The effect of people
Carbon Dioxide (C02) concentrations in the atmosphere increased by over 40% from 278 ppm (parts per million) in 1750 to 400 ppm in 2015.
Figure 1: The diagram above shows concentrations of carbon dioxide and oxygen in the atmosphere. Atmospheric concentration of a) carbon dioxide in parts per million by volume from Mauna Loa (MLO, light green) in the Northern Hemisphere and the South Pole (SPO, dark green) and of b) changes in the atmospheric concentration of O2 from the northern hemisphere (ALT, light blue) and the southern hemisphere (CGO, dark blue).
Most of the fossil fuel CO2 emissions take place in the industrialised countries north of the equator. Consistent with this, the measurement stations in the Northern Hemisphere record slightly higher CO2 concentrations than stations in the Southern Hemisphere. As the difference in fossil fuel combustion between the hemispheres has increased, so has the difference in concentration between measuring stations at the South Pole and Mauna Loa (Hawaii, northern hemisphere).
The atmospheric CO2 concentration increased by around 20 ppm during 2002–2011. This decadal rate of increase is higher than during any previous decade since direct atmospheric concentration measurements began in 1958.
CO2 uptake by photosynthesis occurs only during the growing season, whereas CO2 release by respiration occurs nearly year-round. This means both the Mauna Loa and South Pole concentrations show an annual cycle, with more CO2 in the atmosphere in winter. However, as there is far more land mass and therefore vegetation in the Northern Hemisphere, the annual cycle is more pronounced at Mauna Loa.
Past changes in atmospheric greenhouse gas concentrations can be determined with very high confidence from polar ice cores. During the 800,000 years prior to 1750, atmospheric CO2 varied from 180 ppm during glacial (cold) up to 300 ppm during interglacial (warm) periods. Present-day (2011) concentrations of atmospheric carbon dioxide far exceed this range. The current rate of CO2 rise in atmospheric concentrations is unprecedented with respect to the highest resolution ice core records which cover the last 22,000 years.
Atmospheric oxygen is tightly coupled with the global carbon cycle. The burning of fossil fuels removes oxygen from the atmosphere. As a consequence of the burning of fossil fuels, atmospheric O2 levels have been observed to decrease slowly but steadily over the last 20 years (shown by the blue lines in the graph above). Compared to the atmospheric oxygen content of about 21% this decrease is very small and has no impact on health; however, it provides independent evidence that the rise in CO2 must be due to an oxidation process, that is, fossil fuel combustion and/or organic carbon oxidation, and is not caused by volcanic emissions or a warming ocean releasing carbon dioxide (CO2 is less soluble in warm water than cold).
Having recognised that industrial and agricultural emissions of greenhouse gases have been changing the climate, some people have been thinking about ways in which we could manipulate the Earth’s radiation budget to try and reduce the rate at which the climate is changing. Ideas have ranged from putting shiny material into the outer atmosphere to increasing the productivity of tiny shell building animals in the oceans.
One idea, from a group at Edinburgh University, is to have a fleet of wind powered ships, pumping salt water into the atmosphere (Figure 2). If you watched the ‘cloud in a bottle’ demonstration, you’ll know that cloud droplets need something to condense on to. By pumping salt water in to the atmosphere, you could, in theory, make more, smaller cloud droplets which reflect more of the Sun’s light, cooling the Earth. Concerns include whether, by making the planet cloudier in one place, there might be impacts on rainfall in other places.
Figure 2: A proposed ‘climate engineering’ solution: a wind powered ship spraying salt water into the atmosphere. Stephen Salter, Institute for Energy Systems, University of Edinburgh. Artist’s impression (© John MacNeill).
Figure 3. Time-series of global mean near-surface air temperature from 3 different climate research centres. The black line shows the dataset maintained by the Met Office Hadley Centre and Climate Research Unit at UEA. The grey shading shows the 95% confidence interval on this time-series. The red line shows the data from the US National Oceanographic and Atmospheric Administration (NOAA National Climate Data Center and the blue line shows data from NASA’s Goddard Institute for Space Studies. Data supplied by the Met Office © British Crown copyright 2013, the Met Office.)
Figure 3 shows global temperatures from 1850-2014. You can see that temperatures rose in the first half of the 20th century. This was in part due to the increase in the amount of energy we were getting from the Sun, in part due to the lack of major volcanic eruptions in that time and in part due to greenhouse gases, whose concentration was already increasing in the atmosphere.
Between 1950-1970, global temperatures didn’t change much. This was because, as well as greenhouse gases, industrial processes emit other pollutants, or ‘aerosols’ – such as sulphates. As well as reflecting sunlight themselves, these act as cloud condensation nuclei, making the planet cloudier and therefore cooler. In Europe at least, legislation to clean the air and reduce acid rain has reduced this factor.
Since 1970, the greenhouse gas related warming, together with associated feedback mechanisms such as the melting of polar ice has dominated, with global temperatures rising rapidly.
How does the energy balance of the Earth compare to other planets?
Many scientists have studied Mars and its atmosphere is well understood. Although it’s only around half the size of the Earth, with no oceans and a thin atmosphere composed almost entirely of CO2, it rotates at almost exactly the same rate as the Earth and with a similarly tilted axis. Mars’ weather is dominated by dust storms, the carbon and water cycles and thermal tides driven by the movement of the Sun’s light around the planet.
In the Martian winter, temperatures can be low enough (-133 to -128°C) for CO2 to condense, forming snow and cloud around the pole. This doesn’t have much of an effect on the latent heat fluxes, but does affect the planet’s albedo.
Unlike in the Earth’s atmosphere, very little incoming or outgoing radiation is absorbed by the Martian atmosphere. Mars has much more CO2 than the Earth, but hardly any other greenhouse gases, meaning that whilst one wavelength band is almost entirely absorbed by the atmosphere, the rest of Mars’ heat escapes to space. Mars’s greenhouse effect only warms the planet’s surface by 5 degrees Celsius. This means that, when there isn’t a dust storm blowing at least, Mars’ atmosphere doesn’t have a big effect on the planet’s energy budget.
Every 3-5 years though, things change significantly when there is a major dust storm. Up to 78% of the Sun’s radiation can be reflected or absorbed and never reaches Mars’ surface, leaving the sky a reddish brown. The absorbed solar radiation is emitted as heat in the atmosphere, producing a sort of anti-greenhouse effect, warming the atmosphere but cooling the surface.
Update: 13.00 (BST) Monday 27 June
We have added the x-axis to the graph in figure 1.