This energy is transferred to the atmosphere by warming the air in contact with the surface (thermals), by evapotranspiration and by longwave radiation that is absorbed by clouds and greenhouse gases. The atmosphere in turn radiates longwave energy back to Earth as well as out to space. (Source: IPCC 2007 b p 94, based on the work of Kiehl and Trenberth (1997) ) .
This process by which energy is recycled in the atmosphere to warm the Earth's surface is known as the greenhouse effect and is an essential factor determining the Earth's climate and its suitability to support life. Earth’s average atmospheric temperature at ground level (roughly 15oC on average) is much warmer and less variable than it would be if there were no greenhouse gases in the atmosphere (estimated at a chilly -18oC). A planet like Mars with a very thin atmosphere is very much colder than it would be if it had an atmosphere like Earth, whereas Venus with its much thicker atmosphere is extremely hot.
Note that under stable conditions, in the long term, the total amount of radiation energy entering the Earth’s upper atmosphere from the Sun will be balanced by the amount of energy being radiated back into space from Earth. In such periods, outgoing energy equals incoming energy and the Earth maintains a constant average temperature over time. However, the recent evidence suggests that the Earth is presently absorbing more than it is emitting into space. An indication of this is the 0.74oC rise in the Global Mean Temperature over the past century reported by the IPCC. Since 1955, the warming of the top 700 m of the ocean has accounted for more than 80 per cent of the absorbed energy. This net increase in energy absorption is believed to have been caused by increases in atmospheric greenhouse gas concentrations known as the enhanced greenhouse effect.
It is important to acknowledge that not all scientists are convinced that the increase in concentration of greenhouse gases is the principal cause of the rising global temperatures in recent times. Factors that have contributed to climate variation in the past include such things as changes in the Earth’s orbit around the sun, changes in solar output, continental drift, volcanic activity, changes in the reflectivity of the Earth’s surface and human activity (such as the heat island effect around major cities).
The intensity of the radiation reaching the upper atmosphere from the sun is not a constant figure but varies. Some of this variation is cyclic in nature. In the long term, on the scale of hundreds of thousands of years, significant variations in incoming solar radiation are likely to have been caused by the Earth moving closer or further from the Sun and by other variations in the Earth’s orbit. Such long period oscillations, of which Milankovich Cycles are the best known, are considered to be part of the explanation for the cycle of alternating deep ice ages and warming phases that the earth has experienced over the last few million years.
Of more immediate relevance to the global warming of the last century is the question of just how much of the warming, if any, can be explained by variations in the sun’s activity. Some of this variation in sun’s activity is cyclic in character; the most obvious is the 11-year solar cycle but many scientists believe that other more complex cycles of activity can be identified in the long term records of sunspot observations (Figure 3).
Figure 3: These two pictures of the Sun show how the number of sunspots changes. The picture on the left was taken near solar max in March 2001. It shows many sunspots. The picture on the right was taken near solar min in January 2005. It doesn't have any sunspots.
(Source: UCAR - Images courtesy SOHO/NASA/ESA: [Accessed October 2008])
Since the time when it first became possible to directly measure total solar output (luminosity) using instruments on satellites there have been at least three cycles of 11-year sunspot activity. Close observations show that luminosity has not varied more than 0.1% to 0.2% throughout these cycles. On general grounds, climate scientists calculate that it would require a much bigger variation than this (of at least 0.5%) in order for change in solar output to make a serious direct impact on global temperature. To quote from one recent well regarded study:
‘Variations in the Sun’s total energy output (luminosity) are caused by changing dark (sunspot) and bright structures on the solar disk during the 11-year sunspot cycle. The variations measured from spacecraft since 1978 are too small to have contributed appreciably to accelerated global warming over the past 30 years. … detailed analysis of these small output variations has greatly advanced our understanding of solar luminosity change, and this new understanding indicates that brightening of the Sun is unlikely to have had a significant influence on global warming since the seventeenth century. Additional climate forcing by changes in the Sun’s output of ultraviolet light, and of magnetized plasmas, cannot be ruled out. The suggested mechanisms are, however, too complex to evaluate meaningfully at present.’
(Source: Foukal et al (2006) , Nature, Vol 443 pp 161-166 ]).
In summary, the magnitude of the influence of variation in the intensity of solar irradiance reaching the Earth’s upper atmosphere from the sun on global mean temperature over the past century is likely to have been much less than the influence of the so-called enhanced greenhouse effect based on rising greenhouse gas emissions, (see Graph 4). The IPCC 2007report expresses it this way:
‘since 1750, it is extremely likely [at least 95% probable] that humans have exerted a substantial warming influence on climate. This … is likely [at least 90% probable] to be at least five times greater than that due to solar irradiance changes.’
(Source: IPCC 2007 b p 131 ).
Figure 4: Carbon dioxide concentration in the air (based on Law Dome ice core estimations and on measurements at the Mauna Loa observatory), global air temperature anomaly (based on the estimations of the Hadley Centre in UK, and sunspot activity (Solar Influence Data Centre, Belgium) since 1850. Note: The fluctuations in the Mauna Loa CO2 levels are due to annual seasonal variation in the growth of vegetation in the northern hemisphere land masses.
(Source:Wikipedia [Accessed October 2008]).
Outcomes
Students learn to describe and evaluate arguments concerning the greenhouse debate