The Earth’s Energy Budget
Earth’s Energy Budget is the balance between the radiation arriving from the sun and that which is sent back into space.
On average, 340 watts per square meter of solar energy arrives at the top of the atmosphere. Earth returns an equal amount of energy back to space by reflecting some incoming light and by radiating heat (thermal infrared energy). Most solar energy is absorbed at the surface, while most heat is radiated back to space by the atmosphere. Earth’s average surface temperature is maintained by two large, opposing energy fluxes between the atmosphere and the ground – the greenhouse effect. Increases in greenhouse gas concentrations upsets this balance and causes surface temperatures to rise.
Influences that change the balance between incoming and outgoing energy in the climate system are termed ‘forcings’. Forcings may be natural such as volcanic eruptions or they may be manmade which includes air pollution and greenhouse gases.
The Earth’s Atmosphere
The diagram opposite show the composition of Earth’s atmosphere by volume. The numbers are mainly from 1987, with carbon dioxide and methane from 2009, and do not represent any single source. Credit: NASA Public domain.
By volume, the dry air in Earth’s atmosphere is about 78.09 percent nitrogen, 20.95 percent oxygen, and 0.93 percent argon. Trace gases account for the other 0.03 percent, including the greenhouse gases carbon dioxide, methane, nitrous oxide and ozone
In a paper published in 1861, Tyndall explained the differences between air temperatures at midday and evening and also the difference at the top of a mountain compared with the bottom to be due to the absorption and emission of heat by water vapour. He went on ‘if, as the above experiments indicate, the chief influence be exercised by aqueous vapour, every variation of this constituent must produce a change of climate. Similar remarks would apply to the carbonic acid [carbon dioxide] diffused through the air’.
So the idea of a greenhouse effect was born – although the term does not appear until used by an English scientist, John Henry Poynting in 1907.
Some five years later Arrhenius published the results of his study into whether concentrations of CO2 in the atmosphere could be the cause of the different planetary glaciations that the planet had gone through over time. He concluded that a reduction in the atmospheric CO2 levels to half the then existing ones would result in a drop in the temperature of the planet of between 4 and 5degC, which could lead to a massive cooling like the one that takes place during glaciations.
Research into climate warming gathered pace from the middle of the 20th century and the influence of human activity in burning coal was made clear to readers of the March 1912 issue of Popular Mechanics as noted in the image below. While being uncertain as to the future consequences the author ends by noting that the consuming furnaces of modern industry ‘add to the carbon dioxide in the earth’s atmosphere so that men in generations to come shall enjoy milder breezes and live under sunnier skies’!
Greenhouse Gases compared
It has become clear that the presence of increasing amounts of carbon dioxide and other greenhouse gases has contributed to the global temperature increases noted since the time that the Popular Mechanics article was published. The gases that contribute most to the Earth’s greenhouse effect are water vapour (H2O), carbon dioxide (CO2), nitrous oxide (N2O), methane (CH4) and a group of fluorinated gases.
It may seem surprising that water vapour is the top of the list contributing about 60% of the warming effect given all the publicity afforded to CO2 which contributes between 9 and 26%. But water vapour does not control the Earth’s temperature, rather it is in fact controlled by the temperature as it is a condensable gas able to form liquid water when the temperature decreases. Nonetheless, the presence of water vapour is necessary for life as John Tyndall, the Irish physicist stated in a lecture in 1862 ‘The removal, for a single summer night, of the aqueous vapour from the atmosphere which covers England, would be attended by the destruction of every plant which a freezing temperature could kill’.
CO2 merits its attention as, unlike water vapour, it is spread more or less evenly over the globe and human activities are increasing its concentration.
As of 2019 the concentration of CO2 in the atmosphere was nearly 412 parts per million (ppm) as compared with 280 ppm at the beginning of the Industrial Age – a 47% increase. In the period 2010 to 2019 the growth rate was 2.40 ppm which demonstrates an acceleration from the rate of 0.85 ppm between 1960 and 1969.
Most of the emissions have been due to CO2 from burning fossil fuel as evidenced in part by the increasing presence of carbon’s lighter isotope 12C rather than 13C – the former being associated with living things (which fossils were), the latter with mineral carbon in the earth’s crust. Land use activities such as deforestation and agriculture can result in emissions to, or removal from the atmosphere. (Figure from From PBL Netherlands Environmental Assessment Agency)
‘Background’ Concentration of CO2
Observations of CO2 concentrations are measured at Mauna Loa, Hawaii – a remote location distant from factors that might skew the results. The ‘Keeling Curve’ (named after Dr. Keeling) shows a steady rise from 310 ppm in 1960 to around 415 ppm in 2020, with an annual cycle moving up and down by around 2.4 ppm. Credit to Scripps Institution of Oceanography.
Although CO2 levels are increasing in our atmosphere, one has to look back to the Permian period around 300 million years ago to find them as low as they are today.
Carbon dioxide’s lifetime cannot be represented with a single value because the gas is not destroyed over time, but instead moves among different parts of the ocean–atmosphere–land system. Given the importance of CO2 in relation to climate change it is used as a basis for measuring the potential of other greenhouse gases by way of the Global Warming Potential (GWP) which is the ratio of global warming—or radiative forcing— from one unit mass of a greenhouse gas to that of one unit mass of CO2 over a period of time (time is important as some can continue to reside in the atmosphere for thousands of years after they have been emitted). Thus, CO2 has a GWP of 1. On this basis values for some greenhouse gases over a period of 100 years are Methane (CH4) 23, Nitrous Oxide (NO2) 296 and Chloropentafluoroethane (CFC-115) 7,000.
Methane’s characteristics as regards its affect on the climate differ markedly from those of CO2 in that despite being present in much lower concentrations (2,000 parts per billion as compared with 400 parts per million for CO2), it has a greater warming potential (a GWP of 23 over 100 years), and a shorter life – around 12 years. Methane levels are also increasing as measured at the remote Mauna Loa station and shown below – data shown in orange are preliminary. All other data have undergone rigorous quality assurance.
Although the creation of methane is associated with fossil fuel it only accounts for about 25% of human-caused global emissions. Other, often surprising factors, such as rice cultivation and waste water contribute significant amounts. Enteric fermentation is fermentation that takes place in the digestive systems of animals with methane being expelled by burping.
Nitrous Oxide N2O
Nitrous Oxide forms part of the Earth’s natural nitrogen cycle. It is a long-lived greenhouse gas with an average life of 114 years and a global warming potential that is 100 times that of methane. Over the past 150 years, increasing atmospheric N2O concentrations have contributed to stratospheric ozone depletion and climate change, with the current rate of increase estimated at 2 per cent per decade. The graph below shows the increase in N2O measured from air collected approximately weekly in glass containers for analysis or averages from air sampled semi-continuously at a Global Monitoring Division (GMD) baseline observatory.
As seen from the diagram below agriculture is the main source of N2O accounting for around two-thirds with manure at 23% and synthetic nitrogen fertilisers at 13%. The largest non-agricultural source is fuel combustion (17%, when including indirect emissions of N2O from NOx emissions)
This group of man-made gases includes hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulphur hexafluoride (SF6) and nitrogen trifluoride (NF3). Although they had the benefit of being substitutes for ozone-depleting chlorofluorocarbons (principally used as a refrigerant), they are potent greenhouse gases, some with a 100 year GWP well into the thousands despite generally only being emitted in smaller quantities.
As noted in the paper on ‘Radiative Forcing of Climate’ in the IPCC 6th Assessment report “Although potential CFC replacements are less (or in some cases, not at all) damaging to the ozone layer, the GWPs of several of them are still substantial, however, over periods greater than about 20 years most of the substitutes should have a markedly smaller impact on global wanning than the CFCs they replace, assuming the same emissions”.
As regards their lifetime in the atmosphere these range from up to 270 years for HFCs to around 50,000 for some PFCs.
The rapid growth in the atmospheric concentration of SF6, NF3, and several widely used HFCs and PFCs between years 1978 and 2015 is shown on a logarithmic scale in the right-hand graph below (ppt = part per trillion). Source United States Environmental Protection Agency.
The trend in F-gas emissions was also considered in the report by the PBL Netherlands Environmental Assessment Agency as shown below. They state, however, ‘these are very heterogeneous source categories, with large differences in growth rates for the different constituents, and often with very large uncertainties in emissions, at country level and per gas of the order of 100% or more‘.
If emissions of greenhouse gases were stopped, would the climate return to the conditions of 200 years ago?
This question was posed by The Royal Society in the UK – and their answer?
No. Even if emissions of greenhouse gases were to suddenly stop, Earth’s surface temperature would require thousands of years to cool and return to the level in the pre-industrial era.
Further, they stated ‘If global emissions were to suddenly stop, it would take a long time for surface air temperatures and the ocean to begin to cool, because the excess CO2 in the atmosphere would remain there for a long time and would continue to exert a warming effect’.
A multitude of natural and human influences will determine future climates with their impact being assessed by simulating the climate system using computer-based simulation – modelling – which is the topic of the next page.